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.     

Cluster and band structure ab initio calculations on the adsorption of CO on acid sites of the TiO2(110) surface

The interaction of CO with the cationic sites of the TiO₂(110) surface has been investigated with cluster models and band structure calculations. The analysis of Hartree-Fock and correlated wavefunctions has shown that CO adsorbs at a distance of 4.4–4.5 bohr and a binding energy of 0.7–0.8 eV at low coverage. The bond strength is determined by the CO polarization and the CO б-donation to the surface while the electrostatic attraction is almost exactly cancelled by the Pauli repulsion. The adsorption is accompanied by two important measurable features, a considerable blue shift of the CO vibrational frequency and an increase of the CO 5б ionization potential. Both these effects have been analyzed in detail. We found that the CO ω shift is largely due to a combination of the electrostatic Stark effect and of the repulsion occurring when the CO molecule stretches in the presence of the rigid surface. The change in the 1π–5б binding energies, on the other hand, has an entirely electrostatic origin. Neither the ω shift nor the 5б binding energy shift are determined by the б-donation mechanism. Nevertheless, the occurrence of a charge transfer from CO to the empty levels of the Ti centers is well documented by (a) the energy and dipole moment change associated to this mechanism, (b) the expectation value of a projection operator which measures the charge associated with a given orbital, and (c) the CO dynamic dipole moment. The same analyses also rule out the occurrence of a Ti-to-CO back donation.     

A theoretical investigation of adsorbate-induced surface relaxation effects using cluster models: Al on Si(111)

Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the Hartree-Fock level on AlSi_n subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T₄ site resulting in Al adsorption at the T₄ site to be 5 kcal/mol more stable than at the H₃ site and considerably more stable than adsorption at the T₁ site. However, we only have confidence in this result after performing for the T₄ site a partial geometry optimization on the AlSi₅ subcluster in the AlSi₂₆H₂₄ cluster and by including appropriate correlation corrections.     

On the nature of the SO4/Ag(111) and SO4/Au(111) surface bonding

The nature of sulfate-Ag(111) and sulfate-Au(111) surface bonding has been investigated at the SCF + MP2 level of theory. Convergence of binding energy with cluster size is investigated and, unlike neutral adsorbates, large clusters are required in order to obtain reliable binding energies. In the most stable adsorption mode, sulfate binds to the surface via three oxygen atoms (C_3v symmetry) with a binding energy of 159.3 kcal/mol on Ag(111) and 143.9 kcal/mol on Au(111). The geometry of adsorbed sulfate was optimized at the SCF level. While the bond length between sulfur and the oxygens coordinated to the surface increases, the sulfur-uncoordinated oxygen bond length decreases. This weakening and strengthening of the bonds, respectively, is consistent with bond order conservation in adsorbates on metal surfaces. Although a charge transfer of 0.4 electrons towards the metal is observed, the adsorbate remains very much sulfate-like. The molecular orbital analysis indicates that there is also some charge back-donation towards unoccupied orbitals of sulfate. This results in an increased electron density around sulfur as revealed in the electron density difference maps. Analysis of the Laplacian of the charge density of free sulfate provides a suitable framework to understand the nature of the different charge transfer processes and allows us to establish some similarities with the CO- and SO₂-metal bondings.     

Adsorption and diffusion of a Si adatom on the H/Si(1 1 1) surface: comparison with the H/Si(0 0 1) surface

Using a first-principles method, we investigate the adsorption and diffusion of a Si adatom on the H-terminated Si(1 1 1) substrate, which would be useful in understanding the initial stages of Si homoepitaxy using a H surfactant. The adatom substitutes H atom(s) to form a monohydride structure or a dihydride structure. In forming the monohydride structure, the energy barrier for H substitution is absent. The adatom migrates on the surface with alternating its chemical state between monohydride and dihydride. These behaviors of the adatom are quite similar to those on the H/Si(0 0 1)2 × 1 surface, despite the significant difference in the substrate structure between both orientations. The resulting diffusion barrier is 1.30 eV, which is also comparable to that on the H/Si(0 0 1)2 × 1 surface.     

Anchoring sulphur-headgroup organic molecules at Cu(1 0 0): Tailoring the interface electronic states

Sulphur-headgroup organic molecules have been chemisorbed on Cu(1 0 0) as self-assembled monolayers (SAMs) in highly-ordered two-fold symmetry structures, and the electronic states induced at the interface have been measured by photoemission: a close similarity of the main interface states for methane-thiolate and mercaptobenzoxazole on Cu(1 0 0) in the same p(2 × 2)-phase is observed. The bonding states for methane-thiolate/Cu(1 0 0) in the p(2 × 2) and c(2 × 2) structures have been compared to ab-initio calculation of the total density of states (DOS) for the S/Cu(1 0 0) system in the same phases. The major role of the S-Cu bonding to determine the density of state evolution at the interface is brought to light. The observed differences in the two phases depend mainly on the charge distribution associated to the different molecular packing, with a minor role of the radical group.     

Optical anisotropy at the W(1 1 0) surface

We report ab initio calculations of the anisotropic dielectric function of tungsten (1 1 0) surface using the linear muffin-tin-orbital method. The calculated anisotropy in the optical spectrum, for polarization of light parallel to the surface, exhibits three dominant broad structures at 3.00, 4.01 and 5.34 eV successively positive, negative and then positive. The first peak is clearly assigned to p → d interband transitions in surface atomic sites whereas the two others have their origin in interband transitions in bulk like atoms. Our results, including the interlayer relaxation effect on the surface optical response, are compared to recent reflectance anisotropy measurements.     

First-principles study of the interaction of oxygen with the SnO2(1 1 0) surface

First-principles calculations based on density functional theory in the generalised gradient approximation, together with pseudopotentials and plane-wave basis sets, have been used to investigate the energetics of oxygen adsorption on stoichiometric and weakly and strongly reduced SnO₂(1 1 0) surfaces. It is shown that, if the surface species formed by oxygen adsorption are restricted to be charge neutral, then oxygen cannot be exothermically adsorbed from the gas phase on the stoichiometric surface. A variety of molecular and dissociative modes of adsorption are examined on the reduced surface produced by removing all bridging oxygens and on the weakly reduced surface that results from removal of only a fraction of these oxygens, with the adsorbed species being in both the singlet and the triplet states, and we identify a number of modes not discussed before in the literature. We use the calculated adsorption energies to propose a tentative assignment of these adsorption modes to the peaks observed in temperature programmed desorption experiments on the SnO₂ and TiO₂(1 1 0) surfaces.     

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.     

Study of scanning tunneling microscopy images and probable relaxations of the SrTiO3(100) surface by electronic structure calculations

Partial electron density plots were calculated for a model SrTiO₃(100) surface with √5 × √5 ordered oxygen vacancy to examine why the bright spots of the scanning tunneling microscopy (STM) images of SrTiO₃(100) observed in ultrahigh vacuum (UHV) correspond to the oxygen vacancy sites. Possible dependence of the image on the polarity and magnitude of the bias voltage was also discussed on the basis of partial electron density plot calculations. Our study strongly suggests that the UHV STM imaging involves the lowest-lying d-block level of every two Ti³⁺ centers adjacent to an oxygen vacancy, the tip-sample distance involved in the UHV STM experiments is substantially larger than that involved in typical ambient-condition STM imaging, and the Ti⁴⁺ and Ti³⁺ sites of SrTiO₃(100) are reconstructed.     

Surface chemistry of alkyl and perfluoro ethers: a FTIR and ab initio study of adsorption and decomposition of CF3OCF3 on an Al2O3 surface

The adsorption and thermal decomposition of perfluorodimethyl ether, (CF₃)₂O, on a high-surface-area Al₂O₃ surface was investigated by FTIR under both vacuum and pressure conditions. IR spectra in the 4000-1050 cm⁻¹ range were collected and the spectral assignments were assisted by quantum chemical ab initio calculations. The spectral evidence indicated that (CF₃)₂O decomposed to form adsorbed fluoroformate, FCOO (ads). Increases of temperature (up to 525 K) caused the FCOO (ads) to convert to hydrogen formate, HCOO (ads). Surface hydroxyl groups participated in the decomposition of (CF₃)₂O and the conversion of FCOO (ads) to HCOO (ads). A decomposition mechanism is proposed.     

Many body effects in the electronic and optical properties of the (1 1 1) surface of diamond

The (1 1 1) face is the cleavage surface of diamond. Understanding its properties is very important for the growing technological interest on the chemistry of diamond surfaces. Within DFT the most stable reconstruction is the Pandey chain model, the atoms on the chain being neither buckled nor dimerised. However this geometry gives rise to a semi metallic band structure in contrast with experimental findings that show the presence of a gap ranging from 0.5 to 2 eV. Here we show that the same equilibrium geometry and thus the same metallic band structure is found relaxing the surface using screened exchange (sX) or Hartree-Fock (HF) functionals. We will discuss in detail how breaking the equivalence of the atoms on the chain affects the band structure and we will show that a buckling would yield a semiconducting surface, but is energetically unfavorable. A semiconducting character can be restored, within the equilibrium geometry, if quasiparticle corrections are carefully included within an iterative GW scheme. The result of the theoretical reflectance anisotropy spectra (RAS) at a DFT-RPA level are also presented and discussed. As expected, a strong anisotropy signal is found at low energies due to transitions between surface states inside the fundamental gap.     

Theoretical study on the temperature-induced structural transition of the Si(1 1 3) surface

The temperature-induced structural transition of the Si(1 1 3) surface is investigated by ab initio calculations. In this study, it is found that the room-temperature phase and the high-temperature phase have the 3 × 2 interstitial structure and the 3 × 1 interstitial structure, respectively. The existence of the 3 × 2 and 3 × 1 interstitial structures is supported by the analysis of scanning tunneling microscopy (STM) images and the calculation of surface core level shifts using final state pseudopotential theory. The theoretical STM images of interstitial structures are in good agreement with the STM images suggested by experiments. The analysis of STM images provides an insight into the characteristics of domain boundaries observed frequently in experiments. It is also found that the domain boundary can be formed by local 3 × 1 interstitial structures on the 3 × 2 interstitial surface. The theoretical analysis of the surface core level shifts reveals that the surface core levels in experiment originate from the interstitial structures. The lowest values in the surface core level shifts are found to be associated with the 2p core level shifts of the interstitial atoms.     

The surface science of semiconductor processing: gate oxides in the ever-shrinking transistor

Due to the extreme dimensional scaling required by Moore's law, Si device technology is increasingly subject to the limitations imposed by the intrinsic physics and chemistry of surfaces and interfaces. In this review we outline ways in which fundamental surface science has contributed an understanding to the microelectronics community and discuss areas where surface science may impact future development. We focus on the example of silicon dioxide (SiO₂) on silicon, since this interface lies at the heart of modern transistor technology and has therefore received a great deal of attention in recent years. We highlight a number of experimental and theoretical approaches that have elucidated the fundamental phenomena associated with the formation and evolution of this critical technological interface, revealing the remarkable interdependence of science and technology that now characterizes this rapidly evolving industry.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Theoretical studies on the interaction of oleoyl sarcosine with the surface of apatite

Quantum chemical methods were used to study interactions of models of oleoyl sarcosine (N-oleoyl-N-methyl-amino carboxylic acid) with the Ca²⁺ ion and the surface calcium of apatite. Geometry optimizations revealed that oleoyl sarcosine can form either bidentate or tridentate structures with Ca²⁺ ions. Modification of the functional group in the sarcosine model to investigate its influence on the interaction energies showed that the interaction energies vary with the group. Comparison of interactions of the Ca²⁺ ions and the surface calcium of apatite showed the interaction energies to change with the functional group in a similar way.     

An angle-scanned photoelectron diffraction (XPD) study of the growth and structure of ultrathin Fe films on Au(001)

We report a study of the growth and structure of Fe films on Au(001) at room temperature using angle-resolved photoelectron spectroscopy (ARXPS, AlK) and polar-scan photoelectron diffraction (XPD, AlK), exploiting the forward scattering (FS) enhancement of photoelectrons along atomic chains. The structure of the Fe 3p and 2p XPD polar diagrams and the development of the FS features with film growth evidence that Fe grows pseudomorphically in a nearly perfect layer-by-layer mode with bcc (001) structure rotated by 45° about the surface normal. At least up to 4 and probably up to 6 monolayers Fe, a segregated Au monolayer (surfactant layer) exists on top of the Fe film. This follows from the comparison of a simple model for the development of the substrate and film FS enhancements with the experimental data. By using angular shifts of the Fe 3p and Fe 2p bcc-[111] and bcc-[101] FS peaks we could determine the Au(on top)---Fe and Fe---Fe interlayer distances for 1 and 2 ML thick films to be 1.71(0.04) Å and 1.48(0.08) Å, respectively, showing that very thin films have a slightly expanded bcc structure (bct). The regular bcc angle positions are observed above 4–6 ML.     

A theoretical analysis of adsorption and dissociation of CH3OH on the stoichiometric SnO2(110) surface

A theoretical analysis based on the Hartree–Fock pseudopotential method and a density-functional theory calculation using a hybrid combination of general gradient approximation with pseudopotential procedure has been carried out to study the adsorption and dissociation of methanol on the stoichiometric SnO₂(110) surface. The dependence of the results upon model system and computing method is discussed. An optimization procedure of adsorbate and substrate atom positions on a six-layer slab model has been selected to characterize the corresponding geometric parameters, adsorption energy and charge-transfer processes related with the molecularly adsorbed CH₃OH and dissociative channels to yield methoxy or methyl fragments. In the high-coverage limit (θ=1), we find that dissociation of the methanol molecule via the heterolytic cleavage of the C---O bond is favoured. At lower coverage (θ=1/2), this channel and the molecularly adsorbed methanol present similar adsorption energies.     

Adsorption and diffusion of a single Pt atom on γ-Al2O3 surfaces

Motivated to better understand the interactions between Pt and γ-Al₂O₃ support, the adsorption and diffusion of a single Pt atom on γ-Al₂O₃ was studied using density functional theory. Two different surface models with atoms of various coordination (3–5) were used, one derived from a defected spinel structure, and another derived from the dehydration of boehmite (AlOOH). Adsorption energies are similar for the two surfaces, about −2 eV for the most stable sites, and involve Pt binding to surface O atoms. An unusually strong trapping geometry whereby Pt moves into the surface was identified over the boehmite-derived surface. In all cases the surface transfers 0.2–0.3 e⁻ to the Pt atom. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. The similarity of the two surfaces is attributed to the similar local environments of the surface atoms, as corroborated by geometry analysis, density of states, and Bader charge analysis. Calculated activation barriers (0.3–0.5 eV) for the defected spinel surface indicate fast diffusion and a kinetic Monte Carlo model incorporated these barriers to determine exact diffusion rates and behavior. The kinetic Monte Carlo results indicate that at low temperatures (<500 K) the Pt atom can become trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. Finally we modeled the adsorption of Pt on hydrated surfaces and found adsorption to be weaker due to steric repulsion and/or decreased electron-donating ability of the surface.