We propose a method for mapping the composition of a surface by using an amplitude modulation atomic force microscope operated without tip-surface mechanical contact. The method consists in exciting the first two modes of the microcantilever. The nonlinear dynamics of the tip motion, the coupling of its first two modes, and the sensitivity of the second mode to long-range attractive forces allows us to use this mode to probe compositional changes while the signal from the first mode is used to image the sample surface. We demonstrate that the second mode has a sensitivity to surface force variations below 10−11 N.

The authors acknowledge helpful discussions with A. San Paulo. This work was financially supported by then Dirección General de Investigación Científica y Técnica of Spain (PB98-0471) and the European Commission (MONA-LISA, G5RD-CT-2000-00349).

Peer reviewed