Reaction of nanometer-sized Cu particles with a SiO2 substrate

The thermal stability of nanometer-sized Cu particles on a 400–500 nm thick SiO2 layer on top of a Si(100) substrate was studied after annealing in ultrahigh vacuum up to 620¿°C. Atomic force microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and Auger electron spectroscopy measurements clearly show that Cu-silicide islands are formed. A direct reaction of Cu with the SiO2 support is assumed, which is facilitated by a fairly strong metal-support interaction and by the wetting behavior of the silicide islands. Exposure to air at room temperature results in regeneration of the annealed Cu/SiO2 system

Charge exchange in low-energy He+ ion scattering from solid surfaces

We present a model for neutralization of He+ based on resonant charge transfer from a surface valence band to the He 2s level, followed by Auger deexcitation or autoionization, thus creating a He atom in the ground state. If a He+ ion approaches a surface, it is energetically favorable for the He-surface system to screen the 1s core hole by putting an electron in the 2s level. By taking into account this Coulomb interaction Q we are able to explain the trend in the neutralization behavior of 1–5-keV He+ ions scattered from clean metal surfaces. It is shown that the neutralization probability of He+ is mainly determined by the work function and the surface local density of states

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen-terminated diamond(111)/tungsten-carbide interface have been studied using ultra-high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten-carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer-scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model

Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface

A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-Müller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers