Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

Metastable magnetic configurations of (V, Cr, Mn and Fe) on Ir(0 0 1) surfaces

We present a theoretical study of the electronic and magnetic structure of the 3d-transition metals (M = V, Cr, Mn and Fe) in several overlayer systems. The electronic as well as magnetic structures are investigated for pseudomorphic overlayers (M/Ir(0 0 1)), ordered alloyed overlayers of the type M_0.5Ir_0.5/Ir(0 0 1) and ordered binary surface alloys of V, Cr, Mn and Fe transition metals on Ir(0 0 1) substrates. The calculations are performed with a self-consistent tight-binding method using the unrestricted Hartree-Fock approximation within the Hubbard model. We obtained metastable c(2 × 2) configurations for V, Cr and Mn and a p(1 × 1) configuration for Fe pseudomorphic overlayers. However, ferrimagnetic configuration has been obtained for the ordered surface alloys M_0.5Ir_0.5 and the binary alloyed overlayers on Ir(0 0 1) surfaces.     

Theoretical study of Ti and Fe surface alloys on Al(0 0 1) substrate

Accurate density-functional calculations are performed to investigate the formation of Ti and Fe ultrathin alloys on Al(0 0 1) surface. It is demonstrated that a deposition of Ti monolayer on Al(0 0 1) substrate leads to the formation of Al₃Ti surface alloy with Ti atoms arranged according to the L1₂ stacking, distinct from the D0₂₂ structure characteristic of a bulk Al₃Ti compound. A quest for the reason of this distinct atomic arrangement led us to the study of the surface structure of Al₃Ti(0 0 1) compound. It is concluded that even the Al₃Ti(0 0 1) surface is terminated with three layers assuming a L1₂ stacking and hence this stacking fault can be classified as a surface-induced stacking fault. Several possibilities of Fe atoms distributed in the surface region of Al(0 0 1) have been examined. The most stable configuration is the one with the compact Fe monolayer on Al(0 0 1) and covered by one Al monolayer. Lastly, our calculations show that there is no barrier for the penetration of Fe adatoms below the Al(0 0 1) surface; however, such a barrier is present for a Ti-alloyed Al(0 0 1) surface.     

Ground-state properties of arsenic deposited on the Ge(0 0 1)(2 × 1) surface

We present results of ab initio calculations of structural, electronic and vibrational properties of the Ge(0 0 1) surface covered with a monolayer of arsenic. The fully occupied π_u bonding and π_g antibonding electronic states due to the As-As dimer formation are quite close in energy and their ordering is same as that found on the Si(0 0 1) surface. Using our calculated atomic and electronic structures, surface lattice dynamics was studied by employing a linear response approach based on density functional perturbation theory. A comparison of the phonon spectrum of the Ge(0 0 1)/As(2 × 1) surface with that of the clean Ge(0 0 1)(2 × 1) surface indicates the presence of several new characteristic phonon modes due to adsorption of As atoms.     

Kinetics of residual gas adsorption on Ge(1 1 1) surface in low-energy electron backscattering

Low-energy (0.4-1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10⁻⁷ Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.     

First-principles study on the half-metallicity of zinc-blende CrP(1 1 0) surface

We have investigated the half-metallicity and magnetism at the (1 1 0) surface of CrP by using the all-electron full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA). From the calculated local density of states (LDOS), we found that the (1 1 0) surface of CrP preserves the half-metallicity, but the band gaps (∼1.1 eV) of the minority states for the surface Cr and P atoms are much reduced from the bulk value (∼1.9 eV). The magnetic moment of the P is coupled antiferromagnetically to that of the Cr. The magnetic moment of surface Cr atom is calculated to be 3.31μ_B which is increased by 10% compared to the bulk value, 3.00μ_B.     

Theoretical studies of electronic states and phonon modes on the TiC(0 0 1)(1 × 1) surface

We present a comprehensive picture of structural and electronic properties of the TiC(0 0 1)(1 × 1) surface. Our investigations are based on first-principles calculations within the local-density approximation of the density-functional theory. Good agreement has been observed between our calculation and experimental data for the atomic geometry of the surface. In particular, the calculated bond lengths between the first-layer C and the second-layer Ti (d_1C-2Ti = 2.188 Å) and between the first-layer Ti and the second-layer C (d_1Ti-2C = 2.031 Å) are in good agreement with the corresponding experimental values of 2.25 Å and 2.14 Å, respectively. We have also identified surface electronic states and provided clear support for previously available photoemission measurements. We have further calculated surface phonon modes at the zone centre and at the zone-edge point X using a linear response scheme based on the ab initio pseudopotential method. Our calculated surface phonon results are in excellent agreement with electron energy loss spectroscopy results.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Adsorption of water on TiN (1 0 0), (1 1 0) and (1 1 1) surfaces: A first-principles study

We use first-principles density functional theory-based calculations in the analysis of the interaction of H₂O with (1 0 0), (1 1 0) and (1 1 1) surfaces of TiN, and develop understanding in terms of surface energies, polarity of the surface and chemistry of the cation, through comparison with H₂O adsorption on ZrN. While water molecule physisorbs preferentially at Ti site of (1 0 0) and (1 1 1) surfaces, it adsorbs dissociatively on (1 1 0) surface of TiN with binding stronger than almost 1.32 eV/molecule. Our analysis reveals the following general trends: (a) surfaces with higher energies typically lead to stronger adsorption, (b) dissociative adsorption of H₂O necessarily occurs on a charge neutral high energy surface and (c) lower symmetry of the (1 1 0) plane results in many configurations of comparable stability, as opposed to the higher symmetry (1 0 0) and (1 1 1) surfaces, which also consistently explain the results of H₂O adsorption on MgO available in literature. Finally, weaker adsorption of H₂O on TiN than on ZrN can be rationalized in terms of greater chemical stability of Ti arising from its ability to be in mixed valence.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Ga-induced superstructures on the Si(1 1 1) 7 × 7 surface

Monolayer Ga adsorption on Si surfaces has been studied with the aim of forming p-delta doped nanostructures. Ga surface phases on Si can be nitrided by N₂⁺ ion bombardment to form GaN nanostructures with exotic electron confinement properties for novel optoelectronic devices. In this study, we report the adsorption of Ga in the submonolayer regime on 7 × 7 reconstructed Si(1 1 1) surface at room temperature, under controlled ultrahigh vacuum conditions. We use in-situ Auger electron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction to monitor the growth and determine the properties. We observe that Ga grows in the Stranski-Krastanov growth mode, where islands begin to form on two flat monolayers. The variation in the dangling bond density is observed during the interface evolution by monitoring the Si (LVV) line shape. The Ga adsorbed system is subjected to thermal annealing and the residual thermal desorption studied. The difference in the adsorption kinetics and desorption dynamics on the surface morphology is explained in terms of strain relaxation routes and bonding configurations. Due to the presence of an energetic hierarchy of residence sites of adatoms, site we also plot a 2D phase diagram consisting of several surface phases. Our EELS results show that the electronic properties of the surface phases are unique to their respective structural arrangement.     

Coverage-dependent magnetization of 3d transition-metal adatoms on Co(0 0 1) in the submonolayer regime

The electronic structure of 3d transition-metal atoms on face-centered cubic Co(0 0 1) substrate is determined within ab initio density functional calculations in the gradient corrected approach. Calculations are performed for ordered surface configuration with coverage equal to 0.25, 0.5, 0.75 and 1 ML. For Ni and Fe a ferromagnetic coupling with the Co atoms is always obtained independently of the concentration. Moreover the values of the magnetic moments remain similar. For Mn a ferromagnetic coupling is obtained for low-coverage whereas an in-plane antiferromagnetic coupling is found for a complete Mn overlayer on Co(0 0 1). Also, for Sc, Ti, V and Cr a drastic modification of the magnetic map is observed when we go from low-coverage to the monolayer. Cr (Mn) adatoms present antiferromagnetic (ferromagnetic) coupling with Co(0 0 1) for x = 0.25 whereas an in-plane antiferrimagnetic coupling is obtained for x = 1.00.     

Simulation of STM images of p(3 × 2)/Na/Ge(0 0 1) surface

Adsorption of Na on the Ge(0 0 1) surface is known to be a cause of surface reconstruction. It is expected to find one Na atom per unit cell of the reconstructed surface, however, the precise atomic configuration of this system is still a matter of controversy. Consequently, the aim of our present theoretical study is to examine the atomic structure of stable p(3 × 2)/Na/Ge(0 0 1) surfaces with and without the possible change of the number of Ge atoms in the surface layer (so-called mass transport). Structural and electronic properties of the considered system are investigated using the local-orbital density functional method. Our considerations are completed by a simulation of STM images of the structures following from molecular dynamics calculations.     

Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 × 1) NaCl atomic structure for sample bias voltage V_s < −2.5 V and the bright protrusions should be assigned to the Cl⁻ ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)-p(2 × 2) surface as the result of weak interface interaction between the dangling bonds coming from valence π states of the Ge substrate and the 3p states of the interfacial Cl⁻ ion. These results are consistent with the experiments of STM, LEED and EELS.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

DFT study on surface properties and dissolution trends of Al(1 0 0) surfaces doped with Zn, Ga, In, Sn and Pb

In this paper, the properties and dissolution trends of the surfaces doped with different metal atoms on the Al(1 0 0) surface were investigated by the density functional theory calculations. A surface impurity model was proposed by replacing the topmost surface layer Al atoms by Me (Me = Zn, Ga, In, Sn and Pb) atoms with the coverage of 1/9, 1/4, 1/2, and 3/4 monolayer, respectively. Results show that the surface energy of Me-Al(1 0 0) surfaces depends primarily on the nature of the impurity atom species and the monolayer coverage. The work function of Me-Al surfaces is smaller than that of pure Al(1 0 0) surface, and decreases almost linearly with the amount of Me. It is found that the Me-Al alloys are more easily dissolvable than the pure Al, due to the fact that the electrochemical dissolution potential shifts were negative for all Me-Al(1 0 0) surfaces with respect to the clean pure Al(1 0 0) surface.     

Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy

The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of ΔE_V = 0.07 ± 0.05 eV, and a conduction band offset of ΔE_C = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices.     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).