Search results for "amorphous"
showing 10 items of 790 documents
Molecular dynamics simulation of epitaxial growth of the Si(001) surface
1988
Abstract Molecular beam epitaxy on a Si(100) substrate has been studied using a molecular dynamics method with the Stillinger-Weber model potential. At high substrate temperature, 800 K, well ordered crystalline layers are found to grow underneath an amorphous overlayer of approximately 5 A thick. A limiting temperature for epitaxial growth is found to be 480 K, below which the growth does not produce ordered layers. When the sample deposited below 480 K is heated up to 800 K and the deposition is started again the original adatoms start to form ordered atomic layers. Thus the collisions of the deposited atoms in addition to the substrate temperature seem to play an essential role in the gr…
Role of the strain in the epitaxial regrowth rate of heavily doped amorphous Si films
2008
Solid phase epitaxial regrowth (SPER) of p -doped preamorphized Si was studied by time resolved reflectivity. Strain and dopant concentration were opportunely varied by implanting neutral (Ge) and isovalent (B, Ga) impurities in order to disentangle the two different effects on SPER. Larger SPER rate variations occurred in strained doped Si with respect to undoped samples. The generalized Fermi level shifting model was implemented to include the role of the strain and to fit the experimental data over a large range of temperature for p - and n -type doping. We introduced a charged defect, whose energy level is independent of the dopant species. © 2008 American Institute of Physics.
Cathodoluminescence decay kinetics in Ge+, Si+, O+ implanted SiO2 layers
2003
Abstract Cathodoluminescence spectral shapes and respective band decay times show no similarity between luminescence centers in different crystal and amorphous modifications of SiO2 and GeO2. On the other hand, the additionally produced red luminescence centers (650 nm) by oxygen implantation into SiO2 layers are of the same nature as in stoichiometric SiO2 and are attributed to the non-bridging oxygen hole center (NBOHC). On the other hand, the elevated blue luminescence (460 nm) in Si implanted SiO2 belongs to the silicon related oxygen deficient center (SiODC) as in stoichiometric layers also. Ge implantation into SiO2 and thermal post-annealing leads to a huge violet luminescence (400 n…
Bimodal Acidity at the Amorphous Silica/Water Interface
2015
International audience; Understanding the microscopic origin of the acid base behavior of mineral surfaces in contact with water is still a challenging task, for both the experimental and the theoretical communities. Even for a relatively simple material, such as silica, the origin of the bimodal acidity behavior is still a debated topic. In this contribution we calculate the acidity of single sites on the humid silica surface represented by a model for the hydroxylated amorphous surface. Using a thermodynamic integration approach based on ab initio molecular dynamics, we identify two different acidity values. In particular, some convex geminals and some type of vicinals are very acidic (pK…
Polymer-Functionalised Nanograins of Mg-Doped Amorphous Calcium Carbonate via a Flow-Chemistry Approach
2019
Calcareous biominerals typically feature a hybrid nanogranular structure consisting of calcium carbonate nanograins coated with organic matrices. This nanogranular organisation has a beneficial effect on the functionality of these bioceramics. In this feasibility study, we successfully employed a flow-chemistry approach to precipitate Mg-doped amorphous calcium carbonate particles functionalized by negatively charged polyelectrolytes&mdash
X-ray irradiation effects on fluorine-doped germanosilicate optical fibers
2014
International audience; We report an experimental investigation on the effects of fluorine codoping on the radiation response of Ge-doped Optical Fibers (OFs) obtained by three different drawing conditions. The OFs were irradiated with 10 keV X-rays up to 300 Mrad and studied by online Radiation-Induced-Attenuation (RIA) measurements. Confocal Micro- Luminescence (CML) and Electron Paramagnetic Resonance (EPR) were also employed to investigate the permanent radiation-induced-defects. The variation of the Germanium-Lone-Pair-Center (GLPC) and Non-Bridging- Oxygen-Hole-Centers (NBOHC) concentration with the radiation dose is investigated by CML, whereas the ones of the induced Ge(1), Ge(2) an…
Crystalline nanorods as possible templates for the synthesis of amorphous biosilica during spicule formation in Demospongiae.
2009
In tandem: High-resolution TEM shows that during the initial stages of demosponge spicule formation, a primordial crystalline structure is formed within the axial filament. The recently developed electron diffraction tomography technique (ADT) reveals that the nanorods have a layered structure that matches smectitic phyllosilicates. These intracellular nanorods have been considered as precursors of mature spicules. High-resolution microscopy shows that, during the initial stages of demosponge spicule formation, a primordial crystalline structure is formed within the axial filament. The recently developed electron diffraction tomography technique reveals that the nanorods have a layered stru…
Preface: Special Issue on Structure in Glassy and Jammed Systems
2016
This special issue presents new developments in our understanding of the role of structure in dynamical arrest and jamming. Articles highlight local geometric motifs and other forms of amorphous order, in experiment, computer simulation and theory.
To make a glass—avoid the crystal
2016
Colloidal model systems allow for a flexible tuning of particle sizes, particle spacings and mutual interactions at constant temperature. Colloidal suspensions typically crystallize as soon as the interactions get sufficiently strong and long-ranged. Several strategies have been successfully applied to avoid crystallization and instead produce colloidal glasses. Most of these amorphous solids are formed at high particle concentrations. This paper shortly reviews experimental attempts to produce amorphous colloidal solids using strategies based on topological, thermodynamic and kinetic considerations. We complement this overview by introducing a (transient) amorphous solid forming in a thoro…
Aging effects in glassy polymers: a Monte Carlo study
1996
Abstract By means of dynamic Monte Carlo simulation the physical aging of a glassy polymer melt is studied. The melt is simulated by a coarse-grained lattice model, the bond-fluctuation model, on a simple cubic lattice. In order to generate glassy freezing an energy is associated with long bonds, which leads to a competition between the energetically favored bond stretching and the local density of the melt at low temperatures. The development of this competition during the cooling process strongly slows down the structural relaxation and makes the melt freeze in an amorphous structure as soon as the internal relaxation time matches the time scale of the cooling rate. Therefore the model ex…