Charge transfer across the molecule/metal interface using the core hole clock technique

Abstract

Interfacial charge-transfer is a topic with wide relevance in many research fields, and there is intense scientific interest to characterize and understand the nature and mechanisms of charge transfer across various interfaces, including molecule/metal, molecule/semiconductor and molecule/molecule interfaces. Charge transfer at semiconductor interfaces constitutes the primary step in many novel photovoltaic devices, including dye-sensitized semiconductors, assemblies of inorganic semiconductors with conjugated polymers, and quantum confinement devices. In molecular electronics, the movement of charge through molecules and solid-state structures is relatively well documented, but charge transfer across the molecule/metal junction is still poorly understood, and remains an important area of study. Charge transfer dynamics occurring in the several femtosecond regime has recently been successfully investigated, and is based on core-level excitation and decay. Resonant photoemission spectroscopy, or the core-hole clock technique, is analogous to the pump–probe technique, with an intrinsic time scale based on the lifetime of core-holes. It is the shortness of this time scale, comparable to tens of femtoseconds used in laser spectroscopies, that gives this approach a unique place in the study of electron-transfer dynamics. Moreover, this approach has the advantage of atomic specificity unique to core spectroscopies, as this technique involves the excitation of core electrons. In this review, we summarize recent studies on charge transfer dynamics across molecule/metal interfaces by the core-hole technique. Beginning with simple models of relevant excitation and decay processes involving the core-hole clock technique, we explain the rationale and advantages of this technique. The case studies that will be examined, focus on several molecule/metal systems. The description of these studies illustrates the suitability of the core-hole clock technique in the study of charge transfer across the molecule/metal interface. Detailed analyses of these studies further reveal the critical issues in organic electronics and molecular electronics: interfacial charge transfer dynamics in different systems and the factors influencing the transfer process, such as molecular orbitals, molecular orientation and the coupling of molecule and metal. Charge transfer across the molecule/molecule interface is also briefly explored.