Optimizing Hot Electron Harvesting at Planar Metal–Semiconductor Interfaces with Titanium Oxynitride Thin Films

Abstract

Understanding metal–semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2–x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2–x ) creates a metal–semiconductor contact. Time-resolved pump–probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (N FC ≈ 2.8 × 1019 m–3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (N HE ≈ 1.6 × 1018 m–3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal–semiconductor interface using only the native oxide of titanium oxynitride.

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“…The observed relaxation occurred in the order of tens to hundreds of picoseconds (ps), suggesting strong electron–phonon coupling leading to rapid lattice heating. 38,39 This demonstrates excellent light-to-heat conversion by neat TiN. The result aligns with the observed increase in catalyst bed temperature with decreased Ni loading under inert conditions amongst the prepared (NiO loaded) samples.…”