Morphology and electronic structure of aromatic molecules adsorbed on Au(111)

Abstract

Organic semiconducting thin films of pentacene, p-sexiphenyl and naphtho[2,3- a]pyrene were characterized on a Au(111) substrate. The morphology of the organic adsorbates on Au(111) have been investigated with STM. Coverage-dependent structures have been found with pentacene on Au(111). Four different low coverage structures as well as multiple one-dimensional rows, with coverage-dependent separation, have been imaged. Annealed pentacene films have revealed the monolayer structure. Complex multilayer polymorphs have also been observed and modeled in high-coverage films. An unusual alternating p-sexiphenyl domain has been observed with a herringbone unit cell similar to that found in the bulk. High-resolution images have shown a single molecular enantiomer making up chiral domains of naphtho[2,3-a]pyrene on Au(111). The binding environments of the organic semiconductor molecules on Au(111) have been determined utilizing temperature programmed desorption. Multiple binding energy environments have been observed for several of the organic films. These differing environments have been attributed to the stronger interactions of the molecules in direct contact with the Au(l 11) compared with the molecules in the multilayer. XPS and UPS have been used to characterize the energetics of the immediate interface at the metal/organic heterojunction. XPS has been used to ensure no chemical reaction of the organic on the metal. UPS He I has measured the work function shift of the interface to determine the magnitude of the interface dipole. Large dipoles of up to leV have been determined at these interfaces. These work function shifts are completed after deposition of approximately one monolayer. He II spectra have been used to monitor the growth of valence states in the organic with increasing film thickness.