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AFM - Where are its Uses?


An atomic force microscope (AFM) is a type of high resolution scanning probe microscope that has a resolution that you can measure in fractions of a nanometer. This scientific technology was pioneered in 1986 by Nobel Prize Winner Gerd Binnig along with Calvin Quate and Christoph Gerber. 


Atomic Force Microscopy

One of the most important tools for imaging on the nanometer scale, AFM uses a cantilever with a sharp probe that scans the surface of the specimen. When the tip of the probe travels near to a surface, the forces between the tip and sample deflect the cantilever according to Hooke’s law.


AFM will measure a number of different forces depending on the situation and the sample that you want to measure. As well as the forces, other microscopes can include a probe that performs more specialised measurements, such as temperature. The force deflects the cantilever, and this changes the reflection of a laser beam that shines on the top surface of the cantilever onto an array of photodiodes. The variation of the laser beam is a measure of the applied forces. 


Contact & Non-Contact Modes

There are two primary modes of operation for an atomic force microscope, namely contact mode and non-contact mode depending on whether the cantilever vibrates during the operation.


In contact mode, the cantilever drags across the sample surface and it uses the deflection of the cantilever to measure the contours of the surface. To eliminate the noise and drift that can affect a static signal, low stiffness cantilevers are used, but this allows strong attractive forces to pull the tip to the surface. To eliminate this attraction, the tip is in contact with the surface where the overall force is repulsive.


In non-contact mode, the tip vibrates slightly above its resonance frequency and does not contact the surface of the sample. Any long range forces, like van der Waals forces, decrease the resonant frequency of the cantilever. A feedback loop system helps to maintain the oscillation amplitude constant by changing the distance from the tip to the sample. Recording the distance between the tip and sample at each point allows the software to construct a topographic image of the sample surface.


Most samples will form a layer of moisture on the surface if stored at ambient conditions, and this can make it difficult to measure the sample accurately. If the probe tip is close enough to detect the short-range forces then it is close enough to stick to the moisture. 


AFM Advantages

The atomic force microscope is a powerful tool that is invaluable if you want to measure incredibly small samples with a great degree of accuracy. Unlike rival technologies it does not require either a vacuum or the sample to undergo treatment that might damage it. At the limits of operation however, researchers have demonstrated atomic resolution in high vacuum and even liquid environments.


AFM Disadvantages

One of the major downsides is the single scan image size, which is of the order of 150x150 micrometers, compared with millimeters for a scanning electron microscope.


Another disadvantage is the relatively slow scan time, which can lead to thermal drift on the sample.


As the technology matures, researchers are relying on there being progress instrumentally, requiring improved signal-to-noise ratio, decreased thermal drift, and better detection and control of tip-sample forces, including the use of sharp probes.

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