Resonant coupling and negative differential resistance in metal/ferrocenyl alkanethiolate/STM structures

Abstract

Recent experimental studies demonstrated that self-assembled molecules sandwiched between metallic contacts can perform logic functions based on negative differential resistance (NDR). To understand the mechanism of NDR, the electronic structure and transport properties of one such junction, ferrocenyl alkanethiolate attached to a gold surface and probed with a scanning tunneling microscope tip, are investigated by large scale ab initio calculations. The $I\text{\ensuremath{-}}V$ characteristics show strong NDR features at both positive and negative biases, in good agreement with the experimental data. The voltage-dependent transmission, potential drop profile, and molecular level alignment under bias suggest that the ferrocenyl group acts like a quantum dot and that the NDR features are due to resonant coupling between the highest occupied molecular orbital and the density of states of gold leads. The strength of the individual NDR peaks can be tuned by changing the tunneling distance or using suitable spacer layers.