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.     

The structure of methylthiolate and ethylthiolate monolayers on Au(111): Absence of the (√3 × √3)R30° phase

Surface structures of self-assembled methylthiolate and ethylthiolate monolayers on Au(111) have been imaged with STM. For saturation coverage of 0.33 ML at room temperature, the well-known (√3 × √3)R30° phase routinely observed for longer chain alkanethiolates does not appear under any conditions for adsorbed methylthiolate and ethylthiolate. Instead, both thiolate species organize themselves into a well-ordered 3 × 4 structure. We thus conclude that the stable structure for saturation coverage of methylthiolate/ethylthiolate on Au(111) at RT is 3 × 4, not (√3 × √3)R30° as generally believed. For coverage less than 0.33 ML, a striped-phase with short-range order is observed for methylthiolate. Fourier transform of the STM image from the striped-phase produces a clear (√3 × √3)R30° “diffraction” pattern. This strongly indicates that the (√3 × √3)R30° diffraction pattern for methylthiolate monolayers reported in literature is likely from the striped-phase, rather than from a true (√3 × √3)R30° lattice in real space. Consequently, theoretical modeling that reproduces the (√3 × √3)R30° structure for methylthiolate monolayers should be re-examined.     

Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

The atomic structures of Au and Ag co-adsorption-induced √21 × √21 superstructure on a Si(111) surface, i.e., (Si(111)-√21 × √21-(Au, Ag)), where the Si(111)-5 × 2-Au surface is used as a substrate, have been investigated using reflection high-energy positron diffraction (RHEPD) and photoemission spectroscopy. From core-level spectra, we determined the chemical environments of Ag and Au atoms present in the Si(111)-√21 × √21-(Au, Ag) surface. From the rocking curve and pattern analyses of RHEPD, we found that the atomic coordinates of the Au and Ag atoms were approximately the same as those of the Au and Ag atoms in other Si(111)-√21 × √21 surfaces with different stoichiometries. On the basis of the core-level and RHEPD results, we revealed the atomic structure of the Si(111)-√21 × √21-(Au, Ag) surface.     

Metallization of the β-SiC(100) 3 × 2 surface: A DFT investigation

Using density functional theory (DFT) we report results for the electronic structure and vibrational dynamics of hydrogenated silicon carbide (001) (3 × 2) surfaces with various levels of hydrogenation. These results were obtained using density functional theory with a generalized gradient exchange correlation function. The calculations reveal that metallization can be achieved via hydrogen atoms occupying the second silicon layer. Further increase of hydrogen occupation on the second silicon layer sites results in a loss of this metallization. For the former scenario, where metallization occurs, we found a new vibrational mode at 1870 cm^? 1, which is distinct from the mode associated with hydrogen atoms on the first layer. Furthermore, we found the diffusion barrier for a hydrogen atom to move from the second to the third silicon layer to be 258 meV.     

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.     

Coverage dependence of glycine adsorption on the Ge(100) − 2 × 1 surface

A combination of infrared spectroscopy, X-ray photoelectron spectroscopy and density functional theory has been used to investigate the adsorption behavior of glycine at the Ge(100) ? 2 × 1 surface under ultrahigh vacuum conditions. Comparison of experimental and simulated IR spectra indicates that at 310 K, glycine adsorbs on Ge(100) ? 2 × 1 via O–H dissociation, with some fraction of the products also forming an N dative bond to a neighboring germanium atom. O–Ge dative bonding is not observed. As coverage increases, the surface concentration of the monodentate O–H dissociated adduct increases, while that of the N dative-bonded species appears constant. XPS data support and clarify the IR findings and reveal new insights, including the presence at higher coverage of a minor product that has undergone dual O–H and N–H dissociation. These findings are supported by the calculated energy diagrams, which indicate that the reaction of a glycine molecule on the Ge(100) ? 2 × 1 surface via O–H dissociation and interdimer N dative bonding is both kinetically and thermodynamically favorable and that N–H dissociation of this adduct is feasible at room temperature given incomplete thermal accommodation along the reaction pathway.     

Anomalous scaling behaviour of cobalt cluster size distributions on graphite,epitaxial graphene and carbon-rich (6√3 × 6√3)R30°

The influence of the underlying interface on adsorption of cobalt (Co) is investigated by comparing the nucleation and growth of Co at room temperature on three carbon (C) surfaces, i.e. highly oriented pyrolytic graphite (HOPG), epitaxial graphene/SiC(0001) (hereafter abbreviated as EG) and precursor of EG i.e. C-rich (6√3 × 6√3)R30°/SiC(0001) (hereafter abbreviated as 6√3). On all three surfaces, Co adopts Volmer–Weber growth mode via formation of three-dimensional dome-shaped nanoclusters. Co clusters formed on 6√3 surface are smaller but denser than Co/HOPG or Co/EG. Scaling analysis reveals a critical nucleus size, i* = 1 (atom) and the smallest stable cluster (i* + 1) would be a dimer. Co/HOPG and Co/EG have the same order of magnitude for their cluster densities and sizes. Scaling analyses however show that the i* for Co/EG (i* = 3) is larger than Co/HOPG (i* = 0) and in this respect the smallest stable cluster would be tetramer and monomer respectively. This difference is attributed to the influence of an interface situated between graphene and SiC bulk. It appears that EG is more inert than HOPG towards the adsorption of Co and may act as a better substrate to host Co clusters.     

Structural analysis of Si(111)-√21 × √21-(Ag,Cs) surface by reflection high-energy positron diffraction

We have investigated the Si(111)-√21 × √21-(Ag, Cs) superstructure using reflection high-energy positron diffraction. Rocking curve analysis based on the dynamical diffraction theory reveals that Cs atoms are located at a height of 3.04 Å above the underlying √3 × √3-Ag structure and that they form a triangular structure with a side length of 10.12 Å. The structure of the Si(111)-√21 × √21-(Ag, Cs) surface is significantly different from those of the Si(111)-√21 × √21-Ag and Si(111)-√21 × √21-(Ag, Au) surfaces, probably because of the different electronic structures of the alkali and noble metal atoms.     

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.     

Growth of the Au-induced c(8 × 2) structure on vicinal Ge(001)

We have studied the formation of Ge(001) c(8 × 2)–Au surfaces on vicinal samples by scanning tunneling microscopy. The vicinal samples are tilted 1° toward [110]. The c(8 × 2)–Au surface is prepared by depositing 0.75 ± 0.05 ML of Au onto a germanium surface held at 800 K. The anisotropy introduced by the atomic steps of the vicinal surface and the preferential etching of S_B steps during Au deposition is sufficient to introduce a preferred growth direction for the c(8 × 2)–Au phase. The result is a sample on which 78% of the surface is populated by Au-induced chains oriented parallel to the step direction. These parallel Ge(001) c(8 × 2)–Au domains are separated by single or multiple height D_A steps (0.28 nm high).     

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.     

A Cu(1 1 1)(√3×√3)R30°-Sb structure by impact collision ion-scattering spectroscopy

We have investigated the structures of Cu(1 1 1)(√3×√3)R30°-Sb using time of flight-impact collision ion-scattering spectroscopy. The experimental data and computer simulations support a structural model for the Cu(1 1 1)(√3×√3)R30°-Sb structure in which Sb atoms displace up to 1/3 of the first layer of Cu atoms and incorporate them into the first Cu layer with the Sb atoms displaced outward 0.40 Å with respect to the first-layer Cu atoms. The outermost first layer of Sb and Cu atoms shift from fcc- to hcp-hollow sites (only the top layer of Sb and Cu atoms occupies hcp hollow sites).     

First principles calculations of the Sc adsorption on Si(001)-c(2 × 4)

We have investigated the energetics and the atomic structure of the adsorption of Sc on the Si(001)-c(2 × 4) surface using first principles total energy calculations, within the periodic density functional. The Sc adsorption has been studied at high symmetry sites considering different concentrations. We have first explored the one atom case and then increased the coverage up to a 0.25 of a monolayer of Sc. For the adsorption of one Sc atom we have obtained that the most stable configuration corresponds to the adsorption in the trench between two Si dimers, at the C1 (cave) site. The interaction of the adsorbed Sc with the Si dimers induces a decrease of the dimers buckling amplitude. On the other hand Si dimers without interaction with the adsorbate have buckling amplitudes similar to those of the clean Si surface. When the Sc coverage is increased to two Sc atoms, the most stable structure corresponds to the adsorption at two consecutive V (valley-bridge) sites along the trench between Si dimers, resulting in the weakening of some of the Si dimers bonds. This result indicates that the formation of one dimensional Sc chains on the silicon surface is energetically and kinetically favorable.     

First principles total energy calculations of the adsorption of germane and digermane on Si(0 0 1)-c(2 × 4)

First principles total energy studies are performed to investigate the energetics, and the atomic structure of the adsorption of germane (GeH₄), and digermane (Ge₂H₆) on the Si(0 0 1)-c(2 × 4) surface. It has been observed experimentally that adsorption of Ge₂H₆ is a dissociative process, which first yields GeH₃ and then GeH₂ fragments as products. We first study the adsorption of GeH₂ considering two different models; the intra-row and the on-dimer geometries. Our results show that the on-dimer site is more stable than the intra-row geometry by 0.44 eV. This is not a surprise since in the absence of H atoms, adsorption in the on-dimer site leaves no dangling bonds. In contrast, when the GeH₂ fragment is considered together with two H atoms, the intra-row geometry is favored energetically as compared with the on-dimer site, in good agreement with experiment. Similar results have been previously obtained for the adsorption of SiH₂ on Si(0 0 1). Digermane adsorption is explored according to two different geometries. In the first one, we have considered the adsorption as two GeH₃ fragments, while in the second, we have considered the adsorption as two GeH₂ fragments plus 2 H fragments. In good agreement with experiments, it is found that the latter geometry is energetically more favorable.     

NMR and electric impedance study of lithium mobility in fast ion conductors LiTi2 − xZrx(PO4)3 (0 ≤ x ≤ 2)

Materials of the LiTi_2 − xZr_x(PO₄)₃ series (0 ≤ x ≤ 2) were prepared and characterized by powder X-ray (XRD) and neutron diffraction (ND), ⁷Li and ³¹P Nuclear Magnetic Resonance (NMR) and Electric Impedance techniques. In samples with x < 1.8, XRD patterns were indexed with the rhombohedral R3̅c space group, but in samples with x ≥ 1.8, XRD patterns display the presence of rhombohedral and triclinic phases. The Rietveld analysis of the LiTi_1.4Zr_0.6(PO₄)₃ neutron diffraction (ND) pattern provided structural information about intermediate compositions. For low Zr contents, compositions deduced from ³¹P MAS-NMR spectra are similar to nominal ones, indicating that Zr⁴⁺ and Ti⁴⁺ cations are randomly distributed in the NASICON structure. At increasing Zr contents, differences between nominal and deduced compositions become significant, indicating some Zr segregation in the triclinic phase. The substitution of Ti⁴⁺ by Zr⁴⁺ stabilizes the rhombohedral phase; however, electrical performances are not improved in expanded networks of Zr-rich samples. Below 300 K, activation energy of all samples is near 0.36 eV; however, above 300 K, activation energy is near 0.23 eV in Ti-rich samples and close to 0.36 eV in Zr-rich samples. The analysis of electrical data suggests that the amount of charge carriers and entropic terms are higher in Zr-rich samples; however, the increment of both parameters does not compensate lower activation energy terms of these samples. As a consequence of different contributions, the bulk conductivity of Zr-rich samples, measured at room temperature, is one order of magnitude lower than that measured in Ti-rich samples.     

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&nbsp;–&nbsp;xEux)2(WO4)3 solid solutions are studied in a wide range of europium...