Surface diffraction studies of organic thin films

In order to focus on the role played by the sulfur head group in the formation of self-assembled monolayers of alkanethiols on gold we have studied the adsorption of the shortest thiol radical, obtained by dissociative adsorption of (CH3S)2, on the Au(111) surface. Upon application of a specific dosing/annealing protocol we have observed the formation of a superstructure in addition to the basic (√3 x √3)R30° structure. Though low energy atom diffraction results were not enough to identify the correct unit cell structure of the superlattice, X-ray diffraction peak positions were consistent with a 3x4 periodicity. Preliminary analysis of photoelectron data suggests the atop site to be the most stable one, which is contrary to all the theoretical predictions.; Motivated by the need to know the structure of thin films of organic semiconductor materials at the semiconductor/metal interface that are useful to understand the charge injection mechanism at the source and drain contact in electronic devices such as, for instance, a field effect transistor, we also studied the structure of pentacene thin films grown by supersonic molecular beam deposition on two Ag(111) single crystal surfaces with different average step distances. While the initial stage of the growth is similar on the two different surfaces, regardless of the pentacene kinetic energy and substrate temperature, thicker films show different structural and growth properties. The ultra thin film phase has the same structure on both surfaces, showing, however, as expected much larger domain sizes on the substrate with larger average terrace width. In spite of the inferior quality of the ultra-thin layer grown on the narrower terraces, upon continuing the growth, a well ordered multilayer structure is obtained. This, however, occurs in a narrow range of deposition conditions, i.e. at a relatively high kinetic energy of the molecules (∼5 eV) and a low substrate temperature (200 K). On the other, almost ideally flat surface, the order of the pentacene multilayers is, surprisingly, much poorer in spite of the fact that the measured width of the diffraction peaks of the first monolayer was the narrowest we have ever measured for an organic thin film.