What You Need To Know About The Atomic Force Microscope

The atomic force microscope was considered a major breakthrough for scientists as it is a thousand times more precise than the earlier versions. It is also known as the scanning force microscope (SFM). It was invented in 1986 by the trio, Quate, Gerber and Binnig. It is used for scanning, measuring and manipulation of matter at the nanoscale. It changes the concept of microscopes as we know it. We have all looked through microscopes in school and seen tiny particles become magnified to a large extent. The earlier microscopes worked on the basis of looking through the lens and seeing magnified images. The atomic force microscope is different in the sense that instead of you looking at the matter through the lens, there is a probe that feels the matter and sends images for us to see.

It uses a fine semiconductor or ceramic tip over a surface. This tip is positioned at the end of a microscale cantilever. The cantilever beam deflects according to the attraction or repelling of the tip to the surface. There is a laser spot which measures this deflection. This laser reflects the deflection at an angle from the end of the cantilever into a collection of photodiodes. The precise scanning is facilitated by piezoelectric elements which allow for tiny and accurate movements on the surface. These movements are controlled by electronic commands.

There are different ways in which this microscope can work. It can have the tip in constant contact with the surface. This is known as the static mode. It can also work with the tip in constant movement by tapping on the surface so that it does not remain in constant touch with the surface. This is the dynamic mode. In the static mode, when the tip is constantly touching the surface, there is a risk of the tip getting snapped into the surface if the attractive forces are very high. This mode is usually used when the forces are mostly repulsive. The force between the surface and the tip is kept constant by upholding a constant deflection during scanning.

 

In the dynamic mode, the tip is manipulated near the surface through electronic commands. The tip interacts with the surface of the sample and provides certain forces. These forces determine the oscillation or movement of the tip. The changes in the movement, resonance frequency and amplitude are measured. These changes provide information about the characteristics of the sample. The dynamic mode has two schemes of operation. These are the amplitude modulation and the frequency modulation. The amplitude modulation is used more commonly. The frequency modulation requires very high sensitivity and therefore more stiff cantilevers need to be used.

 

The atomic force microscope has some advantages over the earlier scanning electron microscope (SEM). It generates a true three-dimensional profile of the sample. It does not require the addition of any special substances like carbon coating which could damage the sample. It can be used under normal conditions. Living organisms can also be studied under this microscope. A disadvantage of the atomic force microscope is that it has a maximum image generation which is much lesser than the SEM. Another thing is that if the tip is not accurately chosen for the required resolution, then the image may not be correctly produced.