Showing papers on "Chemical state published in 1993"

Chemistry Science of Change

Abstract: The atomic nature of matter chemical equations and reaction yields chemical periodicity and the formation of simple compounds chemical reactions the gaseous state condensed phases chemical equilibrium acid-base equilibria solubility equilibria thermochemistry spontaneous change and equilibrium redox reactions and electrochemistry electrochemistry and cell voltage chemical kinetics fundamental particles and nuclear chemistry quantum mechanics and the hydrogen atom many-electron atoms and chemical bonding molecular orbitals and spectroscopy co-ordination complexes structure and bonding in solids silicon and solid-state materials chemical processes chemistry of ogens from petroleum to pharmaceuticals polymers - natural and synthetic.

Surface chemical bonding of (NH4)2Sx‐treated InP(001)

Abstract: (NH4)2Sx‐treated InP(001) surfaces were analyzed by using synchrotron radiation photoelectron spectroscopy to characterize the S‐passivated surfaces and elucidate the solution etching mechanism. Polysulfide chemical states were observed for the first time in both the P 2p and In 4d spectra. Monosulfide and polysulfide states were also evident in the S 2p spectra. Etching models consistent with the experimental results were discussed.

Spectroscopic studies of surface copper spinels. Influence of pretreatments on chemical state of copper

Abstract: The nature of copper species of a series of alumina-supported copper samples was investigated by the kinetics of reduction, x-ray photoelectron spectroscopy, temperature-programmed desorption-mass spectrometry of chemisorbed NO and infrared spectroscopy of chemisorbed NO and CO probes. The role of these copper species was also examined in the catalytic decomposition of NO. The surface properties of copper were found to depend strongly on both Cu loading and calcination treatment

Determination of the structure of GaAs(100)-S with chemical-state-specific photoelectron diffraction.

Abstract: The chemical passivation of a semiconductor surface is an important step in electronic device manufacturing. This paper describes the application of chemical-state-specific photoelectron diffraction to solve the surface structure of sulfur-passivated GaAs(100). The results show that the GaAs(100)-S surface is terminated with ordered Ga-S-Ga bridge bonds in the [011] azimuth, and on top of this is a disordered arsenic sulfide overlayer. Complex surface structure such as GaAs(100)-S would not be resolved with conventional techniques such as low-energy electron diffraction, x-ray-absorption near-edge structure, surface-extended x-ray-absorption fine-structure, or vibrational spectroscopies

AES and XPS characterization of SiNx layers

Abstract: The properties of SiN x layers are largely influenced by they composition and the chemical state of the constituents. In the present study SiN x layers obtained by plasma-enhanced and low-pressure chemical desorption methods were characterized by AES and XPS. In the case of the most frequently applied AES characterization, the peak shape and position of the Si LVV line largely depend on the measurement parameters

X‐ray photoelectron spectroscopic studies of the chemical nature of as‐prepared and NaOH‐treated porous silicon layer

Abstract: The effect of various surface treatments of a porous silicon layer (PSL) including etching in NaOH solution on the chemical nature of the surface was studied using x‐ray photoelectron spectroscopy. As‐prepared PSL, which is formed by anodic oxidation of silicon in ethanolic HF solution, is covered with silicon oxide. NaOH treatment removes the surface oxide almost completely. Chemical states of surface silicon at the NaOH‐treated PSL surface are very close to that at the HF‐treated silicon surface. Surface oxygen on the NaOH‐treated PSL surface seems to be in the form of Si‐OH.

High resolution ion induced X-ray spectroscopy for chemical state analysis

Abstract: Satellite lines of K X-ray lines from K and Cr, and L lines from In are observed for impact by 1.4 MeVu1H+, 4He2+, 12C4+, 16O4+ and 35Cl8+. A curved crystal spectrometer was used with a position sensitive proportional counter with improved linearity of the backgammon readout. The distribution of Cr Kα satellite lines depends on the chemical state of Cr, being peaked at lower energy for the toxic valency 6 compounds than for valency 3 and metallically bound Cr. Fluorescence excitation of the Cr diagram line is observed for a stainless steel sample from enhancement by Fe X-rays.

Chemical States of Bromine Atoms on SiO2 Surface after HBr Reactive Ion Etching: Analysis of Thin Oxide

Abstract: Low-energy ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS) have been used to determine the nature of Br atoms on very thin thermal silicon dioxide (approximately 5 nm) after HBr reactive ion etching (RIE). The result of ISS clarified that the etched surface was covered with 1 monolayer Br. The Br atoms on the etched SiO2 surface were found, from the result of XPS analysis, to have two chemical adsorption states. The experiment of atomic Br exposure showed that one adsorption state was on the as-grown SiO2 surface and the other state was at the damaged sites induced by ion bombardment.

High Spatial Resolution XAFS and Its Imaging Applications

Abstract: A synchrotron X-ray microprobe has been applied to XAFS measurements over a small region. Either an ellipsoidal mirror or a modified Kirkpatrick-Baez configuration is used as an X-ray focusing element in combination with a double crystal monochromator. The X-ray beam size at the sample is around 5 µm with a photon flux of 107 to 108 photons/s/300 mA. Minerals in thin sections of rock samples have been analyzed. For analysis of practical samples, fluorescence detection is useful and effective. Deformation of the XAFS spectrum due to a self-absorption effect has been experimentally minimized using the small exit-angle condition; it can also be numerically corrected using physical parameters. Chemical state imaging has also been carried out using the chemical shift of the absorption edge. Lateral distributions of magnetite and hematite in a sintered iron ore have been obtained.

197Au Mössbauer study of copper refinery anode slimes

Abstract: Copper refinery anode slimes are abundantly produced during the electrolytic refining of copper. Although the slimes contain significant and economically recoverable amounts of gold and silver, the chemical state of the gold has not been fully identified. In the present work, the chemical form of gold in a copper anode, in a raw slime, and in slimes treated by different leaching procedures has been investigated by Mossbauer spectroscopy with the 77.3 keVγ-rays of197Au. The Mossbauer spectrum of the anode is typical of a dilute Au:Cu alloy. The spectrum of the raw slime consists of two components, namely, a single, rather broad line with an isomer shift (IS) of about −0.3 mm/s relative to a Pt metal source and a quadrupole doublet with an IS of + 1.2 mm/s and a quadrupole splitting of 5.0 mm/s. The single line component can be attributed to a gold-rich alloy, with an approximate composition of Au60Ag{n40} or Au80Cu20 if it is a binary alloy, or to a ternary Au-Ag-Cu alloy of appropriate composition. The parameters of the quadrupole doublet match those of Ag3AuSe2 (fischesserite) or related Ag2−xAuxSe compounds. In these compounds, the gold atoms are coordinated by two selenium atoms in a linear arrangement, as is typical for Au(I). It was found that the ratio between the concentrations of the metallic phase and the selenide strongly depends on the leaching conditions. The measurement of the Lamb-Mossbauer factor of fischesserite is also reported.

Metallic clusters surface interaction: The case of Pd/SiOx/n-Si(100)

Abstract: Atomic Force Microscopy (A.F.M.) and Rutherford back scattering (R.B.S.) have been employed to study the nucleation and growth phenomena of Palladium clusters cattered onto a thin (20 A) layer of SiOx, grown by plasma oxidation onto Si(100) doped B surface. X ray photoemission (XPS) of the 3d core level of palladium has permitted to follow the clusters silica interaction. Intrinsic final state size effect and extrinsic initial chemical state are discussed for clusters in the range of 10 to 104 atoms.

Structural investigations of electrical conducting plasma polymerized films from 2-iodothiophene

Abstract: Thin plasma polymerized films were produced in a microwave (2.45 GHz) plasma discharge using 2-iodothiophene as the initial monomer. The structure of these electrically conducting films (10−6–10−1 S/cm), the chemical state of the doping element iodine and the elemental homogeneity at the surface and in the films have been investigated by XPS, FTIR and AES. An interesting relationship was found that indicates some similarity to conventional polythiophene, prepared by electrodeposition. Besides covalently bound iodine, considerable amounts of I 3 − and I 5 − were detected and also a small number of O-I bonds were identified.

Chemical state analysis of light elements by undulator-radiation- excited X-ray fluorescence

Abstract: An undulator-radiation-excited X-ray fluorescence analysis system has been developed at an undulator beamline of the Photon Factory. As an analytical application demonstrating the capabilities of this system, the chemical state of boron implanted with a high dosage of BF 2 + into Si(100) was analyzed by measuring both the B Kα soft X-ray emission (SXE) and X-ray absorption near edge structure (XANES) with fluorescence detection. Comparing the SXE and fluorescence XANES spectra of B doped in Si with relevant reference boron compounds, it is confirmed that the chemical state of the B atoms as implanted in Si is metallic despite the fact that B is implanted as BF + 2 . Oxidation of B doped in Si by annealing at 900° C in atmosphere is also observed.

Heteroelemental diatomic secondary ions as a probe for molecular states

Abstract: The complete chemical characterization of environmental particles requires a depth resolved analysis of the chemical state of the elements Using dynamic SIMS, a depth-resolved determination of the chemical state of chromium was derived from measurements of ten different chromium salts and oxides The evaluation is based on the signals of Cr 2 + , CrO + and Cr 2 O + which are the most intense molecular ions The valence state of chromium can be determined for concentrations far less than 1 atom% The transferability of this kind of analysis is shown for manganese compounds Additionally binary mixed metal clusters are shown to indicate binding partners of compounds in mixtures

Surface structure - catalytic function in nanophase gold catalysts

Abstract: Catalysts consisting of ultra-fine gold particles supported on iron oxide have been synthesized by the coprecipitation method. Subsequent to preparation, each sample was heat treated in air at four different temperatures, ranging from 473 K to 773 K. Steady state carbon monoxide oxidation was carried out over each sample. Upon extended reaction, catalyst deactivation took place over three of the catalysts whose respective surface compositions (e.g., gold to iron atomic ratio) were altered appreciably from their initial state. Surface structure analyses performed on all the unreacted catalysts have revealed variations in physical properties (e.g., degree of crystallinity and particle size). In addition, lattice parameters of gold were observed to increase up to 20 % from the bulk value. In contrast, XPS showed both gold and iron to be in essentially the same chemical states for all catalysts, irrespective of heat treatment temperatures. The importance of surface sensitive parameters to catalytic function are discussed.

Chemical state resolved structure analysis using near edge x-ray standing waves.

Abstract: A surface structure analysis technique that provides chemical state discriminated structural information has been developed using x-ray standing wave measurements with photons in the x-ray absorption near edge structure. This near edge x-ray standing wave technique was applied to the surface structure analysis of the partially oxidized surface of a well-defined sulfur-passivated GaAs(111)B system. Oxidized sulfur atoms were about 22% of the surface sulfur atoms and were randomly distributed, while the unoxidized sulfur atoms in a Ga-S bonding state remain at the first layer As sites though they also disordered

A study of oxygen‐implanted Si0.5Ge0.5 alloy by XPS and thermodynamic analysis

Abstract: A thermodynamic study of oxygen-implanted Si 0.5 Ge 0.5 alloy based on an XPS analysis is reported for the first time. The alloy was grown by molecular beam epitaxy (MBE) and was implanted with high-dose oxygen ions (up to 1.8×10 18 cm -2 ) at an energy of 200 keV. The changes of Si, Ge, and O activities with concentration of oxygen were calculated using the THERMO-CALC program, and chemical potentials of these elements throughout the oxygen-buried region were plotted out according to the chemical state information obtained from XPS depth profiling results

The Chemical State of Gallium in Working Alkane Dehydrocyclodimerization Catalysts. In situ Gallium K-Edge X-Ray Absorption Spectroscopy.

Abstract: Ga species in H-ZSM5 zeolites catalyze rate-limiting dehydrogenation steps during the conversion of alkanes to aromatics. Specifically, they promote the recombinative desorption of H-adatoms as H2 and thus inhibit undesired cracking reactions. Here, we describe studies of the physical and chemical state of Ga using in-situ X-ray absorption at the Ga K-edge. Ga+3 species initially present in fresh catalysts reduce at temperatures below 770 K during hydrogen pretreatment or propane reactions. Reduced Ga is present in highly dispersed form without Ga nearest neighbors, probably as a monomeric hydride species coordinated to basic oxygens within zeolite channels. These hydride species are driven to release molecular hydrogen by the high surface hydrogen fugacities that develop during propane dehydrocyclodimerization on H-ZSM5. Reduced Ga species reoxidize to Ga+3 when samples are cooled to room temperature in flowing H2, suggesting that active forms of Ga exist only at reaction conditions. This work illustrates the critical need for in-situ techniques to establish the chemical form of catalytic sites and the misleading conclusions that can arise from the exclusive use of pre- and postreaction characterization to suggest the nature of the catatlytic sites.

XPS study of Si bombarded with low-energy reactive ions

Abstract: Surface atomic composition and chemical state have been investigated for Si(100) bombarded with 5-keV ions of the second period elements in the periodic table. The XPS observations revealed that the surface chemical changes can be divided into three types, i.e., (1) formation of a thin layer composed of the implanted atoms (B + , C + ), (2) formation of new compounds between silicon and the bombarding ions (N + , O + ), and (3) no chemical change (F + , Ne + ). These changes are discussed in connection with irradiation-induced surface segregation of the implanted atoms and thermochemical stability of the Si compounds formed on the target surface.

Structural Characterization of Adsorbed Dinitrogen on Copper-Ion-Exchanged Mordenite through XAFS Measurements

Abstract: X-ray absorption spectroscopy has been used to elucidate the structure of copper-ion-exchanged mordenite in the calcined state. Information about the chemical state and the local structural geometry of the exchanged copper ion has been elucidated by studing the XAXES and EXAFS spectroscopy. These findings show spectral features which would be assigned to Cu(OH)2-like species at 298 K, to CuO phase at 473 K, and to Cu(I) species at 673 K. Cu(I) species is present in an amorphous-like or microcrystalline structure. It is found that N2 molecules are adsorbed strongly on Cu(I) species and give a strong absorption band at 2299 cm-1 in infrared spectroscopy. The analysis of the EXAFS for the adsorbed N2 is performed, and demonstrates that this state is a bent type.

Chemical State of Bi 2 O 3 -doped ZnO Varistors Studied by a DV-Xα Method

Abstract: Auger KL 23 L 23 transitions for oxygen in ZnO were consisted of three peaks whose relative intensities depended on the contents of Bi. This Auger transition was studied by a discrete-variational Xα method. AES profiles observed were compared with those calculated from oxygen partial density-of-state, and it was shown that the electronic relaxation of initial state with a hole at inner O1s core played an important role in the AES transition process for oxygen.

Application of X-ray spectroscopy to the study of electrochemically formed surface oxide films

Abstract: Many analytical techniques can provide information regarding the chemical state, structure, and properties of materials. This paper focuses on two; X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), and their application to the study of electrochemically formed oxide films. A brief review of the phenomena underlying these techniques is provided, along with a description of the commonly used means for implementing them. Their capabilities and limitations are discussed, with an emphasis on the study of passive film composition and oxidation state. A summary of the behavior of Cr in oxide films on AI-Cr alloys is presented as an example. The coordinated use of both XPS and XAS is shown to be useful in achieving full understanding of materials systems such as electrochemically formed oxide films.

Synchrotron radiation studies of small particles and supported catalysts

Abstract: The authors used XAFS to determine the local structure around small CuO clusters on alumina, between room temperature and 400 degrees C. They measured the spectra at the copper and sulphur K edges to obtain information on the structure and chemical state of the clusters.

Characterization of thin Chemical/Native Oxides on Si (100) by Auger and Angle-Resolved XPS

Abstract: Chemical/native oxides grown on Si(100) after several standard wet cleans are characterized by Angle-resolved X-ray Photoelectron Spectroscopy (ARXPS), and Auger Electron Spectroscopy using sputter depth profiles Target Factor Analysis (TFA) was used to separate the Si LVV Auger peak into three components identified by their lineshapes and positions as Si, SiO2, and SiO2 Auger depth profiles were used to quantify the thickness of the oxides, the depth distribution, and amount of SiOx in the interface region ARXPS was used to study the chemical state distribution in the native oxides as a function of depth The depth distribution function from the Auger data was converted to an angle-resolved format for direct comparison to the angle-resolved XPS data With this comparison, the SiOx lineshape is correlated to a 3:1 mixture of Si 3+ and Si 2+ oxidation states