Nonlinear Phase Imaging of Gold Nanoparticles Embedded in Organic Thin-Films

The phase detection in the dynamic mode of the atomic force microscopes is a known technique for mapping nanoscale surface heterogeneities. We present here an additional functionality of this technique, which allows high-resolution imaging of embedded inorganic nanoparticles of diameter and inter-particle distances of a few nanometers. The method is based on a highly nonlinear tip-sample interaction occurring markedly above the nanoparticles, giving thus a high phase contrast between zones with and without nanoparticles. A relationship between the tip-sample interaction strength and the phase signal is established in experiments and from calculations conduced with the model developed by Havilland et al., which is based on solving a combined equation of motion for both cantilever and surface while taking into account the time-varying interaction forces. The nonlinear phase behavior at the origin of the sub-nanometer spatial resolution is found to be the result of a local mechanical stiffening of the surfac...