Force Field Spectroscopy in Three Dimensions

Abstract

Atomic resolution images in noncontact atomic force microscopy (NC-AFM) reflect planes of constant frequency shift. To draw conclusions on the chemical activity at specific surface sites, however, the force acting between tip and sample should be known locally rather than the frequency shift. This is not an easy translation due to the nonlinear nature of the relationship between the two. To overcome this problem, several groups have developed an extension to NC-AFM, dynamic force spectroscopy, which allows the precise, distance-dependent measurement of tip–sample forces. The forces are determined from frequency shift vs. distance curves by mathematical analysis. By combining many of these curves in a raster grid, the full three-dimensional surface force field can be probed with atomic resolution as it extends into vacuum. This chapter reviews experiments performed on NiO, NaCl, KBr, and graphite that illustrate the strengths and weaknesses of the different experimental approaches as well as the type of results that can be obtained.