How AFM Works 1-1 AFM Principle

Описание: NanoWorld
Nano, from the Greek word for 'dwarf', corresponds to a prefix denoting a factor of 10^-9. Thus, a nanometer is one-billionth of a meter, which is the length scale at which intermolecular force and quantum effect take hold. To put the nanoscale in a more understandable perspective, consider that the size of an atom relative to an apple is similar to the size of an apple relative to the planet Earth! Atomic Force Microscopes (AFMs) give us a window into this nanoscale world.

AFM Principle
Surface Sensing
An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip cause the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface.

detection Method
A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of reflected beam) is recorded by the PSPD.

Imaging
An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features.