Anisotropic diffusion of Cu adatoms on strained Cu (1 1 1) surface

Diffusion of Cu atoms on a strained Cu (1 1 1) surface was studied by molecular dynamic simulation using an EAM potential. The anisotropic diffusion behaviour is found when the uniaxial strain is imposed on the surface, which does not exist under the biaxial strain. The migration of the adatom is suppressed along the tensile strain direction. The results suggest that different island morphology can be obtained by controlling anisotropic diffusion of adatoms on the strained surfaces during film growth.     

Formate stability and carbonate hydrogenation on strained Cu overlayers on Pt(1 1 1)

Formate (HCOO) synthesis, decomposition and the hydrogenation of carbonate (CO₃) on Cu overlayers deposited on a Pt(1 1 1) single crystal are investigated to examine the reactivity of a Cu surface under tensile strain with defects present.Formate is synthesized from a 0.5 bar mixture of 70% CO₂ and 30% H₂ at varying temperatures, and the evolution is followed with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Subsequent TPD reveals decomposition of the formate species into CO₂ and H₂ at 420 ± 5 K for strained Cu at sub-monolayer to monolayer coverages. This is a significantly lower decomposition temperature than obtained earlier on pristine Cu(1 1 1) (460 K), as well as for thicker Cu layers where we assign an observed decomposition peak at 440 ± 5 K to relaxed, but defect-rich Cu(1 1 1). However, the thermal stability of formate on strained and defect-rich Cu is similar to previous results obtained for supported, and lattice-strained, Cu nanoparticles.The hydrogenation of carbonate produced by 0.3 bar CO₂ exposure at room temperature was monitored with XPS and TPD showing a significant loss of carbonate when subjected to 0.2 bar H₂ at room temperature. However, the presence of formate on the surface, or any other hydrogenation product, could not be established during or after H₂ exposure by PM-IRRAS, EELS or TPD. Even so, the results suggest that carbonate and its hydrogenation may constitute a relevant pathway to methanol production.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Electronic surface states of Cu(1 1 0) surface

Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.     

Carbon-doped high-mobility hole gases on (0 0 1) and (1 1 0) GaAs

Since Stormer and Tsang have introduced the first two-dimensional hole gas (2DHG) in the GaAs/AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20 years. Both acceptor atoms have significant disadvantages, i.e. either high-diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament-doping source, we prepared high-quality 2DHGs in the (0 0 1) and the nonpolar (1 1 0) crystal plane with carrier mobilies beyond 10⁶ cm²/Vs in quantum well and single interface structures. Low-temperature magnetoresistance measurements recover a large number of fractional QHE states and show a pronounced beating pattern from which the Rashba induced spin-splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (1 1 0) and (0 0 1) wafers with transport features in qualitative agreement to our findings on (1 1 0) substrates.     

Systematical study of dimer diffusion on metal fcc(0 0 1) surfaces

We study systematically the dimer diffusion on a series of metal fcc(0 0 1) surfaces. The atomic interactions are modeled by the realistic model potentials including embedded-atom method potential, surface-embedded-atom method potential, and Rosato-Guillopé-Legrand potential. Based on the results of the static calculations and the molecular dynamics simulations, three different kinds of fcc(0 0 1) surfaces can be distinguished named hard, middle, and soft. On the different kind of surfaces, not only the dominant diffusion mechanism but also the physical model for exchange mechanism is different. In addition, besides the conventional hopping and exchange mechanisms, some other interesting diffusion mechanisms for dimers are observed in our molecular dynamics simulations such as exchange rotation mechanism, cooperative hopping mechanism, and cooperative exchange mechanism.     

Surface deformation phenomena induced by electron-beam irradiation in strained InGaAs/AlGaAs layers on GaAs (1 0 0) and (3 1 1)B substrates

The strained InGaAs/AlGaAs layer structures have been grown on GaAs ( 10 0) and (3 1 1)B substrates in a horizontal low-pressure metalorganic vapor-phase epitaxy system at a temperature of 800°C. In the surface observation using a high-resolution scanning electron microscope, we have found that surface deformation phenomena induced by electron-beam irradiation in strained In_0.36Ga_0.64As,/Al_0.3Ga_0.7As layers on GaAs (1 0 0) and (3 1 1)B substrates. The change of the surface morphology was observed in real time on the display of SEM with the accelerating voltage of 30 kV and the irradiated time of 60–120 s. The surface deformation through mass transport seems to be the cause of the residual strain relaxation due to electron-beam irradiation.     

Surface phonons of GaP(1 1 0) and InAs(1 1 0)

The dispersions of low energy surface phonon modes of GaP(1 1 0) and InAs(1 1 0) measured with inelastic He-atom scattering along the and 0 0 1 directions are presented. Aside from the Rayleigh mode, additional distinct acoustic modes are observed as well as indications of optical modes. Contrary to results for GaAs(1 1 0), a rocking mode was not observed. The experimentally determined phonon dispersions are in excellent agreement with recent ab initio calculations by C. Eckl, et al. [1].     

Surface diffusion of carbon atom and carbon dimer on Si(0 0 1) surface

Carbon (C) atom and carbon dimer (C2) are known to be the main projectiles in the deposition of diamond-like carbon (DLC) films. The adsorption and diffusion of the C adatom and addimer (C2) on the fully relaxed Si(0 0 1)-(2 × 1) surface was studied by a combination of the molecular dynamics (MD) and Monte Carlo (MC) simulation. The adsorption sites of the C and C2 on the surface and the potential barriers between these sites were first determined using the semi-empirical many-body Brenner and Tersoff potential. We then estimated their hopping rates and traced their pathways. It is found that the diffusion of both C and C2 is strongly anisotropic in nature. In addition, the C adatom can diffuse a long distance on the surface while the adsorbed C2 is more likely to be confined in a local region. Thus we can expect that smoother films will be formed on the Si(0 0 1) surface with single C atoms as projectile at moderate temperature, while with C2 the films will grow in two-dimensional islands. In addition, relatively higher kinetic energy of the projectile, say, a few tens of eV, is needed to grow DLC films of higher quality. This is consistent with experimental findings.     

Adsorption and NiO(1 0 0) formation by atomic and molecular oxygen on Ni(1 1 0)

The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0).     

Spiral-like motion of the magnetization direction during reorientation in Ni-films on Cu(0 0 1)

The magnetization reorientation from in-plane to out-of-plane, occurring in Ni-films on Cu(0 0 1) around 9 to 10 ML, was studied in situ during growth by second harmonic generation. s→S and p→S polarization combinations of incident fundamental and reflected second harmonic light were used to show that reorientation proceeds in a spiral-like turn of the magnetization from parallel into the direction perpendicular to the steps, in accordance with reports in the literature that reorientation is driven by magneto-elastic forces.     

Surface alloying and anomalous diffusion of Bi on Cu(1 1 1)

We have used Low-Energy Electron Microscopy (LEEM) to study the formation and structure of the surface alloy of Bi on the Cu(1 1 1) surface. As a function of Bi coverage we find a progression from a dilute surface alloy to a structure, which coexists with a Bi overlayer when the Bi coverage exceeds 1/3 ML. At a temperature of 401 K, an unusual formation of alloy domains is observed. The origin of this effect is traced down to the mobility of Bi adatoms, which is found to strongly depend on the Bi coverage of the surface alloy on which they reside.     

Comparative dynamical study of the HD and isotopomers interacting with Pd(1 1 1) and Cu(1 1 0) surfaces

We study the dynamics of HD and H₂ molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(ν_i=0) is slightly lower than for H₂(ν_i=0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H₂ molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H₂. The theoretical apparent rotational temperature, T_rot, of HD molecules scattered from Pd(1 1 1) at impact energy , is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical T_rot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed.     

The effect of embedded Pb on Cu diffusion on Pb/Cu(1 1 1) surface alloys

We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect.     

Observation of hydrogen adsorption on 6H-SiC(0 0 0 1) surface

We have used coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) to investigate the adsorption of atomic hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface. It has been found that the saturation coverage of hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface is about 1.7 ML. Upon saturated adsorption of atomic hydrogen, the √3×√3 surface structure changes to the 1×1 structure. The data of the CAICISS measurements have indicated that as a result of the hydrogen adsorption, Si adatoms on the √3×√3 surface move from T₄ to on-top sites.     

Near-surface stresses in silicon(0 0 1)

We report here on a detailed atomistic stress analysis of the near-surface stresses on the 2 × 1 surface reconstruction on Si(0 0 1). We find that the surface stresses are spatially periodic with fairly large amplitude. We additionally investigate the stresses in the neighborhood of an S_A-type step.     

Adsorption of Ag on Si(1 0 0) surface

The chemisorption of one monolayer Ag atoms on an ideal Si(1 0 0) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The adsorption energies (E_ad) of different sites are calculated. It is found that the adsorbed Ag atoms are more favorable on C site (fourfold site) than on any other sites on Si(1 0 0) surface, the polar covalent bond is formed between Ag atom and surface Si atom, a Ag and Si mixed layer does not exist and does form an abrupt interface at the Ag–Si(1 0 0) interface. This is in agreement with the experiment results. The layer-projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated. Comparing with the Au/Si(1 0 0) system, the interaction is weaker between Ag and Si than between Au and Si.     

Diffusion of single adatom Cu on Cu (0 0 1) and (1 1 0) surfaces

With static relaxation, the surface diffusion activation energies of a single Cu adatom migrated by both atomic exchange and hopping mechanisms and the forces acted on the diffusing adatom from other atoms of Cu (0 0 1) or (1 1 0) surface are calculated by using the MAEAM. When adatom migrated on Cu (0 0 1) or (1 1 0) surface, the increment curves of the system energy by hopping mechanism are symmetrical and the saddle points are in the midpoints of the migration path, but the ones by the exchange mechanism are dissymmetrical and the saddle points are always close to the initial hole positions of the adatom and away from the initial equilibrium positions of the exchanged atom. From minimization of both the diffusion activation energy and the force acted on the diffusing adatom from other atoms, we found that, on Cu (0 0 1) surface the favorable diffusion mechanism is hopping mechanism, however, on Cu (1 1 0) surface, hopping via long bridge is easier than the exchange mechanism but the hopping via short bridge is more difficult than the exchange mechanism.     

Acetaldehyde photochemistry on TiO2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO₂(1 1 0) with minor decomposition channels yielding butene on the reduced TiO₂ surface and acetate on the oxidized TiO₂ surface. Acetaldehyde adsorbed on oxidized TiO₂(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.