Static and dynamic profiles of tethered polymer layers probed by analyzing the noise of an atomic force microscope

We have analyzed the thermal noise of the cantilever of an atomic force microscope, which is dominated by Brownian motion. The noise power spectra display the resonance properties of the cantilever, and are influenced by the cantilevers immediate environment. Fitting Lorentzians to the noise power spectra, we can derive a friction coefficient, an effective spring constant, and an effective mass. The dc force onto the cantilever is obtained from the static offset. When approaching the cantilever to surfaces covered with tethered polymer layers, we find a strong increase in the friction coefficient. We ascribe this to the breathing mode. At shorter distances there is an increase in spring constant indicative of an elastic interaction. The dc force and the effective spring constant show markedly different behavior.