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Tapping AFM
From Wikipedia, the free encyclopedia
Tapping AFM is one of several different imaging modes used in atomic force microscopy (AFM). AFM is a type of scanning probe microscopy. These microscopes do not use light, as in conventional microscopes, but rather scan a sharp probe over a surface to image extremely small surface structures. With an AFM it is possible to image a surface at 10 nm resolution, (10 × 10−9 m) and generate a 3D map of the sample surface.
The AFM scanning head is a known as cantilever, generally a thin rectangular lever some hundreds of micrometres long and few micrometres wide. Several different shapes and sizes are available. One end of the cantilever is affixed to an inflexible base, usually called a "chip", attached to the AFM tip holder. The opposite side of the cantilever is completely free. On this free side a very fine-pointed tip is mounted perpendicular to the longitudinal axis of the cantilever. Generally the operating distance of the AFM tip is approached in two subsequent stages. In the first (coarse) stage the AFM tip is moved rapidly to the sample using precise stepping motors. After this movement a second (fine) approach is made by moving the sample with a piezoelectric actuator up toward the tip. At this stage, due to the extremely short distance between the tip and the sample surface, one can easily detect the effects of the repulsive/attractive atomic forces that give the name to this device.
In tapping mode the cantilever is let free to oscillate up and down at its resonant frequency. This is due to the effect of the specific forces acting on the cantilever when the tip comes extremely close to the surface, Van der Waals force or dipole-dipole interaction. A Tapping AFM evaluation image is therefore provided by imaging the force of the oscillating contacts of the tip with the sample surface. This is an improvement on conventinoal contact AFM, in which the cantilever just draggs across the surface at constant force and can result in surface damage.