Search

Metal surface oxidation is a well-known phenomenon, but the oxidation states of metal surfaces have not been observed

Real-time observation of an electrochemical reaction at a solid–liquid interface was performed using fluorescence-yield wavelength-dispersive soft X-ray absorption spectroscopy (XAS). A dedicated e...

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

Nanoparticulate ZnO materials were fabricated using microwave assisted hydrothermal synthesis from mixture of zinc acetate and ammonia at variable pH. The particle size and shape of resulting materials changes with the pH of precursor solution. The obtained nanoparticulate ZnO are of single phase nature with particle size ranging between 100 and 3000 nm, and are photoelectrochemically active in water oxidation upon UV illumination. Their photoelectrochemical activity, depends primarily on the ZnO particle surface orientation which changes with the particle size. The highest photoelectrochemical activity results from simultaneous presence of {0− 110} and {− 2110} oriented faces. The presence of {0001} oriented faces apparently suppresses the photoelectrochemical activity. The results indicate that the photoelectrochemical activity of ZnO is mainly related to a presence of {0−110} oriented surface.

Accepted: 2015-07-18, 資料番号: SA1150178000

The authors performed FACET (Investigation on Mechanism of Faceted Cellular Array Growth) experiments under long duration microgravity on the International Space Station (ISS) in 2010. The temperature and concentration distributions in the melt during the growth were precisely measured with high spatial resolution. Negative temperature gradient as well as negative concentration gradient ahead of the S/L interface can be expected to be the driving forces of the morphological instability. It is evident that the conventional model based on the frozen temperature approximation is insufficient to explain the growth mechanism of the faceted cellular array.

The viscosities of molten InSb, GaSb, and $$\mathrm{{In}}_{{x}}\mathrm{{Ga}}_{1-{x}}\mathrm{{Sb}}$$ ( $$x=0.2$$ and 0.4) were measured as a function of temperature using an oscillating viscometer for fundamental understanding of the physical properties for fabricating high quality InGaSb multicomponent semiconductor crystals. The measured values showed good Arrhenius linearity for InSb, GaSb, and $$\mathrm{{In}}_{{x}}\mathrm{{Ga}}_{1-{x}}\mathrm{{Sb}}$$ samples. The absolute values of the viscosity for InSb and GaSb agreed with a previous study. Also, it is suggested that the absolute values of the viscosity among the compounds are quite similar, and the results can be associated with their crystal structures.

Anin situobservation experiment of faceted cellular growth was carried out using transparent organic alloy, salol -t-butyl alcohol, in microgravity conditions on the International Space Station. The temperature and solute concentration fields in the vicinity of the solid-liquid and the growth rate were simultaneously measured by microscopic interferometers.

To assess the surface reactivity of strained Si wafers, open circuit potential (ocp) measurement and scanning surface potential microscopy (SPoM) were carried out. Surface conditions of the wafers were varied by changing the structure of the wafers, i.e., the Ge content of the SiGe underlayer and the thickness of the strained Si layer. The ocp value of the strained Si was negative compared with that of nonstrained Si, and it shifted in the negative direction with an increase in Ge content of the SiGe layer and/or in the thickness of the strained Si layer, indicating that the degree of strain and/or the density of defects at the Si wafer surface affect its reactivity. In addition, the ocp value shifted in the positive direction to reach the value of the nonstrained Si, possibility due to stress relaxation, when the thickness of strained Si exceeds a certain value, which is expected to be the “critical” thickness. The SPoM analysis confirmed that the negative potential shift takes place locally at the surfa...

The effect of lattice strain in strained Si wafer surfaces was investigated with the redox potential as an index parameter for the evaluation of the surface reactivity. The redox potential was calculated by density functional theory calculations. The Gibbs free energies of strained Si, SiO2, H2O, and H2, which are components of the equation that represents the redox potential, were estimated from the calculated energies. The results suggested that the strain in the Si lattice shifts the redox potential in the negative direction, and the amount of shift was estimated to be approximately 10 mV when the degree of strain in Si was 3%. In addition, the Si HOMO energy, which was related to the redox potential, increased with the lattice strain, indicating that the increase in HOMO energy induces the potential shift and possibly the surface reactivity.

Accepted: 2016-06-15, 資料番号: SA1160271000

The oxidation mechanism of aldehydes, which are commonly used as reductants for an electroless deposition process, was studied by using Density Functional Theory (DFT) calculations. The reaction pathway of the three aldehydes, i.e., formaldehyde, acetaldehyde and glyoxylic acid, with different functional groups, were examined by calculating energy profiles of all intermediate species. It was indicated that the pathway in an isolated system proceeds via dianion-free intermediate species. Taking the solvation effect into consideration, it was indicated that the oxidation reactions of the three aldehydes preferably proceed at the solid/liquid interface. In combination with a Cu metal cluster as a model of metal surface, it was also indicated that the oxidation reactions proceed preferentially at the Cu surface. It was expected that the adsorption/desorption energy at the Cu surface of glyoxylic acid, which has an electron-accepting carboxyl group, was smaller and substituent effect lead to its high reducibility.

Using first-principles total energy calculations within density functional theory (DFT), we investigated the surface properties, especially focusing upon the surface reactivity, of strained Si layer deposited onto SiGe underlayer, in comparison with normal (non-strained) Si surface. We used a cluster model as the surface of the strained Si and normal Si, and results of our calculation indicate that, when the Si possessed strained condition, the total energy is destabilized and the value of the gap between HOMO and LUMO became smaller than that of normal Si. In addition, we estimated the reactivity of the strained Si and normal Si surfaces with trace metal ion species in solution. As a result of the adsorption energy calculation of Cu 2+ , Ni 2+ and Fe 2+ to Si (100) surface, it was suggested that the adsorption energy of the metal ion species to the strained Si surface is more stable than that of the normal Si.

Mechanical stress was applied to a Si single crystal wafer to evaluate the changes in its surface properties arising from the lattice strain induced by mechanical stress. The stress was applied quantitatively to the wafer in order to investigate the correlation between the degree of strain and the electrochemical properties of its surface. Finite element method (FEM) structural analysis was used to estimate the degree of strain on the surface, and the open circuit potential was measured in order to investigate the surface electrochemical properties. The analysis suggested that the surface of the wafer was under tensile strain, and the open circuit potential shifted toward the negative direction with the strain. This indicates that mechanical stress applied to a Si surface can change its electronic properties by changing the surface crystal structure. Keywords: Strained silicon, Mechanical stress, Open circuit potential, Electrochemical potential measurement

La5Ti2Cu1−x Ag x S5O7, a solid solution of La5Ti2CuS5O7 and La5Ti2AgS5O7, is a semiconductor studied for sunlight-driven water splitting. In this study, the band structure and effective mass of La5Ti2Cu1−x Ag x S5O7 solid solutions are analyzed on the basis of density functional theory (DFT) calculations to reveal the effect of the composition on the band structure and the anisotropy in the conductivity. The band gap of La5Ti2Cu1−x Ag x S5O7 materials exhibits a bowing with respect to the value of x. The effective masses of holes and electrons in La5Ti2Cu1−x Ag x S5O7 are estimated from the band structure. Both holes and electrons have smaller effective masses in the b-axis direction than in the other directions, testifying the highly anisotropic electronic properties. The wave functions at the valence band maximum and the conduction band minimum are both delocalized along the b-axis only, forming parallel channels for electron and hole conduction. These results support the experimental observation that photoexcited carriers tend to migrate one dimensionally. The outcome of the theoretical calculation is qualitatively consistent with the experimental observations and helps to understand the electronic properties of La5Ti2Cu1−x Ag x S5O7 with varying compositions.

著者人数: 15名, 資料番号: SA1003113000

not Available.

Nanometer-scale defects on the surface of a Si wafer, fabricated by nanoindentation technique, were examined using Raman microscopy and scanning surface potential microscopy to determine the quantitative relation between surface potential shift and Raman shift. Formation of cracks has not observed at the defect sites, indicating that the plastic and elastic deformations were a major issue and that the residual stress from the plastic deformation was observed. The Raman peak of the array of defects formed by nanoindentation shifted to a lower wavenumber as the indentation force was increased. The results indicated that the amount of tensile strain increased with the indentation force, that is, tensile stress was dominant over the condition in the surrounded area, in the indentation-induced defects. Correspondingly, the surface potential of the defect arrays shifted to the negative direction with an increase in indentation force. These results suggest that the tensile stress at the defect site, indicated by the Raman shift, causes the negative shift in the surface potential, which is related to chemical reactivity. Furthermore, Raman shift of a single indent on a silicon-on-insulator (SOI) wafer was observed, which revealed an enhancement in the source-to-noise ratio of the Raman spectrum at the Si defect site, because the ratio of the volume of the defect site to that of the unstrained site increased due to the thin Si layer on the SOI wafer.

The effect of mechanical strain on the surface properties of Si wafer was investigated as a model experiment to evaluate the correlation between the degree of the strain and the electrochemical properties of the Si wafer surface. Finite element structure analysis was applied to estimate the degree of strain, and the open circuit potential measurement was carried out to investigate the surface electrochemical property. The results of these experiments suggested that the mechanical stress induces strain to the Si crystal structure to change the surface reactivity.