Investigation of the morphology of the van der Waals surface of the InSe single crystal

The morphology of the (0001) van der Waals surfaces of the layered single crystal In_1.03Se_0.97, which were prepared using different techniques, has been investigated by scanning probe microscopy methods. It has been assumed that the van der Waals surface prepared with the use of an adhesive tape oxidizes in air due to the chemisorption of acid agents on dangling bonds of the metal and selenium. An analysis of the current-voltage characteristics of the tunneling current has shown that the composition of natural oxides represents a mixture of phases of the In₂O₃ oxide and wide-band-gap selenium oxides. In the InSe surface prepared by cleavage with subsequent exposure in air for approximately 2 min, the scanning with a tunneling microscope has revealed a surface ordering in the form of a corrugation of a complex profile with a fine structure. The last fact reflects the charge density redistribution after the chemisorption of gas molecules from air on this surface and its relaxation to the state with a minimum energy. Atoms of the basal plane are observed on the InSe(0001) van der Waals surface prepared by cleavage in an oxygen-free medium. The surface corrugation is absent. Point defects cause a disturbance of the periodic potential of the single crystal, which extends over a distance equal to four lattice spacings and appears as a shadowing. A technique has been proposed for producing In₂O₃ oxide nanostructures on the surface of the single crystal of the layered semiconductor InSe with the use of an atomic-force microscope probe as a nanoindenter. The ability of the probe to operate in gaseous and liquid media significantly extends the capabilities of the method.     

四角晶相HfO2(001)表面原子和电子结构研究

采用基于第一性原理的密度泛函理论研究了四角晶相二氧化铪(t-HfO₂)体相及 其(001)表面的原子几何与电子结构.理论计算结果表明，t-HfO₂(001)表面不会 产生重构现象.与体相电子结构相比, t-HfO₂(001)表面态密度明显高于体相态 密度.其次，表面原子的态密度更靠近费米能级(E_F)，价带往低能量处移动，并 有表面态产生.计算结果表明了t-HfO₂表面禁带宽度明显低于体相的禁带宽度. t-HfO₂(001)的表面态产生以及表面禁带宽度减小是由于Hf原子与O原子的配位 数减少，表面原子周围的环境发生变化而引起的. 关键词： 密度泛函理论 2(001)')" href="#">t-HfO₂(001) 表面电子结构     

Carbon on Pt(111): Characterization and influence on the chemisorptive properties

Fully dehydrogenated carbon deposits in the monolayer range have been characterized on the Pt(111) face by LEED, Auger and Δφ observations. These techniques pointed to the graphitic character of such deposits at least beyond about 0.5 monolayer of carbon. The gradual deactivation of the Pt surface towards CO or C₆H₆ adsorption with increasing carbon coverage has been explained by the nucleation and the growth of numerous inactive graphitic islands. The loss of chemisorptive properties only occurs for a complete carbon monolayer which may be viewed as a passive film masking the metal surface. Some weakening in the binding energy of CO, C₆H₆ and H₂ has been evidenced on a surface partially covered with carbon. This additional effect of carbon has been discussed mainly in relation with Δφ measurements.     

As3d core level studies of (GaMn)As annealed under As capping

The surface of a Ga_0.95Mn_0.05As layer subjected to low temperature annealing under As capping has been studied by core level photoemission with focus on As3d spectrum. By detailed comparison with the surface of pure GaAs subjected to the same surface treatment, the As spectral component of the reacted surface layer has been identified. The relative intensity of this component is consistent with the notion of an MnAs monolayer terminating the annealed (GaMn)As surface.     

Specific features of the electronic,spin, and atomic structures of a topological insulator Bi<Subscript>2</Subscript>Te<Subscript>2.4</Subscript>Se<Subscript>0.6</Subscript>

The specific features of the electronic and spin structures of a triple topological insulator Bi₂Te_2.4Se_0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi₂Te_2.4Se_0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi₂Te_2.4Se_0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 Å. Stability of the Dirac cone of the Bi₂Te_2.4Se_0.6 compound to deposition of a Pt monolayer on the surface is shown.     

Investigation of epitaxial Nd1.85Ce0.15CuO4 − y film surface by low energy electron diffractometry

The surface of epitaxial Nd_1.85Ce_0.15CuO_{4 ? y} (001) (NCCO) film has been studied by low energy electron diffractometry (LEED) and photoelectron spectroscopy. Ar⁺ ion etching of a surface with subsequent annealing in oxygen at atmosphere pressure has been found to lead to the ordered structure restoration of surface layers with the symmetry type and lattice parameters corresponding to the NCCO phase. Annealing in vacuum at temperatures close to the boundary of thermodynamic phase stability results in the formation of epitaxial Ce_0.5Nd_0.5O_1.75 phase on a surface that is indicated in the LEED pattern as additional spots corresponding to the surface lattice (√2 × √2)R45°.     

Depth profile of the lattice constant of the Cu-poor surface layer in (Cu2Se)1−x(In2Se3)x evidenced by grazing incidence X-ray diffraction

The surface of CuInSe₂ or related compounds is an essential part of heterojunctions for photovoltaic applications. It is generally accepted that the reconstruction of the surface of slightly Cu-deficient film results in a Cu-depleted defect layer. Since this surface layer generally exhibits a CuIn₃Se₅ stoichiometry the existence of a defect chalcopyrite phase CuIn₃Se₅ has also been proposed. Although efforts have been made to identify this phase structurally by means of grazing incidence X-ray-diffraction (GIXRD), it has not been detected so far. However, the existence of a Cu-depleted surface layer fits well in the existing theory, which models the functioning of photovoltaic devices. In this contribution we present for the first time evidence for a Cu-depleted surface layer measured by X-ray diffraction. A careful analysis of GIXRD-data at incident angles from 0.3 to 10° by comparing the measured shape and width with simulated spectra confirms structural changes in the surface layer. The thickness of this layer varies from 5 to 60 nm with the integral Cu-content of the film. Hence GIXRD provides a unique means for non-destructive depth profiling.     

Surface structure of γ-Fe2O3(111)

The surface structure of γ-Fe₂O₃(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10^? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe₂O₃-like layer.     

PASSIVATION OF THE InP(100) SURFACE USING (NH4)2Sx

InP(100) surface treated with (NH₄)₂S_x has been investigated by using photolumines-cence(PL), Auger electron spectroscopy and X-ray photoelectron spectroscopy, It is found that PL intensity increased by a factor of 3.3 after (NH₄)₂S_x passivation and the sulfur remained on the surface only bonded to indium, not to phosphorus. This suggests that the sulfur atoms replace the phosphorus atoms on the surface and occupy the phosphorus vacancies.     

Mössbauer studies of the surface and bulk magnetic structure of scandium-substituted Ba-M-type hexaferrites

A direct comparison of the magnetic structures of a surface layer and of the bulk of Ba-M-type hexagonal ferrites with iron ions partially replaced by Sc diamagnetic ions (BaFe_12?x Sc_xO₁₉) has been made by simultaneous Mössbauer spectroscopy with detection of gamma rays, characteristic x-ray emission, and electrons. It has been found that, if the magnetic lattice of a Ba-M-type hexagonal ferrite is weakly diluted by Sc diamagnetic ions, a ～300-nm thick macroscopic layer forms on the surface of a BaFe_11.4Sc_0.6O₁₉ crystal, in which the iron-ion magnetic moments are noncollinear with the moments in the bulk. The noncollinear magnetic structure forms in the near-surface layer of BaFe_12?x Sc_xO₁₉ crystals because the exchange interaction energy is additionally reduced by the presence of such a “defect” as the surface. This is the first observation in ferromagnetic crystals of an anisotropic surface layer whose magnetic properties, as predicted by Néel, differ from those of the bulk.     

Laser-induced desorption of polyatomic molecules with a CO2 laser

Laser-induced thermal desorption (LITD) has been increasingly employed as a tool for investigating surface processes. In LITD, a pulsed laser beam that is focused onto a surface induces a rapid temperature rise that causes desorption. In spite of the success enjoyed by the CO₂ laser in studies of diatomic molecules, its use with polyatomic molecules is shown to be severely limited by laser-induced dissociation. In desorption experiments with CH₃OH, HCOOH, CH₃NH₂ and NH₃ dissociation occurs only when the laser frequency coincides with an infrared absorption band of the molecule. Fragmentation may take place either on the surface or in the dense gas phase present above the surface during the laser pulse.     

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E ₂(high) and A ₁(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 10³) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.     

First nucleation steps during deposition of SiO2 thin films by plasma enhanced chemical vapour deposition

The initial nucleation stages during deposition of SiO₂ by remote plasma enhanced chemical vapour deposition (PECVD) have been monitored by XPS inelastic peak shape analysis. Experiments have been carried out on two substrates, a flat ZrO₂ thin film and a silicon wafer with a native silicon oxide layer on its surface. For the two substrates it is found that PECVD SiO₂ grows in the form of islands. When the SiO₂ particles reach heights close to 10 nm they coalesce and cover completely the substrate surface. The particle formation mechanism has been confirmed by TEM observation of the particles grown on silicon substrates. The kinetic Monte Carlo simulation of the nucleation and growth of the SiO₂ particles has shown that formation of islands is favoured under PECVD conditions because the plasma species may reach the substrate surface according to off-perpendicular directions. The average energy of these species is the main parameter used to describe their angular distribution function, while the reactivity of the surface is another key parameter used in the simulations.     

Effect of NH3 adsorption on the atomic structure of Si(111) √3 × √3 - Al and Si(111) √3 × √3 - Ag surfaces

The room temperature (RT) adsorption of ammonia (NH₃) on Si(111)√3 × √3-Al and Si(111)√3 × √3-Ag surfaces has been studied using LEED and AES. The transformation from Si(111)√3 × √3-Al surface structure to Si(111)1 × 1-(Al, H) upon NH₃ exposure has been found to be similar to the previously observed structural transformation induced by exposure in the atomic hydrogen. It has been demonstrated that the transformation is caused by hydrogen atoms which are generated by NH₃ dissociation on the Si(111)√3 × √3-Al surface. It has been estimated that about 0.1 ML of ammonia molecules is needed to complete the structural transformation. No interaction of NH₃ with the Si(111)√3 × √3-Ag surface has been found. The dissociation of NH₃ molecules is believed to be impossible on this surface     

Self-similar evolution of the surface morphology of a stressed amorphous alloy foil

A time-ordered sequence of topographic images of a stressed amorphous Fe₇₀Cr₁₅B₁₅ ribbon is presented. It is shown that the surface of this material (unlike polycrystalline metal foil) has a fractal structure due to the nonequilibrium conditions of its formation. As a tensile stress of about 500 MPa is applied to the surface, the fractal dimension of the surface increases from 1.21±0.02 to 1.34±0.03, then drops to 1.12±0.03, and finally increases to 1.22±0.02. In about 1.5 hours, a complex surface morphology characterized by a roughness amplitude of several tens of nanometers evolves into a regular pattern of shear bands with amplitude of about 300 nm. Self-affine changes in surface morphology are explained by competition between several processes, including crack propagation, surface smoothing, and self-diffusion.     

Effect of surface roughness on surface plasmon resonance absorption

The absorption of light by surface plasmons has been studied using the method of attenuated total reflection. The reflectance from a quartz-Ag interface has been measured as a function of angle and surface structure for the wavelength region from 3600 to 6000 . It is shown that the reflectance minimum for a smooth Ag film is changed in both angular position and spectral half-width by roughening the Ag surface with CaF₂ underlayers. Dispersion curves are presented which show that the wave vector of a surface plasmon propagating on an irregular surface is greater than that of an equally energetic surface plasmon propagating on a planar surface.     

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Fe_82.2B_17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H₂. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mössbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mössbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe₂O₃ by boron atoms at the surface and by suppressed carbide formation.     

Mössbauer study of Brazilian columbite

Columbite mineral from Lavra de Golconda in Governador Valadares, Minas Gerais, Brazil has been characterized by X-ray diffraction and electron microscopy. It has short prismatic nonconducting crystals with noninherent impurities of quartz, and exhibits one set of perfect cleavage. The reflectance is lowest normal to the cleavage surface, almost black. The surface is uneven and has small particles in a regular manner. On the other cleavage surface there is a regular pattern of white adherent impurity. The chemical composition of (Fe_0.32Mn_0.68)Nb₂O₆ was determined by mass spectrometry. The mineral has been studied by Mössbauer spectroscopy down to liquid nitrogen temperature. The isomer shift is 1.15 mm/sec and is almost temperature independent, but, the quadrupole splitting (1.55 mm/s at room temperature) is strongly temperature dependent (2.23 mm/s at liquid nitrogen temperature with large linewidth). This temperature dependence is explained in terms of relatively small crystal-field splitting of the t_2g levels.     

Adatom mobility on the surface planes of a simple metal

An approach using the Density Functional Approximation, which had in an earlier paper been applied to the binding of an adatom to a jellium metal (r_s = 4a₀), has been extended to a partially structured metal surface by introducing a layer of surface atoms to replace an equivalent layer of jellium. The model has been used to estimate the adatom motion energies over several quite close packed planes for a simple surface. An important preliminary step was to determine the general electron density contours for each surface by a simple variational method. The ensuing shielding charge distributions have an important bearing on the adatom motion energy. Adatom energies were calculated at three positions: (1) A above a surface atom, (2) B above a bridge position between two surface atoms and (3) C above a central position between three or four neighboring atoms. The motion energy was taken to be E_B ? E_C. As might have been expected this quantity was larger for the less closely packed planes, although it was always quite small due to the nature of the metal — large r_S, small ion core and typical s-p binding. To a rather surprising degree the strength of the shielding charge, the energies and the positions of the adatoms proved to be quite smooth functions of a parameter chosen to measure the close packing of the surface, namely the square of the interplanar distance divided by the surface area per atom.     

Methanol decomposition on the β-Ga2O3 (1 0 0) surface: A DFT approach

Density functional theory (DFT) cluster model calculations on methanol reactions on the β-Ga₂O₃ (1 0 0) surface have been realized. β-Ga₂O₃ structure has tetrahedral and octahedral ions and the results of gallia-methanol interaction are different depending on the local surface chemical composition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The unsaturated surface oxygen atoms strongly oxidize the methanol molecule. CO₂ and H₂O molecules are produced when methanol reacts with a free oxygen vacancy surface on octahedral gallium sites. On the other hand, H₂CO is found after the reaction of this molecule with a free O vacancy surface on tetrahedral gallium sites. A weak interaction between the remaining CO₂ molecule and the oxide surface was found, being this molecule easy to desorb. Otherwise, H₂CO has a stronger surface bond and it could suffer a later oxidation.