Scanning tunneling microscopy study of the coverage-dependent structures of pentacene on Au(111)

Pentacene thin films on Au(111) have been investigated at various coverages using scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD). Two distinct binding environments, surface bound and bulk-like, were evident from TPD. The surface monolayer and multilayer pentacene molecules had binding energies of 110 and 106 kJ/mol, respectively. STM investigations of surfaces with pentacene coverages of around 0.25 monolayer equivalents (MLE) yielded four different ordered structures with end-to-end nearest neighbor interactions. Increasing the pentacene coverage from 0.25 to 1 MLE leads to ordered single monolayer films with rows of molecules aligned side-by-side. STM images of an annealed pentacene film revealed that these side-by-side rows are the dominant structure for complete monolayer films. A periodic row structure with a large spacing of 61 Angstrom was observed to form in films with coverages greater than a monolayer (1.1-1.5 MLE). Upon deposition of 2 MLE, the widely spaced periodic row structure appears to be covered with a layer similar to the monolayer structure. STM images of these coverage-dependent structures and their relationship to bulk crystalline pentacene are discussed.