The electronic structures of commensurate Ru(0001)–(3 × 3)–4Kr and Ru(0001)–(5 × 5)–Kr using density functional theory

We have calculated the minimum energies for each of three positionings of the adatom unit cells for Ru(0001)–(3 × 3)–4Kr (high Kr coverage) and for Ru(0001)–(5 × 5)–Kr (low Kr coverage). The differences between the results for the low and high-coverage cases may clarify puzzles posed by the experimental results of Narloch and Menzel. The low-coverage solution converges to a structure having Kr in the top site at a height of 3.09 Å above the substrate with the adsorption energy 185 meV. In the high-coverage case, adatom unit cells with a corner Kr at top, fcc hollow, and hcp hollow locations are found to have nearly the same adsorption energy of 175 meV. The height of the corner atom above the substrate is found to be 3.35, 3.54, and 3.50 Å for the top, fcc hollow and hcp hollow sites, respectively. These results are explained by demonstrating that there is an enhancement of the substrate electronic density of states at krypton orbital energies in the low-coverage case.     

Ab initio study of Yb on the Ge(111)–(3 × 2) and Si(111)–(3 × 2) surfaces

The surface reconstruction, 3 × 2, induced by Yb adsorption on a Ge (Si)(111) surface has been studied using first principles density-functional calculation within the generalized gradient approximation. The two different possible adsorption sites have been considered: (i) H₃ (this site is directly above a fourth-layer Ge (Si) atom) and (ii) T₄ (directly above a second-layer Ge (Si) atom). We have found that the total energies corresponding to these binding sites are nearly the same, indeed for the Yb/Ge (Si)(111)–(3 × 2) structure the T₄ model is slightly energetic by about 0.01 (0.08) eV/unitcell compared with the H₃ model. In particular for the Ge sublayer, the energy difference is small, and therefore it is possible that the T₄, H₃, or T₄H₃ (half of the adatoms occupy the T₄ adsorption site and the rest of the adatoms are located at the H₃ site) binding sites can coexist with REM/Ge(111)–(3 × 2). In contrast to the proposed model, we have not determined any buckling in the Ge = Ge double bond. The electronic band structures of the surfaces and the corresponding natures of their orbitals have also been calculated. Our results for both substrates are seen to be in agreement with the recent experimental data, especially that of the Yb/Si(111)–(3 × 2) surface.     

Interaction of Ag with the Si(1 1 1)1 × 1–H surface

Synchrotron radiation based photoemission spectroscopy (SRPES) and low energy electron diffraction (LEED) are used to study the interaction between Ag atoms and the Si(1 1 1)1 × 1–H surface. At an Ag coverage of 0.063 monolayers (ML) on the Si(1 1 1)1 × 1–H surface, the Si 2p component corresponding to Si–H bonds decreases, and an additional Si 2p component appears which shifts to a lower binding energy by 109 meV with respect to the Si bulk peak. The new Si 2p component is also observed for 0.25 ML Ag on the Si(1 1 1)7 × 7 surface. These findings suggest that Ag atoms replace the H atoms of the Si(1 1 1)1 × 1–H surface and form direct Ag–Si bonds. Contrary to the widely accepted view that there is no chemical interaction between Ag particles and the H-passivated Si surface, these results are in good agreement with recent first-principles calculations.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.     

Strained c(4 × 2) CoO(1 0 0)-like monolayer on Pd(1 0 0): Experiment and theory

We report on an interface-stabilized strained c(4 × 2) phase formed by cobalt oxide on Pd(1 0 0). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 × 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1 0 0) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (～9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1 0 0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 × 2) structures have been previously found for nickel and manganese oxides on the same substrate.     

Selective adsorption of coronene on Si(1 1 1)-(7 × 7)

The adsorption of coronene (C₂₄H₁₂) on the Si(1 1 1)-(7 × 7) surface is studied using scanning tunneling microscopy (STM). Upon room temperature submonolayer deposition, we find that the coronene molecules preferentially adsorb on the unfaulted half of the 7 × 7 unit cell. Molecules adsorbed on different sites can be induced to move to the preferential sites by the action of the tip in repeated image scans. Imaging of the molecules is strongly bias dependent, and also critically depends on the adsorption site. We analyze the results in terms of differential bonding strength for the different adsorption sites and we identify those substrate atoms which participate in the bonding with the molecule.     

Spectral and Structural Characteristics of (Lu1 – xEux)2(WO4)3 Tungstates

Physics of the Solid State - The structure, luminescence, and IR absorption spectra of (Lu1 – xEux)2(WO4)3 solid solutions are studied in a wide range of europium...     

Spectral and Structural Characteristics of Molybdates (Lu1 – xEux)2(MoO4)3

Physics of the Solid State - The structure, the photoluminescence, and IR absorption spectra of solid solutions (Lu1 ‒ xEux)2(MoO4)3 have been studied over a wide range of...     

Pressure and compositional dependence of electric conductivity in the (Mg1 − xFex)1 − δO (x = 0.01–0.40) solid-solution

(Mg_1 ? xFe_x)_1 ? δO (x = 0.01–0.43) single crystals (~ 8 mm in diameter) were made by a melt-growth method. Electrical conductivity measurements were carried out as functions of temperature and frequency by a complex impedance method under pressure (~ 43 GPa and ~ 673 K and at 0.1 MPa and ~ 1400 K). Our experimental results show a change in charge transport mechanism in the (Mg_1 ? xFe_x)_1 ? δO solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. The low-temperature conduction mechanism in (Mg_1 ? xFe_x)_1 ? δO solid solution is small polaron. Pressure effect of the electric conductivity was observed and the conductivity increased to 0.5 at log scale of S/m with increasing pressure up to 43.4 GPa. The activation energy was decreased linearly with increasing pressure. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy of 2.37(4) eV for the (Mg_0.99Fe_0.01)_1 ? δO solid solution might correspond to a migration enthalpy of O ions through thermally formed defects. It is proposed that a possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is super ionic conduction.     

Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

Haven Correlation Parameter for Diffusion of Fluorine in Superionic Conductors La1 – ySryF3 – y

Physics of the Solid State - The processes of electric charge transfer (conductivity) and mass transfer (diffusion) in La1 ‒ ySryF3 – y superionic...     

Adsorption of small molecules on a (2 × 1) PdZn surface alloy on Pd(1 1 1)

The adsorption of methanol, formaldehyde, methoxy, carbon monoxide and water on a (2 × 1) PdZn surface alloy on Pd(1 1 1) has been studied using DFT calculations. The most stable adsorption structures of all species have been investigated with respect to the structure and the electronic properties. It was found that methanol is only weakly bound to the surface. The adsorption energy only increases with higher methanol coverage, where chain structures with hydrogen bonds between the methanol molecules are formed. The highest adsorption energy was found for the formate species followed by the methoxy species. The formaldehyde species shows quite some electronic interaction with the surface, however the stable η₂ formaldehyde has only an adsorption energy of about 0.49 eV. The calculated IR spectra of the different species fit quite well to the experimental values available in the literature.     

H adsorption at Ag/Si interfaces in epitaxially grown Ag(1 1 1) films on Si(1 1 1)7 × 7 substrates

The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO₂/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.     

As-rich (2 × 2) surface reconstruction on GaAs(111)A

The atomic structures and the formation processes of the Ga- and As-rich (2×2) reconstructions on GaAs(111)A have been studied. The Ga-rich (2×2) structure is formed by heating the As-rich (2×2) phase, but the reverse change hardly occurs by cooling the Ga-rich surface under the As₂ flux. Only when the Ga-rich (2×2) surface covered with amorphous As layers was thermally annealed, the As-rich (2×2) surface is formed. The As-rich (2×2) surface consists of As trimers located at a fourfold atop site of the outermost Ga layer, in which the rest-site Ga atom is replaced by the As atom.     

H/D Isotope Effect in the Conductivity of CaZr1 – xScxO3 – α in Reducing Atmospheres

Physics of the Solid State - The ionic (proton and deuteron) conductivity of the system CaZr1 – xScxO3 – α (x = 0.03–0.20) is studied...     

Preparation,surface state and band structure studies of SrTi(1 − x)Fe(x)O(3 − δ) (x = 0–1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy

In this report, SrTi_(1 ? x)Fe_(x)O_(3 ? δ) photocatalyst powder was synthesized by a high temperature solid state reaction method. The morphology, crystalline structures of obtained samples, was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), respectively. The electronic properties and local structure of the perovskite STF_x (0 ≤ x ≤ 1) systems have been probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. The effects of iron doping level x (x = 0–1) on the crystal structure and chemical state of the STF_x have been investigated by X-ray photoelectron spectroscopy and the valence band edges for electronic band gaps were obtained for STF_x by ultraviolet photoelectron spectroscopy (UPS). A single cubic perovskite phase of STF_x oxide was successfully obtained at 1200 °C for 24 h by the solid state reaction method. The XPS results showed that the iron present in the STF_x perovskite structure is composed of a mixture of Fe³⁺ and Fe⁴⁺ (SrTi_(1 ? x)[Fe³⁺, Fe⁴⁺]_(x)O_(3 ? δ)). When the content x of iron doping was increased, the amount of Fe³⁺ and Fe⁴⁺ increased significantly and the oxygen lattice decreased on the surface of STF_x oxide. The UPS data has confirmed that with more substitution of iron, the position of the valence band decreased.     

Acetylene adsorption on silicon (100)-(4 × 2) revisited

The aim of this work is to revisit the problem of acetylene adsorption on silicon (100). Extending previous theoretical work and including van der Waals forces explicitly in the simulations we remove existing ambiguities about the adsorption sites. The simulated adsorption energies and scanning tunneling microscopy contours are in good agreement with experimental data, they support the interpretation of a two-dimer feature at the surface as resulting from the adsorption of two individual molecules. It is also found that the simulated apparent heights agree with experimental values, if the actual bandgap of silicon is taken into account.     

Synthesis and Gas-Sensing Properties of MnO2 – x and MnO2 – x/CuO-Coated Multiwalled Carbon Nanotube Nanocomposites

Physics of the Solid State - Nanocomposites based on multiwall carbon nanotubes (MWCNTs) impregnated with manganese oxide MnO2 – x and copper oxide CuO are synthesized and...