The effects of alloying and segregation for the reactivity and diffusion of oxygen on Cu3Au(111).

We report results of the segregation induced by the adsorption of O2 and the barrier of the formation of Cu2O in Cu3Au(111) with an experimental and theoretical approach. Oxidation by a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident-energy dependence of the measured O-uptake curve, dissociative adsorption of O2 is less effective on Cu3Au(111) than on Cu(111). The dissociative adsorption is accompanied by the Cu segregation on Cu3Au(111) as well as on Cu3Au(100) and Cu3Au(110). The obvious growth of Cu2O for a 2.3 eV HOMB cannot be observed and it suggests that the Au-rich protective layers prevent the diffusion of O atoms into the bulk. The theoretical approach based on density functional theory indicates that O adsorption shows the same behavior on Cu3Au(111) and on Cu(111). For the diffusion case, the barrier to diffuse into the subsurface in segregated Cu3Au(111) is higher than in Cu(111). This indicates that the segregated Au-rich layer in Cu3Au(111) works as a protective layer.

Initial stages of Cu3Au(111) oxidation: oxygen induced Cu segregation and the protective Au layer profile.

We report results of our experimental and theoretical studies on the Au concentration profile of Cu3Au(111) during oxidation by a hyperthermal O2 molecular beam at room temperature, using X-ray photoemission spectroscopy (XPS), in conjunction with synchrotron radiation (SR), and density functional theory (DFT). Before O2 exposure, we observe strong Au segregation to the top layer, i.e., Au surface enrichment of the clean surface. We also observe a gradual Cu surface enrichment, and Au enrichment of the second and third (subsurface) layers, with increasing O coverage. Complete Cu segregation to the surface occurs at 0.5 ML O surface coverage. The Au-rich second and third layers of the oxidized surface demonstrate the protective layer formation against oxidation deeper into the bulk.

In situ synchrotron radiation photoelectron spectroscopy study of the oxidation of the Ge(100)-2 × 1 surface by supersonic molecular oxygen beams.

In situ synchrotron radiation photoelectron spectroscopy was performed during the oxidation of the Ge(100)-2 × 1 surface induced by a molecular oxygen beam with various incident energies up to 2.2 eV from the initial to saturation coverage of surface oxides. The saturation coverage of oxygen on the clean Ge(100) surface was much lower than one monolayer and the oxidation state of Ge was +2 at most. This indicates that the Ge(100) surface is so inert toward oxidation that complete oxidation cannot be achieved with only pure oxygen (O2) gas, which is in strong contrast to Si surfaces. Two types of dissociative adsorption, trapping-mediated and direct dissociation, were confirmed by oxygen uptake measurements depending on the incident energy of O2. The direct adsorption process can be activated by increasing the translational energy, resulting in an increased population of Ge(2+) and a higher final oxygen coverage. We demonstrated that hyperthermal O2 beams remarkably promote the room-temperature oxidation with novel atomic configurations of oxides at the Ge(100) surface. Our findings will contribute to the fundamental understanding of oxygen adsorption processes at 300 K from the initial stages to saturated oxidation.

The effect of step geometry in copper oxidation by hyperthermal O2 molecular beam: Cu(511) vs Cu(410).

Steps are known to be often the active sites for the dissociation of O(2) molecules and the nucleation sites of oxide films since they provide paths for subsurface migration and oxygen incorporation. In order to unravel the effect of their morphology on the oxidation of Cu surfaces, we present here a detailed investigation of the O(2) interaction with Cu(511) and compare it with previous results for Cu(410), a surface exhibiting terraces of similar size and geometry but different step morphology. As for Cu(410) we find, by x-ray photoemission spectroscopy performed with synchrotron radiation, that Cu(2)O formation gradually starts above half a monolayer oxygen coverage and that the ignition of oxidation can be lowered to room temperature by dosing O(2) via a supersonic molecular beam at hyperthermal energy. The oxidation rate for Cu(511) comes out to be lower than for Cu(410) at normal incidence, about the same when the O(2) molecules impinge towards the ascending step rise, but higher when they hit the surface along trajectories even slightly inclined towards the descending step rise. These findings can be rationalized by a collision induced absorption mechanism.

Kinetics of oxygen adsorption and initial oxidation on Cu(110) by hyperthermal oxygen molecular beams.

Oxygen adsorption and subsequent oxide formation on Cu(110) using a hyperthermal oxygen molecular beam (HOMB) were investigated using X-ray photoelectron spectrometry. The O-uptake curves, which were determined from the evolution of the O-1s peaks, indicate that simple Langmuir type kinetics can describe dissociative adsorption of O(2) with an incident energy (E(i)) below 0.5 eV under Theta < or = 0.5 ML. The reaction order dependence on E(i) implies two competing dissociation mechanisms, trapping-mediated and directly activated adsorption. Oxidation at Theta > or = 0.5 ML proceeds rather effectively using highly energetic HOMB at E(i) > or = 1.0 eV. The azimuthal dependence of the sticking probability during the effective oxidation using HOMB incidence suggests that the added rows, which consist of the Cu-O structure, shade the reactive hollow sites in the trough where oxygen penetrates into the subsurface. The surface Cu(2)O formed with highly energetic HOMB incidence decomposes with desorbing subsurface oxygen even at room temperature, demonstrating that HOMB can induce a metastable surface structure that cannot be produced in the thermal equilibrium process.

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si.

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni3C/NiCx)/graphene/Ni/Ni silicides (Ni2Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene.