Enhanced surface plasmon effect of Ag/TiO₂ nanodiodes on internal photoemission

Over the last several decades, innovative light-harvesting devices have evolved to achieve high-efficiency solar energy transfer. Understanding the mechanism of plasmon resonance is very desirable to overcome the conventional efficiency limits of photovoltaics. The influence of localized surface plasmon resonance on hot electron flow at a metal-semiconductor interface was observed with a Schottky diode composed of a thin silver layer on TiO₂; subsequent X-ray photoelectron spectroscopy characterized how oxygen in the Ag/TiO₂ nanodiode influenced the Schottky barrier height. Photoexcited electrons generate photocurrent when they have enough energy to travel over the Schottky barrier formed at the metal-semiconductor interface. We observed that the photocurrent could be enhanced by optically excited surface plasmons. When the surface plasmons are excited on the corrugated Ag metal surface, they decay into energetic hot electron-hole pairs, contributing to the total photocurrent. The abnormal resonance peaks observed in the incident photons to current conversion efficiency can be attributed to surface plasmon effects. We observed that photocurrent enhancement due to surface plasmons was closely related to the corrugation (or roughness) of the metal surface. While the photocurrent measured on Ag/TiO₂ exhibits surface plasmon peaks, the photocurrent on Au/TiO₂ does not show any peaks even at the Au surface plasmon energy frequency presumably because of the smoothness of the gold film. We modified the thickness and morphology of a continuous Ag layer using electron beam evaporation deposition and heating under gas conditions and found that morphological changes and the thickness of the Ag film are key factors in controlling the internal photoemission efficiency.