Importance of the indentation depth in tapping-mode atomic force microscopy study of compliant materials

We studied the response of a cantilever tapping on polydimethylsiloxane (PD MS) samples of different crosslink density. It is shown experimentally that the tip deeply penetrates into the compliant PDMS samples. A more complian t material leads to a larger indentation such that at a given set-point rat io the indentation force is nearly constant on samples of different elastic moduli. This confirms the simulations by J. Tamayo and R. Garcia [Appl. Ph ys. Lett. 71, 2394 (1997)] that phase contrast acquired at constant set poi nt does not depend on the sample's modulus if other contrast relevant param eters remain identical. PDMS samples of different crosslink density are dis tinguished in terms of amplitude and phase versus distance measurements if the tip-sample interaction is made substantially large and indentation is t aken into account. (C) 1999 American Institute of Physics. [S0003-6951(99)0 0452-0].