Molecular-level Investigation of Two-Dimensional Thin Film Assembly for Acene Derivatives on Metal Surfaces.

摘要

Given the ever increasing call for faster, smaller and more innovative devices, and the utility that organic electronics has been shown to have, it is of paramount importance to study model systems that will help to quantify the driving forces that result in ordered and disordered molecular assemblies. Specifically, the question of why some organic films order and others exist in a disordered state needs further study so that device designers can reliably predict which systems will have the desired properties for their selected device. One of the best tools for determining the local order and electronic structure at the nanoscale level is the scanning probe microscope. As such, we custom designed and home built a combination scanning tunneling/atomic force microscope to operate in UHV at cryogenic temperatures. Using this instrument and another pre-existing microscope, the structural phase space for thin films of both pentacene, an important organic semiconductor, and 9,10-dibromoanthracene (DBA) on Ag(111) were mapped out. Pentacene displays novel structural and electronic effects at reduced temperatures that are not present for room temperature assembly. Included in these is a shift in the energetic position of the lowest unoccupied molecular orbital of the bilayer film, which is hypothesized as being caused by a more densely packed bilayer film at 53 K vs. room temperature. DBA forms a quasi-hexagonal structure via unique weak Br-Br interactions. While similar halogen interactions have been seen in the past, the steric hindrance caused by the molecular structure of DBA induces Br-Br positions that should normally be repulsive but are attractive in this system. Probing the DBA film induces rotations, reflections, and translations of the film on a scale not previously seen.