The initial stages of the growth of copper on a (1x1) and a (root 31x root 31)R+/-9 degrees alpha-Al2O3(0001) surface

We have studied the first stages of the growth of copper on two different alpha-Al2O3 (0001) alumina substrates, presenting respectively a (1 X 1) and a (root 31 X root 31)R +/- 9 degrees LEED (low energy electron diffraction) pattern. The study was realised by means of surface sensitive X-ray absorption spectroscopy (SEXAFS) at the CuK edge. For equal amounts of deposited copper (50% of an equivalent monolayer), we found quite different copper/alumina interfaces as a function of the substrate reconstruction. In the case of the (1 X 1) surface, we noticed the coexistence of copper dimers and very small copper nanoclusters (about 1 shell) weakly bonded to the substrate via the Al surface atoms. In the case of the (root 31 X root 31)R +/- 9 degrees reconstructed substrate, we detected slightly bigger Cu nanoclusters than those present on the (1 X 1) surface. We also observe that some Cu atoms are incorporated to the substrate, occupying the vacancies of the reconstructed surface. No Cu-O bonds are detected in both interfaces. This result can be understood in the case of the aluminium rich (root 31 X root 31)R +/- 9 degrees surface. However, it is more surprising in the case of the (1 X 1) surface and supports the hypothesis of an Al-terminated(1 X 1) alumina substrate.