A quartz crystal microbalance combined with scanning tunneling microscopy (STM-QCM) was used to investigate the interactions between organic adlayers (C6H6 and C6H5I) on Cu surfaces and a metallic STM tip. STM images of C6H6 covered Cu surface improved when the QCM was simultaneously oscillated during the imaging. In contrast, STM images of C6H5I covered surfaces became noisy when the sample was oscillated. The two systems moreover exhibited frequency changes of opposite signs in response to STM tip contact, indicative of different physical phenomena at the surface. The dependence of the STM image quality and the frequency shift were interpreted in terms of the adsorbate-substrate chemical and physical interactions, and different levels of frictional heating at the interface.

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