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

Lowest order in inelastic tunneling approximation : efficient scheme for simulation of inelastic electron tunneling data

We have developed an efficient and accurate formalism which allows the simulation at the ab initio level of inelastic electron tunneling spectroscopy data under a scanning tunneling microscope setup. It exploits fully the tunneling regime by carrying out the structural optimization and vibrational mode calculations for surface and tip independently. The most relevant interactions in the inelastic current are identified as the inelastic tunneling terms, which are taken into account up to lowest order, while all other inelastic contributions are neglected. As long as the system is under tunneling regime conditions and there is no physisorbed molecule on the surface or tip apex, this lowest order in inelastic tunneling (LOIT) approach reduces drastically the computational cost compared to related approaches while maintaining a good accuracy. Adopting the wide-band limit for both tip and surface further reduces calculation times significantly, and is shown to give similar results to when the full energy dependence of the Green's functions is taken into account. The LOIT is applied to the Cu(111)+CO system probed by a clean and a CO contaminated tip to find good agreement with previous works. Different parameters involved in the calculations such as basis sets, k sampling, tip-sample distance, or temperature, among others, are discussed in detail

Formation of ordered structures of NO on Rh(111)

In this work, several (unreported) structures of NO on Rh(111) are presented for coverages ranging from 0-0.78 ML. The formation of these structures as evidenced experimentally by scanning tunneling microscopy (STM) images is explained using density-functional theory derived values for the lateral interactions and adsorption energies of NO on Rh(111). Kinetic Monte Carlo simulations using the derived lateral interactions are performed to determine structures at finite temperatures. The resulting structures can be directly compared to the molecular arrangements observed in the STM experiment. The configuration of the first three structures that form for increasing coverage is the same both experimentally and theoretically. In all three structures, the molecules only occupy the fcc and hcp adsorption sites. Yet, at high coverage, at which the top adsorption sites become occupied, a discrepancy arises

Electromigration of single metal atoms observed by scanning tunneling microscopy

The authors show in this letter that single metal atoms on a Ni(111) surface can be pushed by electromigration forces from a scanning tunneling microscope tip. This repulsive interaction is obsd. over a length scale of 6 nm. While for voltages above -300 mV the atoms are pulled by the microscope tip, the atoms are pushed away below this threshold. This migration is explained by a resonant scattering of strongly correlated electrons. At small voltages chem. forces are pulling the atom, while for larger voltages the at. manipulation is assisted by the tunneling current. [on SciFinder (R)

Magnetic force microscopy and simulations of colloidal iron nanaparticles

Colloidal iron nanoparticles with a core size of 10.6 nm were examined using magnetic force microscopy. Surprisingly, single nanoparticles were more prominently visible in magnetic force gradient images than clusters. A simple qualitative model is proposed to explain this observation, speculating that the local field produced by a cluster of particles may be sufficient to align their moments in the plane of the cluster, even though the particles are superparamagnetic. An alternative possibility of spin glass formation within clusters is also considered. Calculations performed with nanoparticles represented as single dipoles appear to match experimental data quite well

Experimental observation of vibrational modes on Ag(111) along ΓM and ΓK

We present an off-specular high-resolution electron energy-loss spectroscopy study of the vibrational modes along the ΓM and ΓK directions in the surface Brillouin zone of clean Ag(111). We show experimental data of vibrational modes at the K̄ edge, thereby extending the earlier He-scattering data on the S1 Rayleigh mode and pseudo-Rayleigh mode. In addition, we observed the S3 gap mode and three high-frequency modes along ΓK. According to our knowledge, this is the first experimental evidence for the existence of these high-frequency modes in the (111) surface Brillouin zone of fcc metals. Along ΓM the experimental dataset of the S2 gap mode is extended to smaller Q//, and for the first time the high-frequency vibrational mode close to the top of the projected bulk phonon band is observed. The EELS data is analyzed by comparison to theoretical studies previously reported in the literature. This paper presents new experimental data of the surface vibrational structure, which can stimulate theoreticians to refine their modeling and improve the understanding of the Ag(111) surface lattice dynamics.</p

NO structures adsorbed on Rh(111) : theoretical approach to high-coverage STM images

Theoretical modeling of scanning tunneling microscopy (STM) measurements is used for the interpretation of images of nitrogen monoxide on Rh(111) surfaces in order to gain insight into the factors which control the contrast of an STM image, especially in the case of high coverage overlayers. Topographic images of NO/Rh(111) for different coverages and adsorption positions were calculated. These results were used to analyze the experimental images obtained for the p(2×2)-3NO and p(3×3)-7NO high coverage structures. The theoretical calculations confirm that not all NO molecules present on the surface can be observed experimentally, the image being dominated by the contribution of top NO molecules in the adlayer. In addition, the calculations reveal that destructive interference effects between molecular contributions in the tunnel current play a decisive role for the different contrast of the two high coverage structures. A general discussion of why and how the differences in the adsorbate surface configuration reflect the experimental STM images is given

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