Novel techniques in multi-frequency atomic force microscopy and spectroscopy

Abstract

The capability of measuring material properties of nanostructures simultaneously with their size and shape is very desirable for characterization of novel materials and devices at the nanoscale Here we present two novel techniques for imaging and spectroscopy of mechanical and electrical properties of surface nanostructures simultaneously with topographic imaging. First we present a scanning probe technique that can be used to measure charging of localized states on conducting or partially insulating substrates at room temperature under ambient conditions. Electrostatic interactions in the presence of a charged particle between the tip and the sample is monitored by the second order flexural mode, while the fundamental mode is used for stabilizing the tip-sample separation. Cycling the bias voltage between two limits, it is possible to observe hysteresis of the second order mode amplitude due to charging. Results are presented on silicon nitride films containing silicon nanocrystals. Second we report use of nonlinear tip-sample interactions to convert the frequency components of periodic tip-sample interaction forces to frequencies where they can be resonantly detected by resonant heterodyne mixing. One flexural mode of a cantilever is used for tapping-mode imaging and another flexural mode is used for detection of forces converted in presence of an externally injected mechanical oscillation at the difference frequency of the detecting mode and a harmonic of the tapping mode. Material contrast in attractive and repulsive regimes are demonstrated on samples with polymethyl methacrylate patterns and with deoxyribonucleic acid strands on silicon. The techniques can be implemented using standard force microscopy systemsand cantilevers, which make them potentially useful to a greater scientific community.