Effect of oxygen pressure on the optical and structural properties of Cu:Al2O3 nanocomposite films

Cu:Al2O3 multilayer nanocomposite thin films have been produced by alternating pulsed laser deposition and analyzed by nondestructive synchrotron radiation techniques. The effect of growing in an oxygen atmosphere, or exposing the films to an oxygen atmosphere immediately after the copper deposition, on the formation and properties of Cu nanocrystals (NC's) has been investigated. The optical absorption of the films with Cu deposited in vacuum shows a band with a maximum around 630 nm, which is related to a surface plasmon resonance (SPR) and is evidence of the presence of Cu NCs. When Cu is grown in an oxygen atmosphere, a decrease in the overall optical absorption and a damping of the SPR band are observed as the oxygen pressure is increased. Grazing incidence small-angle x-ray scattering results show that NC's with an average size of ∼5 nm are distributed within the Al2O3 matrix forming discontinuous periodic layers, except for the films with copper deposited at the highest oxygen pressure (6.6x10-3 mbar) which show no evidence of NC formation. X-ray absorption spectroscopy analysis shows that whereas the Cu NC's produced have negligible oxidation when Cu is deposited in vacuum, Cu oxide appears for all the other films. The results suggest that small Cu2O clusters coexist with large weakly oxidized Cu clusters for the films produced in oxygen ambient at intermediate pressures (up to 2.6x10-4 mbar). For the highest deposition oxygen pressure it appears that layers of highly disordered Cu oxides and AlCuO2 have been produced. When Cu NCs are exposed to oxygen after deposition an oxide shell preserving the metallic Cu core is formed