Lateral Force Microscopy

This image shows a typical polished metal surface in contact mode. Polish marks and various forms of particulate material are visible on the surface. In contact AFM one is measuring the surface morphology by keeping the tip deflection constant and thus this image reflects an image taken at nearly constant normal force. As the tip encounters changes in morphology there will be momentary changes in normal force until the scanner feedback mechanism is able to adjust the Z-piezo to maintain the deflection set-point. This is accomplished through the integral and proportional gains.

A constant normal force allows one to measure frictional forces if one is able to measure the torsion of the contact cantilever. This is possible if one sets the scan angle to 90 degrees and the data channel to "friction". While cantilever deflections are measured by the laser beam reflecting from the cantilever moving up and down between the top and bottom halves of the four quadrant photodetector, frictional forces manifest as a lateral motion of the laser beam between the left and right halves. This SPM imaging mode is lateral force microscopy or LFM.

This image shows an LFM image of the same region. Some torsion of the contact probe is seen around morphological features such as the longitudinal polishing scratches, but what is more interesting is a circular region of modified friction in the center of the image that does not seem to correlate with any morphological features. This is due to a modification of the surface that does not alter the morphology of the region-- the presence of an adlayer that modifies surface electrostatic forces.

The contact probe can also be chemically modified itself to allow for chemical force microscopy or CFM. In this case the probe is chemically functionalized to optimize the electrostatic interaction with species on the surface. In this case the torsion friction signal can be used to map those species on the surface. In the case of dip-pen nanolithography or DPN, CFM is a very natural method of imaging the regions functionalized because of the presence of those species on the dip-pen probe.