Desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips

We investigated desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl-Si bonds to result in chlorine atom desorption.     

Kinetics of the N2O-CO reaction on Rh(1 1 0)

Our recent studies of the steady-state kinetics of the N₂O-CO reaction on Rh(1 1 0) indicate that at CO excess, the reaction rate increases with increasing temperature. At N₂O excess, the reaction rate is nearly independent of temperature at T < 520 K and rapidly decreases with increasing temperature at T > 520 K. Our present analysis of the relevant data indicates that the latter feature seems to be related to surface-oxide formation. Following this line, we propose a mean-field kinetic model making it possible to describe and clarify the experiment.     

Construction of modified embedded atom method potentials for Cu, Pt and Cu-Pt and modelling surface segregation in Cu3Pt alloys

In this work, surface segregation to Cu₃Pt surfaces is studied with the modified embedded atom method (MEAM). This work is triggered by the catalytic importance of Cu-Pt alloys, together with the contradictory experimental results for the surface segregation in Cu₃Pt(1 1 1) alloys based on low energy ion scattering (LEIS) [Y.G. Shen, D.J. O’Connor, K. Wandelt, R.J. MacDonald, Surf. Sci. 328 (1995) 21] and low energy electron diffraction (LEED) [Y. Gauthier, A. Senhaji, B. Legrand, G. Tréglia, C. Becker, K. Wandelt, Surf. Sci. 527 (2003) 71]. In order to accurately describe the segregation behaviour in the Cu₃Pt system, a reliable potential, that is also applicable to surface phenomena, is indispensable. Therefore, first, new MEAM parameters are derived, consistently based on ab initio density functional theory (DFT) calculations, according to a method that is a modification of previous work [P. van Beurden, G.J. Kramer, Phys. Rev. B 63 (2001) 165106]. Upon testing, these parameters prove to reproduce very well various surface properties of this system. Next, Monte Carlo (MC) simulations combined with the newly derived MEAM potentials are set up to investigate surface segregation to low index single crystal surfaces. For the Cu₃Pt(1 1 1) surface, our MC/MEAM simulations agree completely with the available LEIS evidence and contradict the unusual depth profile based on LEED. However, the slight Pt enrichment observed in the LEED experiments can be reproduced by assuming a slight Pt excess in the bulk of the sample. The simulated composition depth profile, on the other hand, does not agree with the LEED evidence. Also, for the Cu₃Pt(1 0 0) surface, the MC/MEAM results agree completely with LEIS experiments. For the Cu₃Pt(1 1 0) surface, finally, the MC/MEAM simulations show a somewhat deviating behaviour with respect to the experimental LEIS evidence. The possibility of a missing-row reconstruction is evaluated, but cannot explain the discrepancy for the Cu₃Pt(1 1 0) system. In order to further investigate the deviation from the experiments, additional DFT and MEAM calculations are performed in search of the preferred surface termination for Cu₃Pt(1 1 0). Both DFT and MEAM calculations agree on the pure Cu layer as the most stable surface termination. Although the experiment was extensively tested for reproducibility, it possibly reflects a metastable state. Finally, in view of the importance of small and less orderly particles in catalysis, the newly derived MEAM parameters are used in order to study the segregation to Cu₃Pt vicinal surfaces with {1 1 1} terraces, for which no experimental information is available yet.     

Probing the interactions of Pt, Rh and bimetallic Pt-Rh clusters with the TiO2(1 1 0) support

Pt, Rh, and Pt-Rh clusters on TiO₂(1 1 0) have been investigated by scanning tunneling microscopy (STM), soft X-ray photoelectron spectroscopy (sXPS), and low energy ion scattering (LEIS). The surface compositions of Pt-Rh clusters are Pt-rich (66-80% Pt) for room temperature deposition of both 2 ML of Pt on 2 ML of Rh (Rh + Pt) and 2 ML of Rh on 2 ML of Pt (Pt + Rh). Pt and Rh atoms readily diffuse within the clusters at room temperature, and although diffusion is slower at 240 K, intermixing of Pt and Rh still occurs. The binding energies of surface and bulk states for Rh(3d_5/2) and Pt(4f_7/2) can be distinguished in sXPS studies, and an analysis of these spectra indicates that the surface compositions of the Pt + Rh and Rh + Pt clusters are similar at room temperature but not identical. In addition to sintering, the pure Pt, pure Rh and Pt-Rh clusters become completely encapsulated by titania upon heating to 700 K. sXPS investigations show that annealing the clusters to 850 K induces reduction of titania support to Ti⁺² and Ti⁺³, with the extent of reduction being the greatest for Pt, the least for Rh and intermediate for Pt-Rh. We propose that TiO₂ is reduced at the metal-titania interface on top of the clusters, not at the base of the clusters. Furthermore, the extent of titania reduction is greater for metal clusters with weaker metal-oxygen bonds because oxygen atoms are less likely to migrate to the top of the clusters, and therefore the encapsulating titania is oxygen-deficient.     

Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

On the geometrical and electronic structure of an ultra-thin crystalline silica film grown on Mo(1 1 2)

The atomic structure of a well-ordered silica film grown on a Mo(1 1 2) single crystal substrate is discussed in detail using the experimental and theoretical results available to date. New photoelectron spectroscopy results using synchrotron radiation and ultraviolet spectroscopy data are presented. The analysis unambiguously shows that the ultra-thin silica film consists of a two-dimensional network of corner-sharing [SiO₄] tetrahedra chemisorbed on the unreconstructed Mo(1 1 2) surface. The review also highlights the important role of theoretical calculations in the determination of the atomic structure of the silica films and in interpretation of experimental data.     

STM and XPS characterisation of vacuum annealed nanocrystalline WO3 films

Scanning tunnelling microscopy and X-ray Photoelectron Spectroscopy were conducted on magnetron sputtered WO₃ thin films, following a sequence of ultra high vacuum anneals from 100 °C to 900 °C. Annealing from 100 °C to 400 °C induced an upward surface band bending of about 0.3 eV, attributed to the oxygen migration from the bulk to the surface, but no changes in the surface topography. Chemical changes occurred from 600 °C to 800 °C, associated with the formation of secondary oxide species. STM imaging showed that the film surface consists of amorphous particles 35 nm in size up to 600 °C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 °C. The implications in terms of gas sensing are discussed.     

Interaction of CO with surface PdZn alloys

The adsorption and bonding configuration of CO on clean and Zn-covered Pd(1 1 1) surfaces was studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(1 1 1) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(1 1 1) alloy surfaces compared to clean Pd(1 1 1). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al₂O₃ catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.     

Calculation of the formation energies of isolated vacancy and adatom-vacancy pair at low-index surfaces of fcc metals with MAEAM

The formation energies of isolated vacancy and adatom-vacancy pair (where the two are separated by a large distance) at low-index surfaces of fcc metals calculated by using the modifies analytical embedded atom method (MAEAM). The results predict the prevailing formation of vacancies on the surfaces (1 1 1), (1 0 0) (but Pd), Cu and Ni on the (1 1 0) surfaces at low temperature, and the defect formation energies consistently create in the sequence (1 1 0) → (1 0 0) → (1 1 1). With good accuracy, the calculated energy values coincide with those obtained by the embedded atom methods (EAM) and from experiments. The correctness of the method by which calculated the formation energies of point defects on the surface was proved.