INTERACTION OF OXYGEN WITH NI(110) STUDIED BY SCANNING-TUNNELING-MICROSCOPY

By means of scanning tunneling microscopy (STM), the dynamics of the oxygen (O)-induced reconstruction of the Ni(110) surface has been studied in real time and space by recording atom-resolved images of the clean and O-covered Ni(110) surfaces. It is found that oxygen adsorption at room temperature initiates the nucleation of two different coexisting structures. For low oxygen exposures, it is revealed that O induces a structure consisting of strings along the close-packed [110BAR] direction, growing either out from [001] step edges or in troughs created on large, flat terraces. STM reveals that this structure is indeed a reconstructed one, with O atoms plausibly located at a pseudo-threefold site of the rudimentary (111) face of the Ni rows (strings), rather than a disordered structure with O chemisorbed on the unreconstructed surface, as previously suggested in the literature. Coexisting with this string structure, the well known (3 x 1) and (2 x 1) reconstructions, corresponding to local O coverages of 1/3 and 1/2 ML, respectively, develop locally. These reconstructions are stabilized by -Ni-O- rows running along the [001] direction. The nucleation and growth of these reconstructions prove that they are of the added-row rather than the missing-row type. At higher oxygen exposures, the (2 x 1) added-row structure is completed at the expense of the string structure, and subsequent oxygen exposure induces another added-row (3 x 1) structure corresponding to an O coverage of 2/3 ML. At even higher oxygen exposures, atom-resolved STM images and LEED reveal the existence of a (9 x 5) suboxide structure which has previously been identified as a (9 x 4) structure. The (9 x 5) structure is a two-layer structure, with Ni-Ni interdistances similar to those in a surface layer of NiO(100). Finally, an epitaxial NiO(001) oxide is formed. The nucleation and growth of these ''oxide'' phases are discussed.