HARMONIC RESPONSE OF NEAR-CONTACT SCANNING FORCE MICROSCOPY

One-dimensional harmonic oscillator theory was used to model tapping-m ode atomic force microscope (TMAFM) operation in the near-contact regi on in the presence of gases and liquids. The force derivative of the t ip-sample interaction changes the vibration amplitude and frequency at maximum amplitude of the cantilever. Additionally, the interaction is hydrodynamically damped by fluid motion around,the tip and between th e tip and the surface. Good agreement was found between theoretical an d experimental amplitude as a function of height. For a sample-driven TMAFM operating in fluids, the cantilever can be very soft (spring con stant much less than 1 N/m) and operated at frequencies well above the fundamental. Under these conditions the cantilever and sample appear to act with a high spring constant, much like that used in a gaseous o peration. The tip-sample interaction in the fluid is still mediated th rough the force derivative of the sample. (C) 1995 American Institute of Physics.