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Explain the construction and working of of Atomic Force Microscope. Compare AFM with scanning electron microscope
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AFM consists of microscope cantilever with a sharp tip (probe) at its end used to scan the specimen surface.

The cantilever is typically silicon or silicon nitride with the tip radius of curvature of the orders of nm. Basically, AFM is modified TEM in which limitations of TEM is overcomed. When the tip is bought close to the sample, force between the tip and sample leads to the deflection of the cantilever according to the Hook`s law. Instead of using an electrical signal, the AFM relies on forces between the atom on the tip and in the sample.

The force present in the tip is kept constant and the scanning is done. As the scanning continues, the tip will have vertical movements depending upon the topography of the sample. The force present in the tip is kept constant and the scanning is done. As the scanning continue the tip will have vertical movement depending upon the topography o the sample.

A LASER beam is used to have a record of vertical movement of the needle. This information is later converted into visible from using photo diode. Depending upon the situation, AFM measures different types of forces like a Vander Waal’s forces, capillary force, mechanical contact force etc.

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Advantage:

  • 3D surface profile

  • No sample preparation

  • Can work in ambient environment

Disadvantage :

  • Scanning speed is low

  • As the tip is in contact with sample, it is likely to distort the sample

Application :

  • Can be used to study samples of metals, glasses, plastics, biological systems.

Comparison of AFM and SEM

AFM:

  1. Provider 3D surface profile

  2. No special treatment of sample

  3. Can work in ambient environment

  4. Living cells can be studied

  5. Quality of resolution is restricted by radius of curvature of the tip of the needle

SEM:

  1. Provider 2D surface profile

  2. Special treatment of sample required

  3. Required vacuum

  4. Living cells cannot be studied

  5. Quality of resolution is better

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