The binding energies, geometries, charges and electronic structures of a series of impurity atoms [H-Ar] interacting with the -U lattice in various configurations were assessed by means of density functional theory calculations. Periodic trends governing the binding energy were highlighted and related to the electronic properties of the impurity atoms, with some consideration given to the band-structure of -U. The strongest bound impurity atoms include [C, N, O] and [Si, P, S]. The general trends in the binding energy can be reproduced by a simple parameterisation in terms of the electronegativity (charge-transfer) and covalent radius (elasticity theory) of the impurity atom. The strongest bound atoms deviate from this model, due to their ability to bind with an optimum mixture of covalency and ionicity. This last point is evidenced by the partial overlap of the impurity atom p-band with the hybrid d-/f-band of -U. It is expected that the trends and general behaviour reported in this work can be extended to the interactions of impurity atoms with other metallic systems.
Supersonic molecular beam epitaxy can be fruitfully used to grow films of organic materials, controlling their degree of ordering and their morphologies. This is due to the fine control on the initial state of the compounds in the beam. Very sharp and 'tuneable' energy distributions in the beam can be achieved so that the growth is not governed by processes at thermodynamic equilibrium but by the kinetics. This allows us, in some cases, to 'force' the growth in the desired directions instead of leaving this only to the interplay between the weak interaction forces and to the equilibrium thermodynamics to drive the growth. Comparing the optical response, morphology and structure of films grown under different degrees of supersonicity of the beam we show that one can control the growth in details. The grain size and density of defects can be drastically changed by appropriately tuning the beam's parameters. X-ray diffraction and pole figure analysis of the films are reported and discussed.
A way in which thin films of hydrogenated amorphous silicon (a-Si : H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si : H, the crystallization temperature is reduced by more than 350degreesC. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si : H films (hydrogen content between 20 and 45 at. % H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 10(5) V cm(-1). We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of the resulting polycrystalline silicon.
We measured the dynamics of the fragile glass former m-toluidine in its supercooled phase by the time-resolved optical Kerr effect. Using an experimental time filtering of the internal molecular vibrations a better insight into the fast relaxation processes has been achieved. We analysed the dynamic features of these fast processes and compared them with the mode-coupling theory predictions.
This work deals with the determination of the magnitude of the fractal dimension, from the analysis of the temperature dependence of the dc conductivity in the temperature range 1.5–300 K in germanium, disordered by large fluences of fast reactor neutrons, with the radiation defect concentration making it an insulator near the metal–insulator transition. From the reduced activation energy method and the percolation theory of hopping conductivity, the dielectric constants of the samples are calculated. The localization radius is determined. The obtained fractal dimensions are found to satisfy the scaling theory. The value determined for the exponent t is 1.7.
In conventional calculations of transport in magnetic tunnel junctions, one usually assumes that the transverse momentum of the tunnelling electrons is conserved and that the itinerant electron states are orthogonal to localized states. However, in most of the junctions studied, there is diffuse scattering in both the bulk of the electrodes and the barrier so that the transverse momentum is not conserved, and there are processes that couple localized states at the electrode-barrier interface to the itinerant states in the bulk of the electrodes. While it is in principle possible to include these effects, it leads to lengthy calculations. Here we propose an approximate scheme in which we do not take explicit account of either of the effects mentioned above, but in which we calculate the tunnelling through all the states that exist at the electrode-barrier interfaces. We compare the k∥-resolved density of states and tunnelling currents across a junction in our approximate scheme with that found using the Landauer formalism in the ballistic limit.
Electrical resistivity rho, magnetic susceptibility chi, specific heat C, X-ray photoelectron spectroscopy (XPS) and X-ray and neutron diffraction measurements were made on the compound CeRhSn. The measurements reveal an enhancement of the electronic specific heat coefficient gamma = C(T)/T approximate to 160 mJ mol(-1) K-2 at about 1 K and non-Fermi-liquid-like temperature dependences rho(T) proportional to T-0.75 and chi(T) proportional to T(-0.)5 at low temperatures. At high temperatures, the physical properties indicate Kondo-intermediate-valence behaviour. The XPS measurements are consistent with a Cc valence of 3.07. The neutron diffraction measurements reveal anomalies in the planar Ce-Rh and Ce-Ce bond distances in the vicinity of 100 K which are apparently associated with the Cc valence instability in CeRhSn.
The technological relevance of structural glasses has led to increased efforts to resolve structures and to gain insight into the structure-properties relationship that can be exploited for the design of new materials. During the past few years, there have been a large number of computer simulations to model amorphous structures and to study the mechanical properties of glassy systems. Among the mechanical properties the formation and the evolution of cracks greatly change the performance of every material. The traditional approach to brittle fracture is the study of continuum models, where the stress at the crack tip in an ideally brittle material is singular; since breaking individual bonds at the crack tip is of fundamental importance for crack growth in brittle fracture, it is not clear how accurate a continuum description of brittle fracture is. In strong glasses, which are non-equilibrium systems, the properties depend on the cooling rate at which the sample has been quenched; so the structure is not statistically homogeneous and the local strength may differ from the average value; the stress threshold at which one crack starts to propagate is a random variable and depends almost entirely on the extreme-value statistics of the largest defects. Here we try to connect the theoretical strength, the ageing effects on the mechanical properties and the local fracture toughness distribution in this class of materials.
Amorphous tris(8-hydroxyquinoline) aluminium (Alq(3)) thin films were deposited on fused silica substrates by thermal evaporation. Optical absorption. emission and excitation spectra were measured at room temperature and. for the first time, at 80K, on an Alq(3) film in a spectral range extending from the near ultraviolet to the visible. Spectroscopic features were carefully investigated as a function of temperature.
Thermally stimulated luminescence and emission spectra of cerium-doped Lu2SiO5 have been measured in the temperature interval 10 K less than or equal to T less than or equal to 310 K. Eight glow peaks occur with the most intense peak exhibiting a maximum at 306 K. Data analyses show that the peaks generally obey first-order kinetics with thermal activation energies ranging from 0.085 to 0.907 eV. The relatively short lifetime (160 s) of the intense glow peak at 306 K implies that it does not make a significant contribution to the long-lived room-temperature afterglow routinely observed in this phosphor. Spectral emission of the glow peaks is dominated by Ce3+ de-excitation, although additional emission due to trace amounts of Sm3+ is observed for T < 72 K. Elastic recoil detection measurements in nominally undoped and cerium-doped Lu2SiO5 show that the doped specimen contains 3.5-4.0 times more hydrogen within the first 300 nm of the surface than does the undoped sample. However, spectral emission associated with surface hydrogen defects was not observed. Oxygen vacancies, which trap one or more electrons, are proposed as the most likely defect sites in Lu2SiO5. Perturbations of the six- and seven-oxygen-coordinated sites produce a multipeak glow curve with various activation energies and frequency factors. The glow peaks are intrinsic to the C2/c-structured lattice and are independent of the particular rare-earth dopant ion.
Metal nanocluster composite glasses (MNCGs) have been the subject of both experimental and theoretical investigation because of their peculiar optical properties. In particular, the enhanced third-order optical nonlinearity could be exploited in the all-optical switching device technology. Nevertheless. several factors such as thermal stability. wavelength tunability, response time regime, recycling frequency, laser-induced damage and optical absorption come into play in the definition of the material suitability. In this work, we present some results on MNCG films prepared by ion implantation. Nonlinear refractive index values are also presented for rf sputtered films, evidencing the presence of thermal nonlinearity in the optical response. Figures of merit that must be satisfied for ultrafast all-optical switching application of MNCGs are evaluated for this composite.
In several amorphous or nanocrystalline systems, the local heating associated with a focused laser beam of visible light induces a transformation towards ‘bulk crystal’ order. The various steps of this transformation can be monitored through the change in their Raman spectra, collected by a confocal microscope. In fact, these different steps of the evolution of the Raman spectra, reflecting different crystallization degrees, can be observed also as spatial variations by performing a Raman mapping on film samples that appear homogeneous under optical investigation. These variations, on the scale of micrometres, could reveal, in principle, the topography of the crystallinity of the film or the space variation of a ‘crystallization compliance’. These laser-induced transformations have been studied in transparent oxide films, having thickness of the order of a micrometre, obtained by different deposition techniques; here two examples are presented, concerning tungsten trioxide films made via sputtering techniques and mixed titanium oxide-vanadium oxide films deposited by a sol-gel method.
The Kelvin probe technique can be used as a very accurate tool to measure contact potential differences (CPDs). However, the experimentally observed CPD dependence on the probe- sample distance, the so- called stray capacitance effect, is a parasitic effect not explained in the simple Kelvin probe theory. It can be especially disturbing when using a local Kelvin probe or Kelvin probe microscopy as it hinders quantitative use of CPD measurements. We show that this effect can be described without the expedient of stray capacitors by taking into account the electrostatic induction of metal objects located in the vicinity of the probe- sample system. This model leads to excellent agreement with the experimental variations in the apparent value of CPD with the probe- sample distance and allows us to recover the exact CPD value.
We compare the specific heat and dielectric relaxation (T) data of two epoxy resins, poly[(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol A, with the predictions of the entropy theory of Adam and Gibbs. Specific heat data from temperature-modulated differential scanning calorimetry are used to evaluate configurational entropy S C(T) data experimentally. The systems studied show two secondary (β and γ) relaxations inside the experimentally accessible frequency window of dielectric spectroscopy related to two crossover regions along the trace of the dynamic glass transition. The analysis, bypassing the use of the Vogel-Fulcher-Tamman equation, supports a good description of the structural relaxation time made in terms of the Adam-Gibbs model in the temperature range T g < T < Tβ (or T B), with Tβ the temperature where the structural and secondary β-relaxation times tend to merge, and T B the temperature of deviation from a high-temperature Vogel-Fulcher-Tamman τ(T) dependence.
Low-OH-content silica samples having fictive temperatures in the interval 1000-1500degreesC, have been studied by small-angle X-ray scattering using synchrotron radiations both at room temperature and from 20 to 1500degreesC. The limit for zero-angle X-ray scattering intensity is analysed in term of density fluctuations. We demonstrate that density fluctuations are strongly related to structural relaxation, both depend on thermal history (i.e. the fictive temperature) of the sample, in the temperature range below T-g.
A new analysis of electronic transport in chalcogenide glasses, based on bipolaron hopping in the extended pair approximation, is presented. It is assumed that the relaxation time of the carrier when hopping has a Meyer-Neldel type of temperature dependence instead of a simple activated form. In this way, the experimental data for both dc conductivity and ac conductivity can be fitted over a wide range of temperatures, and for a number of glasses using the same set of parameters.
An analysis of the electronic properties of bulk cubic and hexagonal diamond calculated using the ab initio packages CRYSTAL98 and the Vienna Ab initio Simulation Package is presented. We apply these ab initio methods to the study of cubic diamond, including the calculation of electronic properties (such as the band structure, electronic density of states, the indirect bandgap E-g(indirect), the valence band width and the conduction band width) and mechanical properties (such as the equilibrium lattice constant a(0) and the bulk modulus B). The combination of theoretical techniques that yield results that most accurately match experiment for cubic diamond are then used to calculate the electronic properties of the hexagonal diamond polymorph.
An alternative scenario for the glass transition based on the cooperative nature of nucleation processes and the role of entropic effects is presented. The new ingredient is to relate the dissipation during the relaxation process to the release of strain energy driven by the nucleation of progressively larger cooperative spatial regions. Using an equiprobability hypothesis for the transition between different metastable configurations, we obtain a relation between the free energy dissipation rate and the size of the largest cooperative regions. Th is leads to a new phenomenological relation between the largest relaxation time in the supercooled liquid phase and the effective temperature. This differs from the classical Adam-Gibbs relation in that predicts no divergence of the primary relaxation time at the Kauzmann temperature but a crossover from fragile to strong behavior.
