AFM: Contact Modes
The first and original mode of operation is contact mode. As the tip is raster-scanned across the surface, the lever is deflected as it moves over the surface topography.
There are two methods of imaging in contact mode: constant force or constant height. In constant force mode, the tip is continually adjusted to maintain a specified deflection. It is this adjustment that is used to display an image.
When the tip scans in constant height mode, the sample must be relatively flat for the feedback loop to maintain control during scanning. This is useful for small, high-speed atomic resolution scans.
Because the tip is in hard contact with the surface, the stiffness of the lever needs to be less that the effective spring constant holding atoms together, which is on the order of 1 - 10 nN/nm. Most contact mode levers have a spring constant less than 1 N/m.
Schematic of an AFM imaging in contact mode
Lateral Force Microscopy
Also known as friction force mode
Lateral Force Microscopy (LFM) measures frictional forces on a surface. By measuring the lateral twist of the cantilever, rather than its deflection, areas of higher and lower friction are imaged.