The investigation of hyperthermal gas collisions on heterogeneous surfaces using an in situ scanning tunneling microscope

This thesis describes an ultra-high vacuum scanning tunneling microscope (UHVSTM)that is interfaced to a pulsed molecular-beam source. This is the first such instrument to allow in situ monitoring of a sample during molecular-beam exposure. This apparatus is used to investigate the effect of hyperthermal rare-gas bombardment on alkanethiol self-assembled monolayers. STM images show that close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4-eV argon and 1.3-eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the alkanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment. High-energy, rare-gas collisions generally induce three types of structural transformations: domain boundary annealing, vacancy island migration, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. Migration rates are calculated and compared for molecules in close-packed domains, at domain boundary defects, and along the perimeter of vacancy island defects. The number of nearest-neighbor molecules (within the 5 å¡A lattice distance) is stongly predictive of molecular stability with respect to rare-gas bombardment, and the overall dependence of stability on nearest neighbors is well fit by a simple exponential curve for molecules with 0–5 nearest neighbors. For most observed structural changes the incident direction of the molecular beam does not influence the direction of molecular motion, indicating that for this system, collision-induced migration proceeds through vibrational excitation of the molecular film.