Quantum size effects on the work function of metallic thin film nanostructures

Abstract

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant κ, resulting in a very rich phenomenon of "quantum oscillations" in κ as a function of thickness, L, and bias voltage, Vs. More specifically, we find that the phase of the quantum oscillations in κ vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (|Vs| < 0.03 V) the measurement of κ vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of κ vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.