Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range\r\nof surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the\r\ntip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and\r\ntheoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip\r\napex in the observed contrast.\r\nResults: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum\r\n(UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a\r\nrange of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we\r\neliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of\r\nsilicon dimers, which is a key issue in scanning probe studies of Si(100).\r\nConclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by\r\nvariations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians\r\nworking on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the\r\ntip termination critically affects both the short-range force and dissipated energy.
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