Surface chemistry of pentacene on clean and chemically modified silicon(001) surfaces

Pentacene has received much recent attention as the organic semiconductor with the highest charge carrier mobility to date. Using a variety of surface science techniques, the chemistry of pentacene thin film nucleation and growth was investigated on the clean Si(001) surface as well as Si(001) surfaces terminated with an organic monolayer. On the clean Si(001) surface, the strong chemisorption of the first layer of pentacene causes disruption to the conjugated pi electron system. Pentacene molecules in layers beyond the first layer adsorb molecularly and show characteristics of a delocalized pi system. Chemical modification of the Si(001) surface with cyclopentene, a small organic alkene, provides a strongly bound interfacial monolayer and prevents the dissociation of subsequently deposited pentacene molecules.; The concept of chemically modifying the Si(001) surface prior to pentacene deposition is also extended to different self-terminating monolayers. Nucleation and growth of pentacene films on Si(001) surfaces that were chemically modified with 1,5-cyclooctadiene, 1-dodecene, and 1-hexene was monitored using low energy electron microscopy and diffraction. Termination of the surface with any of these molecules enables the growth of large pentacene grains on the order of 100 mum in diameter. Pentacene molecules adopt crystal structures that are in most cases commensurate in one direction with the underlying Si(001)-2 x 1 reconstruction. On the dodecene-terminated 4° miscut Si(001) surface, anisotropic growth is observed that may indicate a straightforward method for controlling pentacene growth.; The conversion of a soluble photopatternable precursor to pentacene via a thermally induced retro Diels-Alder reaction was also studied. In the presence of a photogenerated acid, the precursor converts to pentacene at a lower temperature and in less time than without acid. Ultraviolet light can be used to photopattern thin films of the precursor such that the exposed regions are preferentially converted while unexposed areas remain unconverted. Areas of unconverted precursor are then removed by rinsing in methanol. These films can be patterned on the order of tens of microns and exhibit carrier mobilities comparable to those of vacuum-deposited pentacene films.