# Characterization of nanomaterials by electron microscopy - Part II
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Today I would like to continue where we left off in my previous post, concluding with the explanation of the other methods of characterization of nanomaterials through electron microscopy.

Another of the characterization techniques that are based on electronic microscopy is proximity microscopy.


# Proximity microscopy.
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This method of characterization is basically based on the scanning of the surface of a sample with a very sharp tip, monitoring the interactions that occur between the tip and the surface of the sample.

During the scan, an image is created based on the tip-sample interaction. The two main types of proximity microscopy are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).
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<center> https://upload.wikimedia.org/wikipedia/commons/thumb/5/55/DNA_nanostructures.png/1024px-DNA_nanostructures.png
[Source](https://en.wikipedia.org/wiki/DNA_nanotechnology)
[CC BY 2.5](https://creativecommons.org/licenses/by/2.5/)      
***Example of an image obtained through proximity microscopy*** </center>
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Proximity microscopes are a useful tool to obtain images in a wide range of atomic dimensions. They differ from other techniques such as SEM and TEM in that they have the possibility of manipulating molecules and nanostructures on the surface. However, it is not possible to perform a chemical analysis of the nanomaterials to be studied. This microscopic technique has contributed the most to the development of nanoscience and nanotechnology since, like TEM, it allows obtaining images with atomic resolution.

# Scanning tunneling microscopy (STM)
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In the STM, a potential difference is applied between a conductive tip and a sample placed at very short distances (in the range of a few nanometers) by means of a set of piezoelectric sensors, so that when the tip is very close to the surface, it establishes a tunnel current where the electrons of the sample flow towards the tip or vice versa (according to the sign of the applied voltage).
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<center> https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/ScanningTunnelingMicroscope_schematic.png/732px-ScanningTunnelingMicroscope_schematic.png
[Source](https://commons.wikimedia.org/wiki/File:ScanningTunnelingMicroscope_schematic.png) 
[CC BY-SA 2.0 AT](https://creativecommons.org/licenses/by-sa/2.0/at/deed.en) </center>
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The construction of the tip is of great importance since it must be thin enough so that the tunnel current is established essentially through a single atom, thus generating an image with atomic resolution.

Although this technique is considered a fundamental advance for scientific research, its applications are limited. This is because it is necessary that both the tip and the surface studied conduct electricity so that only conductive or semiconducting samples can be characterized.

# Atomic force microscopy (AFM)
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An AFM consists of the use of a very sharp tip, located at the end of a microlever, which it uses as a force sensor. The tip approaches the surface while the deflection of the microlever is measured. When the deflection and therefore the force (less than 1nN) exerted by the tip on the sample reaches a limit, the approach stops, and then the tip begins to move in the plane of the sample, keeping the deflection constant. To do this, the tip of the sample is conveniently brought closer or further away by means of a piezoelectric system. This is the simplest way to get an AFM image.
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<center> https://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/AFM_schematic_%28EN%29.svg/567px-AFM_schematic_%28EN%29.svg.png
[Source](https://commons.wikimedia.org/wiki/File:AFM_schematic_(EN).svg)
[CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/deed.en) </center>
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The system consists of a tip at the end of the microlever, which is flexed thanks to the force of tip-sample interaction. Measuring this flexion and thanks to a feedback system, the team uses tubular piezoelectrics capable of regulating the relative position between the tip and the sample within a few Angstroms. In this way, the feedback system uses the measurement signal to keep the deflection of the microlever constant.

The forces that produce the deflection of the microlever are varied, depending mainly on the materials of which the point and the sample are constituted and the distance between point and surface

Finally, it is interesting to mention that the techniques that have been described have become fundamental techniques for the study of matter at the nanometric scale and are used for various purposes, ranging from the mere observation of the morphology of surfaces of very different types of samples, until the characterization of properties of scientific and technological interest, thus complementing each other.
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# Refences.
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> * https://arxiv.org/pdf/1601.00786
> * https://www.researchgate.net/post/What_is_the_difference_between_SEM_and_TEM_techniques2
> * www.bfr.bund.de/cm/.../analytical-methods-for-characterization-of-nanomaterials.pdf
> * https://www.nist.gov/programs-projects/atom-manipulation-scanning-tunneling-microscope
> * https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4024087/
> * http://iopscience.iop.org/article/10.1088/1742-6596/61/1/192/pdf
> * Characterization of Nanomaterials (1st Edition), Neha Mohan Samuel Oluwatobi Oluwafemi Nandakumar Kalarikkal Sabu Thomas - 2018
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