Charge Transport at Metal−Molecule Interfaces:  A Spectroscopic View

How an electron crosses a metal−molecule interface has been a longstanding question in many disciplines. The prevalence of this question is illustrated by the different terms used to describe essentially the same process: interfacial electron transfer, charge injection, charge transport, and electron attachment to name a few. The recent surge of interest in molecule-based electronics has renewed the need for quantitative answers to this question. In molecule-based conventional electronic devices, such as organic light-emitting diodes, the metal−molecule interface determines the charge-injection efficiency. The importance of the interface only increases as device dimensions shrink to the scale of a single molecule or a small group of molecules (i.e., molecular electronics). This account takes an experimentalist's view and discusses recent progress in understanding electron transport at metal−molecule interfaces using two-photon photoemission (2PPE) spectroscopy. A 2PPE experiment probes interfacial electron transport in energy, momentum, and time spaces. The latter is especially important because the transport of an electron across a metal−molecule interface is an inherently dynamic process occurring on femtosecond time scales. Recent 2PPE experiments allow us to quantify the following concepts critical to the understanding of interfacial effects in molecule-based electronics: (1) the alignment of molecular orbitals to the metal Fermi level; (2) charge redistribution and interfacial dipoles; (3) the strength of electronic coupling; and (4) electronic−nuclear coupling and dynamic localization.