DISTANCE-DEPENDENT NOISE MEASUREMENTS IN SCANNING FORCE MICROSCOPY

The changes in the thermal noise spectrum of a scanning-force-microsco pe cantilever upon approach of the tip to the sample were used to inve stigate the interactions between the cantilever and the sample. The in vestigation of thermal noise is the natural choice for dynamic measure ments with little disturbance of the sample. In particular, the small amplitudes involved ensure linear dynamic response. It is possible to discriminate between viscous coupling, elastic coupling and changes in the effective mass. The technique is versatile in terms of substrates and environments. Hydrodynamic long-range interactions depending on t he sample, the geometry and the ambient medium are observed. The depen dence of hydrodynamic interaction on various parameters such as the vi scosity and the density of the medium is described. For sufficiently s oft surfaces, the method is sensitive to viscoelastic properties of th e surface. For example, the viscous coupling to the surface is strongl y increased when the surface is covered with a swollen 'polymer brush' .