Short-range Crystalline Order Research Articles

Insight on MWW siliceous zeolites: From 2D precursors toward 3D structure

ITQ-1 layered zeolites, with MWW framework, were prepared using different synthesis and calcination procedures and fully characterized by means of X-ray diffraction, scanning electron microscopy, N2 sorption and vibrational and nuclear magnetic resonance spectroscopies. Exploring different compositions of the precursor gel, the role of the Organic Structure Directing Agent (OSDA) was definitely disclosed. We proved that the concentration of OSDA in the synthesis gel affects the short-range crystalline order of zeolite crystals. In particular, diluted precursor gels led to the formation of low-density materials in the form of hollow spheres, with partially disordered layers, thinner crystals and high defectivity. In general, all ITQ-1 samples had the same crystalline structure but different morphology, either rose-like or hollow spheres. Moreover, the calcination procedure had an impact on the structural defects of the ITQ-1 zeolite: the higher the removal rate of the OSDA, the higher the defectivity degree of the zeolite.

Berry curvature contributions of kagome-lattice fragments in amorphous Fe–Sn thin films

Amorphous semiconductors are widely applied to electronic and energy-conversion devices owing to their high performance and simple fabrication processes. The topological concept of the Berry curvature is generally ill-defined in amorphous solids, due to the absence of long-range crystalline order. Here, we demonstrate that the Berry curvature in the short-range crystalline order of kagome-lattice fragments effectively contributes to the anomalous electrical and magneto-thermoelectric properties in Fe–Sn amorphous films. The Fe–Sn films on glass substrates exhibit large anomalous Hall and Nernst effects comparable to those of the single crystals of topological semimetals Fe3Sn2 and Fe3Sn. With modelling, we reveal that the Berry curvature contribution in the amorphous state likely originates from randomly distributed kagome-lattice fragments. This microscopic interpretation sheds light on the topology of amorphous materials, which may lead to the realization of functional topological amorphous electronic devices.

The influence of Sb doping on the local structure and disorder in thermoelectric ZnO:Sb thin films

Thermoelectric transparent ZnO:Sb thin films were deposited by magnetron sputtering, with Sb content varying between 2 and 14 at%. As evidenced by X-ray diffraction analysis, the films crystallize in the ZnO wurtzite structure for lower levels of Sb-doping, developing a degree of amorphization for higher levels of Sb-doping. Temperature-dependent (10–300 K) X-ray absorption spectroscopy studies of the produced thin films were performed at the Zn and Sb K-edges to shed light on the influence of Sb doping on the local atomic structure and disorder in the ZnO:Sb thin films. The analysis of the Zn K-edge EXAFS spectra by the reverse Monte Carlo method allowed to extract detailed and accurate structural information in terms of the radial and bond angle distribution functions. The obtained results suggest that the introduction of antimony to the ZnO matrix promotes static disorder, which leads to partial amorphization with very small crystallites (∼3 nm) for large (12–14 at%) Sb content. Rutherford backscattering spectrometry (RBS) experiments enabled the determination of the in-depth atomic composition profiles of the films. The film composition at the surfaces determined by X-ray photoelectron spectroscopy (XPS) matches that of the bulk determined by RBS, except for higher Sb-doping in ZnO films, where the concentration of oxygen determined by XPS is smaller near the surface, possibly due to the formation of oxygen vacancies that lead to an increase in electrical conductivity. Traces of Sb–Sb metal bonds were found by XPS for the sample with the highest level of Sb-doping. Time-of-flight secondary ion mass spectrometry obtained an Sb/Zn ratio that follows that of the film bulk determined by RBS, although Sb is not always homogeneous, with samples with smaller Sb content (2 and 4 at% of Sb) showing a larger Sb content closer to the film/substrate interface. From the optical transmittance and reflectance curves, it was determined that the films with the lower amount of Sb doping have larger optical band-gaps, in the range of 2.9–3.2 eV, while the partially amorphous films with higher Sb content have smaller band-gaps in the range of 1.6–2.1 eV. Albeit the short-range crystalline order (∼3 nm), the film with 12 at% of Sb has the highest absolute Seebeck coefficient (∼56 μV/K) and a corresponding thermoelectric power factor of ∼0.2 μW·K−2·m−1.

Influence of Size Purification and Self-Assembly on the Photoluminescence of Silicon Nanocrystal Ensembles

Silicon nanocrystals (SiNCs) synthesized through a nonthermal plasma reaction are purified through density-gradient ultracentrifugation in mixed organic solvents to achieve highly monodisperse fractions. These fractions show a reduction in photoluminescent (PL) linewidth, increased band alignment, and the emergence of short-range crystalline order in dried films. The films display a significant PL enhancement that we suggest arises from nanocrystal interactions in densely packed arrays and we exploit this to achieve photostable emission. We also explore how this PL enhancement emerges, from the scale of individual nanocrystals up to clusters of more than a thousand, and we examine the influence of different polymers on the self-assembly and PL stability of ensemble structures.

Influence of Size Purification and Self-Assembly on the Photoluminescence of Silicon Nanocrystal Ensembles

Silicon nanocrystals (SiNCs) synthesized through a nonthermal plasma reaction are purified through density-gradient ultracentrifugation in mixed organic solvents to achieve highly monodisperse fractions. These fractions show a reduction in photoluminescent (PL) linewidth, increased band alignment, and the emergence of short-range crystalline order in dried films. The films display a significant PL enhancement that we suggest arises from nanocrystal interactions in densely packed arrays and we exploit this to achieve photostable emission. We also explore how this PL enhancement emerges, from the scale of individual nanocrystals up to clusters of more than a thousand, and we examine the influence of different polymers on the self-assembly and PL stability of ensemble structures.

Dry tribology and nanomechanics of gaseous flame soot in comparison with carbon black and diesel soot

Ethylene gas is burnt and the carbon soot particles are thermophoretically collected using a home-built equipment where the fuel air injection and intervention into the 7.5-cm long flame are controlled using three small pneumatic cylinders and computer-driven controllers. The physical and mechanical properties and tribological performance of the collected soot are compared with those of carbon black and diesel soot. The crystalline structures of the nanometric particles generated in the flame, as revealed by high-resolution transmission electron studies, are shown to vary from the flame root to the exhaust. As the particle journeys upwards the flame, through a purely amorphous coagulated phase at the burner nozzle, it leads to a well-defined crystalline phase shell in the mid-flame zone and to a disordered phase consisting of randomly distributed short-range crystalline order at the exhaust. In the mid-flame region, a large shell of radial–columnar order surrounds a dense amorphous core. The hardness and wear resistance as well as friction coefficient of the soot extracted from this zone are low. The mechanical properties characteristics of this zone may be attributed to microcrystalline slip. Moving towards the exhaust, the slip is inhibited and there is an increase in hardness and friction compared to those in the mid-flame zone. This study of the comparison of flame soot to carbon black and diesel soot is further extended to suggest a rationale based on additional physico-chemical study using micro-Raman spectroscopy.

Medium-range order in amorphous silicon investigated by constrained structural relaxation of two-body and four-body electron diffraction data

The structures of four types of amorphous silicon are examined by an experimentally constrained structural relaxation method (ECSR). Experimental selected area electron diffraction data and fluctuation electron microscopy normalized diffraction variance data were used as constraints to guide a Monte Carlo relaxation procedure towards best fit models. A Tersoff potential was also used to further restrict the space of possible solutions. The materials examined were self-ion-implanted silicon and pressure-amorphized silicon, both in their as-prepared and thermally annealed states. In the fitted models for these materials regions containing two types of medium-range order were identified. One type involves formation of paracrystallites with cubic and hexagonal structures, where both short-range crystalline and medium-range order are present. The other type of medium-range order appears in the form of extended crystalline planes without associated short-range crystalline order. These two types can coexist. It is observed that the best fit models for both as-prepared samples contain approximately 10–15% paracrystalline ordered regions, reducing to about 5–10% in the annealed materials. None of the models are true continuous random networks. We conclude that, with long computational times and with a suitable potential function, the ECSR procedure provides a powerful, although at present semi-quantitative, tool for determining the structural form of medium-range order in thin amorphous materials.

Overlayer Surface-Enhanced Raman Spectroscopy for Studying the Electrodeposition and Interfacial Chemistry of Ultrathin Ge on a Nanostructured Support

Ultrathin films of germanium (Ge) have been electrodeposited onto surface-enhanced raman spectroscopy (SERS)-active, polycrystalline gold (Au) nanoparticle film electrodes from aqueous solutions containing dissolved GeO2. An overlayer SERS strategy was employed to use the SERS-activity of the underlying Au electrode to enhance the Raman signatures separately for the Ge phonon mode and vibrational modes of surface groups. Electrochemical and spectroscopic data are presented that demonstrate monolayer-level detection of the electrodeposited material and the preparation of crystalline Ge films exhibiting quantum-confinement effects. Potential-dependent Raman spectra are shown that identify electrodeposition conditions where Ge films can be deposited with either long- or short-range crystalline order. Raman spectra collected with electrodeposited Ge films immersed in solutions containing CN-(aq) did not indicate a significant presence of pinholes that exposed the underlying Au(s) substrate. Raman spectra were also collected that identified a potential-dependence for Ge hydride formation at the interface of these films. Separate spectra were collected for the oxidative dissolution of Ge in solution and the complete dry oxidation of Ge to GeOx in air. These data sets cumulatively represent the first demonstration of the overlayer SERS strategy to follow surface chemical processes at crystalline, nanostructured, Ge materials in situ and in real time.

Vortex pinning in Nb thin films modulated by nanospheres

Superconducting Nb thin films deposited on top of self-assembled spherical nanoparticles have been investigated by transport measurements. Colloid polystyrene nanospheres have been organized by convective self-assembly into colloid crystals. These crystals serve as modulation templates for the Nb thin films which are deposited on top. These periodic arrays provide a natural topographic pattern to modulate laterally the superconducting order parameter in deposited Nb films. The nanoparticle diameter effectively controls the matching fields and flux pinning properties of these films. The superconducting phase boundary and matching effects have been studied for Nb films grown on the templates formed by polystyrene particles with diameter d=202 and d=357nm. The triangular colloid lattice geometry induces strong modulation in thin Nb layers. In combination with the short range crystalline order of the templates, this leads to geometrical vortex ordering effects and vortex density dependent matching phenomena.

Enhancement of Phototransduction Protein Interactions by Lipid Surfaces

The G protein cascade of vision depends on two peripheral membrane proteins: the G protein, transducin (G(t)), and cGMP phosphodiesterase (PDE). Each has covalently attached lipids, and interacts with transduction components on the membrane surface. We have found that their surface interactions are critically dependent on the nature of the lipid. Membranes enhance their protein-protein interactions, especially if electrostatic attraction is introduced with positively charged lipids. These interactions are less enhanced on highly curved surfaces, but are most enhanced by unsaturated or bulky acyl chains. On positively charged membranes, G(t) assembles at a high enough density to form two-dimensional arrays with short-range crystalline order. Cationic membranes also support extremely efficient activation of PDE by the GTPgammaS (guanosine 5'-O-(thiotriphosphate)) form of Galpha(t) (Galpha(t)-GTPgammaS), minimizing functional heterogeneity of transducin and allowing activation with nanomolar Galpha(t)-GTPgammaS. Quantification of PDE activation and of the amount of Galpha(t)-GTPgammaS bound to PDE indicated that G(t) activates PDE maximally when bound in a 1:1 molar ratio. No cooperativity was observed, even at nanomolar concentrations. Thus, under these conditions, the one binding site for Galpha(t)-GTPgammaS on PDE that stimulates catalysis must be of higher affinity than one or more additional sites which are silent with respect to activation of PDE.

The hydrogen diffusion in disordered systems

This paper analyzes the experimental data and presents a critical review of the existing approaches to the description of hydrogen diffusion in disordered amorphous alloys. It is noted that the available theories ignore the role of the short-range order in hydrogen diffusion. A diffusion model, which is based on the approach developed by us earlier, has been proposed for disordered crystalline alloys. In terms of this model specific features of the amorphous state are allowed for by homogeneous local distortions of voids, i.e. a kind of frozen fluctuations of the free volume. General expressions for the chemical diffusion coefficient of hydrogen in amorphous metals and binary alloys having FCC-like short-range crystalline order have been derived. It was shown that the diffusion coefficient may depend on a structural characteristic of disordered systems—the mean square of static displacement of atoms in the vicinity of a void—and also on the paired radial distribution function of atoms. The analysis of the proposed model suggests that the crystalline disorder causes an increase in the diffusion coefficient, which grows (unlike in crystals) linearly with the hydrogen concentration.

Numerical studies of the phase diagram of layered type-II superconductors in a magnetic field

We report on simulations of layered superconductors using the Lawrence-Doniach model in the framework of the lowest-Landau-level approximation. We find a first-order phase transition with a $B(T)$ dependence which agrees very well with the experimental ``melting'' line in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ The transition is not associated with vortex lattice melting, but separates two vortex liquid states characterized by different degrees of short-range crystalline order and different length scales of correlations between vortices in different layers. The transition line ends at a critical end point at low fields. We find the magnetization discontinuity and the location of the lower critical magnetic field to be in good agreement with experiments in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ Length scales of order parameter correlations parallel and perpendicular to the magnetic field increase exponentially as $1/T$ at low temperatures. The dominant relaxation time scales grow roughly exponentially with these correlation lengths. The consistency of our numerical results with various experimental features in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ including the dependence on anisotropy, and the temperature dependence of the structure factor at the Bragg peaks in neutron scattering experiments is demonstrated.

Numerical investigation of the dynamics of a thin-film type-II superconductor with and without disorder

The equilibrium dynamics of a thin film type II superconductor with spherical geometry are investigated numerically in a simulation based on the lowest Landau level approximation to the time-dependent Ginzburg-Landau equation. Both the static and time-dependent density-density correlation functions of the superconducting order parameter have been investigated. As the temperature is lowered the correlation length, the length-scale over which the vortices have short-range crystalline order, increases but the introduction of quenched random disorder reduces this correlation length. We see no signs of a phase transition in either the pure or the disordered case. For the disordered system there is no evidence for the existence of a Bragg glass phase with quasi long-range correlations. The dynamics in both the pure and disordered systems is activated, and the barrier of the relaxation mechanism grows linearly with the correlation length. The self-diffusion time scale of the vortices was also measured and has the same temperature dependence as that of the longest time scales found in the time dependent density-density correlation function. The dominant relaxation mechanism observed is a change in orientation of a correlated region of size of the correlation length. A scaling argument is given to explain the value of the barrier exponent.

Simple Ginzburg-Landau Theory for Vortices in a Crystal Lattice

We study the Ginzburg-Landau model with a nonlocal quartic term as a simple phenomenological model for superconductors in the presence of coupling between the vortex lattice and the underlying crystal lattice. In mean-field theory, our model is consistent with a general oblique vortex lattice ranging from a triangular lattice to a square lattice. This simple formulation enables us to study the effect of thermal fluctuations in the vortex liquid regime. We calculate the structure factor of the vortex liquid nonperturbatively and find Bragg-like peaks with fourfold symmetry appearing in the structure factor even though there is only a short-range crystalline order.

Thermal conductivity of hard carbon prepared from C 60 fulleren

We report measurements of thermal conductivity in 30–350 K range of hard fullerene-based carbon. The material has been prepared from C 60 fullerene under pressure and has an unusual combination of large hardness and relatively high electrical conductivity. Its thermal conductivity is about 5.5 W/mk at room temperature and decreases almost linearly in the investigated temperature range. The data obtained bear resemblance to the thermal properties of amorphous materials. It is consistent with the structural investigation that allows one to suggest the existence of short-range crystalline order in this transformed substance.

Origin of the extended fine structure in the secondary-electron-emission spectrum of graphite.

An extended fine structure (EFS) has been observed in the electron-excited secondary-electron-emission (SEE) spectrum of graphite. Measurements were performed in the 0--150-eV electron kinetic range as a function of primary-electron energy above and below the threshold for carbon core-level 1s ionization. For primary electron energies of 200, 500, and 1000 eV the spectra display a well-defined fine structure that does not depend strongly on the primary electron energy. However, a dependence of the spectrum on orientation of the sample with respect to the primary electron beam and the electron analyzer was found. The effect of structural disorder on the EFS-SEE spectrum of graphite was investigated by performing in situ measurements as a function of the 2-keV Ar-ion irradiation dose. It was found that the EFS-SEE is very sensitive to imperfections created by the ion beam for very low dose levels of ion irradiation. The sensitivity of the spectrum to short-range crystalline order was assessed by comparing the spectrum obtained for graphite to that of glassy carbon. It is concluded that the EFS-SEE of graphite originates from interband electronic excitation processes involving excitations of valence electrons by primary electrons and secondary electrons of sufficient energy into unoccupied electronic states of high density in the graphite crystal field. This conclusion is supported by comparison with other electron spectroscopic measurements of graphite.

Properties of Damaged Starch Granules. II. Crystallinity, Molecular Order and Gelatinisation of Ball-milled Starches

Wheat and maize starches were ball milled to obtain various levels of damaged starch, with corresponding losses of short-range crystalline order (measured by wide-angle X-ray diffraction) and double-helix content (measured by 13C-CP/MAS-NMR) in the dry starches. Regression analyses indicated that all damaged starch in the dry wheat and maize starches was amorphous. Order in damaged maize starches could be restored by wetting and drying the damaged starches. On wetting, the damaged portions of the granules comprised soluble material and swollen gel that were quantified together as damaged starch using an enzymic assay. The undamaged portion comprised surviving native starch granules and birefringent remnants of larger granules that had been partially damaged. Both the enthalpy and temperature of gelatinisation of the starches, and of the recovered native granules plus remnant material, decreased with increasing levels of starch damage. Starches isolated from millstreams showed the same pattern of behaviour. A model is presented to explain the observed patterns of damage. It is concluded that amorphous starch within the native granules (which does not include crystalline amylopectin) should be distinguished from amorphous damaged starch since they differ in composition and in their physical properties.

Finite-size effects and short-range crystalline order in Si and SiO2 studied by X-ray absorption fine structure spectroscopy

The X-ray absorption near-edge structures from the Si 2p edge have been measured at high resolution, and compared to a theoretical calculation using a cluster multiple-scattering model. The absorption features near threshold are shown to be sensitive to the degree of short-range crystalline order, over a range of up to 40 AA. The real-space scattering cluster calculation establishes the connection between the short-range order, as reflected in cluster size or final-state electron damping, and the line-widths of sharp absorption features near the 2p edge. Results are shown for amorphous and crystalline Si, and silicon oxides in the form of quartz, obsidian, and thin films on single-crystal Si substrates.

Effect of quenched disorder on a two-dimensional classical Wigner crystal

The effect of quenched random fields of different types on translational and orientational order and on dislocation melting of a 2D classical Wigner crystal is considered. Isotropie impurities interact only with the longitudinal plasma mode of a 2D Wigner solid and do not destroy, in the harmonic phonon approximation, the 2D translational order for potentials of the lattice-impurity interaction decaying as r −1 or faster. Charged Coulomb impurities embedded in a 2D crystal, if their concentration does not exceed the critical value, may induce the low-temperature reentrant melting. Ionized remote donors destroy the solid phase, unless charge fluctuations are strongly suppressed. For the substrate-generated anisotropic random pinning force there is only a short-range crystalline order, though the sizes of the crystalline regions may be arbitrarily large.

Nature of the pulsed laser process for the deposition of high T c superconducting thin films

The pulsed laser thin-film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry. The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substrates.The factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material. In addition, x-ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short-range crystalline order in the films. Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process. Instead, binary and ternary suboxides are emitted from the target pellet. These suboxides most likely have unit sticking coefficient to the substrate which could contribute to the preservation of the film stoichiometry. The velocity distribution of several neutral species (e.g., BaO) indicates that particles have energies of several eV. Thus the effective temperatures of the emitted species are ∼15×103 K, and these energetic particles may facilitate growth of the crystalline films at low substrate temperatures.