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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

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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

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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

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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.

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The Optimal SEM Image for Nanomanipulation

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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.[1]). This means using FEG sources. Working at short r_{wd} gives a narrow beam (Eq.[2]), 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.[3] b) while low energy usually make the image susceptible to drift due to charging and similar effects.

In-situ TEM manipulation

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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.

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

  1. eq SEM beam diameter
  2. eq SEM beam diameter
  3. fig INTRO 3 e depth