Linear Chains of Styrene and Methyl-Styrene Molecules and their Heterojunctions on Silicon: Theory and Experiment

TL;DR

This study combines experimental STM imaging and theoretical modeling to analyze the structure, electronic properties, and electron transport of styrene and methyl-styrene molecular chains and their heterojunctions on silicon surfaces.

Contribution

It provides a comprehensive understanding of the molecular chain structures, electronic contrast variations, and transport mechanisms, integrating experimental data with theoretical calculations.

Findings

STM profiles explained by theory including contrast changes

Electron transport mainly occurs via direct molecule-to-molecule transmission

Localized electronic states form at heterojunctions in the LUMO band

Abstract

We report on the synthesis, STM imaging and theoretical studies of the structure, electronic structure and transport properties of linear chains of styrene and methyl-styrene 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 methyl-styrene 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 HOMO and 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 methyl-styrene 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.