Atomic and electronic properties of furan on the Si(0 0 1)-(2 × 2) surface

The atomic and electronic properties of the adsorption of furan (C₄H₄O) molecule on the Si(1 0 0)-(2 × 2) surface have been studied using ab initio calculations based on pseudopotential and density functional theory. We have considered two possible chemisorption mechanisms: (i) [4 + 2] and (ii) [2 + 2] cycloaddition reactions. We have found that the [4 + 2] interaction mechanism was energetically more favorable than the [2 + 2] mechanism, by about 0.2 eV/molecule. The average angle between the CC double bond and Si(1 0 0) surface normal was found to be 22°, which is somewhat smaller than the experimental value of 28°, but somewhat bigger than other theoretical value of 19°. The electronic band structure, chemical bonds, and theoretical scanning tunneling microscopy images have also been calculated. We have determined a total of six surface states (one unoccupied and five occupied) in the fundamental band gap. Our results are seen to be in good agreement with the recent near edge X-ray absorption fine structure and high resolution photoemission spectroscopy data.     

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.     

Ultra thin V2O3 films grown on oxidized Si(1 1 1)

The growth of V₂O₃(0 0 0 1) has been investigated by scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Direct evaporation of vanadium onto the Si(1 1 1)-7 × 7 substrate gives rise to massive surface intermixing and consequent silicide formation. In order to obtain the vanadium oxide with good quality, the 7 × 7 surface was initially partially oxidized which leads to a smooth oxygen–silicon surface layer which in turn prevents a complete vanadium–silicon alloy formation. Finally a vanadium oxide film of V₂O₃ stoichiometry was created. The grown film exposes single crystalline areas of stepped surfaces which appear azimuthally randomly-oriented.     

Core-level shifts of InP(1 0 0)(2 × 4) surface: Theory and experiment

Surface core-level shifts (SCLSs) of the (2 × 4)-reconstructed InP(1 0 0) surface with the established mixed In–P dimer structure have been investigated by first-principles calculations and photoelectron spectroscopy. Theoretical values were calculated using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy functional. The obtained theoretical values are quite similar within both approximations. The found differences originate in the tiny structural differences. It is concluded that the expansion or contraction of the crystal lattice has smaller effect on the SCLSs than the geometrical details of the reconstruction, which suggests that the Madelung potential has the dominant effect on the SCLSs. The results support the presence of a P 2p peak at higher binding energy (BE) compared to bulk peak, as proposed with recent measurements [P. Laukkanen, J. Pakarinen, M. Ahola-Tuomi, M. Kuzmin, R. E. Perälä, I. J. Väyrynen, A. Tukiainen, V. Rimpiläinen, M. Pessa, M. Adell, J. Sadowski, Surf. Sci. 600 (2006) 3022], and reveal several hitherto not reported SCLSs. The calculated SCLSs reproduce the measured spectra within reasonable accuracy. Furthermore, the atomic origins of the InP(1 0 0)(2 × 4) SCLSs are solved. In particular, it is shown that the lowest SCLS of P 2p of the InP(1 0 0)(2 × 4) arises from the topmost In–P dimers.     

Structural properties of Cu clusters on Si(1 1 1):Cu2Si magic family

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 × 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)‘5.5 × 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates 20 Cu atoms with 20 Si atoms being expelled from the corresponding 7 × 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu₂Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 × 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 × 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images.     

Atomic and electronic structure of group-IV adsorbates on the GaAs(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the atomic and electronic structure of the group-IV adsorbates (C, Si, Ge, Sn, and Pb) on the GaAs(0 0 1)-(1 × 2) surface considered in two different models: (i) non-segregated Ga-IV-capped structure and (ii) segregated structure in which the group-IV atoms occupying the second layer while the As atom floats to the surface. The non-segregated structure is energetically more favorable than the segregated structure for Sn and Pb, whereas it is the other way around for C, Si, and Ge.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

GaAs(0 0 1) (2 × 4) to c(4 × 4) transformation observed in situ by STM during As flux irradiation

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As₄ flux. The relationship between the As-rich (2 × 4) and c(4 × 4) surfaces is observed throughout the gradual evolution of the reconstruction transformation. The results suggest that during the initial stage of the transformation, Ga-rich As-terminated variations of the c(4 × 4) form in order to accommodate excess mobile Ga produced by pit formation. These transient structures later planarize, as excess Ga is incorporated at step/island edges. Successive imaging of the same sample area during As₄ irradiation allows point-by-point adatom binding to be analysed in a way inaccessible to MBE–STM systems relying on sample quenching and transfer.     

A core level and valence band photoemission study of the (1 1 1) and ( ) surfaces of 3C–SiC

A core level and valence band photoemission study of thick 3C–SiC(1 1 1) and 3C–SiC( ) epilayers grown by sublimation epitaxy is reported. The as introduced samples show threefold 1×1 low-energy electron diffraction patterns. For the Si face and reconstructed surfaces develop after in situ heating to 1100°C and 1300°C, respectively. For the C face a 3×3 reconstruction form after heating to 980°C. A semiconducting behavior is observed for the and 3×3 reconstructed surfaces while the reconstruction show a Fermi edge and thus a metallic-like behavior. The surface state on the surface is investigated and found to have Λ₁ symmetry and a total band width of 0.10 eV within the first surface Brillouin zone. For the Si 2p and C 1s core levels binding energies and surface shifted components are extracted and compared to earlier reported results for 6H– and 4H–SiC.     

Progressive changes in atomic structure and gap states on Si(0 0 1) by Bi adsorption

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(0 0 1) by adsorption of different amounts of Bi coverage. For the 1/4 ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(0 0 1) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2 ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the p_z orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1 ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the p_z orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

CF interaction with Si(1 0 0)-(2 × 1): Molecular dynamics simulation

In this study, the interaction of CF with the clean Si(1 0 0)-(2 × 1) surface at normal incidence and room temperature was investigated using molecular dynamics simulation. Incident energies of 2, 12 and 50 eV were simulated. C atoms, arising from dissociation, preferentially react with Si to form Si-C bonds. A Si_xC_yF_z interfacial layer is formed, but no etching is observed. The interfacial layer thickness increases with increasing incident energy, mainly through enhanced penetration of the silicon lattice. Silicon carbide and fluorosilyl species are formed at 50 eV, which is in good agreement with available experimental data. The level of agreement between the simulated and experimental results is discussed.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

Density-functional study of the CO chemisorption on bimetallic Pd–Sn(1 1 0) surfaces

We study the ordered PdSn c(2 × 2), (2 × 1), and PdSn₂ (3 × 1) overlayers deposited on Pd(1 1 0) by using first-principles density-functional calculations. It appears that the two PdSn structures are almost degenerate in the energy. Pd–Sn surfaces we consider do not display the marked buckling with Sn atoms displaced towards vacuum that is common for Pt–Sn surfaces. Low-coverage CO chemisorption at these overlayers and on analogous surface structures on Pd₃Sn is considered. It is shown that inclusion of an empirical correction to the CO adsorption energy changes the stable adsorption site from the long-bridge to the top one in most cases. The adsorption energy decreases with the number of Sn atoms in the vicinity of the adsorption site, and this property correlates well with the position of the centre of gravity of the local Pd d-electron band, and also with the variation of the local density of d-electron states at the Fermi level. The centre-of-gravity value is used to assess the core-level shifts for Pd atoms in various geometries. Most of the calculated data compare rather well with the recent measurements on Pd–Sn overlayers at Pd(1 1 0) as well as with other data on related bimetallic systems.     

Effect of hydrogenation on the electronic structure of the P/Si(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory (DFT), have been performed to investigate the effect of hydrogenation on the electronic properties of P/Si(0 0 1)-(1 × 2) surface. In parallel with this, the electronic band structure of the hydrogenated and non-hydrogenated P/Si(0 0 1)-(1 × 2) surface have been calculated for half- and full-monolayer P. For the mixed Si-P dimer structure, we have identified two occupied and one unoccupied surface state, which correspond to 0.5 ML coverage of P. When this surface is terminated with H, we see that two occupied states completely disappeared and that one unoccupied state is shifted towards the conduction band. A similar calculations for the 1 ML coverage of P have been also carried out. It is seen that the unoccupied state C₁ appeared in the P/Si(0 0 1)-(1 × 2) surface is passivated when this surface is terminated with the H atoms. To explain the nature of the surface states, we have also plotted the total and partial charge densities at the point of the Surface Brillouin Zone (SBZ).     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

The effect of defects on the hydrogenation in Mg (0 0 0 1) surface

Density-functional theory was presented to investigate the hydrogen dissociation on a pure, Pt-doped, vacancy and oxide Mg(0 0 0 1) surface. Our results show that the energy barriers are 1.05, 0.39, 0.93 and 1.33 eV for H₂ dissociation on the pure, Pt-doped, vacancy and oxide Mg surface, respectively. The calculation results imply that the initial dissociation of H₂ is enhanced significantly for the Pt-doped Mg(0 0 0 1) surface, negligible for the vacancy model and weekend for the oxide model. The density of state results shows that, following the dissociation reaction coordinate, the H–H interactions are weeker for the Pt-doped model while interactions become stronger for the oxide model. It is suggested that the dissociation process is facilitated when Pt atom acts as catalyst and oxide overlayers delay hydrogen adsorption on the Mg layer. The present study will help us understand the defect role being played for the improvement or opposition effect in absorption kinetics of H₂ on the Mg(0 0 0 1) surface.     

Adsorption of molecular SiO2 on a clean Si(1 0 0) surface

We have investigated the adsorption of molecular (gaseous) SiO₂ on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. The SiO₂ molecule is found to be chemisorbed on various sites on the Si surface and the most energetically favourable structure is on top of the dimers. The minimum energy pathways for the various adsorption channels indicate that the reaction is barrierless in all cases. The corresponding vibrational spectrum is also calculated and the adsorbed molecules are, as expected, found to have red-shifted vibrational frequencies. The energetically favourable adsorption sites and adsorption energies are comparable to the results found for SiO.     

The structure and lattice dynamics of RbBr(1 0 0) and RbI(1 0 0) single crystal surfaces: A tensor low energy electron diffraction analysis

The structure and lattice dynamics of RbBr(1 0 0) and RbI(1 0 0) single crystal surfaces cleaved under UHV conditions were investigated by means of low energy electron diffraction (LEED) at temperatures of 156 K and 183 K, respectively. Since RbBr and RbI are insulators the experiments were carried out with a microchannel plate LEED system at very low primary currents (5 nA). For both materials four different diffraction orders could be observed. Diffraction patterns were recorded over an energy range from 30 eV to 220 eV in increments of 2 eV and I(V) curves for each spot were extracted. The I(V) curves were analyzed using the tensor LEED approach. For both alkali halide substrates surface structures of (1 × 1) periodicity close to the truncated bulk structure were found. For RbBr, the first interlayer distance is reduced by about 2.2%, where the Rb⁺ cations in the topmost layer are shifted inwards by 0.06(3) Å, and the anions also exhibit an inward shift which is however smaller (0.04(3) Å). The root mean square vibrational amplitudes are enlarged by a factor of 1.3 for Rb⁺ and 1.25 for Br⁻, respectively. For RbI(1 0 0) the cations of the topmost layer are shifted inwards by 0.07(3) Å and the anions outwards by 0.02(1) Å. The vibrational amplitudes of the ions are not enlarged as for RbBr but close to the corresponding bulk values.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.