Thermal oxidation of copper leads to the spontaneous growth of single crystalline and pristine CuO nanowires which is an attractive materials platform for photoelectrochemical and photochemical applications. However, our limited understanding of CuO nanowire surfaces remains a bottleneck for its widespread adaptation as a material for energy conversion. One approach to probe the CuO nanowire surface is to investigate changes to its chemical and electrical signature upon deposition of a few monolayers (< 2 nm) of Al₂O₃; a chemically and electrically passivating film. Exactly how Al₂O₃ might achieve this passivity is the subject of this talk.

We progressively map, ex situ, the effect of adding few monolayers of Al₂O₃ on the surface chemistry and electrical characteristics of a single CuO nanowire. The CuO nanowire is contacted electrically via electron beam lithography (Figure 1a), prior to the deposition experiments. Using atomic layer deposition (ALD), Al₂O₃ is deposited using trimethyl aluminum (TMA) and H₂O as reactants. A deposition rate of 1Å/cycle is obtained. The chemical changes on the nanowire surface are studied by X-ray photoelectron spectroscopy (XPS) and electrical properties of the nanowire are recorded after every monolayer deposition.

XPS fine spectra of Cu 2p reveal a clear reduction of Cu²⁺ to Cu¹⁺ after only a single pulse of TMA. The O 1s fine spectra reveals the formation of Al-O and Cu⁺¹-O bonds and removal of adsorbed O species after 1 cycle of Al₂O₃ (Figure 1b). The Al 2p fine spectra shows a clear Al-O bond formation after ~ 3 cycles of ALD. Thus, a clear surface reduction of CuO nanowire and subsequent formation of Al₂O₃ is noted in XPS studies. This change in surface chemistry manifests itself in the electrical characteristics as a detectable photocurrent response under ambient (760 Torr) conditions; where, for a pristine CuO nanowire, no such response is previously observed (Figure 1c). Current-voltage characteristics are mapped as a function of temperature and discussed in light of the changes to surface properties observed via XPS.