Nanotechnology/Print version

The Opensource Handbook of Nanoscience and Nanotechnology

Part 1: Introduction

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Introduction to Nanotechnology

Nanotechnology, often shortened to "nanotech," is the study of the control of matter on an atomic and molecular scale. Generally, nanotechnology deals with structures of the size 100 nanometers or smaller in at least one dimension, and involves developing materials or devices within that size. Nanotechnology is very diverse, encompassing numerous fields in the natural sciences.

There has been much debate on the future implications of nanotechnology. Nanotechnology has the potential to create many new materials and devices with a vast range of applications, such as in medicine, electronics and energy production. On the other hand, nanotechnology raises many of the same issues as with any introduction of new technology, including concerns about the toxicity and environmental impact of nanomaterials, and their potential effects on global economics, as well as speculation about various doomsday scenarios. These concerns have led to a debate among advocacy groups and governments on whether special regulation of nanotechnology is warranted.

This open source handbook on nanoscience and nanotechnology is divided into the following chapters, each dealing with a particular facet of nanotechnology:

• Chapter 1: INTRODUCTION - A general overview.
• Chapter 2: SEEING 'NANO' - How to observe what is going on on the nanoscale.
• Chapter 3: PHYSICS -ON THE NANOSCALE - How do things move and what are the important forces on the nanoscale.
• Chapter 4: NANOMATERIALS - The typical nanostructures and their functionality
• Chapter 5: NANOSYSTEMS - Fundamental electronic, optical, mechanical, and fluidic systems that can be made with nanostructures.
• Chapter 6: NANOENGINEERING - How to make nanostructures integrated into useful devices.
• Chapter 7: NANO-BIO PRIMER - An important part of nanotechnology is to interface with living organisms.
• Chapter 8: ENVIRONMENTAL NANOTECHNOLOGY - The environmental issues regarding this emerging technology.
• Chapter 9: NANO AND SOCIETY - Should we worry about nanotechnology?

References

See also notes on editing this book Nanotechnology/About#How_to_contribute.

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Perspective

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A perspective on Nanotechnology

Nanotechnology in the Middle Ages?

The Duke TIP eStudies Nanotechnology course will be adding more to this section (this will be completed by 22 Jun 08)

One of the first uses of nanotechnology was in the Middle Ages. It was done by using gold nanoparticles to make red pigments in stained glass showing that nanotechnology has been around for centuries. The gold when clumped together appears gold, but certain sized particles when spread out appear different colors. Reference: The Nanotech Pioneers Where are they taking us? By Steven A Edwards

In the year 1974 at the Tokyo Science University, Professor Norio Taniigrichi came up with the term nanotechnology.

Nanotechnology was first used to describe the extension of traditional silicon machining down into regions smaller than one micron (one millionth of a meter) by Tokyo Science University Professor Norio Taniguchi in 1974. It is now commonly used to describe the engineering and fabrication of objects with features smaller than 100 nanometers (one tenth of a micron).

Nanotechnology has been used for thousands of years, although people did not know what they were doing. For example, stained glass was the product of nanofabrication of gold. Medieval forgers were the first nanotecnologists in a sense, because they, by accident, found out a way to make stained glass.

Reference Nanotechnology A GENTLE INTRODUCTION TO THE NEXT BIG IDEA By Mark Ratner & Daniel Ratner

In 2001, the federal government announced the National Nanotechnology Intiative to coordinate the work of different U.S. agencies and to provide funds for research and accelerate development in nanotechnology. This was spearheaded by Mahail Roco and supported by both president Clinton and Bush.

References The Nanotech Pioneers Where are they taking us? By Steven A. Edwards http://www.nano.gov/html/about/docs/20070521NNI_Industrial_Nano_Impact_NSTI_Carim.pdf

A Vision

Richard Feynman was a man of great importance to the field of nanotechnology. He was a man with a vision. He believed that with research we could change things on a small scale. In his famous speech There's Plenty of Room at the Bottom in 1959, Richard Feynman discussed the possibility of manipulating and controlling things on a molecular scale in order to achieve electronic and mechanical systems with atomic sized components. He concluded that the development of technologies to construct such small systems would be interdisciplinary, combining fields such as physics, chemistry and biology, and would offer a new world of possibilities that could radically change the technology around us.

Miniaturization

A few years later, in 1965, Moore noted that the number of transistors on a chip had roughly doubled every other year since 1959, and predicted that the trend was likely to hold as each new generation of microsystems would help to develop the next generation at lower prices and with smaller components. To date, the semiconductor industry has been able to fulfill Moore's Law, in part through the reduction of lateral feature sizes on silicon chips from around 10 micrometers in 1965 to 45-65 nm in 2007 via changing from the use of optical contact lithography to deep ultraviolet projection lithography.

In 1974 in Japan, Norio Taniguchi coined the word "nano-technology" to describe semiconductor processes such as thin film deposition and ion beam milling exhibiting characteristic control on the order of a nanometer: "‘Nano-technology’ mainly consists of the processing of separation, consolidation, and deformation of materials by one atom or one molecule."

Since Feynman's 1959 speech the arts of "seeing" and "manipulation" at the nanoscale have progressed from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to various forms of scanning probe microscopy including scanning tunneling microscopy (STM) developed by Binnig and Rohrer at IBM Zurich and atomic force microscopy (AFM) devloped by (Binnig and Quate?) The STM, in particular, is capable of single-atom manipulation on conducting surfaces and has been used to build "quantum corrals" of atoms in which quantum mechanical wave function phenomena can be discerned. These atomic-scale manipulation capabilities prompt thoughts of building up complex atomic structures via manipulation rather than traditional stochastic chemistry. (Note: this pragraph is still rough and references are needed.)

Motivated by Feynman’s beliefs building things nanoscale top-down, Eric Drexler devoted much of his research to making a universal assembler. The American engineer Eric Drexler has speculated extensively about the laboratory synthesis of machines at the molecular level via manipulation techniques, emulating biochemistry and producing components much smaller than any microprocessor via techniques which have been called molecular nanotechnology or MNT.

Successful realization of the MNT dream would comprise a collection of technologies which are not currently practical, and the dream has resulted in considerable hyperbolic description of the resulting capabilities. While realization of these capabilities would be a vindication of the hype associated with MNT, concrete plans for anything other than computer modeling of finished structures are scant. Somehow, a means has to be found for MNT design evolution at the nanoscale which mimics the process of biological evolution at the molecular scale. Biological evolution proceeds by random variation in ensemble averages of organisms combined with culling of the less-successful variants and reproduction of the more-successful variants, and macroscale engineering design also proceeds by a process of design evolution from simplicity to complexity as set forth somewhat satirically by John Gall: "A complex system that works is invariably found to have evolved from a simple system that worked. . . . A complex system designed from scratch never works and can not be patched up to make it work. You have to start over, beginning with a system that works." A breakthrough in MNT is needed which proceeds from the simple atomic ensembles which can be built with, e.g., an STM to complex MNT systems via a process of design evolution. A handicap in this process is the difficulty of seeing and manipulation at the nanoscale compared to the macroscale which makes deterministic selection of successful trials difficult; in contrast biological evolution proceeds via action of what Richard Dawkins has called the "blind watchmaker" comprising random molecular variation and deterministic survival/death.

Technological development and limits

The impact on society and our lives of the continuous downscaling of systems is profound, and continues to open up new frontiers and possibilities. However, no exponential growth can continue forever, and the semiconductor industry will eventually reach the atomic limit for downsizing the transistor. Atoms in solid matter are typically one or two hundred picometers apart so nanotechnology involves manipulating individual structures which are between ten and ten thousand atoms across; for example, the gate length of a 45 nm transistor is about 180 silicon atoms long. Such very small structures are vulnerable to molecular level damage by cosmic rays, thermal activity, and so forth. The way in which they are assembled, designed and used is different from prior microelectronics.

New ways

Today, as that limit still seems to be some 20 years in the future, the growth is beginning to take new directions, indicating that the atomic limit might not be the limiting factor for technological development in the future, because systems are becoming more diverse and because new effects appear when the systems become so small that quantum effects dominate. The semiconductor devices show an increased diversification, dividing for instance processors into very different systems such as those for cheap disposable chips, low power consumption portable devices, or high processing power devices. Microfabrication is also merging with other branches of science to include for instance chemical and optical micro systems. In addition, microbiology and biochemistry are becoming important for applications of all the developing methods. This diversity seems to be increasing on all levels in technology and many of these cross-disciplinary developments are linked to nanotechnology.

Diversification

As the components become so small that quantum effects become important, the diversity will probably further increase as completely new devices and possibilities begin to open up that are not possible with the bulk materials of today's technology.

The nanorevolution?

The visions of Feynman are today shared by many others: when nanotechnology is seen as a general cross disciplinary technology, it has the potential to create a coming "industrial" revolution that will have a major impact on society and everyday life, comparable to or exceeding the impact of electricity and information technology.

Nanocomponents, Tools, and Methods

A positive spiral

As an emerging technology, the methods and components of nanotechnology are under continuous development and each generation is providing a better foundation for the following generation.

Seeing 'nano'

With regards to the methods, the Scanning tunneling microscope (STM) and Atomic Force Microscope (AFM) were developed in the 1980s and opened up completely new ways to investigate nanoscale materials. An important aspect was the novel possibility to directly manipulate nanoscale objects. Transmission and scanning electron microscopes (TEM and SEM) had been available since the 30s, and offered the possibility to image as well as create nanodevices by electron beam lithography.

New nanomaterials

Several unique nanoscale structures were also discovered around 1990: the Carbon-60 molecule and later the carbon nanotubes. In recent years, more complex nanostructures such as semiconductor nanowire heterostructures have also proven to be useful building blocks or components in nanodevices.

So what can I use this 'nano' for?

The applications of such nanocomponents span all aspects of technology: Electronics, optics/photonics, medical, and biochemical, as well as better and smarter materials. But to date few real products are available with nanoscale components, apart from traditional nanoscale products, such as paint with nanoparticles or catalytic particles for chemical reactors.

Prototype devices have been created from individual nanocomponents, but actual production is still on the verge. As when integrated electronics were developed, nanotechnology is currently in the phase where component production methods, characterization methods, tools for manipulation and integration are evolving by mutual support and convergence.

Difficult nanointegration

A main problem is reliable integration of the nanoscale components into microsystems, since the production methods are often not compatible. For fabrication of devices with integrated nanocomponents, the optimal manipulation technique is of course to have the individual components self-assembling or growing into the required complex systems. Self assembly of devices in liquids is an expanding field within nanotechnology but usually requires the components to be covered in various surfactants, which usually also influence the component properties. To avoid surface treatments, nanotubes and whiskers/wires can be grown on chips and microsystems directly from pre-patterned catalytic particles. Although promising for future large scale production of devices, few working devices have been made by the method to date.

The prevailing integration technique for nanowire/tube systems seems to be electron beam lithography (EBL) of metal structures onto substrates with randomly positioned nanowires deposited from liquid dispersions. By using flow alignment or electrical fields, the wire deposition from liquids can be controlled to some extent. The EBL method has allowed for systematic investigations of nanowires' and tubes' electrical properties, and creation of high performance electronic components such as field-effect transistors and chemical sensors. These proof-of-principle devices are some of the few but important demonstrations of devices nanotechnology might offer. In addition, nanomechanical structures have also recently been demonstrated, such as a rotational actuator with a carbon nanotube axis built by Fennimore et al.

A more active approach to creating nanowire structures is to use Scanning probe microscopy(SPM) to push, slide and roll the nanostructures across surfaces. SPM manipulation has been used to create and study nanotube junctions and properties. The ability to manipulate individual nanoscale objects has hence proven very useful for building proof-of-principle devices and prototypes, as well as for characterizing and testing components.

Top-down manufacturing takes bulky products and shrinks them to the nano scale, vs. bottom-up manufacturing is when individual molecules are placed in a specific order to make a product. The bottom-up self-assembly method may be important for future large scale production as well as many of the different approaches to improve the top-down lithographic processes. Such techniques could hence become important factors in the self-sustaining development of nanotechnology.

Hot and hyped

Suddenly everything is 'nano'

There's no question that the field of nanotechnology has quite a sense of hype to it - many universities have created new nanotech departments and courses. But there is also a vision behind the hype and emerging results - which are truly very few in industrial production, but nevertheless hold promise for a bright future. In the hype, many things that were once chemistry, microtechnology, optics, mesoscopic or cluster physics, have been reborn as nanotechnology.

Nanotech is old

You can find nanotechnology in the sunscreen you use in the summer, and some paints and coatings can also be called nanotech since they all contain nanoparticles with unique optical properties. In a way, nanoparticles have been known in optics for hundreds of years if you like to take a broad perspective on things, since they have been used to stain and color glasses, etc. since the middle ages. Nano-size particles of gold were used to create red pigments.

Catalysis is a major industrial process, without which not many of the materials we have around us today would be possible to make, and catalysis is often highly dependent on nanoscale catalytic particles. In this way thousands of tons of nanotechnology have been used with great benefit for years.

Nanoscale wires and tubes have only recently really been given attention with the advent of carbon nanotubes and semiconductor nanowires, while nanoscale films are ever present in antireflection coatings on your glasses and binoculars, and thin metal films have been used for sensitive detection with surface plasmons for decades. Surface plasmons are excitations of the charges at a surface. Nanowires actually were observed in the middle ages - well, they did not have the means to observe them, but saw whiskers grow from melted metals.

The better control over the nanostructure of materials has led to optimization of all these phenomena - and the emergence of many new methods and possibilities.

An example

Take for instance nano-optics: The surface plasmons turn out to be very efficient at enhancing local electrical fields and work as a local amplifier for optical fields, making a laser seem much more powerful to atoms in the vicinity of the surface plasmon. From this comes the surface enhanced raman spectroscopy which is increasingly used today because it makes it possible to do sensitive raman spectroscopy on the large majority of samples that would otherwise be impossible to make such spectra on. In addition, photonic crystals, fancy new quantum light sources that can make single photons on demand and other non-classical photon states are being developed, based on nanotechnology.

The future

There are definitely future scientific applications and commercial potential of all these new methods to handle light, use it for extremely sensitive detection and control its interaction with matter - and so it seems nanotechnology, being about making smaller versions of existing technology as well as new technology, is worth a bit of hype.

References

See also notes on editing this book Nanotechnology/About#How_to_contribute. .

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Overviews

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Internet Resources

Handbooks and Encyclopedias

These are only accessible for subscribers (which is one reason this Wikibook on Nanotechnology was started):

• Encyclopedia of Nanoscience and Nanotechnology, 10-Volume Set
• Handbook of Theoretical and Computational Nanotechnology, 10-Volume Set
• Springer Handbook of Nanotechnology

Websites and newsletters

• International Council on Nanotechnology (ICON) a multi-stakeholder group dedicated to the safe, responsible and beneficial development of nanotechnology. ICON serves as an aggregator for news and research related to nanotechnology environment, health and safety.
• ICON Virtual Journal of Nanotechnology Environment, Health and Safety compiles papers in peer-reviewed journals that address EHS issues in nanotechnology.
• Virtual journal of nanotechnology compiles nano-related papers in peer reviewed journals that do not specialize solely in nanotechnology.
• ACS Nanotation
• Nanotechweb.org
• Nanoforum
• Nanowerk
• nanoRISK
• Physorg.com
• Foresight Nanotech Institute
• Center for Responsible Nanotechnology
• A-Z of nanotechnology
• Google Directory - Nanotechnology (sourced from the Open Directory Project)
• Scientific American Nanotechnology Page
• NanoEd Resource Portal managed by the National Center for Learning and Teaching in Nanoscale Science and Engineering (NCLT)
• NanoHub.org
• UnderstandingNano

Search engines

There are many ways to find information in scientific literature and some that even specialize in nanotechnology. Apart from the free search engines and useful tools such as Google scholar and Google Desktop, there are several more dedicated commercial services:

• ICON Virtual Journal of Nanotechnology Environment, Health and Safety (VJ-NanoEHS) compiles papers in peer-reviewed journals that address EHS issues in nanotechnology.
• ISI - Web of Science Database contains peer reviewed journals, their references and citations. It also has very useful tools such as 'find related papers' that searches for papers sharing the same references as the entry you're looking at. This is the database behind the compilation of the journal citation factors.
• Knovel - Online Handbook Collection and Database is an extensive collection of handbooks and tables.
• PROLA - the Physical Review Online Archive searches Physical Review journals.
• Rubber Bible Online is a physical chemistry handbook which contains tables of physical and chemical data.
• Spin AIP Scitation searches related journals.
• Web of Science by ISI generates the impact factors (see journals below).
• Virtual Journal of Nanotechnology collects nanotech related papers from non-nano specialized journals.
• Derwent Patent database

Peer reviewed Journals

Overview of the nanotechnology related journals and their impact factors (2007 values):

Nanotechnology Related Journals

Name	Web	Impact Factor	ISSN	Comments
ACS Nano	[9]	N/A	1936-0851	general nanotech journal
Advanced Functional Materials	[10]	7.5	1616-301X	?
Advanced Materials	[11]	8.2	0935-9648	?
American Journal of Physics (AJP)	[12]	0.9	0002-9505	?
Applied Physics A: Materials Science & Processing	[13]	1.9	0947-8396	?
Applied Physics Letters (APL)	[14]	3.6	?	?
AZojono - Journal of Nanotechnology Online	[15]	N/A	?	Free access journal
Chemical Reviews	[16]	22.8	0009-2665	?
Current Nanoscience	[17]	2.8	1573-4137	Reviews and original research reports
Fullerenes, Nanotubes, and Carbon Nanostructures	[18]	0.5	1536-383x	all areas of fullerene research
IEEE Transactions on Nanotechnology	[19]	2.1	1536-125X	physical basis and engineering applications of nanotechnology
International Journal of Nanomedicine	[20]	N/A	1176-9114	?
International Journal of Nanoscience	[21]	N/A	0219-581X	New nanotech journal (Feb 2002)
Japanese Journal of Applied Physics	[22]	1.2	1347-4065	?
Journal of Applied Physics	[23]	2.2	?	?
Journal of Biomedical Nanotechnology	[]	N/A	?	JBN is a peer-reviewed multidisciplinary journal providing broad coverage in all research areas focused on the applications of nanotechnology in medicine, drug delivery systems, infectious disease, biomedical sciences, biotechnology, and all other related fields of life sciences.
Journal of Experimental Nanoscience	[24]	N/A	1745-8080	New nanotech journal(March 2006)
Journal Of Microlithography Microfabrication And Microsystems	[25]	N/A	1537-1646
Journal of Micromechanics and Microengineering	[26]	1.9	0960-1317	?
Journal of Nano Research	[27]	N/A	1661-9897
Journal of Nanomaterials	[28]	N/A	?	science and applications of nanoscale and nanostructured materials
Journal of Nanoparticle Research	[29]	2.3	1388-0764	?
Journal of Nanoscience and Nanotechnology	[30]	2.0	?	JNN is a multidisciplinary peer-reviewed journal covering fundamental and applied research in all disciplines of science, engineering and medicine. JNN publishes all aspects of nanoscale science and technology dealing with materials synthesis, processing, nanofabrication, nanoprobes, spectroscopy, properties, biological systems, nanostructures, theory and computation, nanoelectronics, nano-optics, nano-mechanics, nanodevices, nanobiotechnology, nanomedicine, nanotoxicology.
Journal of Physical Chemistry A	[31]	2.9	?	?
Journal of Physical Chemistry B	[32]	4.1	?	?
Journal of Physical Chemistry C	[33]	N/A	?	Nanomaterials and Interfaces, Nanoparticles and Nanostructures, Surfaces, Interfaces, Catalysis, Electron Transport, Optical and Electronic Devices, Energy Conversion and Storage
Journal of the American Chemical Society (JACS)	[34]	7.9	?	Multidisciplinary chemistry journal
Journal of Vacuum Science & Technology A (JVSTA)	[35]	1.3	?	Vacuum, Surfaces, Films
Journal of Vacuum Science & Technology B (JVSTB)	[36]	1.4	?	Microelectronics and Nanometer Structures: Processing, Measurement, and Phenomena
Langmuir	[37]	4.0	?	Research in the fields of colloids, surfaces, and interfaces
Micron	[38]	1.7	?	Journal for Microscopy
Materials Chemistry and Physics	[39]	1.9	0254-0584	materials science, including nanomaterials and opto electronics
Materials Science and Engineering: C	[40]	1.3	0928-4931	Biomimetic and Supramolecular Systems
Materials Science and Engineering: R: Reports	[41]	17.7	0927-796X	Invited review papers covering the full spectrum of materials science and engineering
Materials Today	[42]	N/A	1369-7021	materials science and technology
Microfluidics and Nanofluidics	[43]	2.2	1613-4982	all aspects of microfluidics, nanofluidics, and lab-on-a-chip science and technology
Microscopy Research and Technique	[44]	1.6	?	?
Nano	[45]	N/A	1793-2920	New nanotech journal (July 2006)
Nano Letters	[46]	9.6	?	General nanotechnology journal
Nanomedicine	[47]	2.8	?	?
Nanopages	[48]	N/A	1787-4033	Since sept 2006.
Nano Research	[49]	N/A	?	First issue july 2008
Nano Research Letters	[50]	wait	1931-7573	articles with open access
Nanotechnology	[51]	3.3	?	Journal specializing in nanotechnology
NanoToday	[52]	N/A	?	Is this peer reviewed or more a news/reviews journal?
Nature	[53]	31.434	?	One of the major journals in science
Nature Biotechnology	[54]	22.8	?	advances in life sciences
Nature Materials	[55]	19.8	?	covers a range of topics within materials science
Nature Methods	[56]	15.5	?	tried-and-tested techniques in the life sciences and related area of chemistry
Nature Nanotechnology	[57]	14.9	?	mix of news, reviews, and research papers
Nanotoxicology	[58]	N/A	1743-5404	Research relating to the potential for human and environmental exposure, hazard and risk associated with the use and development of nano-structured materials
Open Nanoscience Journal	[59]	N/A	1874-1401	Open access journal with research articles, reviews and letters.
Physical Review Letters (PRL)	[60]	6.9	?	One of the top physics journals
PLoS Biology	[61]	13.5	1544-9173	Peer reviewed open access bio journal
PLoS ONE	[62]	N/A	?	Peer reviewed open access science journal
Proceedings of the National Academy of Sciences(PNAS)	[63]	10.2	?	multidisciplinary scientific serial: biological, physical, and social sciences.
Recent Patents on Nanotechnology	[64]	N/A	1872-2105	?
Science	[65]	26.4	?	One of the major journals in science
Solid-State Electronics	[66]	1.3	?	?
Small Journal	[67]	6.4	1613-6810	New nanotech journal
Smart Materials and Structures	[68]	1.5	0964-1726	since 1992
Thin Solid Films	[69]	1.7	0040-6090	Thin-film synthesis, characterization, and applications.
Ultramicroscopy	[70]	2.0	?	Microscopy related research.
Virtual Journal of Nanotechnlogy	[71]	N/A	1553-9644	Collecting nanotech related papers from non-nano spcialized journals

• Impact factors are only guides to how much a papers is referenced in the years just after publication.
• Please add comments about the journals and update impact factors!
• The Physics and Astronomy Classification Scheme PACS2006 and Nanoscale Science and Technology - Collection of Applicable Terms from PACS 2006

Conferences

• NSTI Nanotech 2008
• TNT - Trends in Nanotechnology 2007;2006
• MNE - Micro and Nanoengineering 2007;2008
• Virtual Conference on Nanoscale Science and Technology
• Foresight Unconference Vision Weekend NanoBioInfoCognoSocioPhysical technologies
• [72]
• International Microprocess and Nanotechnology Conference, Japan
• Nanosafe 2008: International Conference on Safe production and use of nanomaterials

Nanotech Products

Please add more products, comments and more info about the products if you have any!

See also the List of nanotechnology applications in wikipedia

Woodrow Wilsom Center for International Scholars is starting a Project on Emerging Nanotechnologies (website should be under construction at www.nanoproject.org) that among other things will try to map the available 'nano'products and work to ensure possible risks are minimized and benefits are realized.

Emerging products

• 2008 MultiProbe’s AFM Nanoprober is now qualified for 32nm technology nodes. [73]
• Intel will make products with 45 nm linewidth transistors available from 2008 [74]
• Batteries are increasingly incorporating nanostructures.
• Flexible, cheaper, or more luminous Flat screen displays
• Pressure-sensitive mobile devices [75]

Available in 2006

• Surface coatings: TCnano, Nanocover, Stay clean.

Available in 2005

• Molybdenum disulfide catalytic nanoparticles in Brimm catalysts[76] made by Haldor Topsøe
• Forbes top ten nanoproducts in 2005[77]
  • Apples IPod with sub 100nm elements in its memory chips
  • Choleterol reducing nanoencapsulated oil,Shemen Industries Canola Active.
  • Nanocrystals improve the consistency of chocolate[78]
  • Zelen Fullerene C-60 Day Cream [79]
  • Easton Stealth CNT baseball bat
  • Nanotex textiles once again
  • ArcticShield polyester socks from ARC Outdoors with 19nm silver particles that kill fungs to reduce odor.
  • NanoGuard developed by Behr Process for improved paint hardness.
  • Pilkingtons self-cleaning 'Activ Glass'.
  • NanoBreeze Air Purifier from NanoTwin Technologies, where the UV light from a fluorescent tube cleans the air by photochemical reactions in nanoparticles.

Available in 2004

• Cold cathode carbon nanotube emitters for X-ray analysis by Oxford instruments[80][]
• Forbes has an overview in 2004 of what they consider the top ten nanotech products:
  • Footwarmers with nanporous aerogel for 3-20 times lighter than comparable insulating materials used in shoes (produced by Aspen Aerogels).
  • Matress covers with nanotex fibres that can be washed (Simmonos bedding company).
  • Better golf drivers with carbon nanotube enforced metal composites (produced by Maruman & Co) and nanocomposite containing golf balls (produced by NanoDynamics)
  • The company 'Bionova' apparently adds some nanoproducts to their 'personalized product line'.
  • EnviroSystems make a nanoemulsive disinfectant cleaner, called EcoTru, that is EPA Tox category 4 registered (meaning very safe to use)
  • EnviroSystems also make a spray-on version of this product.
  • BASF makes a nanoparticle coating for building materials called Mincor, that reduces their wettabililty.
  • A nanostructured coating produced by Valley View, called Clarity Defender, improves visibility through windscreens in rain. Another company, Nano-Film, makes a similar coating on sunglasses.
  • w:Flex-Power makes a gel containing nanoscale liposomes for soothing aching muscles
  • 3M espe Dental adhesive with silica nanoparticle filler.

Available in 2003

• NanoGuard Zink Oxide nanoparticles for sunscreens FDA approved
• Forbes 2003 top ten nanoproduct [81] includes:
  • High performance ski wax, Cerax Nanowax [82].
  • Nanotex textiles in ski jackets from Ziener[83]
  • Nanotex textiles
  • Plenitude Revitalift antiwrinkle cream by L'Oréal contains nanocapsules with vitamin A [84]
  • organic light-emitting diodes (OLEDs) in Sony camera flat screen display
  • Nanofilm coatings for ani-reflection and scratch resistance [85]
  • Zink oxide nanoparticles in Sunscreen by BASF [86]
  • carbon nanotube enforced tennis rackets [87] and nanopolymer enforced tennis balls [88]

Available in 2000

Nanotex makes textiles where the clothing fibres have been coating in nanoscale fibres to change the textile wettability. This makes the textile much more stain resistant.

Companies making nanotech research equipment

• MultiProbe Manufacturer of a 1-to-6 head Atomic Force nanoprobing tool used in failure analysis, that combines multi-scan fault isolation imaging with nanoprobing electrical capabilities. For process technology node measurements of 32nm, 45nm, 65nm, 90nm or larger.
• Veeco AFM and related equipment
• Zyvex nanomanipulation equipment
• Nanofactory in-situ TEM manipulation equipment
• SmarAct nanomanipulators
• Capres micro four point conductance measurement probes
• ImageMetrology SPIP software for SPM analysis
• QuantumWise software for simulating nanosystems
• [89] AFM and related equipment

Products that have been nanostructured for decades

• Catalysts
  • Haldor Topsøe
• Computer processesors are increasingly made of nanoscale systems
  • Intel

Non-nanotech products and a warning

Not everything that says nano is nano - and given the hype surrounding nanotechnology you will see an increasing number of 'nano' products that have nothing to do with it. It is worrying when sometimes problems arise with non-nano products and this adds to the 'scare' that is present in the public, fuelled by the newspapers where they are just waiting for a nice scandal... an example was the product Magic Nano from a German company that made a number of users sick when inhaling the aerosol cleaning product - which in the end turned out to have nothing 'nano' in it. There is good reason to be very alert to such issues. Not all countries have legislation in place to secure the consumers against the possible dangers present in nanoparticles and some products could end being marketed before having been tested well enough. Though this example turned out to be 'non-nano', we will probably meet new cases shortly that are truly 'nano'. On this background environmental and health aspects will be an important part of this book.

Suppliers

Nanomaterials

• Sigma-Aldrich
• Zyvex
• overview of companies making nanoparticles and related equipment

Nanolithography

• NIL Technology sells stamps for nanoimprint lithography (NIL) and provides imprint services.

Quantum Dots

• Evident technologies

A nano-timeline

Overview of some important events in nanotechnology

See also History of Nanotechnology in Wikipedia

A Nanotechnology Timeline

Year	Development
Medieval	Observation of metal whisker growth and nanoparticles used for staining glass
1900	Max Planck proposes energy quantization.
1905-30	Development of quantum mechanics
1927	Heisenberg formulated his uncertainty principle
1933	The first First electron microscope was built by Ernst Ruska
1952	First carbon nanotubes observation by Radushkevich and Lukyanovich
1953	DNA structure discovered by James D. Watson and Francis Crick
1959	Feynmanns talk There is plenty of room at the bottom
1965	Proposal of Moores Law
1981	Invention of STM by Gerd Binnig and Heinrich Rohrer
1985	Invention of AFM by Binnig, Quate and Gerber
1985	Buckyball discovery by Harry Kroto, Robert Curl, and Richard Smalley
1986	K. Eric Drexler publishes his book Engines of Creation, in which he discusses both the potential huge benefits and the potential dangers of nanotechnology. He talks about a future of nanotechnology defined by molecular manufacturing, where self-replicating nanobots/assemblers are engineered to carry out practical applications.
1989	Don Eigler pushed around xenon atoms to spell IBM	–	1991	Rediscovery of carbon nanotubes by Sumio Iijima

A nano-scale overview

Just to get a sense of proportion

A Nano-scale overview

Scale	typical elements
1 m	1 m is 1.000.000.000 nanometers ( 10^9 nm )
200 µm	About the size of the smallest letters you can write with a very very sharp pencil and a very very steady hand.
100 µm	Typical thick hair
10-1000 µm	Cells in living organisms can have many sizes, and neurons can be much longer. In frog embryos (Tadpoles) the initial embryo cells can be up to 1000µm.
8 µm	Red blood cell
1 µm	Bacteria
100 nm	Virus
5-100 nm	The range for nanotechnology systems built from atomic/molecular components (quantum dots, nanoparticles, diameter of nanotubes and nanowires, lipid membranes, nanopores...).
10 nm	Size of typical Antibody molecules in living organisms immune defence
6-10 nm	Thickness of a cell membrane, and typical pore size in membrane.
2.5 nm	The width of DNA (but it depends on the conditions)
1 nm	The size of a C60 buckyball molecule or glucose molecule.
0.3 nm	The size of a water molecule.
1 Å = 0.1 nm	Roughly the size of hydrogen atom.
0.7 Å = 70 pm	The best resolution in AFM achieved so far where they managed to image individual orbitals in an atom.

• Distances between objects can be measured with sub Å precision with STM, laser interferometry and its even done continuously in a standard airbag acceleration sensor chip that costs a few dollars and senses the vibrations of a micro-inertial mass element with femtometer precision (10^-15 m).

Bibliography

• G. Ali Mansoori, Principles of Nanotechnology, Molecular-Based Study of Condensed Matter in Small Systems, (New Jersey: World Scientific, 2006).
• Monthioux, Marc; Kuznetsov, Vladimir L. (2006). "Who should be given the credit for the discovery of carbon nanotubes?". Carbon 44. doi:10.1016/j.carbon.2006.03.019. Retrieved on 2007-07-26.

References

See also notes on editing this book Nanotechnology/About#How_to_contribute.

the nanotechnology pioneers by Steven A. Edwards
Engines of Creation 2.0: The Coming Era of Nanotechnology by K. Eric Drexler

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About the Book

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Vision

We hope to use the Wikibooks format to make an Open Source Handbook on Nanoscience and Nanotechnology, freely accessible for everyone, that can be updated continuously.

Wikipedia is growing fast and one of the most visited websites on the net – a valuable resource of information we all use.

In science and technology we often need more detailed information than what can be presented in a brief encyclopedic article – and here wikibooks.org, a sister project to Wikipedia, can help us with this newly started handbook.

Though the book is still in its infancy, it has been elected book of the month December 2006, and we hope this will provide PR and more people contributing to the project!

The plan to create the book:

1: First to create smaller articles to ‘cover’ the entire area of nanotechnology and achieve a well defined structure the book (some parts could be revised thoroughly in this process,for instance the materials chapter).

2: Once the structure is reasonably well defined, to begin refining the articles with in-depth material so we reach lecture-note level material.

3: Since everybody can contribute, a continuous contribution of material is expected and a backing group of editors is needed to maintain a trustworthy level of information.

An voluntary editorial board is being put together to oversee the book, support, contribute and follow its development.

Discussion about the content of the book can be found on the main talk page talk:Nanotechnology

As with Wikipedia, we hope to see a solid information resource continuously updated with open source material available for everyone!

Editing hints

References in Wikibooks

Add references whenever possible, with reference lists at the end of each page. Please try to make links to the articles with the DOI (digital object identifier) because that gives a uniform and structured access for everyone to the papers.

All papers get a DOI - a unique number like a bar code in a supermarket. All DOIs are registered by www.doi.org and in the reference list you can add links like https://doi.org/10.1039/b504435a so people will be able to find it no matter how the homepage of the journal or their own library changes.

The References section has an example reference.

Add links to the Wikipedia whenever possible - and for the beginning I will rely extensively on Wikipedia's pages on the subjects, simply referring to these. This textbook could be simply a gathering of Wikipedia pages, but an encyclopedia entry is brief, and for a handbook it is preferable to have more in-depth material with examples and the necessary formulas. So, some information in this textbook will be very much like the Wikipedia entries and we might not need to write it in the book but can simply refer to Wikipedia, but the hope is that this will be more a text book as is the intention with Wikibooks.

Multiple references, see w:Help:Footnotes

Links

There's a shorthand way to make links to Wikipedia from Wikibooks: [[w:Quantum_tunneling|Wikipedia on Quantum Tunneling]] gives the link Wikipedia on Quantum Tunneling.

History

The book was started by Kristian Molhave (wiki user page) 13. Apr. 2006. Initially it was named Nanowiki, and later changed to Nanotechnology. Kristian is currently slowly uploading material to the book and looking for people who would like to contribute that can and substantial material to specific sections under the GNU license. I hope we can make an 'editorial panel' of people each keeping an eye on and updating specific sections.

The Summer 2008 Duke Talent Identification Program (TIP) eStudies Nanotechnology students will be adding to the content of this Wikibook. From June-Aug 2008 there will be content additions with references that will add to this great resource.

Authors and Editors

• The Opensource Handbook on Nanoscience and Nanotechnology was started by Kristian Molhave(wiki user page)

Editors

• An editorial board is currently being organized.

Support and Acknowledgments

Starting this book is supported by the Danish Agency for Science, Technology and Innovation through Kristian Mølhave's talent project ’NAMIC’ No. 26-04-0258.

How to Reference this Book

I am not currently sure how work on wikibooks or wikipedia can be referenced reliably in published literature.

Three suggestions:

1) Reference the references from the wikibook. Wikibooks are not intended to be the publication channel for new results, but should be based on published and accepted information with references and these references can be used. But this of course does not give credit to the book, so I recommend then adding an acknowledgement about the book to give it PR and credit.

2) Reference the book with a specific page and date - the previous versions of the pages are all available in the history pane and can easily be accessed by future users. You can also hit "permanent version" on the left side of the webpage (it is under "toolbox"). That sends you specifically to the selected version of the wikipage with a link to it that will never change.

3) Reference the PDF version and its version number. Once the book achieves a reasonable level, PDF versions will become available for download and they will have a unique version number and can be retrieved.

Other suggestions are most welcome!

Edwards, Steven A.,The Nanotech Pioneers Christiana, USA: Wiley-VCH 2006, pg 2

Reaching Out

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Teaching Nanotechnology

Teachers' Toolbox is a Wikibook on teaching methods and ways to improve teaching. The toolbox is intended to give you an overview of methods you can use when teaching in general.

If you know of places that have teaching material available on the net, please add a link to the list below:

• Nanotechnology
• Nano-to-Macro Transport Processes, Fall 2004
• nanostructured materials and interfaces

Outreach projects

There are several nanotechnology related outreach projects. Here are some examples to give ideas:

• Nanoscale Science Education
• Citizens school of nanotechnology
• Webcast outreach lecture
• Exploring the nanoworld
• K-14 outreach
• Cornell Museum

Demonstration experiments

There is a dedicated section for nanotechnology in the Wikiboook on Science Show, which is a cross disciplinary collection of demonstration experiments. The is growing steadily and we will begin to add to the English version soon. Please add to these books with any demonstration experiments and ideas you have!

There are others available on the net:

• Nano-Tex: Testing New Nano Fabrics
• Nano kits and for instance the exercise moving a wall

References

See also notes on editing this book about how to add references Nanotechnology/About#How_to_contribute.

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Part 2: Seeing 'Nano'

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The eyes in nanotech

Without being able to 'see' the nanoscale objects, nanotechnology would be very difficult. In this part, the different microscope techniques are reviewed along with various spectroscopic and diffraction methods that can tell us more about the nanoscale structure of matter.

the electromagnetic spectrum

Visible light is only a part of the electromagnetic spectrum and useful information about different physical interactions in nanostructures can be acquired from the different parts of the electromagnetic spectrum.

Seeing 'nano' can be done in different ways, but not with the naked eye which normally cannot see things much smaller than 100µm (though a single atom can be seen if it lights up in a dark room). Instead of our eyes, we use various instruments to 'see' for us, and they 'see' different things depending on how they are made:

Microscopy

Microscopy uses microscopes to create an image of the specimen. The image is rarely an image as you see it with your eyes, but rather how some physical probe interacts differently with the specimen as function of position on it. The physical probe can be an AFM cantilever, a beam of light or electrons, or something completely different.

Overview of Microscopes

Overview of the different types of microscopes

Optical: The beam from a light source is focused onto a sample and either the transmitted or scattered light is collected by an objective lens and the image is magnified onto a camera or to the observer's eye. The resolution can be down to about 200 nm, and the microscopes can be fairly cheap, small and easy to use.

Transmission Electron Microscope (TEM): Electrons from a very bright electron source are directed to a very thin sample that is transparent to the high energy electrons (100-300 keV) and the electron beam is then magnified by electromagnetic lenses and sent onto a fluorescent screen or a camera to observe the image. The resolution can be less than 0.1 nm on expensive high-end instruments where even individual atoms can be imaged. The samples must be very thin (typically less than 200 nm) and the whole system must be under high vacuum.

Scanning Electron Microscope (SEM): A focused electron beam is scanned over a sample and the scattered electrons are detected. The detector current is used to give an image depending on the electron beam position on the sample. The resolution can be down to about 5 nm and the sample can be much larger than in the TEM because the electrons do not have to pass through the sample.

Scanning Probe Microscopes (SPM) move a very sharp probe across a sample in a raster pattern while recording how the probe interacts with the sample. The typical SPMs are the AFM, STM and SNOM:

Atomic Force Microscope (AFM): An almost atomically sharp tip is protruding from a cantilever and is scanned over the sample. When the cantilever deflects, a laser beam reflected off the backside of the cantilever will change directions and this will be measured by a photodetector. The laser position can be used to control the force between the tip and the sample, and the AFM is often used to measure both topography and forces on the nanoscale. The resolution is normally down to about 1 nm, but even subatomic resolution is possible. The AFM can work with both dry and wet, conducting and isolating samples.

Scanning Tunneling Microscope (STM): An atomically sharp tip is moved within atomic distance of a sample that has a voltage applied to it. When the tip-sample distance becomes so small that the electron clouds of the tip and sample touch, electrons can much more easily tunnel between the two and this gives rise to a tip-sample current (often a few pA at a 1V bias voltage). This current can be used to maintain a fixed tip-sample distance when the tip is scanned over the sample, and this can give images of conducting surfaces with atomic resolution.

Scanning Near-field Optical Microscope (SNOM): As electrons can tunnel between electrical conductors in the STM, photons can tunnel between optical guiding structures. The SNOM uses a narrow light guide to measure how the optical electromagnetic field changes as the guide is moved across the sample. For instance, light can be sent from below the sample and then scattered into the scanning light guide above it. The resolution can be much smaller than the wavelength of light.

Point-Projection Microscopes: The Field Emission Microscope (FEM), Field Ion Microscope (FIM) and the atom probe are examples of point-projection microscopes where ions are excited from a needle-shaped specimen and hit a detector. The Atom-Probe Tomograph (APT) is the most modern incarnation and allows a three-dimensional atom-by-atom (with chemical elements identified) reconstruction with sub-nanometer resolution.

Spectroscopy

Spectroscopy uses spectrometers to tell how radiation interacts with the specimen as function of the energy/wavelength of the radiation

Diffraction

Diffraction uses radiation to observe how it is scattered in different directions from the specimen. This can be used to tell about the order of the atoms in the sample.

Surface analysis

Many of these methods are used for 'macroscopic' surface analysis where the outmost nanometers of a material is being studied over larger areas. The methods can be combined with microscopes to give spectrometrical information from a well defined location on the sample - for instance when doing diffraction measurements in a TEM or level spectroscopy in an STM on a single atom.

References

See also notes on editing this book about how to add references Nanotechnology/About#How_to_contribute.

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Optical Methods

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Optical Microscopy

The Abbe diffraction limit

Observation of sub-wavelength structures with microscopes is difficult because of the Abbe diffraction limit. Ernst Abbe found in 1873 that light with wavelength λ,travelling in a medium with refractive index n and converging to a spot with angle φ will make a spot with radius

$d={\frac {0.61\lambda }{nsin\phi }}$

The denominator nsinφ is called the numerical aperture (NA) and can reach about 1.4 in modern optics, hence the Abbe limit is roughly d=λ/2. With green light around 500nm the Abbe limit is 250nm which is large compared to most nanostructures or biological cells with sizes on the order of 1μm and their internal organelles being much smaller. To increase the resolution, shorter wavelengths can be used such as UV and X-ray microscopes. These techniques offer splendid resolution but are expensive, suffer from lack of contrast in biological samples, and also tend to damage the sample.

Resources

• Microscope optics
• Microscopy Primer
• Olympus Interactive Java Tutorials on Microscopy
• Nikon MicroscopyU with interactive tutorials on microscopy techniques

The optical microscope

Sketch of an optical microscope

Bright Field

The light is sent to the sample in the same directions as you are looking - most things will look bright unless they absorb the light.

Dark Field

Light is sent towards the sample at an angle to your viewing direction and you only see light that is scattered. This makes most images appear dark and only edges and curved surfaces will light up.

Laser Scanning Confocal Microscopy (LSCM)

Confocal laser scanning microscopy is a technique that allows a much better resolution from optical microscopes and three dimensional imaging. A review can be found in Paddock, Biotechniques 1999

Using a high NA objective also gives a very shallow depth of focus and hence the image will be blurred by structures above or below the focus point in a classical microscope. A way to circumvent this problem is the confocal microscope, or even better the Laser Scanning Confocal Microscope (LSCM). Using a laser as the light source gives better control of the illumintaion, especially when using fluorescent markers in the sample. The theoretical resolution using a 1.4 NA objective can reach 140nm laterally and 230nm vertically while the resolution quoted in ref is 0.5×0.5×1μm. The image in the LSCM is made by scanning the sample in 2D or 3D and recordning the signal for each point in space on a PC which then generates the image.

X-ray microscopy

X-ray microscopy uses X-rays to image with much shorter wavelength than optical light, and hence can provide much higher spatial resolution and use different contrast mechanisms. X-ray microscopy allows the characterization of materials with submicron resolution approaching the 10's of nanometers. X-ray microscopes can use both laboratory x-ray sources and synchrotron radiation from electron accelerators. X-ray microscopes using synchrotron radiation provide the greatest sensitivity and power, but are unfortunately rather large and expensive. X-ray microscopy is usually divided into two overlapping ranges, referred to as soft x-ray microscopy (100eV - 2keV) and hard x-ray microscopy (1keV-40keV). All x-rays penetrate materials, more for higher energy x-rays. Hence, soft x-ray microscopy provides the best contrast for small samples. Hard x-rays do have the ability to pass nearly unhindered through objects like your body, and hence also give rather poor contrast in many of the biological samples you would like to observe with the x-ray microscope. Nevertheless, hard x-ray microscopy allows imaging by phase contrast, or using scanning probe x-ray microscopy, by using detection of fluorescent or scattered x-rays. Despite its limitations, X-ray microscopy is a powerful technique and in some cases can provide characterization of materials or samples that cannot be done by any other means.

Infrared spectrometry (FTIR)

vIdentification of the functional groups present in a nanomaterial is a frequent requirement in nanoscience and nanotechnology research. Among other tools, FT-IR has found much popularity among researches due to its versatility, relative ease of use and ability to use as a quantification tool.

Atoms in a chemical bonds constantly vibrate. This vibration can be analogue to a system with two masses attached to a spring. The vibration frequency depend upon the weight of the masses and the spring constant of the connecting spring. In the same way, depending on the masses of the atoms that contributes to a bond and cohesiveness of the bond, frequency differ. Since bonds have atoms with different shapes and sizes and different strength, each combination of atoms in an each type of bond has a unique harmonic frequency. This natural frequency lies in the range of infrared region and therefore a spectroscopic method that use IR can be devised to analyze bond vibrations.

When the IR radiation with the same harmonic frequency of the bond shines upon the bond. The bond vibration is amplified by increased transfer of energy from the IR radiation. When range of IR frequencies given to the material, it only absorb IR frequencies that corresponds to the natural frequencies of the bonds that exist in the sample. Others are not absorbed and can be analyzed using an Infrared spectrometer, which tells you the frequencies that are absorbed by the sample. This provides important information about the functional groups present in the sample. This is exactly what FT-IR does.

As FT-IR can be used to get information about functional groups present in nanomaterials. This is particularly useful in cases such as when one attempts to surface modify nanomaterials to increase affinity, reactivity or compatibility. Analyzing the FT-IR of a nanomaterial would tell you what groups present and then appropriate surface modification strategy be decided based on the groups present. Further, it can also be useful in characterizing the surface modification has taken place, as new groups should emerge if the reaction is successful.Identification of the functional groups present in a nanomaterial is a frequent requirement in nanoscience and nanotechnology research. Among other tools, FT-IR has found much popularity among researches due to its versatility, relative ease of use and ability to use as a quantification tool.

Atoms in a chemical bonds constantly vibrate. This vibration can be analogue to a system with two masses attached to a spring. The vibration frequency depend upon the weight of the masses and the spring constant of the connecting spring. In the same way, depending on the masses of the atoms that contributes to a bond and cohesiveness of the bond, frequency differ. Since bonds have atoms with different shapes and sizes and different strength, each combination of atoms in an each type of bond has a unique harmonic frequency. This natural frequency lies in the range of infrared region and therefore a spectroscopic method that use IR can be devised to analyze bond vibrations.

When the IR radiation with the same harmonic frequency of the bond shines upon the bond. The bond vibration is amplified by increased transfer of energy from the IR radiation. When range of IR frequencies given to the material, it only absorb IR frequencies that corresponds to the natural frequencies of the bonds that exist in the sample. Others are not absorbed and can be analyzed using an Infrared spectrometer, which tells you the frequencies that are absorbed by the sample. This provides important information about the functional groups present in the sample. This is exactly what FT-IR does.

As FT-IR can be used to get information about functional groups present in nanomaterials. This is particularly useful in cases such as when one attempts to surface modify nanomaterials to increase affinity, reactivity or compatibility. Analyzing the FT-IR of a nanomaterial would tell you what groups present and then appropriate surface modification strategy be decided based on the groups present. Further, it can also be useful in characterizing the surface modification has taken place, as new groups should emerge if the reaction is successful.

Raman Spectroscopy

• Raman scattering
• Resonance Raman spectroscopy

Surface Enhanced Raman Spectroscopy (SERS)

• Surface Enhanced Raman Spectroscopy (SERS)
• Surface-enhanced Raman spectroscopy: a brief retrospective

References

See also notes on editing this book Nanotechnology/About#How_to_contribute.

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Electron Microscopy

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Electron microscopy

An overview:

Electron microscopes uses electrons instead of photons, because electrons have a much shorter wavelength than photons and so allows you to observe matter with atomic resolution.

There are two general types of electron microscopes: the Scanning Electron Microscope (SEM) that scans an electron beam over the surface of an object and measures how many electrons are scattered back, and the Transmission Electron Microscope (TEM) that shoots electrons through the sample and measures how the electron beam changes because it is scattered in the sample.

A very simple sketch of a Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) compared to an optical transmission microscope and a cathode ray tube (CRT) TV screen - both systems have many things on common with the electron microscope. The optical microscope uses lenses to control the lights pathway through the system and is in many ways built up like a TEM - only the TEM uses electromagnetic lenses to direct the beam of electrons. The CRT uses electromagnetic lenses as the TEM and SEM to control the electron beam, and generates an image for the viewer by scanning the beam over a fluorescent screen - in the same way the a SEM generates an image by scanning the electron beam over a small sample.

Using electron beams however requires working in a vacuum environment, and this makes the instruments considerably larger and expensive. All electron microscope work under at least low pressures and usually in high vacuum chambers to avoid scattering the electrons in the gas. In environmental electron microscopes, differential pumping systems are used to actually have gasses present by the sample together with the electron beam.

Introduction to Electron Microscopy

For imaging of nanoscale objects, optical microscopy has limited resolution since the objects are often much smaller than the wavelength of light. The achievable resolution for a wavelength $\lambda$ is often given by the diffraction limit $d$ as

$d=0.61{\frac {\lambda }{(NA)}}$ (r.g., diffraction limit)

with numerical aperture $NA$, which can be approximated by the largest angle of incidence $\alpha$ of the wavefront towards the sample, $NA\approx \alpha$.

Since $\alpha \ll 1$ for the present purposes, we can approximate $\sin(\alpha )\approx \tan(\alpha )\approx \alpha$ and hence $NA\approx \alpha \approx r_{a}/r_{wd}$ where $r_{a}$ is the radius of the objective lens aperture and $r_{wd}$ the working distance.

Optical microscopes can often reach a resolution of about $d=200$ nm. For nanoscale resolution this is unfortunately not sufficient to distinguish for instance a single nanotube from two adhering to each other, since they have diameters of less than 100 nm.

The figure below gives an overview typical magnifications achievable by the different electron microscopes compared to a light microscope.

The different methods for microscopy cover a range of magnification roughly indicated by the bars in the figure. The resolution of optical microscopy is limited to about 200 nm. a) SEM image of the head of an ant facing a microfabricated chip with a pair of microfabricated grippers. The grippers are barely visibly at the tip of the arrow. b) SEM image of a gripper approaching a large bundle of carbon nanotubes. c) Closeup in SEM of the gripper and nanotubes. d) TEM image of a carbon nanotube suspended between two grippers. e) TEM closeup of the shells of carbon atoms in a carbon nanotube. On the nanometer scale this particular carbon nanotube does not show a well defined carbon shell structure.

Electron optical systems use electrical and magnetic fields to control the electron beam. Although the law of refraction in optics is exchanged with the Lorentz force in electrodynamics, the electron optical system has similar diffraction limits as optical systems, since they depend on the wave nature of the electron beam.

One can achieve a considerable improvement in resolution with instruments such as the transmission electron microscope and the scanning electron microscope that use electrons with De Broglie wavelength much smaller than that of visible light. The De Broglie wavelength λ of an electron with momentum p is

$\lambda ={\frac {h}{p}}={\frac {h}{\sqrt {2m_{e}E_{b}}}},$ (Eq. De Broglie wavelength)

where $h$ is Plancks constant. The electron has rest mass $m_{e}$ and energy $E_{e}=m_{e}c^{2}=511keV$.

If an electron with charge $q_{e}$ is accelerated from rest by an electrical potential $U$, to the electron beam energy $E_{b}=q_{e}U$, it will have a wavelength of 1 nm at 1 eV decreasing to 1 pm at 100 keV where it will be travelling with 50% the speed of light.

This chapter will briefly review fundamental issues for electron microscopy that are similar for SEM and TEM: the limitations imposed by the electron optical beam system in the microscope column; the interaction of the electron beam with the sample; the standard image formation method in SEM and TEM. These issues are essential to understand the results and limitations reached in SEM and TEM microscopy.

For further details, please refer to reviews of electron microscopes and their applications, such as Goldstein et al. that contains a thorough review of SEM, while Goodhew and Humphreys is a more general introduction to both SEM and TEM.

The Electron Optical System

For high resolution imaging, a well focused beam is required, just as in optical microscopy. Due to the short wavelength of electron beams with keV energies, as given by the #Eq de Broglie wavelength, the properties of the electron optical system and the electron emitter mainly defines the limits on the achievable beam diameter. The current density in the electron beam can be approximated by a Gaussian distribution of current density j [A/m²] as function of radius, r, from the beam center

$j(r)=j_{0}e^{-{\frac {r^{2}}{r_{0}^{2}}}},$

with radius determined by $r_{0}$, giving a the full width half maximum $FWHM=2v(ln2)r0$. Integrating $I_{b}=\int \int j(r)rdrd\theta$ gives the total beam current

$I_{b}=j_{0}\pi r_{0}^{2}.$

The electron optics impose a limit on the achievable beam current density and radius by the brightness of the electron emitter $\beta _{e}$, which is conserved throughout the system .

Brightness, ß, is a measure of the current per area normal to the beam direction and per element of solid angle . At the center of the Gaussian beam,

$\beta ={\frac {j_{0}}{\pi \alpha ^{2}}}$

and the brightness is related to the current density in eq SEM Gaussian beam profile. The emitter brightness $\beta _{e}$ is determined by the type of electron emitter and the beam energy $E_{b}$

$\beta _{e}={\frac {j_{e}E_{b}}{\pi \Delta E}}$

with emission current density for W-filament sources about $j_{e}$~3 A/cm², for LaB6 sources about 100 A/cm², while field emission guns (FEG) can reach 10⁵A/cm². The energy spread of the electrons from the sources are about ΔE~1 eV and slightly lower for FEGs. Due to conservation of the brightness in the system, the beam diameter depends on current as

$r_{0}={\frac {1}{\pi }}{\frac {r_{wd}}{r_{a}}}{\sqrt {\frac {I_{b}}{\beta _{e}}}}.$

The ideal beam probe size determined by the conservation of brightness cannot be obtained in a real system. Effects such as aberration will make the minimum achievable beam diameter larger. Equation #eq SEM beam diameter however seem to adequately describe the beam diameter for the present discussion. Apart from the additional beam widening contributions, the image detection method imposes limits on useful values for the parameters in Eq. SEM beam diameter which differ for SEM and TEM.

Electron Range

The electron optical system sets limitations to the achievable primary beam current and radius. The expected image resolution set by the primary beam cannot be reached if the signal detected for imaging is caused by electrons scattered far in the sample. The trajectory of an electron penetrating a bulk solid is a complex trajectory due to multiple elastic and inelastic collision events. As the primary electron (PE) penetrates into the sample it will gradually change direction and loose energy in collisions. The mean free path due to elastic and inelastic collisions, $\lambda _{mfp}$, depends on the atomic number of the material and the PE energy. At 100 keV $\lambda _{mfp}=150nm$ for carbon and 5 nm for gold . For samples thinner than $\lambda _{mfp}$ the main part of the PE will pass relatively unaffected through the sample, which is the basis for TEM.

Overview of electron electron scattering processes in bulk and tip-shaped specimens. The PE are scattered within the interaction volume, defined the electron range in the material. The range is longer than the mean free path $\lambda _{mfp}$. The SE have a very short range, and only those created within that range from the surface can escape the material. This defines the SE escape depth.

SEM can be used for thicker specimens. The electrons that escape from the sample in a new direction compared to the PE due to elastic collisions are called backscattered electrons (BSE).

For samples thicker than $\lambda _{mfp}$, the volume interacting with the scattered PE defines the range of the electrons in the material, and this is considerably larger than the minimum achievable primary beam diameters.

The electron range is about 1 µm at 10 keV for carbon, decreasing with higher atomic number for the material. Both the high energy PE and BSE generate secondary electrons (SE) by inelastic scattering events. The SE are generally defined as having energy below 50 eV while the BSE have energies up to the PE energy. The range of SE is typically 1 nm for metals and about 10 nm for insulators .

The short range of the SE make the yield of SE highly dependent on the energy lost by the PE within the SE range from the surface, and this makes high Z substances efficient generators of SE. The main emission of SE takes place in the region where the PE strikes the surface and within the SE escape depth from this region.

The electron range increases with beam energy. The internal structure of the EEBD deposits can be examined at high electron beam energies in SEM. At 5 kV with shallow penetration depth, the surface of the tips is clearly visible while at higher energies a core of more dense material becomes increasingly visible. At 100 keV and above, TEM images can achieve atomic resolution where the lattice planes in nanocrystals such as the gold nanocrystal in (c). The gold crystal is embedded in amorphous carbon with no clear lattice pattern.

Scanning electron microscopy (SEM)

• Wikipedia: Scanning electron microscopy (SEM)

In a scanning electron microscope a beam is scanned over the sample surface in a raster pattern while a signal is recorded from electron detectors for SE or BSE. The PE energy is kept relatively low (1-30 keV) to limit the interaction volume in the specimen that will contribute to the detected signal. Especially low energy PE will provide high sensitivity to surface composition as they cannot penetrate far into the sample.

The figure above showed the effect of PE penetration depth of a carbonaceous nanostructure with a gold core, where only the surface is visible at low PE energies, while the carbon becomes increasingly transparent and the core visible at high PE energies.

The low energy SE can easily be attracted and collected by a positively charged detector and are hence an efficient source for an image signal. The standard SE detector is an Everhart-Thornley (ET) detector where a positively charged grid attracts the SE and accelerates them to sufficiently high energies to create a light pulse when striking a scintillator. The light pulse is then amplified by a photomultiplier. Despite the complex construction, the ET detector is remarkably efficient, but requires large $r_{wd}$ for effective collection of the SE by the charged grid.

Another SEM detector is the in-lens detector, where SE passing through the column aperture are accelerated towards a solid state detector. The in-lens detector complements the ET by being more efficient at short $r_{wd}$.

Environmental SEM (ESEM)

Simple sketch of an Environmental Scanning Electron Microscope (ESEM), where a differential pumping system with two pressure limiting apertures between the ultra high vacuum SEM column and the low vacuum sample chamber allows high pressures up to 10 hPa around the sample. This is enough to have liquid water at moderate cooling of 5 deg. C.

The ESEM makes it possible to use various gasses in the sample chamber of the microscope since there are narrow apertures between the sample chamber and the gun column, and a region in between that is connected to a differential pumping system. Pressures up to about 10 Torr are normally possible in the sample chamber.

The standard Everly-Thornhart SE detector would not work under such conditions since it would create a discharge in the low pressure gas. Instead a "gaseous secondary electron detector (GSD)" is used, as shown in the figure below. The GSD measures the current of a weak cascade discharge in the gas, which is seeded by the emission of electrons from the sample.

Two examples of images from an ESEM. Taken with a Philips XL-30 FEG. The first shows a electron beam deposited nanowire between two microelectrodes that has burnt after sustaining a high bias current. The other shows a multiwall carbon nanotube sample. Shorter working distances often improves image quality and so does a low beam current but it also increases the image acquisition time

In the ESEM one can work with for instance water vapour or argon as the environmental gas, and it is possible to have liquid samples in the chamber if the sample stage is cooled sufficiently to condense water.

Transmission electron microscopy (TEM)

• Transmission electron microscopy (TEM)
• High Resolution Transmission electron microscopy (HRTEM)

A Philips EM 430 TEM

When the specimen thickness is about the mean free path, $\lambda _{mfp}$, TEM can be used to achieve high resolution images such as the image above where the atomic lattice of a gold nanocrystal is visible. Since the detected electrons are transmitted PE where the energy can be in the 100 keV range, the resolution is not limited by the issues regarding secondary electrons. The electron beam optics can be optimized for higher current densities (Eq. #eq SEM current density) at higher energies compared to SEM.

To achieve optimal imaging conditions for the thin TEM samples, the working distance has been made short. In most TEMs, the space for the sample holder is only about (5 mm)³ between the two objective lenses for the incoming and transmitted beam. Before reaching a CCD camera, the transmitted beam is sent through several magnification lenses to achieve the high magnification (500.000X is not unusual).

The image formation in TEM can be based on several principles, but practically all images used in this work were made by phase contrast imaging, here called High Resolution TEM or HRTEM. At sufficiently high brightness, electron sources can produce coherent electron beams due to the point-like emitter surface area and small energy spread . The coherent electron beam can be considered as a spherical wave propagating from the emitter and out through the electron optical system, much like a laser beam would propagate through an optical system.

The HRTEM images are often based on the interference of the electron wavefront after it has passed through the sample and reaches a CCD detector to give a phase contrast image of the sample. The image will have a resolution determined of course by the wavelength of the electrons (Eq. #eq SEM de broglie wavelength) but mainly by the imperfections of the electron optics which also perturbs the wavefront. The optimal imaging condition is for a sample thickness about $\lambda _{mfp}$, where the wavefront is only slightly perturbed by passing through the sample. TEM instruments are normally easily capable of resolving individual shells of a carbon nanotubes. The fine-tuning of the electron optical system to the required resolution can be achieved in about 30 min for many microscopes.

TEM images of the same nanostructure using standard 'bright field' TEM vs HAADF STEM. The sample is a gold nanoparticle containing environmental electron beam deposited rod.

Electron Holography

In special TEM microscopes, the diffracted beam can be combined with a part of the original electron beam from the electron gun, and the image that is recorded is an interference pattern that depends on how much the phase of the diffracted beam was changed. By recording such images, one can measure how the electron wave function changes as it passes through or nearby a nanostructure - and this allows you to measure the electric and magnetic fields surrounding nanostructures.

Electron Tomography

By recording numerous TEM images of an object at many different angles, these images can in a computer be combined to create a three-dimensional model of the object. The technique is time consuming but allows you to see nanostructures in 3D.

References

Scanning probe microscopy

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Section on AFM
Section on STM
Section on SNOM

Scanning probe microscopy

Scanning probe microscopy covers the methods where a sharp tip is scanned over a surface in a raster pattern and the interaction with the surface is recorded in each pixel to form an image of the interaction. There are a multitude of methods and interactions in SPM. Broadly speaking, there are three main categories:

Overview of the main types of Scanning Probe Microscope types: Scanning Tunneling Microscope (STM) - using the tunneling current I between the outermost atom of a conducting probe within an atomic distance from a substrate to map out the sample topography and electrical properties. Atomic Force Microscope (AFM) - using the van der Waals forces or contact forces between a tip and the sample to measure the sample topography or mechanical properties. Scanning Near-field Optical Microscope (SNOM) - using the scattered light through a sub-wavelength aperture to form an image.

• In scanning tunneling microscopy (STM), one uses an atomically sharp metallic tip and records the minute tunneling current between the tip and the surface, when the tip is hovering so close to the surface that electrons can move between the surface and the tip.
• In Atomic force microscopy (AFM), a cantilever with a sharp tip - somewhat like the needle of an old record player - is scanned over the surface and the topography or surface softness can be recorded.
• In Scanning near-field optical microscopy (SNOM) a probe with a smalle aperture is scanned over the surface collecting the light comming from regions much smaller than the wavelength of the light used.

Atomic force microscope (AFM)

• Upcoming page in this wiki book on AFM

• Atomic force microscope (AFM)
• Kelvin force microscopy
• Magnetic force microscope
• Scanning voltage microscopy
• Magnetic resonance force microscope
• Lateral force microscopy

Scanning tunneling microscopy (STM)

• Upcoming page in this wiki book on STM
• Scanning tunneling microscopy (STM)

Scanning Near-field optical microscopy (SNOM)

• Upcoming page in this wiki book on SNOM

Wiki links:

• Scanning Near-field optical microscopy (SNOM)
• Evanescent wave coupling

Resources

• A Practical Guide to Scanning Probe Microscopy by Rebecca Howland and Lisa Benatar
• homebrew atomic-resolution microscope

References

See also notes on editing this book about how to add references Nanotechnology/About#How to contribute.

---

Atomic force microscope (AFM)

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Atomic Force Microscopy (AFM)

This is a new page and we hope you will help proof reading it and add to it!!

The relation between torsional spring constant and lateral spring constant is in doubt. Please check ("Normal and torsional spring constants of atomic force microscope cantilevers" Green, Christopher P. and Lioe, Hadi and Cleveland, Jason P. and Proksch, Roger and Mulvaney, Paul and Sader, John E., Review of Scientific Instruments, 75, 1988-1996 (2004), DOI:http://dx.doi.org/10.1063/1.1753100) and ("Lateral force calibration in atomic force microscopy: A new lateral force calibration method and general guidelines for optimization" Cannara, Rachel J. and Eglin, Michael and Carpick, Robert W., Review of Scientific Instruments, 77, 053701 (2006), DOI:http://dx.doi.org/10.1063/1.2198768) for details.

Typical AFM setup. The deflection of a microfabricated cantilever with a sharp tip is measured be reflecting a laser beam off the backside of the cantilever while it is scanning over the surface of the sample.

Resources

• A Practical Guide to Scanning Probe Microscopy by Rebecca Howland and Lisa Benatar

Methods in AFM

A wealth of techniques are used in AFM to measure the topography and investigate the surface forces on the nanoscale:

For imaging sample topography:

• Contact mode, where the tip is in contact with the substrate. Gives high resolution but can damage fragile surfaces.
• Tapping / intermittent contact mode (ICM), where the tip is oscillating and taps the surface.
• Non-contact mode (NCM), where the tip is oscillating and not touching the sample.

For measuring surface properties (and imaging them):

• Lateral force microscopy (LFM), when the tip is scanned sideways it will tilt and this can be measured by the photodetector. This method is used to measure friction forces on the nanoscale.
• Force Modulation Microscopy. Rapidly moving the tip up and down while pressing it into the sample makes it possible to measure the hardness of the surface and characterize it mechanically.
• Electrical force microscopy. If there are varying amount of charges present on the surface, the cantilever will deflect as it is attracted and repelled. kelvin probe microscopy will normally be more sensitive than measuring s static deflection.
  • Kelvin probe microscopy. By applying an oscillating voltage to an oscillating cantilever in non-contact mode and measuring the charge induced oscillations, a map can be made of the surface charge distribution.
  • Dual scan method - an other kelvin probe method described below.
• Magnetic Force Microscopy. If the cantilever has been magnetized it will deflect depending on the magnetization of the sample.
• Force-spectroscopy or force-distance curves. Moving the cantilever up and down to make contact and press into the sample, one can measure the force as function of distance.
• Nanoindentation. When pressing the cantilever hard into a sample it can leave an imprint and in the force distance curve while doing indentation can tell about the yield stress, elastic plastic deformation dynamics.
• Liquid sample AFM. By immersing the cantilever in a liquid one can also image wet samples. It can be difficult to achieve good laser alignment the first time.
• Electrochemical AFM.
• Scanning gate AFM
• Nanolithography
  • Dip-pen lithography

Reviews of Atomic Force Microscopy

SEM image of a typical AFM cantilever

• Force measurements with the atomic force microscope: Technique, interpretation and applications. Surface Science Reports 59 (2005) 1–152, by Hans-Jurgen Butt, Brunero Cappella,and Michael Kappl. 152 pages extensive review of forces and interactions in various environments and how to measure and control these with AFM.

Cantilever Mechanics

Cantilever has width w, thickness t, length L and the tip height from the cantilever middle to to the tip is h.

The typical geometry of an AFM cantilever. Length l, thickness t, width w, and tip height h is measured form the middle of the beam

When the cantilever is bent by a point force in the z-direction $F_{N}$ at the tip will deflect distance z(x) from the unloaded position along the x-axis as

$z(x)={\frac {1}{2}}{\frac {F_{N}L}{EI}}\left(x^{2}-{\frac {1}{3L}}x^{3}\right)$

with cantilever length $L$, Youngs modulus $E$, and moment of inertia $I$.

The tip deflection is

$\Delta _{z}=z\left(L\right)={\frac {1}{3}}{\frac {F_{N}}{EI}}L^{3}$

giving a spring constant $k_{N}$ from

$F_{N}=k_{N}\Delta _{z}$

so

$k_{N}=3{\frac {EI}{L^{3}}}$

The angle of the cantilever in the x-z plane at the tip $\theta _{x}$, which is what gives the laser beam deflection will be

$\theta _{N}=z^{\prime }\left(L\right)$

$z^{\prime }\left(x\right)={\frac {1}{2}}{\frac {FL}{EI}}\left(2x-{\frac {1}{L}}x^{2}\right)$

$z^{\prime }\left(L\right)={\frac {1}{2}}{\frac {F}{EI}}L^{2}={\frac {3}{2}}{\frac {z\left(L\right)}{L}}$

The difference between a hinged and fixed beam's angle of deflection and the cantilever tip. The fixed beam will give a larger deflection signal

giving the relation

$\theta _{N}={\frac {3}{2}}{\frac {\Delta _{z}}{L}}$

between the tip deflection distance and tip deflection angle. This is a factor 3/2 bigger than the result we would expect if the beam was stiff and hinged at the base, showing us that we get a bigger deflection of the laser beam when the beam is bending than when its straight.

AFM cantilever, with deflection angles and detector setup. The Z-deflection from the sample Z topography is giving a deflection in the xz-plane and measured by the top-bottom detector pair. Lateral forces on the cantilever give both torsion (yz-plane deflection and Left/ritgh detector signal) and a lateral deflection in the xy-plane that cannot be measured by the detector.

The AFM detector signal

The cantilever can bend in several ways, which is detected by the quadrant photo detector that most AFMs are equipped with. Normal topography signal is given by 'normal' deflection of the cantilever tip in the x-z direction, $\theta _{xz}=\theta _{N,}$ and detected by the left-right (or A-B) detector coupling quadrants as $V_{LR}=V_{1}+V_{3}-V_{2}-V_{4}$.

Lateral forces applied to the tip will bend the cantilever in the x-y and x-z plane too. Lateral deflection cannot be detected by the quadrant detector since it doesn't change laser beam deflection, and deflection is also rather small, as we shall see. Lateral forces also twist the cantilever tip producing torsional deflection in the y-z direction, $\theta _{yz}=\theta _{tor},$ which in turn produces the lateral force signal from the top-bottom detector measuring $V_{LR}=V_{1}+V_{2}-V_{3}-V_{4}.$

For deflection in the z-direction, 'normal' spring constant relating the force and deflection $F_{N}=k_{N}\Delta _{z}$ is

$k_{N}={\frac {1}{4}}Y{\frac {wt^{3}}{L^{3}}}$

Expressed in the angle of deflection, $\theta _{N}={\frac {3}{2}}{\frac {\Delta _{z}}{L}},$ there is an angular spring constant

$F_{N}=c_{N}\theta _{N}$

with $c_{N}={\frac {2}{3}}k_{N}L$.

AFM cantilever and the forces acting between the tip and the sample.

Contact, Tapping, and Non-contact Mode

If an oscillator experiences an attractive force pulling it away from it rest position, the resonance frequency will drop (at snap-in it will be zero). A repulsive force squeezing it will increase the resonance frequency.

The repulsive and attractive force regimes as the AFM tip approaches the sample.

If an AFM tip is moved to contact with a sample, the resonance frequency is first decreasing slightly due to attractive forces and then increasing due to the repulsive forces. Eventually the repulsive force become so high we cannot oscillate it and we have achieved contact.

Contact mode: Because the tip is in contact, the forces are considerably higher than in non-contact mode and fragile samples as well as the tip can easily be damaged. The close contact on the other hand makes the resolution good and scan speeds can be high.

The varying resonance frequency is the cantilever moves between the attractive end repulsive regions of the force distance curve can be used to measure the cantilever position and to keep it in the attractive or repulsive part of the force distance curve.

The tip oscillation frequency for tapping and non-contact mode AFM are to either side of the tip resonance frequency. The green signal is the oscillation amplitude while the yellow is the phase

Non-contact mode: If we oscillate the cantilever at a higher frequency than its free resonance and use the feedback loop to maintain a oscillation amplitude setpoint slightly lower than that of the free oscillation, it will move the tip down until the attractive forces lower the resonance frequency and makes the oscillation amplitude drop to the setpoint level.

Tapping mode: if we oscillate the cantilever at a lower frequency that its free oscillation, moving it towards the sample will first make it oscillate at a lower frequency which will make the stage move closer to try and raise the oscillation amplitude, and at eventually as it reaches repulsive forces will settle where resonance frequency cannot increase more without giving too high an amplitude.

Typical AFM cantilever properties

Use for	k (N/m)	f (kHz)
Non contact (NC)	10-100	100-300
Intermittent contact (IC)	1-10	20-100
Contact	0.1-1	1-50

Tapping Mode

Tapping-mode (also called intermittent contact mode) is the most widely used operating mode in which the cantilever tip can experience both attractive and repulsive forces intermittently. In this mode, the cantilever is oscillated at or near its free resonant frequency. Hence, the force sensitivity of the measurement is increased by the quality factor of the cantilever. In tapping-mode operation, the amplitude of the cantilever vibration is used in feedback circuitry, i.e., the oscillation amplitude is kept constant during imaging. Therefore it is also referred as amplitude modulation AFM (AM-AFM). The primary advantage of tapping mode is that the lateral forces between the tip and the sample can be eliminated, which greatly improves the image resolution. Tapping mode experiments are done generally in air or liquid. Amplitude modulation is not suitable for vacuum environment since the Q-factor of the cantilever is very high (up to 105) and this means a very slow feedback response.

Lateral Force Microscopy

If the sample is scanned sideways in the y direction, the frictional forces will apply a torque on the cantilever, bending it sideways and this can be used to measure the frictional forces. The lateral force gives both a lateral and torsional deflection of the tip. Only the torsional can be detected in the photodetector.

For sideways/lateral bending, the lateral spring constant is corresponding to the normal spring constant but with width and thickness interchanged

$k_{lat}={\frac {1}{4}}Y{\frac {tw^{3}}{L^{3}}}=k_{N}{\frac {w^{2}}{t^{2}}}$

and a similar eq's for angular deflection as above. With thickness typically much smaller than the width for AFM\ cantilevers, the lateral spring constant is 2-3 orders of magnitude higher than $k_{N}.$

For a sideways, lateral force $F_{lat}$ on the cantilever we will have a sideways deflection determined by

$F_{lat}=k_{lat}\Delta _{y-lat}$

If the lateral force is applied to the AFM tip, $F_{lat}$, it will give a lateral deflection but also apply a torque $\tau$ twisting the beam

$\tau =hF_{tor}$

Twisting an angle $\theta _{tor}$ gives a torional tip deflection of $\Delta _{y-tor}=\theta _{tor}h.$

The relation for the torsional spring constant is (please check this equation)

$F_{tor}=k_{tor}\Delta _{y-tor}$

with

$k_{tor}=G{\frac {wt^{3}}{3h^{2}L}}={\frac {Ywt^{3}}{8h^{2}L}}=k_{N}{\frac {1}{2}}\left({\frac {L}{h}}\right)^{2}$

and then

$\tau =h^{2}k_{tor}\theta _{tor}={\frac {1}{2}}k_{N}L^{2}\theta _{tor}={\frac {1}{2}}k_{N}{\frac {L^{2}\Delta _{y-tor}}{h}}$

From above we have

${\frac {k_{lat}}{k_{tor}}}=2\left({\frac {wh}{tL}}\right)^{2}$

The factor $2\left({\frac {wh}{tL}}\right)^{2}$ is typically $2\left({\frac {20\ast 10}{2\ast 100}}\right)^{2}=2.0$ - so about 1 but larger or smaller depending on whether its a contact or non-contact cantilever.

Friction Loop Scan

Typical signal from a scan with the AFM in lateral force mode - a friction loop scan. At the turning points the tip sticks to the surface and the signal has a linear slope with the detector sensitivity. When the lateral tip-sample force exceeds the static friction force between the sample and substrate, the tip will start to slide with the dynamic friction force and s steady signal.

For optimal torsional sensitivity - but the following is not always correct since it depends highly on the contact forces you need etc: For high torsional sensitivity, $wh>>tL$. Since we are in contact mode AFM, L must be large and t thin for a low $k_{N}$. So better torsional sensitivity means wider cantilevers and definitely large tip heights.

Coupled Lateral and Torsional deflection in the cantilever

But how much will a cantilever bend laterally and how much will it twist when applied a lateral force at the tip? An applied lateral force will move two degrees of freedom with a Hookes law behaviour - the torsion and lateral motion. The applied force $F_{tor}$ is also an applied $F_{lat.}$

The effective spring constant for pushing the tip in the y direction is then

$F_{tor}=k_{eff}\Delta _{y}$

with $\Delta _{y}=\Delta _{y,lat}+\Delta _{y,tor}$ and

$k_{eff}={\frac {1}{{\frac {1}{k_{lat}}}+{\frac {1}{k_{tor}}}}}={\frac {k_{lat}}{1+{\frac {k_{lat}}{k_{tor}}}}}$

and the deflection made in the torsional spring $\Delta _{y,tor}$ is

$\Delta _{y,tor}={\frac {\Delta _{y}}{1+{\frac {k_{tor}}{k_{lat}}}}}$

and this approaches $\Delta _{y}$ when $k_{tor}<k_{lat,}$so the cantilever is more prone to tilting than lateral deflection. The lateral deflection can be found from

$\Delta _{y,lat}k_{lat}=\Delta _{y,tor}k_{tor}\Delta _{y,lat}={\frac {k_{tor}}{k_{lat}}}\Delta _{y,tor}$

The torsional deflection angle is then

$\theta _{tor}={\frac {\Delta _{y,tor}}{h}}={\frac {k_{lat}}{k_{lat}+k_{tor}}}{\frac {F_{tor}}{hk_{eff}}}={\frac {k_{lat}}{k_{lat}+k_{tor}}}{\frac {1+{\frac {k_{lat}}{k_{tor}}}}{k_{lat}}}{\frac {F_{tor}}{h}}={\frac {1}{h}}{\frac {F_{tor}}{k_{tor}}}$

as anticipated from the assumption that the torsional and lateral springs are coupled in series. So when a constant force is applied, the detector signal is a measurement of the force.

Question: during a friction loop scan, the tip is fixed by static friction on the surface and its a constant deflection, both torsional and lateral deflection must be included to find the actual deflected distance of the tip before its pulled free from to slide the surface and the lateral deflection could influence the beginning of the friction loop curve?

Measuring the cantilever dimensions

The vibration frequency of the fundamental mode of the cantilever is an easily measurable quantity in the AFM and can be used to evaluate a cantilever is within specifications. Its given by

$f[Hz]={\frac {t\beta _{i}^{2}}{4\pi L^{2}}}{\sqrt {\frac {Y}{3\rho }}}={\frac {\left(1\ast 10^{-6}\right)\left(1.875\right)^{2}}{4\pi \left(100\ast 10^{-6}\right)^{2}}}{\sqrt {\frac {\left(160\ast 10^{9}\right)}{3\ast 2330}}}=1.\,338\,5\times 10^{5}$

Easily measurable quantities in AFM: length L, resonance freq f, tip length $l_{tip},$width $w$,

Not so easy: thickness t, cross section (often there are inclined sidewalls), force konst $k_{norm},$ tip length from middle of the cantilever since we dont know the thickness.

Noise Sources in AFM

Thermal noise

$E={\frac {1}{2}}kx^{2}={\frac {1}{2}}k_{B}T$

$\left\langle x\right\rangle _{rms}={\sqrt {\frac {k_{B}T}{k_{spring}}}}$

for a 1 N/m cantilever this amounts to $\left\langle x\right\rangle _{rms}={\sqrt {k_{B}300}}=0.6.$Å. So a 1 Å noise level requires ${\frac {k_{B}300}{10^{-20}}}=0.4$ N/m which is not a very low spring constant for a contact mode cantilever.

So thermal noise can become a problem in some AFM cantilevers at room temperature!!

Electrical Force Microscopy

The Kelvin Probe Microscopy Method and Dual Scan Method can be used to map out the electrical fields on surfaces with an AFM.

Kelvin Probe Microscopy Method

The principle of Kelvin probe microscopy (KPM). The lock-in amplifier generates a signal on the tip and the electrostatic tip-surface interaction is readout by the laser and the lock-in amplifier adjusts accordingly.

In the Kelvin probe microscopy (KPM) method a voltage is applied between the AFM tip and the surface. Both a DC and AC voltage is applied to the tip so the total potential difference $V_{tot}$ between the tip and surface is:

$V_{tot}=-V_{S}+V_{DCt}+V_{ACt}\cdot \sin(\omega \cdot t),$

where $V_{S}=V_{S}(x,y)$ is the local surface potential, $x,y$ is the position of the tip, $V_{DCt}$ is the DC signal on the tip, $V_{ACt}$ is the amplitude of the AC signal, and $\omega$ is the frequency of the AC signal.

The frequency of the AC signal is much lower than the resonance frequency of the cantilever (a factor of 10) so the two signals can be separated by a lock-in amplifier. Via the electrostatic forces the setup measures the surface potential. If one assumes that the electrostatic force ($F$) between the tip and surface is given by

$F={\frac {{\frac {\partial C}{\partial z}}\cdot V_{tot}^{2}}{2}},$

where $C$ is the capacitance and $z$ is the distance between the tip and the surface.

If a parallel plate capacitor is assumed

$C={\frac {A_{c}\epsilon _{0}}{z}}$,

where $A_{c}$ is the area of the tip. The derivative of capacitance is

${\frac {\partial C}{\partial z}}=-{\frac {A_{c}\epsilon _{0}}{z^{2}}}$.

Combining the force ($F$) and $V_{tot}$ yields:

$F={\frac {\frac {\partial C}{\partial z}}{2}}\cdot {\Big (}-V_{S}+V_{DCt}+V_{ACt}\cdot \sin(\omega \cdot t){\Big )}^{2}=$

${\frac {\frac {\partial C}{\partial z}}{2}}\cdot {\Big (}(V_{DCt}-V_{S})^{2}+V_{ACt}^{2}\cdot \sin(\omega \cdot t)^{2}+2\cdot (V_{DCt}-V_{S})\cdot V_{ACt}\cdot \sin(\omega \cdot t){\Big )}.$

Using Pythagorean identities

${\big [}\cos(x)^{2}+\sin(x)^{2}=1{\big ]}$

and de Moivre's formula

${\big [}(\cos(x)+i\cdot \sin(x))^{n}=\cos(n\cdot x)+i\cdot \sin(n\cdot x){\big ]},$

We find

$V_{ACt}^{2}\cdot \sin(\omega \cdot t)^{2}=V_{ACt}^{2}\cdot \ {\big (}1-\cos(\omega \cdot t)^{2}{\big )}=$

${\frac {1}{2}}\cdot V_{ACt}^{2}\cdot {\big (}2-2\cdot \cos(\omega \cdot t)^{2}{\big )}=$

${\frac {1}{2}}\cdot V_{ACt}^{2}-{\frac {1}{2}}\cdot V_{ACt}^{2}\cdot \cos(2\cdot \omega \cdot t).$

This inserted in the equation for the force ($F$) gives :

$F={\Big (}{\frac {\frac {\partial C}{\partial z}}{2}}\cdot {\big (}(V_{DCt}-V_{S})^{2}+{\frac {1}{2}}\cdot V_{ACt}^{2}{\big )}+{\big (}2\cdot (V_{DCt}-V_{S})\cdot V_{ACt}{\big )}\cdot \sin(\omega \cdot t)-$

$({\frac {1}{2}}\cdot V_{ACt}^{2})\cdot \cos(2\cdot \omega \cdot t){\Big )}$

$F=k_{1}+k_{2}\cdot \sin(\omega \cdot t)+k_{3}\cdot \cos(2\cdot \omega \cdot t),$

where

$k_{1}={\frac {\frac {\partial C}{\partial z}}{2}}\cdot {\big (}(V_{DCt}-V_{S})^{2}+{\frac {1}{2}}\cdot V_{ACt}^{2}{\big )}$,

$k_{2}=(2\cdot (V_{DCt}-V_{S})\cdot V_{ACt})$, and $k_{3}=-{\frac {1}{2}}\cdot V_{ACt}^{2}$.

The frequency $\omega$ is set by an external oscillator and can therefore be locked by the lock-in amplifier. The signal detected by the lock-in amplifier (the $k_{2}$ part) is minimized by constant varying $V_{DCt}$. When this signal approaches zero, this corresponds to $V_{DCt}=V_{S}$ i.e. mapping $V_{DCt}$ vs. the sample surface $(x,y)$ gives $V_{S}(x,y)$.

Dual Scan Method

The principle of the Dual Scan (DS) method where first a topography line scan is made, then the tip is lifted a distance d and another line scan is made with the source drain voltage turned on.

In the Dual Scan (DS, or sometimes called lift-mode method) one first makes a line scan with no potential on either the AFM tip or the sample in either tapping or non-contact mode. Next, the AFM tip is raised several tens of nanometers (30-70 nm) above the surface. A new line scan is made at this height, but this time with a potential on the sample also in non-contact mode. This is repeated over the desired scan area until the whole area has been scanned. For imaging the surface potential, the phase of the cantilever vibration is mapped out. The principle is shown in the figure where d is the distance between the tip and the surface in the second scan. The phase shift is dependent on the force ($F$) acting on the tip

$\phi =\tan ^{-1}({\frac {k}{Q\cdot {\frac {\partial F}{\partial z}}}})$

where $Q$ is the quality factor of the cantilever, $k$ the spring constant, and $z$ the distance between the tip and the surface.

For small phase shifts the phase can be written as

$\phi \approx {\frac {Q\cdot {\frac {\partial F}{\partial z}}}{k}}.$

The derivative of the force can be written as:

${\frac {\partial F}{\partial z}}={\frac {1}{2}}{\frac {\partial ^{2}C}{\partial z^{2}}}\cdot V_{s}^{2},$

where $V_{s}$ is the surface potential and $C$ is the capacitance between the tip and the surface . The second derivative of the capacitance is

${\frac {\partial ^{2}C}{\partial z^{2}}}={\frac {2A_{c}\epsilon _{0}}{z^{3}}}.$

Combining equations for the phase and the derivative of the force yields the phase dependence of the phase shift

$\phi \approx {\frac {Q\cdot {\frac {\partial ^{2}C}{\partial z^{2}}}\cdot V_{s}^{2}}{k\cdot 2}}.$

To find the surface potential, one must estimate the other parts of equation for the phase. The spring constant ($k$) can be determined if the dimensions (a regular cantilever) and material of the cantilever are known:

$k={\frac {E\cdot w\cdot h^{3}}{4\cdot L^{3}}},$

where $E$ is the Young modulus, $w$ is the width of the cantilever, $h$ the height, and $L$ is the length. The quality factor ($Q$) of the cantilever can be found by measuring the shape of resonance peak. The second derivative of the capacitance can be estimated by assuming that the tip of the AFM is a plate with radius $r$ so the derivative of the capacitance is given by:

${\frac {\partial ^{2}C}{\partial z^{2}}}={\frac {2\cdot \pi \cdot r^{2}\cdot \epsilon _{0}}{z^{3}}},$

where $\epsilon _{0}$ is the vacuum permittivity. This way of estimating the other parts of the equation for the phase is quite accurate according to . One can also estimate the values by measuring them at a surface with a known potential and at different known heights and then one simply calculate backwards for that particular AFM tip.

Discussion

Both the DS and KPM methods have their strengths and weaknesses. The DS method is easier to operate, since it has fewer interlinked parameters, needing to be adjusted. The KPM method is faster, as it does not require two scans (an image with the DS method with a resolution of 512 $\times$ 512 pixels and a scan rate of $0.8$ Hz takes about half an hour). The DS method will normally obtain much better lateral resolution in the potential image compared to the KPM method. This is due to the fact that the signal depends on the second derivative of the capacitance, which in turn depends on the distance in ${\frac {1}{r^{3}}}$ compared to the KPM method where the dependence is only ${\frac {1}{r^{2}}}$. This rapidly reduces the problem of the tip sidewall interaction. On the other hand, the KPM method has better sensitivity because it operates much closer to the surface.

References

• A. Bachtold, M. S. Fuhrer, S. Plyasunov, M. Forero, E. H. Anderson, A. Zettl, and P. L. McEuen; Physical Review Letters 84(26), 6082-6085 (2000).

• G. Koley and M. G. Spencer; Applied Physics Letters 79(4), 545-547 (2001).

• T. S. Jespersen and J. Nygård; Nano Letters 5(9), 1838-1841 (2005).

• V. Vincenzo, and M Palma, and P. Samorí; Advanced Materials 18, 145-164 (2006).

• Veeco, "Electrostatic Force Microscopy for AutoProbe CP Research Operating Instructions", 2001 (Manual).

• D. Ziegler and A. Stemmer; Nanotechnology 22, 075501 (2011).

Suppliers of AFM systems

• Park Systems
• Danish Micro Engineering
• Nanonics Imaging Ltd
• MultiProbe
• NT-MDT Co.
• RHK Technology Inc.
• JPK Instruments
• Veeco Instruments
• Nanotec
• Asylum Research
• Agilent Technologies
• Schaefer Technologie GmbH
• Nanomagnetics Instruments Inc.
• attocube systems AG
• Nano Scan Technology Ltd.
• AraResearch
• Advanced Technologies Center

Suppliers of cantilevers for AFM

• Nanonics Imaging Ltd
• NT-MDT Co.
• Veeco Probes
• BudgetSensors
• Nanosensors
• Nano and More
• Nano World

Overview of properties of various cantilevers

Software for AFM image analysis

• NT-MDT Co.
• FemtoScan Online: a powerful image processing sortware from ATC [7]
• XPMPro Image Analysis from RHK Technology
• WsXM freeware from Nanotec.es
• SPIP from Image Metrology
• Gwyddion Free and Open Source software for SPM
• Punias Force-distance curve analysis software
• Hooke:a free, open platform for single molecule force spectroscopy

References

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Scanning tunneling microscopy (STM)

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Scanning Tunneling Microscopy

Basic overview of the scanning tunneling microscope tip-sample interaction. When the tip is within atomic distance of the sample surface and a small bias voltage about a volt or so is applied, tunneling current can be measured. Adjusting the height of the tip while scanning the tip over the surface with a fixed bias voltage to always maintain a constant tunnel current will map out the sample topography.

A look into the Ultra High Vacuum (UHV) chamber of a UHV Scanning Tunneling Microscope (STM). Several grippers are mounted to move samples back and forth between the holder for multiple samples and the STM microscope which is the tubular gold capped structure held by a spring suspension.

Tips for STM

• KOH etched tungsten tips
• PtIr tips

References

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Scanning Near-field optical microscopy (SNOM)

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Scanning Near-field optical microscopy (SNOM)

The Abbe diffraction limit of optical microscopy can be circumvented if an evanescent wave is used instead of a travelling wave. The SNOM can be compared to a stethoscope : A doctor can locate your heart with a precision of a few cm, despite the fact that the sound of a beating heart he is listening to has a wavelength of the order 100 m. Apparently he has a resolving power of λ/1000 which is far better than what's dictated by the Abbe diffraction limit. A similar setup can be made with light waves: If the light is forced through a sub-wavelength sized aperture, the emitted field will be evanescent and decay exponentially on a length scale usually shorter than the wavelength. With this technique a resolution of 12 nm has been shown back in 1991. SNOM has the ability to make high resolution in all directions (x,y, and z) and can be adapted to fit onto the same laser systems and spectrometers that other microscopes also use.

Images by SNOM are made by scanning the probe over the sample like an LSCM, AFM or STM. The SNOM is a very versatile tool used by both physicists and biologists for many purposes, but the probe only interacts with the sample in close vicinity of the aperture, and hence the sample-probe distance becomes a concern for the fragile samples and probes.

One widespread distance control method is the shear force technique, invented in the 1992, where the SNOM probe is set in vibrations with an amplitude up to a few nm and the motion is detected and used in a feedback loop that senses the minute shear forces that occur when the probe tip is a few nm above the sample surface. Numerous shear force setups have been described in the literature. Both optical and non-optical methods are used to detect the vibrations. Groups using non-optical methods claim the optical methods are sources of stray light that will seriously affect the measurements, while e.g. find optical setups to be advantageous.

References

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Additional methods

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Point-Projection Microscopes

Point-Projection Microscopes are a type of field emission microscope, and consists of three components: an electron source, the object to the imaged, and the viewing screen.

• Field emission microscope
• Field ion microscope
• Atom Probe

Low energy electron diffraction (LEED)

LEED is a technique for imaging surfaces, and has two principle methods of use: qualitative and quantitative. The qualitative method measures relative size and geometric properties, whereas the quantitative method looks at diffracted beams as a way of determining the position of atoms.

• Low energy electron diffraction (LEED)
• LEED intro

Reflection High Energy Electron diffraction

RHEED is similar to LEED but uses higher energies and the electrons are directed to the be reflected on the surface at almost grazing incidence. This way the high energy electrons only penetrates a few atomic layers of the surface.

X-ray Spectroscopy and Diffraction

X-ray Spectroscopy refers to a collection of techniques including, but not limited to X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy.

X-rays can be used for X-ray crystallography.

Auger electron spectroscopy (AES)

Auger Electron Spectroscopy is a technique that takes advantage of the Auger Process to analyze the surface layers of a sample.

Nuclear Magnetic Resonance (NMR)

• Nuclear Magnetic Resonance (NMR) - in a magnetic field the spin of the nuclei of molecules will precess and in strong fields (several tesla) this happens with rf frequencies that can be detected by receiving rf antennas and amplifiers. The precession frequency of an individual nucleus will deviate slightly depending on the its surrounding molecules' electronic structure and hence detecting a spectrum of the radiofrequency precession frequencies in a sample will provide a finger print of the types of molecules in that sample.
• Nuclear quadrupole resonance is a related technique, based on the internal electrical fields of the molecules to cause a splitting of the nuclear magnetic moments energy levels. The level splitting is detected by rf as in NMR. Its is used mainly for experimental explosives detection.

Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR)

Electron Spin Resonance (ESR) measures the microwave frequency of paramagnetic ions or molecules .

Mössbauer spectroscopy

Mössbauer spectroscopy detects the hyperfine interactions between the nucleus of an atom, and the ambient environment. The atom must be part of a solid matrix to reduce the recoil affect of a gamma ray emission or absorption.

Non-contact Nanoscale Temperature Measurements

Heat radiation has infrared wavelengths much longer than 1 µm and hence taking a photo of a nanostructure with e.g. a thermal camera will not provide much information about the temperature distribution within the nanostructure (or microstructure for that sake).

Temperatures can be measured locally by different non-contact methods:

• Spectroscopy on individual quantum dots [90].
• Spectra of laser dyes incorporated in the structure
• Raman microscopy (the temperature influences the ratio of stokes and anti-stokes lines amplitude, the width of the lines and the position of the lines.)
• Transmission electron microscopy can also give temperature information by various techniques [91]
• Special AFM probes with a temperature dependent resistor at the tip can be used for mapping surface temperatures
• Infrared Near-field Microscopy
• Confocal raman microscopy can provide 3D thermal maps [92]

References

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Physics at the Nanoscale

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<<< Prev Part: Seeing 'Nano'
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Chemistry and Biochemistry are in many ways nanotechnologies working with nanoscale structures. In physics the classical laws of motion are not always valid on the nanoscale where a quantum mechanical model often is more suitable, and there are often a wealth of forces that are important that we do not consider very much in classical physics - the surface forces.

This part is about how nanosystems move, the forces that control the motion, and the ways we can model it.

References

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Intro to Nanophysics

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Quantum mechanics and classical mechanics are still valid on the nanoscale - but many assumptions we are used to take for granted are no longer valid. This makes many traditional systems difficult to make on the atomic scale -if for instance you scale down a car the new relation between adhesion forces and gravity or changes in heat conduction will very likely make it perform very poorly if at all - but at the same time a wealth of other new possibilities open up!

Scaling laws can be used to determine how the physical properties vary as the dimensions are changed. At some point the scaling law no longer can be applied because the assumptions behind it become invalid at some large or small scale.

So, scaling is one thing - the end of scaling another, and surfaces a third! For instance at some point the idealized classical point of view on a system being downscaled will need quantum mechanics to describe what's going on in a proper way, but as the scale is decreased the system might also be very different because the interaction at the surface becomes very significant compared to the bulk.

This part will try to give an overview of these effects.

Scaling laws

Scaling laws can be used to describe how the physical properties of a system change as the dimensions are changed.

The scaling properties of physical laws is an important effect to consider when miniaturizing devices. On the nanoscale the mass and heat capacity become very unimportant, whereas eg. surface forces scaling with area become dominant.

Quantized Nano Systems

Quantum wires are examples of nanosystems where the quantum effects become very important.

Break junctions is another example.

Resources

• Simple model of conduction - the classical case

Bulk matter and the end of bulk: surfaces

• Surface states are electronic states on the surface of a material, which can have radically different properties than the underlying bulk material. For instance, a semiconductor can have superconducting surface states.

• Surface reconstruction

The surface of a material can be very different from the bulk because the surface atoms rearrange themselves to lower their energy rather than stay in the bulk lattice and have dangling bonds extending into space where there is no more material. Atoms from the surroundings will easily bind to such surfaces and for example for silicon, more than 2000 surface reconstructions have been found, depending on what additional atoms and conditions are present.

• Surface plasmons

Plasmons are collective oscillations of the electrons in matter, and the electrons on the surfaces can also make local plasmons that propagate on the surface.

The Tyndall Effect

The Tyndall Effect is caused by reflection of light off of small particles such as dust or mist. This is also seen off of dust when sunlight comes through windows and clouds or when headlight beams go through fog. The Tyndall Effect can only be seen through colloidal suspensions. A colloid is a substance that consists of particles dispersed throughout another substance, which are too small for resolution with an ordinary light microscope but are incapable of passing through a semi permeable membrane. The Tyndall Effect is most easily visible through liquid using a laser pointer. The Tyndall Effect is named after its discoverer, the 19th-century British physicist John Tyndall.

References

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Modelling Nanosystems

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The Schrödinger equation

the Schrödinger equation

$H(t)\left|\psi \left(t\right)\right\rangle =\mathrm {i} \hbar {\frac {\partial }{\partial t}}\left|\psi \left(t\right)\right\rangle$

where $\mathrm {i}$ is the imaginary unit, $t$ is time, $\partial /\partial t$ is the partial derivative with respect to $t$, $\hbar$ is the reduced Planck's constant (Planck's constant divided by $2\pi$), $\psi (t)$ is the wave function, and $H\!\left(t\right)$ is the Hamiltonian operator.

Hartree-Fock (HF) or self-consistent field (SCF)

In computational physics and chemistry, the Hartree–Fock (HF) method is a method of approximation for the determination of the wave function and the energy of a quantum many-body system in a stationary state.

The Hartree–Fock method often assumes that the exact, N-body wave function of the system can be approximated by a single Slater determinant (in the case where the particles are fermions) or by a single permanent (in the case of bosons) of N spin-orbitals. By invoking the w:variational method, one can derive a set of N-coupled equations for the N spin orbitals. A solution of these equations yields the Hartree–Fock wave function and energy of the system.

Especially in the older literature, the Hartree–Fock method is also called the self-consistent field method (SCF). In deriving what is now called the Hartree equation as an approximate solution of the Schrödinger equation, Hartree required the final field as computed from the charge distribution to be "self-consistent" with the assumed initial field. Thus, self-consistency was a requirement of the solution. The solutions to the non-linear Hartree–Fock equations also behave as if each particle is subjected to the mean field created by all other particles (see the Fock operator below) and hence the terminology continued. The equations are almost universally solved by means of an iterative method, although the fixed-point iteration algorithm does not always converge. This solution scheme is not the only one possible and is not an essential feature of the Hartree–Fock method.

The Hartree–Fock method finds its typical application in the solution of the Schrödinger equation for atoms, molecules, nanostructures and solids but it has also found widespread use in nuclear physics. (See Hartree–Fock–Bogoliubov method for a discussion of its application in nuclear structure theory). In atomic structure theory, calculations may be for a spectrum with many excited energy levels and consequently the Hartree–Fock method for atoms assumes the wave function is a single configuration state function with well-defined quantum numbers and that the energy level is not necessarily the ground state.

For both atoms and molecules, the Hartree–Fock solution is the central starting point for most methods that describe the many-electron system more accurately.

The rest of this article will focus on applications in electronic structure theory suitable for molecules with the atom as a special case. The discussion here is only for the Restricted Hartree–Fock method, where the atom or molecule is a closed-shell system with all orbitals (atomic or molecular) doubly occupied. Open-shell systems, where some of the electrons are not paired, can be dealt with by one of two Hartree–Fock methods:

• w:Restricted open-shell Hartree–Fock (ROHF)
• w:Unrestricted Hartree–Fock (UHF)

history

The origin of the Hartree–Fock method dates back to the end of the 1920s, soon after the discovery of the w:Schrödinger equation in 1926. In 1927 D. R. Hartree introduced a procedure, which he called the self-consistent field method, to calculate approximate wave functions and energies for atoms and ions. Hartree was guided by some earlier, semi-empirical methods of the early 1920s (by E. Fues, R. B. Lindsay, and himself) set in the w:old quantum theory of Bohr.

In the w:Bohr model of the atom, the energy of a state with w:principal quantum number n is given in atomic units as $E=-1/n^{2}$. It was observed from atomic spectra that the energy levels of many-electron atoms are well described by applying a modified version of Bohr's formula. By introducing the w:quantum defect d as an empirical parameter, the energy levels of a generic atom were well approximated by the formula $E=-1/(n+d)^{2}$, in the sense that one could reproduce fairly well the observed transitions levels observed in the w:X-ray region (for example, see the empirical discussion and derivation in w:Moseley's law). The existence of a non-zero quantum defect was attributed to electron-electron repulsion, which clearly does not exist in the isolated hydrogen atom. This repulsion resulted in partial screening of the bare nuclear charge. These early researchers later introduced other potentials containing additional empirical parameters with the hope of better reproducing the experimental data.

Hartree sought to do away with empirical parameters and solve the many-body time-independent Schrödinger equation from fundamental physical principles, i.e., ab initio. His first proposed method of solution became known as the Hartree method. However, many of Hartree's contemporaries did not understand the physical reasoning behind the Hartree method: it appeared to many people to contain empirical elements, and its connection to the solution of the many-body Schrödinger equation was unclear. However, in 1928 J. C. Slater and J. A. Gaunt independently showed that the Hartree method could be couched on a sounder theoretical basis by applying the w:variational principle to an w:ansatz (trial wave function) as a product of single-particle functions.

In 1930 Slater and V. A. Fock independently pointed out that the Hartree method did not respect the principle of antisymmetry of the wave function. The Hartree method used the w:Pauli exclusion principle in its older formulation, forbidding the presence of two electrons in the same quantum state. However, this was shown to be fundamentally incomplete in its neglect of w:quantum statistics.

It was then shown that a w:Slater determinant, a w:determinant of one-particle orbitals first used by Heisenberg and Dirac in 1926, trivially satisfies the antisymmetric property of the exact solution and hence is a suitable w:ansatz for applying the w:variational principle. The original Hartree method can then be viewed as an approximation to the Hartree–Fock method by neglecting exchange. Fock's original method relied heavily on w:group theory and was too abstract for contemporary physicists to understand and implement. In 1935 Hartree reformulated the method more suitably for the purposes of calculation.

The Hartree–Fock method, despite its physically more accurate picture, was little used until the advent of electronic computers in the 1950s due to the much greater computational demands over the early Hartree method and empirical models. Initially, both the Hartree method and the Hartree–Fock method were applied exclusively to atoms, where the spherical symmetry of the system allowed one to greatly simplify the problem. These approximate methods were (and are) often used together with the w:central field approximation, to impose that electrons in the same shell have the same radial part, and to restrict the variational solution to be a spin eigenfunction. Even so, solution by hand of the Hartree–Fock equations for a medium sized atom were laborious; small molecules required computational resources far beyond what was available before 1950.

Hartree–Fock algorithm

The Hartree–Fock method is typically used to solve the time-independent Schrödinger equation for a multi-electron atom or molecule as described in the w:Born–Oppenheimer approximation. Since there are no known solutions for many-electron systems (hydrogenic atoms and the diatomic hydrogen cation being notable one-electron exceptions), the problem is solved numerically. Due to the nonlinearities introduced by the Hartree–Fock approximation, the equations are solved using a nonlinear method such as w:iteration, which gives rise to the name "self-consistent field method."

Approximations

The Hartree–Fock method makes five major simplifications in order to deal with this task:

• The w:Born–Oppenheimer approximation is inherently assumed. The full molecular wave function is actually a function of the coordinates of each of the nuclei, in addition to those of the electrons.
• Typically, relativistic effects are completely neglected. The momentum operator is assumed to be completely non-relativistic.
• The variational solution is assumed to be a w:linear combination of a finite number of basis functions, which are usually (but not always) chosen to be w:orthogonal. The finite basis set is assumed to be approximately complete.
• Each w:energy eigenfunction is assumed to be describable by a single w:Slater determinant, an antisymmetrized product of one-electron wave functions (i.e., orbitals).
• The mean field approximation is implied. Effects arising from deviations from this assumption, known as w:electron correlation, are completely neglected for the electrons of opposite spin, but are taken into account for electrons of parallel spin. (Electron correlation should not be confused with electron exchange, which is fully accounted for in the Hartree–Fock method.)

Relaxation of the last two approximations give rise to many so-called w:post-Hartree–Fock methods.

Greatly simplified algorithmic flowchart illustrating the Hartree–Fock method

Variational optimization of orbitals

The variational theorem states that for a time-independent Hamiltonian operator, any trial wave function will have an energy w:expectation value that is greater than or equal to the true w:ground state wave function corresponding to the given Hamiltonian. Because of this, the Hartree–Fock energy is an upper bound to the true ground state energy of a given molecule. In the context of the Hartree–Fock method, the best possible solution is at the Hartree–Fock limit; i.e., the limit of the Hartree–Fock energy as the basis set approaches completeness. (The other is the full-CI limit, where the last two approximations of the Hartree–Fock theory as described above are completely undone. It is only when both limits are attained that the exact solution, up to the Born–Oppenheimer approximation, is obtained.) The Hartree–Fock energy is the minimal energy for a single Slater determinant.

The starting point for the Hartree–Fock method is a set of approximate one-electron wave functions known as w:spin-orbitals. For an w:atomic orbital calculation, these are typically the orbitals for a hydrogenic atom (an atom with only one electron, but the appropriate nuclear charge). For a w:molecular orbital or crystalline calculation, the initial approximate one-electron wave functions are typically a w:linear combination of atomic orbitals (LCAO).

The orbitals above only account for the presence of other electrons in an average manner. In the Hartree–Fock method, the effect of other electrons are accounted for in a w:mean-field theory context. The orbitals are optimized by requiring them to minimize the energy of the respective Slater determinant. The resultant variational conditions on the orbitals lead to a new one-electron operator, the w:Fock operator. At the minimum, the occupied orbitals are eigensolutions to the Fock operator via a w:unitary transformation between themselves. The Fock operator is an effective one-electron Hamiltonian operator being the sum of two terms. The first is a sum of kinetic energy operators for each electron, the internuclear repulsion energy, and a sum of nuclear-electronic Coulombic attraction terms. The second are Coulombic repulsion terms between electrons in a mean-field theory description; a net repulsion energy for each electron in the system, which is calculated by treating all of the other electrons within the molecule as a smooth distribution of negative charge. This is the major simplification inherent in the Hartree–Fock method, and is equivalent to the fifth simplification in the above list.

Since the Fock operator depends on the orbitals used to construct the corresponding w:Fock matrix, the eigenfunctions of the Fock operator are in turn new orbitals which can be used to construct a new Fock operator. In this way, the Hartree–Fock orbitals are optimized iteratively until the change in total electronic energy falls below a predefined threshold. In this way, a set of self-consistent one-electron orbitals are calculated. The Hartree–Fock electronic wave function is then the Slater determinant constructed out of these orbitals. Following the basic postulates of quantum mechanics, the Hartree–Fock wave function can then be used to compute any desired chemical or physical property within the framework of the Hartree–Fock method and the approximations employed.

Mathematical formulation

The Fock operator

Main article: w:Fock matrix

Because the electron-electron repulsion term of the w:electronic molecular Hamiltonian involves the coordinates of two different electrons, it is necessary to reformulate it in an approximate way. Under this approximation, (outlined under Hartree–Fock algorithm), all of the terms of the exact Hamiltonian except the nuclear-nuclear repulsion term are re-expressed as the sum of one-electron operators outlined below, for closed-shell atoms or molecules (with two electrons in each spatial orbital). The "(1)" following each operator symbol simply indicates that the operator is 1-electron in nature.

${\hat {F}}[\{\phi _{j}\}](1)={\hat {H}}^{\text{core}}(1)+\sum _{j=1}^{N/2}[2{\hat {J}}_{j}(1)-{\hat {K}}_{j}(1)]$

where

${\hat {F}}[\{\phi _{j}\}](1)$

is the one-electron Fock operator generated by the orbitals $\phi _{j}$, and

${\hat {H}}^{\text{core}}(1)=-{\frac {1}{2}}\nabla _{1}^{2}-\sum _{\alpha }{\frac {Z_{\alpha }}{r_{1\alpha }}}$

is the one-electron core Hamiltonian. Also

${\hat {J}}_{j}(1)$

is the w:Coulomb operator, defining the electron-electron repulsion energy due to each of the two electrons in the jth orbital. Finally

${\hat {K}}_{j}(1)$

is the w:exchange operator, defining the electron exchange energy due to the antisymmetry of the total n-electron wave function. This (so called) "exchange energy" operator, K, is simply an artifact of the Slater determinant. Finding the Hartree–Fock one-electron wave functions is now equivalent to solving the eigenfunction equation:

${\hat {F}}(1)\phi _{i}(1)=\epsilon _{i}\phi _{i}(1)$

where $\phi _{i}\;(1)$ are a set of one-electron wave functions, called the Hartree–Fock molecular orbitals.

Linear combination of atomic orbitals

Main articles: w:basis set (chemistry) and w:basis set

Typically, in modern Hartree–Fock calculations, the one-electron wave functions are approximated by a w:linear combination of atomic orbitals. These atomic orbitals are called w:Slater-type orbitals. Furthermore, it is very common for the "atomic orbitals" in use to actually be composed of a linear combination of one or more Gaussian-type orbitals, rather than Slater-type orbitals, in the interests of saving large amounts of computation time.

Various basis sets are used in practice, most of which are composed of Gaussian functions. In some applications, an orthogonalization method such as the w:Gram–Schmidt process is performed in order to produce a set of orthogonal basis functions. This can in principle save computational time when the computer is solving the Roothaan–Hall equations by converting the w:overlap matrix effectively to an w:identity matrix. However, in most modern computer programs for molecular Hartree–Fock calculations this procedure is not followed due to the high numerical cost of orthogonalization and the advent of more efficient, often sparse, algorithms for solving the w:generalized eigenvalue problem, of which the Roothaan–Hall equations are an example.

Numerical stability

w:Numerical stability can be a problem with this procedure and there are various ways of combating this instability. One of the most basic and generally applicable is called F-mixing or damping. With F-mixing, once a single electron wave function is calculated it is not used directly. Instead, some combination of that calculated wave function and the previous wave functions for that electron is used—the most common being a simple linear combination of the calculated and immediately preceding wave function. A clever dodge, employed by Hartree, for atomic calculations was to increase the nuclear charge, thus pulling all the electrons closer together. As the system stabilised, this was gradually reduced to the correct charge. In molecular calculations a similar approach is sometimes used by first calculating the wave function for a positive ion and then to use these orbitals as the starting point for the neutral molecule. Modern molecular Hartree–Fock computer programs use a variety of methods to ensure convergence of the Roothaan–Hall equations.

Weaknesses, extensions, and alternatives

Of the five simplifications outlined in the section "Hartree–Fock algorithm", the fifth is typically the most important. Neglecting electron correlation can lead to large deviations from experimental results. A number of approaches to this weakness, collectively called w:post-Hartree–Fock methods, have been devised to include electron correlation to the multi-electron wave function. One of these approaches, w:Møller–Plesset perturbation theory, treats correlation as a perturbation of the Fock operator. Others expand the true multi-electron wave function in terms of a linear combination of Slater determinants—such as w:multi-configurational self-consistent field, w:configuration interaction, w:quadratic configuration interaction, and complete active space SCF (CASSCF). Still others (such as variational quantum Monte Carlo) modify the Hartree–Fock wave function by multiplying it by a correlation function ("Jastrow" factor), a term which is explicitly a function of multiple electrons that cannot be decomposed into independent single-particle functions.

An alternative to Hartree–Fock calculations used in some cases is w:density functional theory, which treats both exchange and correlation energies, albeit approximately. Indeed, it is common to use calculations that are a hybrid of the two methods—the popular B3LYP scheme is one such w:hybrid functional method. Another option is to use w:modern valence bond methods.

Software packages

For a list of software packages known to handle Hartree–Fock calculations, particularly for molecules and solids, see the w:list of quantum chemistry and solid state physics software.

Sources

• Levine, Ira N. (1991). Quantum Chemistry (4th ed.). Englewood Cliffs, New Jersey: Prentice Hall. pp. 455–544. ISBN 0-205-12770-3.
• Cramer, Christopher J. (2002). Essentials of Computational Chemistry. Chichester: John Wiley & Sons, Ltd. pp. 153–189. ISBN 0-471-48552-7.
• Szabo, A.; Ostlund, N. S. (1996). Modern Quantum Chemistry. Mineola, New York: Dover Publishing. ISBN 0-486-69186-1.

Slater determinant

Two-particle case

The simplest way to approximate the wave function of a many-particle system is to take the product of properly chosen orthogonal wave functions of the individual particles. For the two-particle case with spatial coordinates $\mathbf {x} _{1}$ and $\mathbf {x} _{2}$, we have

$\Psi (\mathbf {x} _{1},\mathbf {x} _{2})=\chi _{1}(\mathbf {x} _{1})\chi _{2}(\mathbf {x} _{2}).$

This expression is used in the w:Hartree–Fock method as an w:ansatz for the many-particle wave function and is known as a w:Hartree product. However, it is not satisfactory for w:fermions because the wave function above is not antisymmetric, as it must be for w:fermions from the w:Pauli exclusion principle. An antisymmetric wave function can be mathematically described as follows:

$\Psi (\mathbf {x} _{1},\mathbf {x} _{2})=-\Psi (\mathbf {x} _{2},\mathbf {x} _{1})$

which does not hold for the Hartree product. Therefore the Hartree product does not satisfy the Pauli principle. This problem can be overcome by taking a w:linear combination of both Hartree products

$\Psi (\mathbf {x} _{1},\mathbf {x} _{2})={\frac {1}{\sqrt {2}}}\{\chi _{1}(\mathbf {x} _{1})\chi _{2}(\mathbf {x} _{2})-\chi _{1}(\mathbf {x} _{2})\chi _{2}(\mathbf {x} _{1})\}$

$={\frac {1}{\sqrt {2}}}{\begin{vmatrix}\chi _{1}(\mathbf {x} _{1})&\chi _{2}(\mathbf {x} _{1})\\\chi _{1}(\mathbf {x} _{2})&\chi _{2}(\mathbf {x} _{2})\end{vmatrix}}$

where the coefficient is the w:normalization factor. This wave function is now antisymmetric and no longer distinguishes between fermions, that is: one cannot indicate an ordinal number to a specific particle and the indices given are interchangeable. Moreover, it also goes to zero if any two wave functions of two fermions are the same. This is equivalent to satisfying the Pauli exclusion principle.

Generalizations

The expression can be generalised to any number of fermions by writing it as a w:determinant. For an N-electron system, the Slater determinant is defined as

$\Psi (\mathbf {x} _{1},\mathbf {x} _{2},\ldots ,\mathbf {x} _{N})={\frac {1}{\sqrt {N!}}}\left|{\begin{matrix}\chi _{1}(\mathbf {x} _{1})&\chi _{2}(\mathbf {x} _{1})&\cdots &\chi _{N}(\mathbf {x} _{1})\\\chi _{1}(\mathbf {x} _{2})&\chi _{2}(\mathbf {x} _{2})&\cdots &\chi _{N}(\mathbf {x} _{2})\\\vdots &\vdots &\ddots &\vdots \\\chi _{1}(\mathbf {x} _{N})&\chi _{2}(\mathbf {x} _{N})&\cdots &\chi _{N}(\mathbf {x} _{N})\end{matrix}}\right|\equiv \left|{\begin{matrix}\chi _{1}&\chi _{2}&\cdots &\chi _{N}\\\end{matrix}}\right|,$

where in the final expression, a compact notation is introduced: the normalization constant and labels for the fermion coordinates are understood – only the wavefunctions are exhibited. The linear combination of Hartree products for the two-particle case can clearly be seen as identical with the Slater determinant for N = 2. It can be seen that the use of Slater determinants ensures an antisymmetrized function at the outset; symmetric functions are automatically rejected. In the same way, the use of Slater determinants ensures conformity to the w:Pauli principle. Indeed, the Slater determinant vanishes if the set {χ_i } is w:linearly dependent. In particular, this is the case when two (or more) spin orbitals are the same. In chemistry one expresses this fact by stating that no two electrons can occupy the same spin orbital. In general the Slater determinant is evaluated by the w:Laplace expansion. Mathematically, a Slater determinant is an antisymmetric tensor, also known as a w:wedge product.

A single Slater determinant is used as an approximation to the electronic wavefunction in Hartree–Fock theory. In more accurate theories (such as w:configuration interaction and w:MCSCF), a linear combination of Slater determinants is needed.

The word "detor" was proposed by S. F. Boys to describe the Slater determinant of the general type, but this term is rarely used.

Unlike w:fermions that are subject to the Pauli exclusion principle, two or more w:bosons can occupy the same quantum state of a system. Wavefunctions describing systems of identical w:bosons are symmetric under the exchange of particles and can be expanded in terms of w:permanents.

Fock matrix

In the w:Hartree–Fock method of w:quantum mechanics, the Fock matrix is a matrix approximating the single-electron w:energy operator of a given quantum system in a given set of basis vectors.

It is most often formed in w:computational chemistry when attempting to solve the w:Roothaan equations for an atomic or molecular system. The Fock matrix is actually an approximation to the true Hamiltonian operator of the quantum system. It includes the effects of electron-electron repulsion only in an average way. Importantly, because the Fock operator is a one-electron operator, it does not include the w:electron correlation energy.

The Fock matrix is defined by the Fock operator. For the restricted case which assumes w:closed-shell orbitals and single-determinantal wavefunctions, the Fock operator for the i-th electron is given by:

${\hat {F}}(i)={\hat {h}}(i)+\sum _{j=1}^{n/2}[2{\hat {J}}_{j}(i)-{\hat {K}}_{j}(i)]$

where:

${\hat {F}}(i)$ is the Fock operator for the i-th electron in the system,

${\hat {h}}(i)$ is the w:one-electron hamiltonian for the i-th electron,

$n$ is the number of electrons and ${\frac {n}{2}}$ is the number of occupied orbitals in the closed-shell system,

${\hat {J}}_{j}(i)$ is the w:Coulomb operator, defining the repulsive force between the j-th and i-th electrons in the system,

${\hat {K}}_{j}(i)$ is the w:exchange operator, defining the quantum effect produced by exchanging two electrons.

The Coulomb operator is multiplied by two since there are two electrons in each occupied orbital. The exchange operator is not multiplied by two since it has a non-zero result only for electrons which have the same spin as the i-th electron.

For systems with unpaired electrons there are many choices of Fock matrices.

Hartree-Fock (HF) or self-consistent field (SCF)

Density Functional Theory

Connection to quantum state symmetry

The Pauli exclusion principle with a single-valued many-particle wavefunction is equivalent to requiring the wavefunction to be antisymmetric. An antisymmetric two-particle state is represented as a sum of states in which one particle is in state $\scriptstyle |x\rangle$ and the other in state $\scriptstyle |y\rangle$:

$|\psi \rangle =\sum _{x,y}A(x,y)|x,y\rangle$

and antisymmetry under exchange means that A(x,y) = −A(y,x). This implies that A(x,x) = 0, which is Pauli exclusion. It is true in any basis, since unitary changes of basis keep antisymmetric matrices antisymmetric, although strictly speaking, the quantity A(x,y) is not a matrix but an antisymmetric rank-two w:tensor.

Conversely, if the diagonal quantities A(x,x) are zero in every basis, then the wavefunction component:

$A(x,y)=\langle \psi |x,y\rangle =\langle \psi |(|x\rangle \otimes |y\rangle )$

is necessarily antisymmetric. To prove it, consider the matrix element:

$\langle \psi |((|x\rangle +|y\rangle )\otimes (|x\rangle +|y\rangle ))\,$

This is zero, because the two particles have zero probability to both be in the superposition state $\scriptstyle |x\rangle +|y\rangle$. But this is equal to

$\langle \psi |x,x\rangle +\langle \psi |x,y\rangle +\langle \psi |y,x\rangle +\langle \psi |y,y\rangle \,$

The first and last terms on the right hand side are diagonal elements and are zero, and the whole sum is equal to zero. So the wavefunction matrix elements obey:

$\langle \psi |x,y\rangle +\langle \psi |y,x\rangle =0\,$.

or

$A(x,y)=-A(y,x)\,$

Pauli principle in advanced quantum theory

According to the w:spin-statistics theorem, particles with integer spin occupy symmetric quantum states, and particles with half-integer spin occupy antisymmetric states; furthermore, only integer or half-integer values of spin are allowed by the principles of quantum mechanics. In relativistic w:quantum field theory, the Pauli principle follows from applying a rotation operator in imaginary time to particles of half-integer spin. Since, nonrelativistically, particles can have any statistics and any spin, there is no way to prove a spin-statistics theorem in nonrelativistic quantum mechanics.

In one dimension, bosons, as well as fermions, can obey the exclusion principle. A one-dimensional Bose gas with delta function repulsive interactions of infinite strength is equivalent to a gas of free fermions. The reason for this is that, in one dimension, exchange of particles requires that they pass through each other; for infinitely strong repulsion this cannot happen. This model is described by a quantum w:nonlinear Schrödinger equation. In momentum space the exclusion principle is valid also for finite repulsion in a Bose gas with delta function interactions, as well as for interacting spins and w:Hubbard model in one dimension, and for other models solvable by w:Bethe ansatz. The ground state in models solvable by Bethe ansatz is a Fermi sphere.

Density Functional Theory

References

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Physical Chemistry of Surfaces

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Physical Chemistry of Surfaces

• Introduction to Surface Chemistry

Surface Forces

Sketch of some important forces involved in surface interactions.

Hydrophobic and hydrophilic surfaces

• Wetablility
• wetting
• Hydrophobe
• Hydrophile
• Super hydrophilicity

Surface tension and the wetting angle on hydrophobic and hydrophilic surfaces

Surface Energy

• Surface energy

Surface Diffusion

• Diffusion

References

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Background material

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Dispersion relations

Dispersion relations are essential in describing a physical system and can often be a bit tricky to comprehend.

References

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Part 4: Nanomaterials

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This part will give an overview of nanomaterials - the production methods, their properties, brief examples of applications and demonstrations of their capabilities.

Apart from a general overview of production methods, we have divided the materials as

• Semiconducting
• Metallic
• Organic

A division that might be up for revision shortly.

One way to classify a material is its electronic structure Overview of the electronic structure of the different fundamental classes of materials. Atoms have discrete energy levels for each electronic state. Electronic transitions by eg. optical excitation can change the state of the atom. Molecules can also have discrete energy levels, but the more complex structure also gives a much more complex diagram of electronic states. In addition, the molecules con rotate and vibrate which modulates the oberserved energy levels. Insulators can be seen as a condensed phase of molecules with little electronic connection between neighboring molecules for conducting a current. Only when excitation is made with an energy above the several eV bandgap will conduction be possible. Semiconductors have a more narrow bandgap and even at room temperature a few conduction electrons will be excited into the conductance band. Doped Semiconductors have higher electrical conductance because added dopants provide conduction electrons. Metals can be considered as ionized metal atoms in a sea of free electrons, giving a high conductivity and high reflectivity of light (as long as it is not too high in frequency).

Another is according to their geometry overview of nanoscale structure geometries

Overview of nanomaterials

A brief overview table of nanostructures

Type	Structure	Production	Properties
Buckyballs/ C60	Buckminsterfullerene C60 -60 Carbon atoms arranged as in football
Single Wall Carbon Nanotubes (SWCNT)	This animation of a rotating Carbon nanotube shows its 3D structure. A single shell of carbon atoms arranged in a cylindrical chicken-wire-like hexagonal structure with diameter from about 2 nm.		Semiconducting or metallic depending on how the carbon lattice is twisted.
Multi Wall Carbon Nanotubes (MWCNT)	Concentric shells of SWCNT, diameter up to hundreds of nm
Silicon Nanowires and heterostructures	Silicon crystals with diameters from a few nm	Typically VLS growth
III-V Nanowires	crystals with diameters from a few nm	typically VLS growth. A wealth of heterostructures can be formed to make tunnel barrier junctions etc.	Semiconducting, often optically active and fluorescing due to direct bandgap (unlike silicon).
Gold nanoparticles
Silica nanoparticles
Platinum nanoparticles	small metallic clusters		Used as catalysts in many reactions

Further reading

• Carbon Nanotube Cookbook

References

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Overview of Production methods

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• Chemical vapor deposition
• Metal Organic Vapor Phase Epitaxy (MOVPE)
• Arc discharge where an electical arc is made between rods of the precursor materials.
• Physical vapor deposition
• Molecular beam epitaxy using solid sources and Chemical beam epitaxy using gaseous sources.
• Epitaxy in general.
• Atomic layer epitaxy where only a few monolayers are deposited and bond to chemisorption sites on the surface.
• Self-assembled monolayers
• metal organic chemical vapor deposition abbreviation as MOCVD
• atomic layer deposition, for depositing uniform thin films one atomic layer at a time (By [http://www.fmnt.fi/index.pl?id=2913&isa=Category&op=show Dr. Tuomo Suntola and co-workers in Finland.)

Types of nanometal synthesis

The most common types of nanometal synthesis deal with 'wet' methods in which metal nanoparticles are produced in a colloid with an organic material of some sort.

Gold nanoparticles can be produced by either:

1) Reduction of HAuCl₄ in a solution of sodium citrate, then boiling it, causing gold nanoparticles to form in a wine-red solution.

2) Mixing HAuCl₄ in water, which produces a solution that is subsequently transferred into toluene using tetraoctylammonium bromide (TOAB), a phase transfer catalyst. Phase transfer catalysts help reactants dissolve in organic (carbon-containing) material where the reactant otherwise couldn't w/o the PTC. Afterwards, the solution is stirred with sodium borohydride, in the presence of certain alkanes, which bind to the gold in the solution, allowing for the formation of gold nanoparticles.

Synthesis of other metal nanoparticles can possibly be achieved by reducing metal salts in organic solvents such as ethanol, or by variations of the above methods which synthesize gold nanoparticles.

References

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Semiconducting Nanostructures

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Nanotubes

Certain compounds are capable of forming nanotubes where the tube consists of round shell of a single layer of atoms in a cylindrical lattice. Carbon nanotubes is the most famous example, but also other materials can form nanotubes such as Boron-nitride, Molybdenum sulfide and others.

Nanotubes can also be made by etching the core out of an shell structured rod, but such tubes will normally contain many atomic layers in the wall and have crystal facets on the sides.

Carbon Nanotubes

Carbon nanotubes are fascinating nanostructures. A sheet of graphene as in common graphite, but rolled up in small tubes rather than planar sheets.

Carbon nanotubes have unique mechanical properties such as high strength, high stiffness, and low density and also interesting electronic properties. A single-walled carbon nanotube can be either metallic or semiconducting depending on the atomic arrangement .

This section is a short introduction to carbon nanotubes. For a broader overview the reader is referred to one of the numerous review articles or books on carbon nanotubes

Geometric Structure

The simplest type of carbon nanotube consists of just one layer of graphene rolled up in the form of a seamless cylinder, known as a single-walled carbon nanotube (SWCNT) with a typical diameter of just a few nanometers. Larger diameter nanotube structures are nanotube ropes, consisting of many individual, parallel nanotubes closed-packed into a hexagonal lattice, and multi-walled carbon nanotubes (MWCNTs) consisting of several concentric cylinders nested within each other.

Multiwall carbon nanotube (MWCNT) sample made by a CVD process using iron containing catalystic particles. The MWCNT are adhering in mats.

Single walled Carbon Nanotube

The basic configuration is thus the SWCNT. Its structure is most easily illustrated as a cylindrical tube conceptually formed by the wrapping of a single graphene sheet. The hexagonal structure of the 2-dimensional graphene sheet is due to the $sp^{2}$ hybridization of the carbon atoms, which causes three directional, in-plane $\sigma$ bonds separated by an angle of 120 degrees.

The nanotube can be described by a chiral vector $C$ that can be expressed in terms of the graphene unit vectors ${\textbf {a}}_{1}$ and ${\textbf {a}}_{2}$ as ${\textbf {C}}=n{\textbf {a}}_{1}+m{\textbf {a}}_{2}$ with the set of integers $(n,m)$ uniquely identifying the nanotube. This chiral vector or 'roll-up' vector describes the nanotube circumference by connecting two crystallographically equivalent positions i.e. the tube is formed by superimposing the two ends of ${\textbf {C}}$.

Based on the chiral angle SWCNTs are defined as zig-zag tubes ($\theta =0deg\leftrightarrow m=0$), armchair tubes ($\theta =30deg\leftrightarrow n=m$), or chiral tubes ($0deg<\theta <30deg$).

Multiwalled Carbon Nanotubes

MWCNTs are composed of a number of SWCNTs in a coaxial geometry. Each nested shell has a diameter of $d={\sqrt {3}}a_{C-C}(m^{2}+n^{2}+mn)^{1/2}/\pi$ where $a_{C-C}$ is the length of the carbon-carbon bond which is 1.42 Å. The difference in diameters of the individual shell means that their chiralities are different, and adjacent shell are therefore in general non-commensurate, which causes only a weak intershell interaction.

The intershell spacing in MWCNTs is $\sim$ 0.34 nm - quite close to the interlayer spacing in turbostratic graphite

Electronic Structure

The electronic structure of a SWCNT is most easily described by again considering a single graphene sheet. The 2-D, hexagonal-lattice graphene sheet has a 2-D reciprocal space with a hexagonal Brillouin zone (BZ).

The $\sigma$ bonds are mainly responsible for the mechanical properties, while the electronic properties are mainly determined by the $\pi$ bands. By a tight-binding approach the band structure of these $\pi$ bands can be calculated

Graphene is a zero-gap semiconductor with an occupied $\pi$ band and an unoccupied $\pi ^{*}$ band meeting at the Fermi level at six $K$ points in the BZ, thus it behaves metallic, a so-called semimetal.

Upon forming the tube by conceptually wrapping the graphene sheet, a periodic boundary condition is imposed that causes only certain electronic states of those of the planar graphene sheet to be allowed. These states are determined by the tube's geometric structure, i.e. by the indices $(n,m)$ of the chiral vector. The wave vectors of the allowed states fall on certain lines in the graphene BZ.

Based on this scheme it is possible to estimate whether a particular tube will be metallic or semiconducting. When the allowed states include the $K$ point, the system will to a first approximation behave metallic. However, in the points where the $\pi$ and the $\pi ^{*}$ bands meet but are shifted slightly away from the $K$ point due to curvature effects, which causes a slight band opening in some cases

This leads to a classification scheme that has three types of nanotubes:

• Metallic: These are the armchair tubes where the small shift of the degenerate point away from the $K$ point does not cause a band opening for symmetry reasons.

• Small-bandgap semiconducting: These are characterized by $n-m=3j$ with $j$ being an integer. Here, the wave vectors of the allowed states cross the $K$ point, but due to the slight shift of the degenerate point a small gap will be present, the size of which is inversely proportional to the tube diameter squared with typical values between a few and a few tens meV

• Semiconducting: In this case $n-m\neq 3j$. This causes a larger bandgap, the size of which is inversely proportional to the tube diameter: $E_{g}=k/d$ with experimental investigations suggesting a value of $k$ of 0.7-0.8 eV/nm

Typically the bandgap of the type 2 nanotubes is so small that they can be considered metallic at room temperature. Based on this it can be inferred that 1/3 of all tubes should behave metallic whereas the remaining 2/3 should be semiconducting. However, it should be noted that due to the inverse proportionality between the bandgap and the diameter of the semiconducting tubes, large-diameter tubes will tend to behave metallic at room temperature. This is especially important in regards to large-diameter MWCNTs.

From a electrical point of view a MWCNT can be seen as a complex structure of many parallel conductors that are only weakly interacting. Since probing the electrical properties typically involves electrodes contacting the outermost shell, this shell will be dominating the transport properties In a simplistic view, this can be compared to a large-diameter SWCNT, which will therefore typically display metallic behavior.

Electrical and Electromechanical Properties

Many studies have focused on SWCNTs for exploring the fundamental properties of nanotubes. Due to their essentially 1-D nature and intriguing electronic structure, SWCNTs exhibit a range of interesting quantum phenomena at low temperature

The discussion here will so far, however, primarily be limited to room temperature properties.

The conductance $~G~$ of a 1-dimensional conductor such as a SWCNT is given by the Landauer formula

$~G=G_{0}\sum _{i}T_{i}~$,

where $~G_{0}=2e^{2}/h=(12.9{\rm {k\Omega )^{-1}}}$;
$~1/G_{0}~$ is the conductance quantum;
and $~T_{i}~$ is the transmission coefficient of the contributing channel $~i~$.

More information on nanotubes

• Wikipedia Carbon nanotubes
• The nanotube site
• Compiled overview of properties of carbon nanotubes

Commercial suppliers of carbon nanotubes and related products

• Nanoledge.com nanotubes and related products - fibers, pellets, resins and dispersions...
• Carbon Solution, Inc. Mainly SWCNT.
• BuckyUSA Fullerens, SWCNT, MWCNT.
• CNI HiPco Carbon nanotubes
• Carbolex SWCNT,sub-gram quantities are sold via sigma aldric
• Sigma-Aldrich SWCNT

"Buckyball"

C₆₀ with isosurface of ground state electron density as calculated with DFT

An w:association football is a model of the Buckminsterfullerene C₆₀

Buckminsterfullerene (IUPAC name (C₆₀-I_h)[5,6]fullerene) is the smallest fullerene molecule in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also the most common in terms of natural occurrence, as it can often be found in soot.

The structure of C₆₀ is a truncated (T = 3) icosahedron, which resembles a soccer ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.

The w:van der Waals diameter of a C₆₀ molecule is about 1 nanometer (nm). The nucleus to nucleus diameter of a C₆₀ molecule is about 0.7 nm.

The C₆₀ molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 angstroms.

Silicon buckyballs have been created around metal ions.

Boron buckyball

A new type of buckyball utilizing boron atoms instead of the usual carbon has been predicted and described by researchers at Rice University. The B-80 structure, with each atom forming 5 or 6 bonds, is predicted to be more stable than the C-60 buckyball. One reason for this given by the researchers is that the B-80 is actually more like the original geodesic dome structure popularized by Buckminster Fuller which utilizes triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemists as it was concluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage undergoes a spontaneous symmetry break yielding a puckered cage with rare Th symmetry (symmetry of a volleyball). The number of six atom rings in this molecule is 20 and number of five member rings is 12. There is an additional atom in the center of each six member ring, bonded to each atom surrounding it.

Variations of buckyballs

Another fairly common buckminsterfullerene is C₇₀, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.

In mathematical terms, the structure of a fullerene is a trivalent convex polyhedron with pentagonal and hexagonal faces. In graph theory, the term fullerene refers to any 3-regular, planar graph with all faces of size 5 or 6 (including the external face). It follows from Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|, |F| indicate the number of vertices, edges, and faces), that there are exactly 12 pentagons in a fullerene and |V|/2-10 hexagons.

20-fullerene (dodecahedral graph)	26-fullerene graph	60-fullerene (truncated icosahedral graph)	70-fullerene graph

The smallest fullerene is the w:dodecahedron--the unique C₂₀. There are no fullerenes with 22 vertices. The number of fullerenes C_2n grows with increasing n = 12,13,14..., roughly in proportion to n⁹. For instance, there are 1812 non-isomorphic fullerenes C₆₀. Note that only one form of C₆₀, the buckminsterfullerene alias w:truncated icosahedron, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes C₂₀₀, 15,655,672 of which have no adjacent pentagons.

w:Trimetasphere carbon nanomaterials were discovered by researchers at w:Virginia Tech and licensed exclusively to w:Luna Innovations. This class of novel molecules comprises 80 carbon atoms (C80) forming a sphere which encloses a complex of three metal atoms and one nitrogen atom. These fullerenes encapsulate metals which puts them in the subset referred to as w:metallofullerenes. Trimetaspheres have the potential for use in diagnostics (as safe imaging agents), therapeutics and in organic solar cells.

Semiconducting nanowires

Semiconducting nanowires can be made from most semiconducting materials and with different methods, mainly variations of a chemical vapor deposition process (CVD).

There are many different semiconducting materials, and heterosrtuctures can be made if the lattice constants are not too incompatible. Heterostructures made from combinations of materials such as GaAs-GaP can be used to make barriers and guides for electrons in electrical systems.

Low pressure metal organic vapor phase epitaxy (MOVPE) can be used to grow III-V nanowires epitaxially on suitable crystalline substrates, sucha s III-V materials or silicon with a reasonably matching lattice constant.

Low pressure metal organic vapor phase epitaxy (MOVPE) can be used to grow III-V nanowires epitaxially on suitable crystalline substrates, sucha s III-V materials or silicon with a reasonably matching lattice constant. Nanowire growth is catalyzed by various nanoparticles, which are deposited on the substrate surface, typically gold nanoparticles with a diameter of 20-100nm.

Nanowire growth is catalyzed by various nanoparticles, which are deposited on the substrate surface, typically gold nanoparticles with a diameter of 20-100nm.

To grow for instance GaP wires, the sample is typically annealed at 650C in the heated reactor chamber to form an eutectic with between the gold catalyst and the underlying substrate.

Then growth is done at a lower temperature around 500C in the presence of the precursor gasses trimethyl gallium and phosphine. By changing the precursor gasses during growth, nanowire heterostructures with varying composition can be made

SEM image of epitaxial nanowire heterostructures grown from catalytic gold nanoparticles

Resources

• wikipedia Semiconducting nanowires
• IOFFE semiconductor physical properties

Nanoparticles

• Nanoparticles
• Quantum dots

Catalytic particles

• Catalyst
• Catalysis
• Haber process

Commercial suppliers of nanoparticles

• Reade has a wide selection of nanoparticles
• sigma-aldrich has dispersions of nanoparticles
• nanophase

Contributors and Acknowledgements

• Jakob Kjelstrup Hansen

References

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Metallic Nanostructures

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Gold

The red color in some stained glasses has through centuries been made from gold nanoparticles, and its also gold particles that make the red color in many pregnancy tests. Gold nanoparticles are used on many technoogies: they have a red color because of a plasmon resonance and the noble metal makes them good for specific binding of molecules with thiol groups.

Copper

When copper is viewed on the nano level, several changes occur. the temperature of the metal decreases, as well as the fatigue limit decreases. Also, the tensile stress and elongation rate of the copper decreases.

this info is from: http://cat.inist.fr/?aModele=afficheN&cpsidt=14408198

References

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Organic Nanomaterials

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References

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Quick Review of nanometals

Nanometal (also called metal nanoparticles) is very attractive and that is because of their size and shape dependent properties. The optical properties (linear and nonlinear) depend on that and they on dominated by something called the collective oscillation of conduction electrons. There are so many ways that you can prepare metal nanoparticles but the most used methods are based on wet chemistry. You can find nanometal being used in medical applications to the weaponry the military use. Nanometals also have a thing called Surface Plasmon Resonance (SPR) this is what cause the change in colors that we see. For example in the 4th century when the Lycurgus cup was created. The cup changes red when the light is shone inside of the cup and green when reflective light is shone on the outside of it. There are so many methods that you can prepare nanometals and the most popular way is by reducing HAuCl₄ (chlorauric acid) in a sodium citrate solution that is boiling and then the formation of gold nanoparticles are revealed by a deep red color that looks like wine in about 10 min.

Types of nanometal synthesis

The most common types of nanometal synthesis deal with 'wet' methods in which metal nanoparticles are produced in a colloid with an organic material of some sort.

Gold nanoparticles can be produced by either:

1) Reduction of HAuCl₄ in a solution of sodium citrate, then boiling it, causing gold nanoparticles to form in a wine-red solution.

2) Mixing HAuCl₄ in water, which produces a solution that is subsequently transferred into toluene using tetraoctylammonium bromide (TOAB), a phase transfer catalyst. Phase transfer catalysts help reactants dissolve in organic (carbon-containing) material where the reactant otherwise couldn't w/o the PTC. Afterwards, the solution is stirred with sodium borohydride, in the presence of certain alkanes, which bind to the gold in the solution, allowing for the formation of gold nanoparticles.

Synthesis of other metal nanoparticles can possibly be achieved by reducing metal salts in organic solvents such as ethanol, or by variations of the above methods which synthesize gold nanoparticles.

References

Part 5: Nanosystems

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This part describes the subfields of nanotechnology with a very technological aim:

• Nanomechanics
• Nano-optics and Nanophotonics
• Nanofluidics
• Nanoelectronics

References

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Nanoelectronics

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Nanoelectronics

Nanoelectronics is expected to be cheaper to fabricate than silicon / gallium / arsenic based electronics. It also might be small enough to not require a power source as it is possible to abstract a small amount of energy from the surrounding heat by a molecular level energy scavenging system

Double barrier systems

• Coulomb Blockade

Coulomb Blockade

• Tunnel Junctions
• Quantum Tunneling

Moletronics / Molecular Electronics

Using molecules for electronics, often called moletronics or molecular electronics , is a new technology which is still in its infancy, but also brings hope for truly atomic scale electronic systems in the future.

One of the more promising applications of molecular electronics was proposed by the IBM researcher Ari Aviram and the theoretical chemist Mark Ratner in their 1974 and 1988 papers Molecules for Memory, Logic and Amplification, (see Unimolecular rectifier ) . This is one of many possible ways in which a molecular level diode / transistor might be synthesized by organic chemistry. A model system was proposed with a spiro carbon structure giving a molecular diode about half a nanometre across which could be connected by polythiophene molecular wires. Theoretical calculations showed the design to be sound in principle and there is still hope that such a system can be made to work.

However one researcher, experimentalist Jan Hendrik Schön, could not wait for the necessary technical progress and at a time when he was publishing one scientific paper a week and winning scholarships, heading for the top in nanotechnology, it was discovered he had fabricated both the experiment where such a device worked and several other potentially important milestones in the field. This incident is discussed by David Goodstein in Physics World [93]. However it seems only a matter of time before something like this proposed elegant solution to the problem demonstrates the behavior of a diode.

Quantum Computing

A quantum computer would be incredibly fast compared to microelectronics. It would also be able to use the properties of quantum mechanics to be in fuzzy states which could represent many numbers at once allowing a massive density of memory. How such devices would be nano-fabricated is however way beyond current technology.

The first working 2-qubit quantum computer was demonstrated in 1998.

In 2006, the first working 12 qubit quantum computer was demonstrated.

Bibliography

• Michel le Bellac, A Short Introduction to Quantum Information and Quantum Computation, Cambridge University Press (2006) ISBN 978-0-521-86056-7.
• Michael A. Nielsen and Isaac L. Chuang, Quantum Computation and Quantum Information, Cambridge University Press (2000) ISBN 978-0-521-63235-5.

Resources on the net

• Keithley.com has some freely available handbooks on low noise measurements in electronics and nanotechnology
• Nanoelectronics Overview at Understandingnano

References

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Nano-optics

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Nanophotonics and nanooptics

With the increasing demand for smaller, faster, and more highly integrated optical and electronic devices; as well as extremely sensitive detectors for biomedical and environmental applications; a field called nano-optics or nano-photonics is emerging - studying the many promising optical properties of nanostructures.

Like nanotechnology itself, it is a rapidly evolving and changing field – but because of strong research activity in optical communication and related devices, combined with the intensive work on nanotechnology, nano-optics appears to be a field with a promising future.

Nanophotonics is seen as a crucial technology for extending Moore's Law into the next few decades. In the past few years nanophotonics researchers worldwide have developed, On Chip Silicon Lasers, Gigahertz Silicon Electro Optic Switches, and Low Loss Highly Integratable Compact Nanowires (With waveguides of 100’s of nanometers' width).

Nanophotonics is mainly expected to play a complementary role to micro/nano electronics on chip and extend the capacity of telecommunication networks into the Terabit/s regime. One of the major emphasis’s in the last few years has been developing on-chip interconnects to break the bottle neck for higher data rates within integrated chips.

In conjugation with Nanofluidics, Nanophotonics is also finding applications in biomedical sensors, Medical diagnosis, etc.

Nanophotonic components such as Microcavities with ultra high life time of trapped photons are expected to find applications in fundamental experimental physics such as gravitational wave detection.

Intel, IBM, Lucent, and Luxtera have highly functional and well funded nanophotonic research groups. A number of universities in: US, UK, Japan, Italy, China, Belgium, etc. have been actively pursuing nanophotonics. Apart from a growing number of hits on the word in publication databases like "Web of Science", which shows it is already getting increased attention, it is also increasingly mentioned in the aims of the funding agencies, which will surely add to the activity in the field as increased economical support becomes available.

Electrooptic modulators

Electro-optic modulators are devices used to modulate, or modify a beam of light. Currently they are mainly used in the information technology and telecommunications industries (e.g. fiber-optic cables). EOM’s have good potential in nanophotonics. Nanoscale optical communication devices will have increased speed and efficiency, once they can be engineered and used. Nano-size electrooptic modulators will be an integral part of a nanoscale communications network.

Photodetector

Photodetectors respond to radiant energy. They are basically sensors of light or other electromagnetic energy. A sensor is a electronic device that converts one type of energy to another for various reasons. Nanoscale size photodetectors will be an integral part of a theoretical nanoscale optical information network.

Electrooptic switches

Electrooptic switches change signals in optical fibers to electrical signals. Typically semiconductor-based, their function depends on the change of refractive index with electric field. This feature makes them high-speed devices with low power consumption. Neither the electro-optic nor thermo-optic optical switches can match the insertion loss, back reflection, and long-term stability of opto-mechanical optical switches. The latest technology combines all-optical switches that can cross-connect fibers without translating the signal into the electrical domain. This greatly increases switching speed, allowing today's telcos and networks to increase data rates. However, this technology is only now in development, and deployed systems cost much more than systems that use traditional opto-mechanical switches.

Photonic crystals

"Photonic crystals are composed of periodic dielectric or metallo-dielectric nanostructures that are designed to affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. Simply put, photonic crystals contain regularly repeating internal regions of high and low dielectric constant." Photonic crystals are used to modify or control the flow of light. Photonic crystals may have a novel use in optical data transmission but are not extremely prominent. They may be used to filter for interference in a fiber optic cable, or increase the quality of the transmission. In addition, they can be used to divide different wavelengths of light. Photonic crystals can already be manufactured at close to the nanoscale.

Sensors

Nanotechnology creates many new, interesting fields and applications for photonic sensors. Existing uses, like digital cameras, can be enhanced because more ‘pixels’ can be placed on a sensor than with existing technology. In addition, sensors can be fabricated on the nano-scale so that they will be of higher quality, and possibly defect free. The end result would be that photos would be larger, and more accurate. As part of a communication network, photonic sensors will be used to convert optical data (photons) into electricity (electrons). Nanoscale photonic sensors will be more efficient and basically receive similar advantages to other materials constructed under the nanoscale.

Multiplexers

A multiplexer is a device for converting many data streams into one single data stream, which is then divided into the separate data streams on the other side with a demultiplexer. The main benefit is cost savings, since only one physical link will be needed, instead of many physical links. In nano-optics, multiplexers will have many applications. They can be used as part of a communication network, as well as utilized on a smaller scale for various modern scientific instruments.

Vanadium dioxide

Vanadium dioxide has the interesting property of changing from a transparent state to a reflective, mirror-like state in less than 100 femtoseconds (1/10 of a trillionth of a second). Vanderbilt University discovered the transition at 68 degrees celsius. The temperature that the transition happens can be changed by adding small amounts of impurities, and it is possible to lower the temperature by as much as 35 degrees celsius. However, there is a size limit, the change will not occur in particles that are smaller than 20 atoms across, or 10 nanometers. This property has many applications. Possibilities are a 'solar shade' window that changes from letting light in, to reflecting light back automatically when the temperature starts rising. Also, nanosensors could be created which could measure the temperature at different locations in human cells. However, most importantly, this transition can be utilized in creating an 'ultrafast' optical switch which could be used in communications or computing. Currently, researchers are seeing if they can put a layer of vanadium dioxide nanoparticles on the end of an optical fiber to create a very high speed link.

Quantum dots

Quantum dots have several applications. One of the first applications found was their ability to emit very specific wavelengths of light. This is different from other light emitting bulbs since quantum dots could be tuned across the visible and ultraviolet spectrums very precisely. Researchers have found that if they put about 2,000 quantum dots together, they would have a finely tuned LED. Researchers have tried for an extremely long time to get these dots to emit light. In the 1990’s someone was able to get a dark red light. Since then other researchers have been able to tune the dots to a higher frequency, thus gaining blue and green light. The applications for this would be beneficial so that we could make full color screens and monitors.

Resources

• Near and far field - near and far field radiation can to some extent be compared to listening to a walkmans earphones; the one carrying the earphones can hear the sound perfectly even though the bass sound wavelength is much larger than the earphone. If you are not wearing the earphones, the high frequency sounds will be much higher than the bass. The bass can only be heard in the near field.
• plasmonics
• Rochester Nano Optics

Bibliography

• Lucas Novotny and Bert Hect, Principles of Nano-Optics, Cambridge University Press (2006).

References

Nanomechanics

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Example of a nanomechanical system: Molecularly sized gears

Nanomechanics

Some of the mechanical aspects of nanotechnology is

• Extreme Youngs modulus materials
• High frequency resonances in nanoscale oscillators

NEMS

Nano-electro-mechanical systems

Mechanics of beams and cantilevers

Cantilevers are essential in many mechanical systems: Nanotubes, nanowires and atomic force microscopes...

The harmonic oscillator

Fundamental in the description of any oscillating systems is the harmonic oscillator and the quantum mechanical version, the Quantum harmonic oscillator.

References

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Nanofluidics

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Flow on the large scale, in Microsystems and in nanosystems show very different behaviour.

In nanofluidic systems you willl have

• Small sample volume
• High area to volume ratio
• Domination of surface forces
• Short diffusion times
• Enhanced reaction kinetics due to short diffusion times
• Relatively large electric double layer

• Microfluidics
• Brownian motion of nanoparticles
• Diffusion
• Fluid dynamics

Nanoscale flow can be enhanced considerably compared to what is predicted by macroscale Knudsen flow or continuum hydrodynamics models - see Science Vol. 312. no. 5776, pp. 1034 - 1037 DOI-link (Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes)

FERROFLUIDS

Ferrofluids are a form of colloidal suspensions. They are tiny iron particles covered with a liquid coating, also surfactant, that are then added to water or oil, which gives them their liquid properties. Ferrofluids are colloidal suspensions. Those are materials with properties of more than one state of matter. In this case, the two states of matter are the solid metal and liquid it is in. This ability to change phases with the application of a magnetic field allows them to be used as seals, lubricants, and may open up further applications in future nanoelectromechanical systems.

Resources

• a wikibook on Microfluidics

• Microfluidics: Fluid physics at the nanoliter scale REVIEWS OF MODERN PHYSICS, VOLUME 77, JULY 2005 by Squires and Quake
• The Fluid Mechanics of Microdevices—The Freeman Scholar Lecture Journal of Fluids Engineering 1999, Vol. 121 / 5

References

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Part 6: Nanoengineering

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>>> Next Part: Nano-bio Primer

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This chapter takes a look into the engineering aspects of nanotechnology: how to integrate the nanostructures with our known technology, the creation of nanomaterials and simple systems in the laboratory, and the development of functional devices that can be used by society.

References

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Top-down and bottom-up approaches

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Top-down and bottom-up approaches

There are two types of approaches for the synthesis of nanomaterial and fabrication of nanostructure.

• top-Down approaches refers to slicing or successive cutting of bulk material to get nano-sized particles.there are two types *attrition ,* milling

• Bottom-up refers to method where devices 'create themselves' by self-assembly. Chemical synthesis is a good example. Bottom-up should, broadly speaking, be able to produce devices in parallel and much cheaper than top-down methods, but getting control over the methods is difficult when things become larger and bulkier than what is normally made by chemical synthesis. Of course, nature has had time to evolve and optimize self-assembly processes that can do wonders.

Microfabrication made smaller

Not much of nanotechnology is based on methods from Microfabrication.

References

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Self assembly

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• Self assembly
• Self assembled monolayers
• Molecular nanotechnology
• Molecular engineering

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Lithography

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Down: Module on EBID

Lithography

• Nanolithography

Electron beam lithography

Electron beam lithography (EBL)

Nano imprint lithography (NIL)

Nanoimprint lithography (NIL)

Focused Ion Beam Techniques

Focused Ion Beam Techniques

Electron Beam Induced Deposition (EBID or EBD)

The highly focused electron beam in a SEM is used for imaging nanostructures, but it can also be used to make nanoscale deposits. In the presence of carbonaceous or organometallic gasses in the SEM chamber, electron beam induced deposition (EBID or electron beam deposition (EBD)) can be used to construct three-dimensional nanostructures or solder/glue nanostructures.

There is module in this handbook dedicated to EBID

References

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Electron Beam Induced Deposition (EBID or EBD)

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Electron Beam Induced Deposition (EBID or EBD)

EBID Background

It was apparent with the first electron microscopes in the 50's, that the presence of an electron beam slowly coated the sample with a carbonaceous substance .

Even earlier, it was observed that electron radiation decomposed organic vapors present in the vacuum chamber, such as oil vapors unavoidable in diffusion pumped vacuum systems or outgassing from the sample itself .

When the background gas is decomposed by irradiation through various ionization and dissociation processes, the gas turns partly into volatile compounds re-emitted into the chamber and partly into solid amorphous carbon. The material properties range from diamond-like-carbon (DLC) to polymers depending on the exact deposition conditions .

The reactions taking place during EBD are not well characterized. Both ionization and dissociation are expected to contribute . The cross-section for both dissociation and ionization are peaked at low electron energies (<50 eV), indicating that secondary electrons are likely to be the main cause of deposition rather than the primary electrons (PE).

By focusing the PE beam in a fixed spot, a thin needle-shaped deposit will grow up towards the electron beam. The tip width can be considerably wider than the PE beam diameter and typically of the order 100 nm. The width is determined by the scattering of the PE in the tip structure which in turn also creates SE escaping through the tip apex and sidewalls causing a wider deposit .

With many potential applications in micro- and nanotechnology, the EBD technique has received increasing attention since the 80's as a method for creating submicron structures. Comparing EBD to electron beam lithography (EBL), the EBD process must be considered "slow" while EBL is "fast" since the required irradiation dose is many orders of magnitude smaller for EBL.

The use of EBD in commercial production of nanostructures is today limited to "supertips" for AFM cantilevers with extreme aspect ratios that cannot readily be achieved by other methods .

For research purposes, where high throughput is not a requirement, the technique appears convenient in several applications. Apart from depositing structures, it has also been used to solder nanocomponents. Both single and multiwalled carbon nanotubes (SWNT and MWNT) have been soldered to AFM cantilevers for stress-strain measurements and to micromechanical actuators for electrical and mechanical tests

Metal deposition by EBID

A large fraction of the EBD publications have been focussing on the use of metal containing precursor gasses. Koops et al. and Matsui et al. pioneered the extensive use of metal containing source gasses to make deposits with high metal contents. They also began scanning the beam in patterns to make three-dimensional structures. Complex three-dimensional structures can be made by EBD with both carbonaceous and metal-containing EBD. Another intriguing possibility is to use EBD to make catalyst particles for subsequent growth of nanowires and tubes .

Compared to the planar and resist-based EBL, the EBD method is slow and difficult to scale to large productions, but on the other hand offers the possibility to create elaborate three-dimensional structures, which cannot readily be made by EBL. The EBD method appears to be a versatile tool capable of constructing nanodevies, contacting nanostructrues to create composite electronic nanostructures, and soldering nanostructures such as carbon nanotubes to microelectrodes.

For electronic applications one would like to achieve as high a conductivity as possible of the deposited material. Metal-containing EBD materials usually contain metallic nanocrystals in an amorphous carbon matrix with a conductance considerably lower than that of the pure metal. The metal content and conductivity of the EBD material can be increased to approach that of bulk metals by several methods:

1: Heating the substrate has been shown to increase the metal content of the deposit. Koops et al. have observed an increase from 40 wt. % at room temperature to 80 wt.% at 100°C. Others, for example Botman et al. have shown the link between deposit composition and conductivity as a function of post-treatment in heated gases.

2: Using a carbon free precursor gas, such as $PF_{3}AuCl$, Hoffman et al. made gold deposits with a resistivity of 22 µΩcm which is only 10 times the bulk value of Gold.

3: Introducing an additional gas such as water vapor while using an environmental scanning electron microscope (ESEM). It is even possible to create desposits with a solid gold core under controlled deposition conditions .

Resources

• Review paper: Focused, Nanoscale Electron-Beam-Induced Deposition and Etching by Randolph et al.
• [http://www.febip.info/ Focused Electron Beam Induced Processes

(FEBIP)]

A Simple Model of EBD

To accurately model the EBD process, one has to resort to Monte Carlo simulations that can incorporate the different scattering effects taking place during the process. Extensive work has been done on models for the deposition of amorphous carbon tips . Generally there is very little available knowledge on:

• The radiation induced chemistry of the metal containing precursor gas. A wealth of reactions are possible, but limited data is available for the conditions and substances used for EBD.

• The chemical content of the produced amorphous carbon in the deposit.

• The current density in the electron beam is rarely well characterized.

Not knowing the chemical details of the deposition process, the exact product of the deposition, or the electron beam, a simple analytical model will also provide insight to the essential parameters. A simple model is reviewed below to provide an understanding of the basic requirements and limitations for the EBD process.

The Rate Equation Model

Electron beam deposition of planar layers on surfaces can be reasonably described by a simple rate equation model .

The model shows the fundamental limitations for growth rate and its dependence on beam current and precursor gas flow. The model calculations and most experiments in this thesis, are based on the precursor gas dimethyl gold acetylacetonate, here abbreviated to DGAA. The vapor pressure of the precursor gas determines the flux of precursor molecules to the surface. The flux rate of molecules F [m²s¹] of an ideal gas at rest is

$F={\frac {P}{\sqrt {2\pi mk_{B}T}}}$

with pressure P, atomic weight m, Boltzmann's constant $k_{B}$ and temperature T. For DGAA which has a vapor pressure of 1.3 Pa at 25 °C, this gives a flux rate $F=10^{18}cm^{2}s^{-1}$

The cross-sections for electron beam induced ionization and dissociation of the precursor gas to form the deposits are generally not known. Cross-sections are usually of the order Ų and peak at low energies, corresponding the low energy of the secondary electrons which are probably the main cause of the deposition.

Sketch of the EBID process

The adsorption of molecules on the target surface and highest density of SE near the surface make it reasonable to assume that the deposition rate dN_{dep}/dt depends on the surface density of adsorbed precursor molecules N_{pre}, the beam current density J, and the effective cross section s_0 as

${\frac {dN_{dep}}{dt}}=N_{pre}\sigma _{0}{\frac {J}{q_{e}}}$

The surface density $N_{pre}$ of the adsorbed precursor molecules in Eq. #eq EBD dep surface rate is the source for the deposited material and depends on both the deposition, ad- and desorption processes as sketched in the figure.

With maximum surface density $N_{0}$ (e.g. one monolayer, since generally more than one monolayer cannot be expected unless the target is cooled compared to the source, to give condensation of the source gas. Then adsorption probability a and lifetime t (s), a rate equation can be written for the precursor surface density as

${\frac {dN_{dep}}{dt}}=aF(1-(N_{pre}/N_{0}))-\sigma _{0}JN_{pre}/t-N_{pre}/q_{e}$

The steady state adsorbate density, $N_{pre}$, is then

$N_{pre}={\frac {aF}{aF/N_{0}+1/\tau +\sigma _{0}J/q_{e}}}$

If each deposition event on average results in a cubic unit cell of deposited material with volume V, the vertical growth rate R [nm/s] is

$R=N_{pre}V\sigma _{0}J/q_{e}$

$=V{\frac {aF}{q_{e}{\frac {\tau aF/N_{0}+1}{\sigma _{0}Jt}}+1}}$

The dependence on precursor flux falls into two cases:

• $\tau aF>N_{0}$ when a monolayer is always present and increasing the flux rate F has little effect on the growth rate R, since the surface is saturated

• $\tau aF<N_{0}$ when less than a monolayer is present, and increasing F will increase the growth rate R.

Increasing the electron beam current will in this model always increase the deposition rate. The rate increases relatively linearly with the electron flux, until it begins to saturate when the source gas flux becomes the limiting factor for the growth rate.

Scheuer et al. have measured the EBD deposition cross section of $Ru_{3}(CO)_{1}2$ to be of the order $\sigma _{0}$ =0.2 Ų and $\tau =1$ s. Using these values, a rough estimate of the growth rate can be calculated. For the estimate, we assume a monolayer is present $(\tau aF=N_{0})$; a sticking efficiency of a=100%; the vapor pressure flux of DGAA; an electron beam diameter of 20 nm; a total beam current of 0.2 nA; and finally that the unit cell volume V for deposition is that of gold. With this set of values, the deposition rate becomes R= 100 nm/s.

The used values make $\sigma _{0}J/q_{e}>>\tau aF/N_{0}$ so the deposition is not limited by the electron beam current but by the gas flux which would have to be 10 times higher to reach saturation. The beam radius will have to be increased to r= 0.5 µm to reach the electron flux limited region and this radius is much larger than the observed resolution in most experiments. Its is important to secure as high as possible flux of precursor gas in the experiments, since this is the main limiting factor in the model whereas the focus of the electron beam is expected to be less important due to the high current density.

Limitations to the model

The rate model is suited for describing deposition of planar layers, but for the case of deposition of tip structures in a real system, several other effects influence the deposition rate:

Scattering of primary electrons in the deposited structure. BSE and SE are emitted through the sidewalls and apex of the structure in a non-uniform way, and the PE/BSE scattering make SE generation take place in a larger region than the PE beam radius, which considerably limits the minimal radius for tip structures.

The figure above illustrate these effects. Simulations are needed to make proper estimates of the influence of scattering, but qualitatively it should cause a lower vertical growth rate as less electrons must be expected to emerge through the upper surface of the structure.

The PE beam is not uniform as considered in the model. In an ESEM, a Gaussian distribution of the PE beam can be expected, and the scattering of the electron beam in the environmental gas creates an low current density "electron skirt" around the PE beam. This should be considered both for the possible contamination in the larger region irradiated with low current density, but also for reducing the current in the primary beam and thus the growth rate.

It was assumed that the source supply precursor gas with the vapor pressure gas flux rate. The rate could be considerably lower if the source material does not have enough surface area to sustain the gas flow or the distance to the source is too large. The fact that many organometallic compounds decompose in contact with water in the case of EEBD could also reduce the source gas flow.

Not all irradiation induced events will result in deposition of material. Substantial amounts of material could be volatile or negatively ionized and carried away, especially in an the ESEM environment. Electron attachment is also taking place in the ESEM and is known to influence the detection of secondary electrons . This could reduce the supply of precursor gas and hence the deposition rate.

Surface diffusion of the precursor gas will influence the supply rate. When depositing in only a small area, surface diffusion of adsorbed molecules from the surrounding area can considerably increase the supply of precursor molecules. This is usually the explanation given why many EBD experiments observe that the tip deposition is faster in the beginning; for then to decrease to a steady state growth rate, when a tip structure is formed which limits the supply by surface diffusion. This could increase the rate at the very beginning of the deposition.

The predicted vertical growth rate from the model must be an upper estimate on the achievable rate, since most unaccounted effects will work to reduce the steady state growth rate.

Summary

Little data is available on the precursor gasses for EBD. A simple rate equation model gives an estimated deposition vertical growth rate of 100 nm/s for the typical precursor gasses. This estimated growth rate is expected to be an upper limit. Especially the flow rate of precursor gas should be as high as possible in the experiment since this is the limiting factor for the deposition rate.

Environmental Electron Beam Deposition (EEBD)

The experimental setup for environmental electron beam deposition (EEBD) with a precursor gas supply either mounted on the sample stage or via an external gas feed system.

The ESEM makes it possible to use various gasses in the sample chamber of the microscope since there are narrow apertures between the sample chamber and the gun column, and a region in between that is connected to a differential pumping system. Pressures up to about 10 Torr are normally possible in the sample chamber.

The standard Everly-Thornhart SE detector would not work under such conditions since it would create a discharge in the low pressure gas. Instead a "gaseous secondary electron detector (GSD)" is used, as shown in the figure below. The GSD measures the current of a weak cascade discharge in the gas, which is seeded by the emission of electrons from the sample.

TEM images illustrating how the morphology of EEBD tips using DGAA as precursor depends on the deposition conditions. (a) Apart from water vapor, all other tested environmental gasses (N2; O2/Ar; H2/He) have resulted in tips containing gold particles embedded in an amorphous carbon containing matrix. (b) When using water vapor as environmental gas, a dense gold core becomes increasingly pronounced as the vapor pressure and beam current is increased. (c) A contamination layer almost void of gold can be deposited on the tip by scanning the beam while imaging. So-called proximity contamination can also occur if depositions are done later within a range of a few μm from the tip. The contamination layer is thicker on the side facing later depositions. (d) Electron irradiation in SEM or TEM causes the contaminated tips to bend irreversibly towards the side with the thickest contamination layer. The tips were deposited from left to right and thus bent towards the last deposition. More information in and .

In the ESEM one can work with, for instance, water vapour or argon as the environmental gas, and is is possible to have liquid samples in the chamber if the sample stage is cooled sufficiently to condense water.

Without precursor gas present in the chamber, the EBD deposition rate is normally negligible in the high vacuum mode as well as in the gas mode of the ESEM.

In environmental electron beam deposition (EEBD), the deposited tips have a shell structure and consist of different material layers each characterized by a certain range of gold/carbon content ratio. Above a certain threshold of water vapor pressure and a certain threshold of electron beam current, the deposited tips contain a solid polycrystalline gold core .

Acknowledgement

This page was started based on the material in:

• Kristian Mølhave's PhD thesis chapter 4 and accompanying papers and .
• Please add material here if you add to the page content!

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Nanomanipulation

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Nanomanipulation

• Wikipedia Nanorobotics
• Nanorobotics introduction
• Video clips of Nanomanipulation on YouTube
• [94] Pick and Place of NW in SEM on YouTube
• Video of the manipulation of a nanowire for TEM observation on YouTube

A slip stick actuator that provides coarse and fine positionoing modes. Coarse positioning provides long range but low precision, while fine positioning provides high precision and short range. The slip stick principle: Slow actuation of the piezo element leads to fine positioning. A combination of rapid contraction and slow extension can make the actuator move in coarse steps Δx because the force on the base becomes larger than the static friction force between the base and base plate. Reversing the direction is done by using slow contractions instead.

AFM manipulation

With AFM nanostructures such as nanotubes and nanowires lying on surfaces can be manipulated to make electrical circuits and measure their mechanical properties and the forces involved in manipulating them.

STM manipulation

Using an STM individual atoms can be manipulated on surface this was first demonstrated by Eigler et al. Here Xe atoms were manipulated on Ni to spell out IBM. This was then extended by Crommie et al., where Fe atoms were moved to create quantum corals. Here the electron standing waves created inside the corral are imaged by the STM tip. The probably demonstrates the highest resolution nanomanipulation.

In-situ SEM manipulation

To monitor a three-dimensional nanomanipulation process, in-situ SEM or TEM manipulation seems preferable. AFM (or STM) does have the resolution to image nanoscale objects, even down to the sub-atomic scale, but the imaging frame rate is usually slow compared to SEM or TEM and the structures will normally have to be planar. SEM offers the possibility of high frame rates; almost nanometer resolution imaging of three-dimensional objects; imaging over a large range of working distances; and ample surrounding volume in the sample chamber for the manipulation setup. TEM has a much more limited space available for the sample and manipulation systems but can on the other hand provide atomic resolution. For detailed studies of the nanowires' structure, TEM is a useful tool, but for the assembly of nanoscale components of a well defined structure, such as batch fabricated nanowires and nanotubes, the SEM resolution should be sufficient to complete the assembly task.

As the STM and AFM techniques opened up completely new fields of science by allowing the investigator to interact with the sample rather than just observe, development of nanomanipulation tools for SEM and TEM could probably have a similar effect for three-dimensional manipulation. Recently, commercial systems for such tasks have become available such as the F100 Nanomanipulator System from Zyvex in October 2003. Several research groups have also pursued developing such systems.

To date the tools used for in-situ SEM nanomanipulation have almost exclusively been individual tips (AFM cantilever tips or etched tungsten tips), sometimes tips used together with electron beam deposition have been used to create nanowire devices. Despite the availability of commercial microfabricated grippers in the last couple of years, little has been reported on the use of such devices for handling nanostructures. Some electrical measurements and manipulation tasks have been performed in ambient conditions with carbon nanotube nanotweezers.

A microfabricated electrostatic gripper inside a scanning electron microscope where it has picked up some silicon nanowires.

Companies selling hardware

• Omniprobe
• Zyvex
• Imina Technologies

The Optimal SEM Image for Nanomanipulation

As the typical SEM image is created from the secondary electrons collected from the sample, compromises must always be made to obtain the optimal imaging conditions regarding resolution and contrast. The contrast in a SEM SE image depends on the variations in SE yield from the different surface regions in the image and the signal to noise level. The resolution depends on the beam diameter and is at least some nm larger due to the SE range.

The optimal solution is always to use as good an emitter as possible (high ß_{e} in Eq.). This means using FEG sources. Working at short r_{wd} gives a narrow beam (Eq.), but will usually shield the standard ET detectors from attracting sufficient secondary electrons. Nanomanipulation often requires working with high resolution between two large manipulator units which further limits the efficiency of signal detection.

The manipulation equipment must be designed to make the end-effector and samples meet at short r_{wd}, and without obstructing the electron path towards the detector. A short r_{wd} also gives a short depth of focus, which can be a help during nanomanipulation because it makes it possible to judge the working distance to various objects by focussing on them. The operator can use this to get an impression of the height of the objects in the setup. Generally, for nanomanipulation, the above considerations indicate an inlens detector often can be advantageous.

Reducing the beam current to narrow the electron beam necessarily limits the number of detected electrons and make the signal-to-noise ratio low, unless one makes very slow scans to increase the number of counts

<footnote>The signal to noise ratio S/N for Poisson distributed count measurements n is S/N=vn and high counts are necessary to reduce noise in the images. </footnote>.

When used for in-situ nanomanipulation one needs a fast scan rate to follow the moving tools (preferably at rates approaching live video) and this requires high beam currents. The acceleration voltage is also important, and too high PE energy can make the sample transparent (such as the carbon coating in Fig. b) while low energy usually make the image susceptible to drift due to charging and similar effects.

In-situ TEM manipulation

TEM offers atomic 3D resolution but the extreme requirements on stability combined with very limited sample space makes the construction of in-situ TEM manipulation equipment quite a task. With such systems, people have observed freely suspended wires of individual atoms between a gold tip and a gold surface; carbon nanotubes working as nanoscale pipettes for metals and a wealth of other exotic phenomena.

Companies selling hardware

• Nanofactory

References

See also notes on editing this book about how to add references Nanotechnology/About#How to contribute.

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Part 7: Nano-Bio Introduction

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Nano-bio Primer

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Bio-nanotechnology

• Nanobiotechnology
• Bionanotechnology
• Nanomedicine
• Basics of biochemistry an E handbook from Lund University

Biological 'units'

• The wiki book on Cell Biology
• Bacteria
• Virus
• Cell biology and another wiki entry on Cell Biology
• The brain and neurons
• Molecular Biology of the Cell a great text book with full text search freely available on the net.
• Cells alive

Biosystem building blocks

• DNA
• proteins, see also wikibook on Proteomics
• liposomes

make up cell membranes and these are the very important barriers that control what can enter and exit a cell. The importance can be seen in the fact that one third of all proteins are membrane proteins.

Cell Energy Supply and Consumption

The cell organelle the mitochondria is the power plant of the cell energy metabolism.

It synthesises ATP, the basic energy source for numerous processes in cells. The synthesis proces is driven by an electric potential (~150mV) across the mithocondrial inner membrane that is maintained by ion pumps in the membrane that make the outside more acidic while the inner matrix region is more alkaline. The gradient is ~0.5 pH units . The membrane potential is not homogeneous over the mitochondria

The Chemiosmotic theory of the ATP synthesis was developed by Peter Mitchell in 1961, who was later rewarded the chemistry Nobel prize for it in 1978.

Current research on Mitocondria was reviewed eg. in Science 28 aug 1998 and 5 mar 1999.

The membrane potential difference is small, but the menbrane is also very thin; approx 5-7nm, giving an electrical field across the membrane of the order 30MV/m - which will make any physicist designing high energy particle accelerators quite envious - the value is huge compared to electrical fields that would make large scale matter break down. Its is on contrast to the otherwise weak forces normally encountered when talking about the physical properties of lipid membranes. Despite being a thin and soft membrane that can easily be ruptured by mechanical contact, it is capable of withstanding extreme electrical fields.

The membrane potential can be observed by using the fluorescent probe JC-1 -or more verbosely 5,5’,6,6’-tetrachloro-1,1’,3,3’-tetraethylbenzimidazolylcarbocyanine iodide. JC-1 is a lipophilic cation that can penetrate the (lipo)membrane of the cell and mitochondria. JC-1 can be excited by green laser light and emit fluorescent green light at 530nm when it is in a monomeric form, but if the membrane potential is increased above a threshold around 80-100mV, JC-1 will aggregate and the fluorescense becomes increasingly orange at 590nm . JC-1 is a very tested and reliable marker for the membrane potential . A recipe for the use of JC-1 can be found in .

The immunesystem

• Immunology
• Antigen, antibody.

Fluorescent Markers

Natural cells and biological structures often have very little contrast when seen in optical microscopes. Take a normal yeast cell and look at it under a microscope and you will only see small balls with no structure. Fluorescent markers and dyes can be used to stain specific substances inside the cell which would otherwise not give an appreciable optical signal. Such markers have for many years been essential to get proper images of biological samples. No matter what type of microscope is used, the fluorescent markers are widely used to enhance the contrast and signal to noise ratio in measurements.

Using fluorescent dyes and recording spectra of light emitted from whole or selected parts of cells can give valuable information , such as:

• The functional and structural characteristics of normal or malignant cells
• The intracellular dynamics of molecules that are naturally occurring, or added to the cell such as drugs.
• Characterization of the interactions between cells, or the cell and its surrounding media.
• Intracellular dynamics of ions such as Ca++, Mg++, or other important variables such as pH or membrane potentials by using fluorescent markers for the chemicals and potentials under investigation.

The fluorescent markers are used for many other techniques than just microscopy. A method called flow cytometry is very efficient for analyzing large numbers of individual cells. One by one, individual cells pass through a thin channel where they are exposed to the exciting laser light and the emitted fluorescense and absorption is detected by light sensors. The technique gives very good statistical results, but does not allow the detection of eg. individual mithocondria inside a well functioning cell but will rather give an average value of the state of all mithocondria in a single cell.

Ressources

• The Handbook — A Guide to Fluorescent Probes and Labeling Technologies

Lengths and Masses in biochemistry

1Da is one atomic mass unit and approximately the mass of a proton. It is about one thousandth of a zeptogram = 10^-24g

• 1 Da Hydrogen
• 8-30 kDa Cytokine
• 950 kDa typical long PMMA polymer chain
• 10.000 DNA base pairs weighs about 40ng
• An organism typically has tens of chromosomes chromosomes, which each can have up to 1000s of genes, and be made from up to 100 million base pairs.

Sizes of small life forms

• Nanobes Nanobes are tiny filamental structures first found in some rocks and sediments. from 20 nm.
• Parvovirus - a family of viruses down to 20 nm
• 'Nanobacteria' or 'Calicifying Nanoparticles (CNP)' - a recently discovered class of nanoparticles that seem related to various calcification processes in the body and may be living [see also new scientist 23 june 2007 p38]. 50-100nm
• Nanoscale microbes (Nanoarchaeota a kind of archaea) [science vol 314 p1933]
• Smallest bacterium ' mycoplasma genitalium' -M. genitalium was also considered to be the organism with the smallest genome. 300 nm
• Nanoarchaeum equitans a thermophile 400 nm
• The largest virus 'mimivirus', that infects amoebaes 400nm
• Possibly the most abundant organism on earth, the marine bacterium 'pelagibacter ubique (SAR11)' 500 nm
• Typical gut bacteria 'E. Coli' 2000-6000 nm.

References

See also notes on editing this book about how to add references Nanotechnology/About#How_to_contribute.

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Biosensors

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Biosensors

Biological sensor functionalisation, receptors and signals

• Assays for measuring the biochemical properties, such as immunesystem responses, specific proteins etc.
• DNA binding protein
• Functional group
• Receptor biochemistry
• Neuron
• The Biotin - Streptavidin binding is the strongest biochemical bond and often used when you need to demonstrate a sensor or bind different biomolecules together.
• Biofouling - by using certain polymers such as PEG or PLL-PEG, cell adhesion to surface can be drastically reduced.

DNA Biosensors

In the future, DNA will find use as a versatile material from which scientists can craft biosensors. DNA biosensors can theoretically be used for medical diagnostics, forensic science, agriculture, or even environmental clean-up efforts. No external monitoring is needed for DNA-based sensing devices. This is a significant advantage. DNA biosensors are complicated mini-machines—consisting of sensing elements, micro lasers, and a signal generator. At the heart of DNA biosensor function is the fact that two strands of DNA stick to each other by virtue of chemical attractive forces. On such a sensor, only an exact fit—that is, two strands that match up at every nucleotide position—gives rise to a fluorescent signal (a glow) that is then transmitted to a signal generator.

Resources

. <http://www.nigms.nih.gov>*Performance limits of nanobiosensors App. Phys. Lett. 88, 233120, 2006.

References

"The Chemistry of Health." The Chemistry of Health (2006): 42-43. National Institutes of Health and National Institute of General Medical Sciences. Web

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Nanomedicine - Targeting diseases

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NanoMedicine

Helping improve humanity is one of the promises of nanotechnology. Much hope and hype has surrounded nanomedicine. Research is actively pursuing the benefits of nanotechnology enabled medicine and the promise of organ specific drug delivery and cancer treatments. But, there is no consensus among the research and medicine community that shows the toxic effects of heavy metal nanoparticles in the body when used as "treatments". This chapter will highlight some of the research findings in the nanomedicine area.

Scientists are working to find "nanostructers" for many different kinds of cures, including Parkin's and Cardiovascular Disease, treatment for cancer, nanomaterial for new artificial limbs, and nanodevices that could restore hearing and vision. Soon, nanomedicine will be able to cure many diseases and illnesses. References: http://www.medicalnewstoday.com/articles/67702.php

The scientists have found a way to deliver molecules within 'wells' of polymers that are a part of a capsule. They are also making ground medicine with particles in the nanoscale to increase effectiveness.

Source- Nanotechnology A gentle introduction to the next big idea, Written by Mark and Daniel Ranter

• eMedicine – The original open access comprehensive medical textbook for all clinical fields.
• Nanomedicine book site

Example #1: Nanosilver

For thousands of years, silver has been known to be a potent bacteria killer. However, due to the fact that silver could not dissolve well, it's efficiency as an antimicrobial has been slim. This problem has been solved by the company Nucryst Pharmaceuticals. To combat it's inability to dissolve, they use a nanocrystalline form of silver to fight off bacteria. Silver ions rapidly kill microbes in a variety of ways which include blocking the cell respiration pathway, interfering with components of the microbial electron transport system, binding DNA, and inhibiting DNA replication.

Their first product with the silver is Anticoat, a dressing for serious burns. Anticoat antimicrobial barrier dressing works to reduce or kill off bacteria, have high absorbency rates, continue working for a full 7 days, and are easy to remove without disrupting the wound. When the dressing is removed, it peels off in one piece and a new coat can be applied. A high absorbency rate is required because many wounds release a lot of body fluids and a good absorbency rate will maintain a healthy wound environment. It is usable for a week and can also be used for serious words. Also, now they are exploring the anti-inflammatory properties of silver for use in atopic dermatitis and certain respiratory conditions.

Example #2: Regenerating Neurons

There is a research team at USC that is working on producing artificial motor neurons. These neurons could serve several functions; including letting people with paralyzed limbs use their limbs again. These fake neurons would essentially take over the functions of the real motor neurons. Using this technology doctors could replace motor neurons since neurons do not grow back.

Example 3: Peptides for wound healing

Researchers at MIT have found liquids called peptides that form a nanoscale barrier in seconds, stopping the flow of blood. later. when the wound is healed, the solution breaks down and can be used by the body as new tissue. The same scientists also reported that a peptide partially restored a hamster's vision.

Examples #6: Malaria

Dr. Subra Suresh, a professor at MIT, using nanotechnology, has studied malaria, an infection spread by mosquitoes, in which tiny parasites infect the red blood cell. Using “laser tweezers” and two nano-sized glass beads fused to the red blood cell surface, he found that infected cells may be as much as 15-times stiffer than normal cells, which causes them to clog up small blood vessels. He is now looking at the effect of different genes in the parasite that may produce this effect as this may allow the finding of a treatment for this worldwide disease. "Tiny tools tackle malaria", 2005. Retrieved on 6-26-2008.</ref> [95]

Example #7: Increased drug dispersion from nanoparticles

One of the greatest prospects of nanomedicine is in drug delivery. On the most basic level current drugs ground into a smaller state have a greater surface area which allows them to dissolve more quickly in the stomach. A variation on this idea is using small crystals of medicine. These crystals allow every molecule to be close to the surface which creates an environment where even the most slow dissolving compounds will dissolve quickly.

Example #8: Drug delivery polymer nanoparticles

Drug delivery is one of the best benefits of nanomedicine. There are many different schemes for improving drug delivery, for example, molecules can be put into nanoscale cavities inside polymers. The polymer can then be swallowed as part of a tablet or pill, and when the polymer opens inside the body, the drugs can be released into the body. More complex schemes have also been developed, such as getting drugs through cell walls and into the cell. Efficient drug delivery is essential, because many diseases depend on processes within the cell, and can only be affected with drugs delivered into the cell.

Example #9: IMEDD (Intelligent MicroEngineered Drug Delivery)

IMEDD (Intelligent MicroEngineered Drug Delivery) is trying to make tiny drug deliver pumps. The researchers at IMEDD are working with Terry Conlisk, who is an engineer at Ohio State, who has made a computer model that helps small drug reservoirs pump out drugs when they are needed. They use this principal to work the pumps, "If a fluid is positively or negatively charged and there is a like charge to the inner surfaces of a channel, the charges will repel each other. The result is that the fluid will flow down the channel." In their experiments, they have been able to put almost 0.5 nL of saline per minute through a channel only 7nm wide. Medical researchers hope to use this technology to push tiny amounts of drugs into the body exactly where they are needed. They accomplish this by a technique developed by a team of scientists and engineers at ASU called photocapillarity. Photocapillarity is defined as "the microfluidic actuation of water in an enclosed capillary or microchannel using light".

Targeting Cancer

Cancer is the focus of many new nanomedicine therapies under development for repairing damaged tissues and treating and detecting cancer by developing medical interventions at the molecular scale that couples nanotechnology, clinical science, and the life sciences. People are developing innovative drug and gene delivery strategies and modulating molecular events can control cell processes such as initiating, enhancing, and maintaining macro- and micro- vasculature to ensure tissue viability, vessel networks within tissue engineered constructs and autologous tissue flaps and grafts. Developing novel synthetic, natural, or hybrid materials to control cell-material interactions, biomechanics, angiogenesis, and the in vivo release of therapeutic agents.

Molecular imaging and therapy - nanotechnology is hope to be the source of new ways to target cancer with fewer side-effects

• Cancer
• Oncology -the study and treatment of cancer

Cancer example #1: Nanoparticles Generate Supersonic Shock Waves to Target Cancer

Researchers from UCM (the University of Missouri-Columbia) and the United States Army have made a nano-sized “bomb”. This bomb can target drug delivery to cancer tumors without damage to any other cells. The nano thermites produce shock waves in the Mach 3 range. Cancer fighting drugs would be administered via a needle, and then a device would send a pulse into the tumor. The pulse would create little holes in the tumor, so the drugs can enter.

Cancer example #2: monitor post-treatment relapse

MNC is working with the Medical School at Swansea to develop a nanoscale sensor that would be put into the body and would be capable of detecting the growth of cancerous cells in patients. It would monitor post-treatment relapse. This way of finding the cancer cells at an early stage would reduce mortality rates dramatically.

Cancer Example #3: Photodynamic therapy

Photodynamic therapy (a type of treatment for cancer that is nothing like chemotherapy) is directed to hit the spot where the cancer is, it is a therapeutic idea. What happens during photodynamic therapy? They would put a metal nanodot or a molecular dot (the particle) inside your body and then they would shine a type of light to illuminate it from the outside. Then the light is absorbed by which ever particle they put in your body. So if you have a metal nanodot and it have absorbed the light the energy will heat the metal nanodot up making every tissue that it near heat up to. But with the molecular dot the light absorbed creates oxygen molecules that are very energetic. And since the oxygen molecules are highly reactive it chemically reacts or destroy the organic molecules that are beside it (example: tumors).

Cancer Example #4: NIH Roadmap's Nanomedicine initiative

NIH Roadmap's Nanomedicine initiative is working on advancing the study of nanotechnology. They hope to be able to detect cancer cells before a tumor develops and accurately destroy them, and have nano-sized pumps in your body that deliver medicines into your body where you need it. They believe they will be able to do this in ten years.