Herringbone and pi-stacked phases of pentacene thin films grown on metal surfaces

The structure of an organic semiconductor thin film is closely related to their optical and electronic properties. The main difference between an organic and an inorganic semiconductor is the intermolecular interaction forces: weak van der Waals interactions for the former and strong covalent bonds in the latter. Among the organic semiconductors studied to date, pentacene has been shown to possess the highest charge carrier mobility, comparable to that of a-Si:H. This thesis focuses on the growth of the pentacene thin films in two distinctively different phases: the pi-stacked, lying-down phase on clean Au(111) and the herringbone, standing-up phase on a passivated metal Au surface. In the pi-stacked phase, pentacene forms metastable submonolayer structures that transform into close-packed monolayer structure at high coverages. The monolayer structure is maintained up to several multilayers and the films grow non-epitaxially. Each close-packed, pi-stacked layer shows a very broad range of azimuthal angles of molecular domains. Relatively strong van der Waals interaction between the metal surface and the monolayer of molecules results in the first lying down layer, which seeds the non-epitaxial growth of pi-stacked multilayer films. Passivating the metal surface with a self-assembled monolayer (SAM) benzyl thiolatea weakens the pentacene-substrate interaction and leads to the growth of a bulk-like herringbone phase with the long molecular axis almost perpendicular to the substrate surface. The films in the herringbone phase possess molecular defects where pentacene molecules are displaced in the z direction and show buckled-up defects with maximum displacement ≤2.5 A. Quantum-chemical calculation suggest that these defects induce both hole and electron levels in the gap and act as shallow traps for charge carriers.