Temperature-dependent phase evolution of copper-oxide thin-films on Au(111)

Abstract

The formation of ultrathin copper oxide layers on an Au(111) surface is explored with scanning tunneling microscopy and density functional theory. Depending on the thermal treatment of as-grown Cu–O samples, a variety of thin-film morphologies is observed. Whereas 1D oxide stripes with Au[11] and Au[10] orientation emerge at 450 and 550 K annealing, respectively, a planar (2 × 2) Cu–O network with specific domain structure develops at higher temperature. The latter is ascribed to a Cu₃O₂ honeycomb lattice with oxygen ions alternatingly located in surface and interface positions. Strain minimization and a thermodynamic preference for Cu-rich edges lead to the formation of structurally well-defined boundaries, delimiting either triangular, elongated or stripe-like Cu₃O₂ domains. The low-temperature phases compirse complex arrangements of hexagonal and square Cu–O units, similar to those found in Cu₂O(111) and (100) surfaces, respectively. The transitions between different thin-film phases are driven by Cu dissolution in the gold crystal and O₂ evaporation and therefore accompanied by a thinning of the oxide layer with increasing temperature.