Low-temperature synthesis of C60 thin films by ionized cluster beam deposition technique

C₆₀ films on Si(1 1 1) have been grown at low substrate temperature of 100°C by ionized cluster beam deposition technique. Fourier transform infrared (FTIR) transmission spectroscopy, Raman measurements and X-ray diffraction are employed to investigate the structure of deposited films. The results show that the acceleration voltage plays an important role in the growth of the films. As the acceleration voltage is moderate (100 V), a pristine C₆₀ film with face-centered-cubic (fcc) crystal structure can be grown. Further increase of the acceleration voltage leads to the formation of amorphous carbon (a-C) in the grown films. When the acceleration voltage is increased to 600 V, the deposited film has a complete amorphous carbon (a-C) structure.     

Growth and melting of metallic nanoclusters in glass: A review of recent investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi-or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T _m on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T _m is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established. The text was submitted by the authors in English.     

Influence of <Emphasis Type="Italic">in situ</Emphasis> photoexcitation on structure of damaged layer in GaAs (001) substrates implanted with Ar<Superscript>3+</Superscript> ions

The influence of photoexcitation on the formation of the defect structure in GaAs crystals implanted with 200 keV Ar⁺ ions at doses of 1 × 10¹³, 3 × 10¹³, and 5 × 10¹³ cm^?2 has been studied by high-resolution Xray diffractometry. It was found that photoexcitation gives rise to annihilation of radiation-induced Frenkel pairs, and, thus, decreases the residual concentration of radiation-induced point defects. It is established that amorphization of the damaged layer proceeds via the formation and growth of clusters of radiation-induced point defects. The vacancy-and interstitial-type clusters are spatially separated—the former are located closer to the crystal surface than the latter. Photoexcitation hinders cluster growth and stimulates diffusion of interstitial defects into the substrate depth.     

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol-gel route

The objective of this study is to understand the effect of low temperature sol-gel synthesis on the microstructural properties of lithium [xLi₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] and silver [xAg₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state ⁷Li and ²⁹Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, ⁷Li NMR shows quadrupolar satellite transitions attributed to LiNO₃, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with ²⁹Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag-L-III XANES spectral studies show a local environment similar to AgNO₃ for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li₂O-0.7SiO₂ composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li⁺ mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H⁺ mobility. Variation in conductivity is explained qualitatively using existing theoretical models.     

An interface clusters mixture model for the structure of amorphous silicon monoxide (SiO)

The present state of research on the structure of amorphous silicon monoxide (SiO) is reviewed. The black, coal-like modification of bulk SiO is studied by a combination of diffraction, microscopy, spectroscopy, and magnetometry methods. Partial radial distribution functions of SiO are obtained by X-ray, neutron and electron diffraction. Disproportionation of SiO into Si and SiO₂ is verified. High resolution TEM gives an upper limit of less than 2 nm for the typical Si cluster size. The Si K-edge electron energy-loss near-edge structure (ELNES) data of SiO are interpreted in terms of the oxidation states Si⁴⁺ and Si⁰. X-ray photoelectron spectroscopy gives first details about possible stoichiometric inhomogeneities related to internal interfaces. The wipe-out effect in the ²⁹Si MAS NMR signal of SiO is confirmed experimentally. The new estimation of the wipe-out radius is about 1.1 nm. First-time W-band, Q-band, and X-band ESR and SQUID measurements indicate an interfacial defect structure. Frequency distributions of atomic nearest-neighbours are derived. The interface clusters mixture model (ICM model) suggested here describes the SiO structure as a disproportionation in the initial state. The model implies clusters of silicon dioxide and clusters of silicon surrounded by a sub-oxide matrix that is comparable to the well-known thin Si/SiO₂ interface and significant in the volume because of small cluster sizes.     

Algorithms for solving atomic structures of nanodimensional clusters in single crystals based on X-ray and neutron diffuse scattering data

New algorithms for solving the atomic structure of equivalent nanodimensional clusters of the same orientations randomly distributed over the initial single crystal (crystal matrix) have been suggested. A cluster is a compact group of substitutional, interstitial or other atoms displaced from their positions in the crystal matrix. The structure is solved based on X-ray or neutron diffuse scattering data obtained from such objects. The use of the mathematical apparatus of Fourier transformations of finite functions showed that the appropriate sampling of the intensities of continuous diffuse scattering allows one to synthesize multiperiodic difference Patterson functions that reveal the systems of the interatomic vectors of an individual cluster. The suggested algorithms are tested on a model one-dimensional structure.     

In situ synthesis of Ag–Cu bimetallic nanoparticles in silicate glass by a two-step ion-exchange route

Ag–Cu bimetallic nanoparticle/silicate glass composites were fabricated by means of a two-step ion-exchange and subsequently thermal treatment. Optical absorption spectroscopy and transmission electron microscopy (TEM) were used to study the influence of preparation conditions on the formation of Ag–Cu bimetallic nanoparticles in silicate glass. The results show that the volume fraction of copper nanoparticles increases with ion-exchange duration when keeping the same annealing temperature and duration in hydrogen atmosphere during the first stage of Cu doped glass. The Cu doped glass is beneficial to the formation of Ag nanoparticles during subsequent Ag⁺ for Na⁺ ion exchange. Extending Cu⁺ for Na⁺ ion exchange duration makes against the introduction of Ag ions into the Cu doped glass and the formation of Ag nanoparticles in the glass matrix. A few Cu₂O nanoparticles were found in the glass matrix due to the oxidization of small Cu nanoparticles.     

X-ray RDF analysis of 1500 Å thick amorphous films

This paper reports the development of a grazing incidence x-ray scattering (GIS) technique which makes possible x-ray diffraction analysis of thin (>100 Å, thus far) amorphous or highly disordered films on substrates. In contrast to earlier x-ray and electron diffraction techniques, it is not necessary to remove the sample from the substrate to obtain radial distribution functions (RDF's) or differential distribution functions (DDF's). This paper discusses the scattering geometry used for these measurements, the application to very thin amorphous films and our results from 1500 Å thick films of evaporated a-GeSe₂, Ag photo-diffused films and sputtered a-Ag-GeSe₂ films. Thin film techniques are necessary for this material because most of the photo-diffused Ag is within 1500 Å from the surface. These will be compared to results from “bulk” glasses of similar compositions prepared by both sputtering and quenching from the liquid.     

X-ray analysis of Self-Organized InAs/InGaAs Quantum Dot Structure

We report on an X-ray study of an InAs/InGaAs/GaAs multi quantum dot stack grown by metalorganic chemical vapor deposition using grazing incidence reflectometry, high-resolution X-ray diffraction, reciprocal space mapping and pole figure analysis. No direct signal from the quantum dots is found by the high-resolution techniques. All rocking curves on different symmetric and asymmetric Bragg reflections can be simulated within the framework of dynamical theory assuming a perfect tretragonally distorted InAs/InGaAs/GaAs multiquantum well system. A pole figure analysis in the vicinity of the (113) and (022) reflections, however, reveals a signal from the quantum dots. There is a considerable indium enrichment in the quantum dots as compared to the wetting layer indicating a strong In-diffusion during their formation. Moreover, a strongly anisotropic diffuse scattering distribution with respect to the [110] and [1-10] directions is found.     

Properties of Si-rich SiO2 films by RF magnetron sputtering

Si-rich silicon oxide (SiO_x, 1<x<2) films were prepared by RF magnetron reactive sputtering or co-sputtering on the Si(1 1 1) substrates. X-ray diffraction patterns showed that the peak of silicon nanocrystals (NCs), separated from SiO_x films, had (1 1 1) preferred orientation. The results of scanning electron microscopy indicated the Si NCs uniting into clusters. We demonstrated that the photoluminescence (PL) peaks at 650 nm was caused by defect center. In particular, we discussed the correlation between the PL and the structure of SiO_x films. The mean size of the Si NCs was estimated to be about 3 nm by the PL peak position.     

Influence of Bi1−xSbx and Sb overlayers on the annealing behaviour of Ag films

Annealing of thin Ag films covered by Bi_1−xSb_x (x = 0.09) and Sb overlayers, respectively, has been investigated by measuring the electrical resistance. Material exchange by diffusion leading to the formation of an intermetallic phase between Ag and Sb was found to be mainly responsible for the annealing effects. By X-ray and electron diffraction the intermetallic compound has been identified as the ε-phase Ag₃Sb. The diffusion processes due to annealing were studied by SIMS. A correlation between the measured resistance and electron micrographs of the cross-sectional film structure was found.     

Structure of the molten silver chloride

The structure of molten AgCl has been studied by neutron and X-ray diffractions. Significant differences in the neutron and X-ray total structure factors have been observed, reflecting different contributions of three partial structures Ag–Ag, Ag–Cl and Cl–Cl. By means of the reversed Monte Calro (RMC) structural modeling and analysis, partial structures have been calculated and then compared with the earlier work [J.Y. Derrien and Dupuy, J. Phys. Chem. Liq. 5 (1976) 71]. The results show that Cl–Cl correlations are similar to those for a close packed structure. Ag–Ag correlations are gas-like and the local structure of unlike pairs is similar to those for a rock salt type structure with vacancy sites.     

Effect of X-ray irradiation on solarization and crystallization of photosensitive glasses containing Ce, Sb, Sn and Ag

The effect of X-ray irradiation on the solarization and crystallization of a lithium silicate glass was investigated in this research. The results showed that the X-ray power has a significant effect on the amount of solarization, and the crystallization temperature of the glass. Applying an X-ray power of 2400 W on glass containing the photosensitive elements of Ce, Sb, Sn and Ag led to reduction of the DTA crystallization peak temperature from 704 to 590 °C. UV spectroscopy showed that solarization of irradiated glasses containing Ce, Sb and Sn ions was responsible for formation of Ag clusters and reduction of crystallization temperature of glasses. Microstructural analysis also showed that the solarized glasses had a finer microstructure relative to that of the non-irradiated one. These differences attributed to changes in crystallization mechanism observed in the non-irradiated glass.     

Cluster self-organization of silicate and germanate systems: Invariant suprapolyhedral cluster precursors and self-assembly of the crystal structures of Li,TR silicates (germanates)

The initial stages of formation of suprapolyhedral clusters (containing polyhedra of different types) in an evolving chemical system are considered. The suprapolyhedral clusters of the chain and cyclic types are used for modeling two-dimensional periodic structures. The developed model is used to search for cluster precursors in the structures of Li,TR silicates (germanates) of the known structure types. The complete threedimensional reconstruction of the self-assembly of Li,TR silicates (germanates) is performed using computer methods (with the TOPOS program package) according to the following scheme: cluster precursor → primary chain → microlayer → microframework (supraprecursor) → … framework. Two types of invariant cyclic cluster precursors composed of the TR polyhedra linked by tetrahedra and the TR polyhedra joined by diorthotetrahedra are identified in five and two structures, respectively. It is revealed that the lithium atoms are located at the centers of all clusters. New types of two-dimensional nets with a hierarchical structure formed as a result of the packing of cyclic four-, six-, and eight-node clusters are described.     

Synthesis and characterization of type-II textured tungsten disulfide thin films by vdWR process with Pb interfacial layer as texture promoter

The growth of type-II textured tungsten disulfide (WS₂) thin films by solid state reaction between the spray deposited WO₃ and gaseous sulfur vapors with Pb interfacial layer has been studied. X-ray diffraction (XRD) technique is used to measure the degree of preferred orientation ‘S’ and texture of WS₂ films. Scanning electron microscopy (SEM) and transmission electron microscopy techniques have been used to examine the microstructure and morphology. The electronic structure and chemical composition were studied using X-ray photoelectron spectroscopy (XPS). The use of Pb interfacial layer for the promotion of type-II texture in WS₂ thin films is successfully demonstrated. The presence of (0 0 3 l), (where l=1, 2, 3, …) family of planes in the XRD pattern indicates the strong type-II texture of WS₂ thin films. The crystallites exhibit rhombohedral (3R) structure. The large value of ‘S’ (1086) prompts the high degree of preferred orientation as well. The stratum of crystallites with their basal plane parallel to the substrate surface is seen in the SEM image. The EDS and XPS analyses confirm the tungsten to sulfur atomic ratio as 1:1.75. We purport that Pb interfacial layer enhances type-II texture of WS₂ thin films greatly.     

Glass-formation and crystallization processes in Ag–Y–Cu alloys

Investigation of glass-formation and crystallization processes of several alloys of Ag–Y–Cu system was conducted. The samples were produced by melt spinning and Cu-mold casting. The structure of ribbon samples was examined by X-ray diffractometry and transmission electron microscopy. Phase transformations were studied by differential scanning and isothermal calorimetry and differential thermal analysis. Despite the large difference in atomic radii between the components in the investigated system, even being produced by melt spinning technique at high cooling rate, only Ag₆₁Y₂₉Cu₁₀ alloy with large supercooled liquid region was found to be X-ray amorphous. A principle for obtaining an alloy with high glass forming ability involving difference in atomic radii between the alloy components is considered. Efficient cluster packing model was also tested.     

A Model Study of the Growth of Crystals Consisting of a Small Number of Metal Atoms by the Hückel Molecular Orbital Method (II) Addition of One Atom to (N + 1)-atomic Clusters

The regularities of the formation of (N + 2)-atomic clusters obtained by the growth of (N + 1)-atomic clusters and the distribution of the (N + 2)-atomic clusters according to their stability are studied. The most stable (N + 2)-atomic cluster is not always obtained from the most stable (N + 1)-atomic cluster. Sometimes it is formed by the least stable (N + 1)-atomic cluster. A necessary condition for the appearance of the most stable (N + 2)-atomic cluster is the formation of a bond between the N + 2nd and the N + 1st atoms. This means that the most stable (N + 2)-atomic cluster may be considered as obtained directly from the N-atomic crystal by jumping over the stage of the (N + 1)-atomic cluster and adding a dimer suitably oriented with respect to the N-atomic crystal. The least stable (N + 2)-atomic cluster is always obtained from the least stable (N + 1)-atomic cluster.     

Arrangement of K Clusters in the K-Doped Zeolite K-LTA

Abstract

K clusters incorporated into zeolite K-LTA causes ferromagnetism below 8 K. Distribution of K⁺ ions in its α-cages was studied by X-ray powder diffraction. On the basis of space group F23, electron-density distribution was determined by Rietveld analysis followed by iteration of analysis by a maximum-entropy method and whole-pattern fitting. K⁺ ions were located in α-cages in such a way that K clusters between adjacent α-cages are not equivalent to each other, which will lead to the ferromagnetism with antiferromagnetic components.     

Optically-induced darkening and crystallization in amorphous Ag–Sb–S films

The amorphous films of Sb₃₃S₆₇ and Ag_x(Sb_0.33S_0.67)_100−x composition, where x was between 0 and 25 at.% Ag, were prepared by different techniques, i.e. by vacuum evaporation, optically-induced silver dissolution into the binary Sb–S chalcogenides deposited by vacuum evaporation or by spin-coating techniques. Ternary Ag–Sb–S amorphous films were also prepared directly by pulsed laser deposition. We have studied effect of silver content in the host materials and Ar⁺ ion laser exposure on optical properties and structure of the films. The optically-induced darkening and crystallization was proved in studied films and their potential application in optical memories could be expected.     

Atomic order in magnetic Mn inclusions in Si crystals: XAS and TEM studies

A Si (1 0 0) crystal implanted with Mn⁺ ions, exhibiting ferromagnetic properties, was studied with XAS and TEM to examine the local atomic order around Mn atoms. The advantage of the XAS technique was its elemental selectivity, which allowed extracting information on the atomic surroundings of Mn atoms even at very low concentrations of this element, incapable of producing a signal in X-ray diffraction. It is very important to find out what is responsible for the ferromagnetism of this new class of materials. The knowledge of the location of Mn atoms in the Si crystal lattice is crucial in developing models of ferromagnetic interactions. The performed studies have proven beyond doubt that Mn atoms are not located randomly in the Si crystalline matrix but form Mn–Si clusters immersed in a strained Si matrix. Assuming the atomic order and dimensions of the cluster found through EXAFS and HRTEM, we have reproduced the electronic structure of Mn atoms by modeling the XANES spectrum in agreement with the experimental one.