Mg clusters on MgO surfaces: study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

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