Single molecule electronics and tunneling in molecules

Covalent tethering of single molecules between a gold surface and a gold nano-contact permits reliable measurement of the electronic properties of single molecules. Understanding of molecular electronics at the single molecule level should pave the way for understanding of more complex, multi-molecule devices. A single molecule study of n-alkanes shows that the electron decay rate, β_N, is strongly affected by the nature of the connections to the molecule, β_N being small and decreasing with applied bias. Mechanical (non-bonded) contacts result in β_N increasing with bias, an outcome completely inconsistent with tunneling theory. The degree to which theory and experiment diverge even for such simple molecular electronic components is illustrated by the observation of contact charging in alkanethiol monolayers. We speculate that standard tunneling theory does not provide a full description of charge transport in even these simple molecules.