Bistability in a H-terminated Si(1 0 0)2 × 1 surface obtained by ab initio transport calculations

We have analyzed electron tunneling due to the electric field from a hydrogen-terminated Si(1 0 0)2 × 1 ultrathin film on a metal substrate by density functional transport calculations. We have obtained a hysteresis loop in the tunneling current, which comes from the existence of two electronic structures. Furthermore, we have clarified that, as a condition of bistable electron transport, a double-barrier potential structure is not necessarily required for zero field, because it can be induced by the electric field.     

Measurement of the surface charge density of CO-saturated Pt(1 1 1) electrodes as a function of potential: the potential of zero charge of Pt(1 1 1)

From measurements of the charge flowing upon immersion, at controlled potential, of a CO-covered Pt(1 1 1) electrode in a 0.1 M HClO₄ solution, the corresponding surface charge density vs. potential curve was obtained, and from this the potential of zero charge (pzc) of the CO-covered Pt(1 1 1) electrode. From these data it was estimated that the error incurred when the potential of zero total charge (pztc) of Pt(1 1 1) electrodes is determined by the CO-charge displacement method is of approximately 50 mV at pH 1 and of approximately 90 mV at pH 3. Furthermore, the experimentally determined pzc of the CO-covered Pt(1 1 1) electrode has allowed us to make an estimation of the potential of zero free charge (pzfc) of Pt(1 1 1) electrodes.     

Nitroxyl free radical binding to Si(1 0 0): a combined scanning tunneling microscopy and computational modeling study

The ultra-high vacuum scanning tunneling microscope (UHV-STM) was used to investigate the addition of the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical to the Si(1 0 0) surface. Room temperature studies performed on clean Si(1 0 0)-2 × 1 confirm the proposed binding of the unpaired valence electron associated with the singly occupied molecular orbital (SOMO) of the molecule with a Si dangling bond. A strong bias dependence in the topography of isolated molecules was observed in the range of −2.0 to +2.5 V. Semiempirical and density functional calculations of TEMPO bound to a three-dimer silicon cluster model yield occupied state density isosurfaces below the highest occupied (HOMO) and unoccupied state densities isosurfaces above the lowest unoccupied molecular orbital (LUMO) which trend in qualitative agreement with the bias dependent STM topography. Furthermore, the placement of TEMPO molecules on dangling bonds was controlled with atomic precision on the monohydride Si(1 0 0) surface via electron stimulated desorption of H, demonstrating the compatibility of nitroxyl free radical binding chemistries with nanopatterning techniques such as feedback controlled lithography.     

Surface electronic structure of α-Mo2C(0 0 0 1)

The electronic structure of α-Mo₂C(0 0 0 1) has been investigated by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. A sharp peak is observed at 3.3 eV in normal-emission spectra. Since the peak shows no dispersion as a function of photon energy and is sensitively attenuated by oxygen adsorption, the initial state of the peak is attributed to a surface state. Resonant photoemission study shows that the state includes substantial contribution of 4d orbitals of the Mo atoms in the second layer. The emissions with constant kinetic energies of 22 and 31 eV above the Fermi level (E_F) are found in normal-emission spectra, and these emissions are interpreted as originating from the Mo N₁N₂₃V and N₂₃VV Auger transitions, respectively.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

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.     

Effect of substrate doping concentration on quantum well states of Pb island grown on Si(1 1 1)

The interaction between the metallic film/island and the semiconductor substrate is important to the electronic properties of metallic nanostructure grown on semiconductor substrate. Here, we report a series of comparison experiments to investigate the effect of doping concentration of Si substrates on the quantum well state (QWS). Using scanning tunneling microscopy, we observed that the apparent QWS energy positions show a strong dependence on the substrate used and on the sample temperature. Further experimental results by varying the height of scanning tunneling microscope tip over the Pb island uncovered that the observed apparent QWS energy position changes mainly come from the partial bias voltage drop on the combined resistance of the Pb wetting layer and the substrate, which is comparable with the vacuum tunneling resistance at low temperatures.     

Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash

In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 °C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a complex nature of this superstructure with double-hexagonal symmetry and (2√3×2√3)-R30° periodicity. Among the possible candidate phases, including metastable FeSi₂ with a CaF₂ structure and FeSi_1+x with a CsCl structure, the best match of the interatomic distances to the measured 14.4 Å × 14.4 Å unit cell dimensions pointed to the hexagonal Fe₂Si (Fe₂Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I-V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to β-FeSi₂.     

Surface core level shift observed on NiAl(1 1 0)

Using high resolution core level spectroscopy, a surface core level shift towards lower binding energy of −0.13 eV is determined for the 2p level of the outwardly relaxed Al surface atoms on NiAl(1 1 0). Density functional theory based calculations with inclusion of final state effects yield a value of −0.14 eV for this shift in excellent agreement with experiment. We show that the initial state approximation yields a value of +0.09 eV, i.e. the inclusion of final state relaxation effects is vital not only to obtain the correct value but even the correct sign for this shift.     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

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.     

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.     

Study of the interactions between the overlayer and the substrate in the early stages of palladium growth on TiO2(1 1 0)

The interaction between palladium and the (1 1 0) surface of a TiO₂ single crystal and the electronic properties of the system were studied by means of photoelectron spectroscopy (core levels and valence band) and resonant photoemission for Pd coverage in the sub- and monolayer range. We performed the metal depositions at room temperature. Similarly to copper, platinum and rhodium, palladium does not reduce titanium, has metallic character already from a quite early deposition, grows with a 3D-island mode and the interactions between palladium and TiO₂ are very weak. Annealing treatments at 400 °C provoke a change in the morphology of the palladium clusters and the formation of islands characterized by a higher ratio between volume and area in contact with the substrate.     

FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

Band bending at the Si(1 1 1)–SiO2 interface induced by low-energy ion bombardment

Experiments employing photoreflectance spectroscopy have uncovered band bending due to electrically active defects at the Si(1 1 1)–SiO₂ interface after sub-keV Ar⁺ ion bombardment. The band bending of about 0.5 eV resembles that for Si(1 0 0)–SiO₂, and both interfaces exhibit two kinetic regimes for the evolution of band bending upon annealing due to defects healing. The healing takes place about an order of magnitude more quickly at the (1 1 1) interface, however, probably because of less fully saturated bonding and higher compressive stress.     

Dependence of surface properties on adsorbate-substrate distance: Work function changes and binding energy shifts for I/Pt(1 1 1)

This paper presents an analysis of the character of the bond of I adsorbed at on-top and 3-fold sites of Pt(1 1 1). At both sites, the bonding is dominated by an ionic interaction supplemented with some covalent character due to donation from the adsorbed I anion to the Pt surface. The way in which the I-Pt interaction affects observed properties has been established. In particular, the origins of the anomalous work function changes induced by the adsorption of I and the shifts of I core level binding energies are explained. It is shown that the magnitudes of the changes in these properties can be directly correlated with the distance of the I from the Pt surface. Thus, these shifts can be interpreted to indicate adsorbate height. The fact that the negatively charged I adsorbate leads to a work function decrease, rather than the increase expected due to the charge of the adsorbate, may appear to be an anomaly. However, it is shown that this decrease arises from electronic reorganizations that cancel the dipole due to the charge of the adsorbate. Furthermore, the electronic terms that contribute to a lowering of the work function are larger as the adsorbate moves closer to the surface.     

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb-Si interfacial structure.     

Sulfur adsorption on Fe(1 1 0): a DFT study

The adsorption of atomic S on the Fe(1 1 0) surface is examined using density functional theory (DFT). Three different adsorption sites are considered, including the atop, hollow and bridge sites and the S is adsorbed at a quarter monolayer coverage in a p(2 × 2) arrangement. The hollow site is found to be the most stable, followed by the bridge and atop sites. At all three sites, S adsorption results in relatively minor surface reconstruction, with the most significant being that for the hollow site, with lateral displacements of 0.09 Å. Comparisons between S-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the S. At the hollow site, the presence of S causes an increase in the surface Fe d-orbital density of states between 4 and 5 eV. However, S adsorption has no significant effect on the structure and magnetic properties of the lower substrate layers.