First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Ab initio calculations of interfacial magnetism in Fe/Mo superlattices

Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

Spin-spiral and domain wall structures in monolayer Fe/W(1 1 0)

The stability of spin-spiral and domain wall structures in an Fe monolayer on a W(1 1 0) substrate is theoretically investigated. By analyzing the exchange parameters obtained from first principles total energy calculations, we find that a competition between the nearest-neighbor ferromagnetic and long-distant antiferromagnetic exchange interactions leads to a stabilization of the spin-spiral structures. When the strong magnetocrystalline anisotropy (MCA) arising from the Fe/W(1 1 0) interface is introduced, however, the formation of the spin-spiral structures is suppressed and the ground state appears to be the ferromagnetic state—as observed in experiments. In addition, the strong MCA is found to play a key role in determining the domain wall structures.     

Adsorption and diffusion of OH on Mo modified Pt(111) surface: First-principles theory

A first-principles study of adsorption and diffusion of OH on Pt and PtMo(111) surfaces is described. It confirms that the dissociation of water is much easier on PtMo than on pure Pt. Furthermore, we also found that OH binds most strongly at Mo atop site with adsorption energy of −3.32 eV, which is ∼1 eV stronger than binding to the pure Pt(111) surface. OH is much more localized on the PtMo alloy surface than on pure Pt. Both the stranger bond and the higher localization of OH contribute to the enhanced fuel cell performance with PtMo electrodes compared to pure Pt.     

Spin polarization in photoelectron spectroscopy from antiferromagnets: Cr films on Fe(1 1 0) from first principles

The spin- and angle-resolved photoelectron spectroscopy from ultrathin Cr films on Fe(1 1 0) is investigated by means of first-principles electronic structure and photoemission calculations. The antiferromagnetic ordering in the Cr films leads in dependence on film thickness to a rapidly decreasing and oscillating photoelectron spin polarization, in reasonable agreement with recent experiments (Dedkov (2007) [1]). The oscillation period is explained by quantum-well states in the Cr film and by a Fermi surface nesting vector. The importance of transition matrix elements is highlighted. The findings point to a noncollinear magnetic structure at the Fe/Cr interface.     

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.     

Barrier height imaging of Si(1 1 1)3 × 1-Ag reconstructed surfaces

We investigated the bias voltage polarity dependence of atomically resolved barrier height (BH) images on Si(1 1 1)3 × 1-Ag surfaces. The BH images were very similar to scanning tunneling microscopy (STM) images in both the empty and filled states. This similarity strongly supports the interpretation that the BH image reflects the vertical decay rate of the surface local density of states (LDOS). Differences in contrast and protrusion shapes between BH and STM images were observed. We attributed these differences to the geometric contribution to the STM image and to the improved spatial resolution of the BH image due to the lock-in technique.     

Electronic surface states on the MOVPE-prepared InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface

In this paper, the InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface was studied in detail, which turned out to be the most suitable to develop an InGaAs/GaAsSb interface that is as sharp as possible. In ultra high vacuum the InGaAs surface was investigated with low-energy electron diffraction, scanning tunneling microscopy and UV photoelectron spectroscopy employing synchrotron radiation as light source. Scanning the Γ−Δ−X direction by varying the photon energy between 8.5 eV and 50 eV, two surface states in the photoelectron spectra were observed in addition to the valence band peaks.     

Half-metallicity characteristic at zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1)

Electronic and magnetic properties of the zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1) semiconductors are studied within the framework of the density-functional theory using the FPLAPW+lo approach. We found that the Cr-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for the Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that at phase transition has occurred. Also the half-metallicity character is preserved at both CrSb/GaSb and CrSb/InAs interfaces. The conduction band minimum (CBM) of CrSb in the minority spin case lies about 0.63 eV above that of InAs, suggesting that the majority spin can be injected into InAs without being flipped to the conduction bands of the minority spin. On the other hand the CrSb/GaSb interface has a greater valence band offset (VBO) compared with the CrSb/InAs interface and the minority electrons have lower contribution in the injected currents and hence more efficient spin injection into the GaSb semiconductor. Thus the CrSb/GaSb and CrSb/InAs heterojunctions can be useful in the field of spintronics.     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.     

Distribution and magnetization of Co near the rutile TiO2 (110) surface: A first-principles study

The room-temperature ferromagnetism demonstrated by Co-doped TiO₂ films remains a challenge to our understanding, notwithstanding intensive experimental and theoretical investigations. We have calculated the binding energy and spin-polarization of Co atoms doped in the rutile TiO₂ (110) surface using first-principles method, aiming to elucidate the relationship between structure and magnetism of Co-doped TiO₂ films. In a defect-free surface the binding energy of Co substituting for Ti reduces slightly with the depth into the surface, suggesting a very minor Co depletion near the surface. More interestingly, the stability of ferromagnetic coupling over anti-ferromagnetic coupling of Co atoms decreases rapidly when it goes from surface to the bulk region. The residual ferromagnetism in the surface will give rise to a non-vanishing average magnetic moment of the thin film. The calculated pairing energy of Co is 0.12 eV/Co in the surface layer and 0.20 eV/Co in the middle layer of the film, an indication that Co atoms have a tendency to aggregate in both regions and that pairing will not modify the Co concentration in the direction vertical to the surface.     

Tunneling spectra for single molecules of HEX-fluorescent dye attached to DNA adsorbed on Cu(1 1 1) surfaces

We used the scanning tunneling microscope (STM) to examine single-stranded deoxyribonucleic acid (DNA) oligomers deposited on a metal surface. Because STM can be used to study the electrical properties of materials via the tunneling spectra, we used it to visualize DNA oligomers at the single molecule resolution. The 5′-hexachloro-fluorescein phosphoramidite (HEX)-labeled oligomers (sequence, AGCTTC) were observed on an atomically flat Cu(1 1 1) surface. At large tip-sample distances at large set-point biases, the lowest unoccupied molecular orbit (LUMO) peak of the empty state can be observed for the dye molecules on the tunneling spectra. When this distance becomes small, similar spectra as for the Cu substrate were observed for the dye molecule on the LUMO-related peak. Cu gave peaks at small bias voltages in the filled state. From comparison of these peaks on each subunit of the molecules, the measured values of dI/dV on HEX were smaller to those on Cu because of the large size of the HEX molecule, but the normalized values of dI/dV/(I/V) were apparently equal. We believe that the tunneling current is able to pass through the HEX molecules to the Cu substrate, thus reflecting the density of the Cu(1 1 1) surface. Molecular size therefore affects the intensity of dI/dV. LUMO-related peaks sometimes cannot be observed for HEX because of conformational differences, but Cu peaks can almost always be observed for HEX molecules. These peaks for the counter ions are almost the same as those for the Cu substrate. Thus, tunneling spectra can assist in the molecular mapping of DNA.     

Study on the adsorption of fluorescein on Ag(1 1 0) substrate

The adsorption of fluorescein on the Ag(1 1 0) surface has been investigated by the first-principles pseudopotential method. Various adsorption geometries have been calculated and the energetically most favorable structure of fluorescein/Ag(1 1 0) was identified. The fluorescein molecule, in most favorable structure, is on hollow site, and the adsorption energy is 2.34 eV. Here the adsorption sites refer to the positions at the first layer of the substrate where the middle carbon atom of the fluorescein molecule is located. The bonding strength of the fluorescein molecule to the Ag substrate is site selective, being determined by electron transfer to the oxygen atoms of the molecule and local electrostatic attraction between the oxygen atoms and the silver atoms.     

First-principles study of CoO films on MnO (1 1 1): Stability and interfacial structure

Based on the generalized gradient approximation, full potential linearized augmented plane-wave (FP-LAPW) calculations have been performed to study the stability and the interfacial structure of CoO/MnO (1 1 1). The surface energy, the strain energy and the binding energy are calculated and discussed. The calculations revealed that the CoO/MnO (1 1 1) is a stable interface structure. Also examined were the electronic properties and the atomic spin magnetic moments of the interface. It was found that the interface exhibited half-metallic property and the atomic magnetic moments were obviously weakened at the interface for metal atoms compared with the corresponding magnetic moments in bulk material.     

Investigation of the Shockley surface state on clean and air-exposed Au (1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) carried out in vacuum and air were used to study the electronic structure of the Au (1 1 1) surface in the range of 0.0-0.7 eV below the Fermi level. The STS experiment carried out in UHV showed the existence of the Shockley surface state (SS) located 0.48 eV below the Fermi level. STS carried out in air showed strong local maximum located 0.35 eV below the Fermi level. This maximum was ascribed to the SS shifted toward lower energy due to carbon and oxygen overlayer. To confirm that the SS could exist on the sample exposed to air we did ultraviolet photoemission spectroscopy (UPS) experiment on air-treated and clean Au (1 1 1). Our results suggest that the SS position initially measured at 0.38 eV below the Fermi level was shifted to 0.27 eV after air treatment. Additionally, the level of contamination was measured using X-ray photoelectron spectroscopy (XPS).     

Electronic structure and electron dynamics at the GaSb(0 0 1) surface studied by femtosecond pump-and-probe pulsed laser photoemission spectroscopy

Transiently excited electron states at the GaSb(0 0 1) surface have been studied by means of time- and angle-resolved photoemission spectroscopy based on a femtosecond laser system. A normally unpopulated surface electron state has been found at ∼250 meV above the valence band maximum with a strong confinement at the center of the surface Brillouin zone. The lifetime of transiently excited carriers at the intergap surface states has been found to be ∼11 ps, associated with rapid carrier diffusion.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

Local electronic edge states of graphene layer deposited on Ir(1 1 1) surface studied by STM/CITS

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to observe electronic structure of the edges of monolayer graphite film deposited on the Ir(1 1 1) surface. The electronic structure derived from the tunnelling spectra revealed peak in electron local density of states very close to the Fermi level. This electronic state was interpreted in terms of localised edge state caused by the topology of the π electrons networks typical for the zig-zag edges. The observed maximum of local density of states at about 0.2 eV above the Fermi level was ascribed to the presence of resonant state caused by the appearance of disclinations centres in the vicinity of the graphite edges.