Transition voltage spectroscopy in vacuum break junction: Possible role of surface states

The recently proposed transition voltage (V-t) spectroscopy (TVS) is becoming an increasingly popular tool to analyze electric transport through molecular devices. Very recently, to get additional insight into TVS, the distance (d) dependence V-t(d) in vacuum break junctions has been investigated experimentally. In this letter, we point out inconsistencies of the initial theoretical interpretation, based on approximate Wenzel-Krammers-Brillouin (WKB)-type results within the standard vacuum tunneling barrier model. We demonstrate that even the exact treatment of this model fails to explain the experimental V-t(d)-dependence, but the agreement with experiment is substantially improved by accounting for electron states (or resonances) at electrodes' surface. These findings suggest that surface states may also play a significant role in molecular devices with sharp electrodes and calls for further investigations. Copyright (C) EPLA, 2012