Molecular commensurability with a surface reconstruction: STM study of azobenzene on Au(111)

Azobenzene derivatives form a unique class of photoactive molecules that have the potential for nanoscale optical applications. We have observed the self-assembly behavior of azobenzene molecules adsorbed to Au(111) using scanning tunneling microscopy (STM). We find that azobenzene creates a surprising variety of surface structures whose commensurability with the underlying Au(111) ``herringbone'' reconstruction is coverage dependent. Two commensurate molecular chain phases exist in the low-coverage regime, and phase conversion between these structures can be induced using the STM tip. In the high-coverage regime we observe incommensurate molecular phases as well as commensurate molecular vacancy ordering. These molecular structures reflect a coverage-dependent competition between molecule-molecule and molecule-substrate interactions.