Modulation of Electrical Conduction Through Individual Molecules on Silicon by the Electrostatic Fields of Nearby Polar Molecules: Theory and Experiment

摘要

We report on the synthesis, scanning tunneling microscopy (STM) andtheoretical modeling of the electrostatic and transport properties ofone-dimensional organic heterostructures consisting of contiguous lines of CF3-and OCH3-substituted styrene molecules on silicon. The electrostatic fieldsemanating from these polar molecules are found, under appropriate conditions,to strongly influence electrical conduction through nearby molecules and theunderlying substrate. For suitable alignment of the OCH3 groups of theOCH3-styrene molecules in the molecular chain, their combined electric fieldsare shown by ab initio density functional calculations to give rise topotential profiles along the OCH3-styrene chain that result in stronglyenhanced conduction through OCH3-styrene molecules near the heterojunction formoderately low negative substrate bias, as is observed experimentally. Undersimilar bias conditions, dipoles associated with the CF3 groups are found inboth experiment and in theory to depress transport in the underlying silicon.Under positive substrate bias, simulations suggest that the differingstructural and electrostatic properties of the CF3-styrene molecules may leadto a more sharply localized conduction enhancement near the heterojunction atlow temperatures. Thus choice of substituents, their attachment site on thehost styrene molecules on silicon and the orientations of the molecular dipoleand higher multipole moments provide a means of differentially tuning transporton the molecular scale.