Scanning Tunnelling Microscopy Studies: Self-Assembly on Graphite
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The two-dimensional self-organization of molecules on surfaces and at interfaces is at the center of vigorous research activity. Thin interfacial films have numerous longstanding technological applications; however, the interest in a fundamental understanding of the self-assembly process has surged in recent years due to the central role envisioned for the controlled bottom-up assembly in future nanoscale molecular engineering. Monolayers physisorbed on the basal plane of graphite can serve as ideal model systems for such studies.
Highly ordered adsorbate monolayers have been generated on graphite substrates for a broad range of molecular species and experimental conditions. On this inert substrate, the dominant dispersion and electrostatic interactions often afford sufficient adsorbate mobility to aid in the self-assembly of thermodynamically favored monolayer structures. A combination of theoretical and experimental approaches, including proximal probes, diffraction-based techniques, and thermal desorption, has been employed to interrogate adsorbate structures and dynamics, and ultimately unravel the delicate balance of forces driving the self-assembly process.
Among experimental techniques, scanning tunneling microscopy (STM) plays a prominent role in elucidating the spatial orientation and conformation of individual adsorbate molecules. The present article focuses mainly on STM studies addressing the role of interactions associated with functional groups and with saturated hydrocarbon chains in driving self-assembly on the basal plane of graphite. The structure and dynamics of the observed ordered, lamellar monolayers are seen to result from a set of competing geometric requirements. In addition, a brief discussion of STM image contrast is included, highlighting the ability of STM to provide information about the role of individual adsorbate electronic states in mediating electron transport. The final section of this article is concerned with recent studies exploiting both geometric and electronic contributions to STM image contrast to address molecular device properties in carefully tailored model systems self-assembled on graphite.