Scanning microdeformation microscopy: experimental investigations on non-linear contact spectroscopy

In ac contact force microscopy, different kinds of set-ups derived for the most part from atomic force microscopes have been developed in the last ten years in spectroscopy mode, in order to investigate the local elastic properties of materials in the nanometre range. In most cases, quantitative results are delicate to obtain due to local forces such as atomic, capillary or adhesion, which are dominant on this scale between the tip and the sample. Generally the non-linear dependence of these forces on the distance is linearized in a small region of the interaction force response. We propose in this study the investigation of the non-linear regime of the tip-sample interaction with our mesoscopic scanning probe microscope, called scanning microdeformation microscope. Due to the operating range, only the Hertzian contact has been considered. This non-linear regime is observed in the case of high static loads on the sample or by increasing the excitation amplitude of the tip. More particularly, we have quantitatively measured with a high heterodyne interferometer the response directly on the sample in the vicinity of the vibrating tip. The detection of the spectra clearly shows the generation of higher harmonics in this situation. Different harmonic signatures have been measured depending on the elastic properties of the materials.