Mesoscopic pattern formation during initial oxidation of Ni(1 1 1) observed by electron emission microscopy

An apparatus available for metastable-atom electron emission microscopy (MEEM) and photoelectron emission microscopy (PEEM) was constructed for the present study, based on our electron trajectory simulations. In MEEM, information on the local electronic states at the topmost layer is selectively obtained. The mesoscopic (μm-scale) images during initial oxidation of Ni(1 1 1) at temperature of 300-700 K were monitored and compared with the spectroscopic data. The MEEM images obtained upon thermal collision of He∗(23S) are shaded in ascending order of brightness: the chemisorbed, clean, and oxidized surfaces. Their brightness is in good agreement with the emission yield estimated from the corresponding electron emission spectra. During nucleation and lateral growth of oxide at 300 K, the PEEM and MEEM images reveal dappled and sometimes periodic patterns, probably due to surface defects that lingered even after the surfaces were cleaned. The evolution of the images suggests that the initial oxidation proceeds via successive multinucleation. It is also found that the μm-scale patterns of oxide strongly depend on the substrate temperature and dose pressure of gaseous oxygen. The oxide formation in the mesoscopic scale is discussed in terms of a diffusion-reaction field.