Linear chains of styrene and methylstyrene molecules and their heterojunctions on silicon: Theory and experiment

We report on the synthesis, scanning tunnel microscope (STM) imaging, and theoretical studies of the structure, electronic structure, and transport properties of linear chains of styrene and methylstyrene molecules and their heterojunctions on hydrogen-terminated dimerized silicon (001) surfaces. The theory presented here accounts for the essential features of the experimental STM data including the nature of the corrugation observed along the molecular chains and the pronounced changes in the contrast between the styrene and methylstyrene parts of the molecular chains that are observed as the applied bias is varied. The observed evolution with applied bias of the STM profiles near the ends of the molecular chains is also explained. Calculations are also presented of electron transport along styrene linear chains adsorbed on the silicon surface at energies in the vicinity of the molecular highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. For short styrene chains this lateral transport is found to be due primarily to direct electron transmission from molecule to molecule rather than through the silicon substrate, especially in the molecular LUMO band. Differences between the calculated position dependences of the STM current around a junction of styrene and methylstyrene molecular chains under positive and negative tip bias are related to the nature of lateral electron transmission along the molecular chains and to the formation in the LUMO band of an electronic state localized around the heterojunction.