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Atomic Force Microscopy

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by

Ceri Bradshaw

on 24 October 2013

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Transcript of Atomic Force Microscopy

Atomic Force Microscopy
The Different Modes of AFM
Contact mode
Non-contact mode
Tapping mode
What is AFM?
Imaging technique that utilises simple principles.

Gives detailed images of atomic surfaces.

One of the highest resolution Scanning Probe Microscopes





By Ceri Bradshaw
Constituents of an AFM
The cantilever
The detection system
The piezoelectric crystal
The basic principle
Keeps a constant force on a cantilever.

The z component changes to ensure the constant force, with this being what is measured to give the image.
The Cantilever
There are 2 different types - either rod like as shown or triangular.

Different properties of cantilever lend themselves to either non-contact or contact methods as explained later.


A system of lasers are used to determine
where the cantilever is.
The Detection System
The Piezoelectric Crystals
These are crystals normally made out of ceramics that contract and expand depending on the voltage.

Gives the output voltage that varies on z position.

Using this voltage, an image is obtained
Contact Mode AFM
Uses repulsive forces.

Comes into close 'contact' with the material to be measured (Still a few angstroms from the material).

Works due to Hooke's Law -> F=-kx. To keep the force constant, you must keep the distance from the material x constant.
Non-Contact Mode AFM
Uses attractive forces.

Keeps the cantilever around 100 angstroms from the material.

Instead of maintaining constant deflection/force on the cantilever, it works by using the resonant frequency of the cantilever.
Obtaining the Resonant Frequency
This is obtained to get the original value.

When the Van der Waals Forces change on
the cantilever, the position of the resonant
frequency changes.

Non-contact mode detects this change,
which is used to give the output.

Note that the cross is slightly to the right of
the peak.
A look at forces between atoms
Forces between atoms follow the
Lenard-Jones 6-12 potential.

This differentiated with regards to
radius gives force.

When repulsive, forces responsible
are short range forces such as the
Pauli Exclusion Principle.

Attractive forces are due to Van der Waals
forces.
Advantages and Disadvantages:
Contact Mode
Advantages;
Can obtain a better resolution
Scanning speeds can be higher
Disadvantages;
Can have problems with fluid distorting the image
More sensitive to lateral forces
Easier to break cantilevers...
Advantages and Disadvantages:
Non-contact Mode
Advantages;
Minimal force is actually exerted on the sample, so better for imaging more fragile surfaces
Not as sensitive to lateral forces
Disadvantages;
Can only be used if there is a minimal fluid layer
Slower scan speed to avoid the effects of fluid layers
Results!
The Calibration Grating
Graphite
Thanks for listening - Any questions?
References
http://www.nisenet.org/sites/default/files/images/AFM-med.jpg,
http://en.academic.ru/pictures/enwiki/49/12-6-Lennard-Jones-Potential.png
http://www.vub.ac.be/META/images/toestellen/afm-stm5.jpg
https://www.asylumresearch.com/Applications/CombinedAFMOptical/LightPathDiagram.jpg
A Vilalta-Clemente and K Gloystein, Principles of Atomic Force Microscopy (2008)
Problems
Initial thoughts were that there were problems with the x/y movement when scanning.

Found out to be problems with the z component feedback loop.
Magnetic Force Microscopy
This was the planned next stage of the experiment.

It's effectively the same principle, with the magnetic force instead of the electrostatic force being kept constant when scanning a magnetic material.

Full transcript