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Dynamic Atomic Force Microscopy (AFM) has opened the way to study dissipation processes at the nanoscale [1]. The phase lag between oscillation and excitation in Amplitude Modulation (AM) AFM has been used for a long time as a source of compositional contrast. Non-contact AFM (technically, AFM operated in the frequency modulation mode, FM-AFM) offers --apart from the usual topography images of constant frequency shift--, another source of contrast for atomic-resolution imaging: the energy required to keep the oscillation amplitude. Recently, a combination of FM-AFM experiments and large-scale ab initio calculations has been able to identify a dissipation channel on different semiconductor surfaces due to single-atom contact adhesion [2,3]. In this contribution, we further explore, in two technologically relevant organic-based systems, the role of adhesion hysteresis as a dominant dissipation process and the atomistic mechanisms involved. Firstly, we investigate the origin of the intramolecular contrast in energy dissipation FM-AFM images of PTCDA molecules adsorbed on metal surfaces. Secondly, we discuss the interaction of a silicon-oxide nanoasperity with a monolayer of sexithiophene molecules, where topography and energy dissipation has been studied experimentally with Amplitude Modulation (AM) AFM [4,5]. [1] R. Garcia and R. Perez, Surf. Sci. Rep. 47, 197 (2002). |
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