Structural stability of stepped nickel surfaces

The structural stability of stepped Ni(755) and Ni(771) surfaces in the temperature range 20–500°C and during adsorption of oxygen and different forms of carbon has been studied using scanning tunneling microscopy and low-energy electron diffraction. A phase transition from the structure with double steps at room temperature to the structure with single steps at a temperature above 350°C has been observed on the clean Ni(755) surface. This transition disappears after oxygen adsorption at a temperature above 350dgC. In this case, the structure on the Ni(755) surface with single steps is stabilized in contrast to Ni(771), which tends to faceting during oxygen adsorption. It has been shown that fullerenes C60 form an array of one-dimensional chains at the upper boundaries of steps of the substrate, when they are adsorbed on the Ni(755) surface.     

Molecular beam epitaxy and reconstructed surfaces

In the last years molecular beam epitaxy (MBE) has become a very important method to create new materials. Scattering and channeling of high-energy ions is a mass-dispersive, surface-sensitive crystallographic technique, particularly suited for the investigation of epitaxial systems. The combination of both techniques allows new insight into some of the fundamental processes at interfaces and surfaces. As an example we discuss the influence of substrate reconstruction on epitaxial growth. We show for the Si/Ge and Si/Si systems that the reordering of the reconstruction-induced displacements at the substrate surface, a necessary condition for epitaxial growth, is critically dependent on the type of reconstruction: Deposition of Ge or Si on Si(100)2×1 at room-temperature relieves the reconstruction, whereas Si(111)7×7 appears unaffected. This difference is discussed in terms of structural models for these surfaces. We shall also discuss the implications of these results with respect to MBE and, in particular, to Si homoepitaxial temperatures.     

半导体硅重构表面及其相变动力学的研究进展

简单介绍了文章作者在半导体硅重构表面及其相变动力学研究方面的进展.近期si(111)(7×7)-(1×1)相变的实验研究发现,将温度升高到相变温度以上时,7×7岛面积以恒定的衰减速率随时间减小至零,且初始面积越大的岛这个衰减速率就越大.文章作者分析了大量的实验事实,由此提出了一个双速相场模型来解释这个重要而令人困惑的现象.模型重点是:在相变过程中,7×7关键结构变化较快,随后的层错消解过程要慢得多.这个模型完美地解释了相关实验现象,说明该模型抓住了关键物理要素,这种相场方法也可以用于其他半导体表面相变研究.     

Diffraction from reconstructed surfaces with incommensurate domain walls

The kinematic approximation method has shown that the peak intensity and the full width at half maximum (FWHM) of stepped surfaces exhibit an oscillatory behavior for changing incident energy. This paper generalizes the kinematic approximation to an (N × 1) reconstructed surface with a distribution of various types of lateral displacements at a step. A particular solution of this model we call the fixed point solution, yields a clear intuitive understanding of these oscillations as well as an exact solution for the step density of any surface. The specific examples of (5 × 1) and (2 × 1) reconstruction are examined to show the striking differences between the reconstructed surface diffraction patterns. These differences make an examination of the half-maximum (HM) intensity position a powerful tool to determine the surface structure for any incommensurate stepped surface.     

Structural stability properties of friedman cosmology

A dynamical system with Robertson-Walker symmetries and the equation of the statep=, 01, considered both as a conservative and nonconservative system, is studied with respect to its structural stability properties. Different cases are shown and analyzed on the phase space (x=R ^D , y=x).     

Structural and optical properties of porous iron oxide

The structural and optical properties of the novel porous iron oxide fabricated by wood template have been investigated. The obtained porous iron oxide was characterized to be α- Fe₂O₃ by Fourier transform infrared and Raman spectroscopy. X-ray absorption fine structure measurement revealed that the bond length of Fe-O₁ of the porous iron oxide has good agreement with that reported for the α- Fe₂O₃ crystal structure while the bond lengths for Fe-O₂ and Fe-Fe deviate slightly from those of the α- Fe₂O₃ crystal structure. Photoluminescence from the porous iron oxide exhibited broad emission bands around 760 and 890 nm, which are believed to be due to the unique nanoscale structure of the porous iron oxide.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Structural and optical properties of lithium borobismuthate glasses

Glasses with compositions 25Li₂O-(75−x)Bi₂O₃-x B₂O₃, with 0?x?30 mol%, have been prepared using the melt quenching technique. The density and the molar volume have been determined. IR spectroscopy is used as a structural probe of the nearest neighbor environment in the glass network. The optical transmittance and reflectance spectrum of the glasses have been recorded in the wavelength range 400-1100 nm. The values of the optical band gap E_g^opt for indirect transition and refractive index have been determined for 0?x?30 mol%. The average electronic polarizability of the oxide ion α_o²⁻ and the optical basicity have been estimated from the calculated values of the refractive indices. Variations in the different physical parameters such as the density, molar volume, optical band gap, refractive index, average electronic polarizability of the oxide ion and optical basicity with B₂O₃ content have been analyzed and discussed in terms of the changes in the glass structure.     

Structural and optical properties of InSe under pressure

Abstract

We have investigated the high pressure behavior of InSe by x-ray powder diffraction and optical measurements. The rhombohedral γ-polytype of InSe (space group R3m) exhibits a strongly anisotropic compressibility characteristic of the layer-type structure. Mode Gruneisen parameters of intralayer modes have been determined by Raman scattering. At 10.3(5) GPa InSe undergoes a phase transition to the rocksalt structure, which remains stable up to at least 30 GPa. Optical reflectivity measurements show the cubic high pressure phase to have metallic character.     

Microscopic theory of optical properties of crystal surfaces

Light-propagation equations at crystal surfaces are solved in the long-wavelength limit, fully accounting for surface anisotropy, inhomogeneity and non-locality of the dielectric susceptibility tensor. Local-field and excitonic effects are also accounted in principle. Equations allowing calculation of internal and external reflectivity coefficients are given and used to analyze experimental data.     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Structural,electronic, stability and reduction properties of perovskite surfaces: The case of rhombohedral BaCeO3

The (100), (110) and (111) surfaces of rhombohedral phase BaCeO₃ perovskite with two kinds of surface terminations are investigated using a periodic DFT + U method. We show that the lowest energy for surface formation via crystal cutting (cleavage energy) corresponds to (100) terminations. Out of all studied terminations, only BaO(100) and BaCeO(110) are stable with respect to precipitation of oxide phases and metals in respective ranges of oxygen chemical potentials, whereas CeO2(100) termination is not stable with respect to CeO₂ precipitation for all temperatures and oxygen partial pressures. Analyzing the electronic properties of the surfaces, we have established that reduction of the cerium oxidation state occurs in response to the local stoichiometry (lack of surface oxygen's, etc.) rather than as a result of breaking of cerium–oxygen bonds and formation of under-coordinated cerium ions. This equally applies to cerium reduction in the case of surface vacancy formation. We have calculated the vacancy formation energies as these can be viewed as a measure of surface activity in the catalytic reaction with various adsorbates. We find that CeO2 termination of the (100) surface and modified O2 termination of the (110) surface (O termination) have the lowest vacancy formation energies.     

Structural stability and electronic properties of GaSb nanowires

The structural stability and electronic properties of four different shapes of GaSb nanowire have been studied by ab-initio method using the generalized gradient approximations. The different structures were two atom linear wire, two atom zigzag wire, four atom square wire and six atom hexagonal wire. The geometry optimization and the stability of all nanowires were investigated. We explore the minimum energy atomic configuration for all the considered shapes. We find that four atom square wire configuration has greater stability in comparison to other shapes. The analysis of density of states and band structures of optimized nanowires predicts that semiconducting nanowires may be metallic or semiconducting. The behavior entirely depends upon the geometrical structure.     

Structural and optical properties of ZnSe thin films stacked with PbSe submonolayers

Structural and optical properties of Zinc Selenide (ZnSe) thin films stacked with multiple Lead Selenide (PbSe) submonolayers (ML) were studied. Thermal evaporation was preferred to produce ZnSe–PbSe thin films with the PbSe ML thickness ranges from 2.5 to 10 nm. Polycrystalline nature of the ZnSe was revealed through high resolution X-ray diffractometer measurement. The development of micro strain at the interfaces with increasing PbSe ML thickness was observed. A cross-sectional TEM image shows well-ordered periodicity and reproducibility of the layer thickness. The enhancement of optical absorption of ZnSe was identified upon stacking of PbSe ML. The evidence for quantum confinement in PbSe ML was revealed by the obtained red shift in band gap (2.5–1.8 eV) values as well as photoluminescence emission at 1,071 nm. The presence of tensile strain in the ZnSe layers upon staking of PbSe ML was discussed by the shift in LO phonon modes in Raman spectra.     

Structural characterization and optical properties of ZnSe thin films

Zinc selenide (ZnSe) thin films (d = 0.11-0.93 μm) were deposited onto glass substrates by the quasi-closed volume technique under vacuum. Their structural characteristics were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The experiments showed that the films are polycrystalline and have a zinc blende (cubic) structure. The film crystallites are preferentially oriented with the (1 1 1) planes parallel to the substrate surface. AFM images showed that the films have a grain like surface morphology. The average roughness, R_a = 3.3-6.4 nm, and the root mean square roughness, R_rms = 5.4-11.9 nm, were calculated and found to depend on the film thickness and post-deposition heat treatment.The spectral dependence of the absorption coefficient was determined from transmission spectra, in the range 300-1400 nm.The values of optical bandgap were calculated from the absorption spectra, E_g = 2.6-2.7 eV.The effect of the deposition conditions and post-deposition heat treatment on the structural and optical characteristics was investigated.     

Structural, optical and electromagnetic properties of aluminum-clay nanocomposites

Aluminum pillared and exchanged bentonite particles were synthesized by the ion exchange method. The characteristics of the particles were investigated by Fourier-transform infrared spectra (FTIR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscope (SEM), electron dispersive X-ray spectrometer (EDS), reflectance spectrophotometer (RS) and electromagnetic transition instrument (ETI). FTIR spectra showed a successful incorporation of Al complexes into the clay interlayer. The TGA result demonstrated an improvement in thermal stability of the Al-pillared clay compared with the untreated particles. SEM and EDX results showed the presence of aluminum aggregates on the surface of clay. It was also found that Al ions affect electromagnetic properties of the clay particles.     

Structural and optical properties of Li substituted CuO nanoparticles

Copper oxide (CuO) is a favorable material for photovoltaic application where lattice defects/distortions play a significant role for shaping its optical and several other physical properties. In this study, pristine and lithium (Li) substituted CuO nanoparticle (1.0, 3.0, 5.0 and 7.0 mol% of Li) have been prepared via an eco-friendly and cost-effective sol–gel method and a systematic study on the effect of Li ion substitution has been drawn. The Rietveld refinement results confirmed the Li ion substitution obsessed structural alteration from monoclinic to tetragonal symmetry (C2/c?→?I4/mmm). It was observed that the C2/c and I4/mmm synchronized phases continues up to 7%. Such structural alteration leads to fascinating optical properties due to destruction of parent phase. Moreover, lithium cations inhibit the crystal growth, which created various types of vacancy/defect states that essentially need to be investigated using SEM, FTIR and Raman spectroscopy. Moreover, we have demonstrated that the native lattice alterations brought by size misalliance amid the host [Cu²⁺ (0.73 Å)] and the dopant [Li?+?(0.76 Å)] and the presence of oxygen vacancies created via Li substitution in CuO also results in the increment of deep level emission in photoluminescence spectra. The obtained results confirmed that Li ion substitution remarkably enhance optical properties, making such materials promising for device applications.     

Structural,electronic and optical properties of fluorite-type compounds

An oscillator chain with dynamical traps and additive white noise is considered. Its dynamics are studied numerically. New type nonequilibrium phase transitions are shown to arise in the case when the trap effect is pronounced. Locally they manifest themselves in distortion of the symmetry of particle arrangement. Depending on the system parameters, the particle arrangement is characterized by the corresponding distributions taking either a bimodal form, or a twoscale one, or a unimodal onescale form that, however, deviates substantially from the Gaussian distribution. The particle velocities also exhibit a number of anomalies, in particular, their distribution can be extremely wide or take a quasi-cusp form. A large number of various cooperative structures and superstructures are found in the visualized time patterns. In a certain sense their evolution is independent of the individual particle dynamics, enabling us to regard them as dynamical phases.     

Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.