Search results for "Silicon"
showing 10 items of 1391 documents
Structure and diffusion of oxygen and silicon interstitials in silicon
1999
Abstract Ab initio quantum chemical simulation of silicon interstitials and oxygen-related defects Oi, V–O2, and V–O4 in oxygen-containing silicon was performed using the embedded molecular cluster model. The defect geometry and electronic structure were studied. The migration activation energy for Oi defect was estimated as 2.73 eV at the atmospheric pressure, and 2.70, 2.68, and 1.92 eV for the lattice compressed by 0.25, 0.37, or 5.0 per cent, respectively. The activation energy of silicon interstitial is not changing with pressure. The molecular cluster used to simulate V–O4 defect with C2v symmetry was shown to have only slight deviation from D2d at atmospheric pressure, a strong devia…
Qualitative and quantitative characterization of the in vitro dehydration process of hydrogel contact lenses
2007
Purpose: To investigate the in vitro dehydration process of conventional hydrogel and silicone-hydrogel contact lens materials. Methods: Eight conventional hydrogel and five silicone-hydrogel contact lenses were dehydrated under controlled environmental conditions on an analytical balance. Data were taken at 1-min intervals and dehydration curves of cumulative dehydration (CD), valid dehydration (VD), and dehydration rate (DR) were obtained. Several quantitative descriptors of the dehydration process were obtained by further processing of the information. Results: Duration of phase I (r 2 5 0.921), CD at end of phase I (r 2 5 0.971), time to achieve a DR of � 1%/min (r 2 5 0.946) were stron…
Tetraaryl-methane analogues in group 14—V. Distortion of tetrahedral geometryin terms of through-space π–π and π–σ interactions andNMR sagging in ter…
1998
Abstract 44 members of thecompound series Ph4−nMRn (M=Si, Ge, Sn, Pb; R=o-, m-, p-Tol; n=0–4) were synthesized (15 newcompounds). The crystal structures of Ph3Sn (o-Tol) and PhSn (o-Tol)3 were determined and compared to 16 known structures. Subject to the distanced (M–C), an interplay between through-space π–π repulsion and π–σ attraction leads to either elongated or compressed tetrahedral geometry. 29 Si-, 119 Sn- and 207 Pb-NMR chemical shifts were determined in solution and in the solid state.73 Ge chemical shifts were measured only in solution. Anupfield or downfield sagging of the chemical shifts along each series is rationalized in terms of a π–σcharge transfer which is constrained by…
The spatial distribution of non-linear effects in multi-photon photoemission from metallic adsorbates on Si(1 1 1)
2001
Multi-photon excitations from thin metallic films on silicon substrates have been observed utilising photoemission electron microscopy. The photoelectrons have been excited by means of high power femtosecond laser pulses with a photon energy below the work function threshold. The strong spatial variations of the non-linear effects became directly visible in electron emission from the adsorbed thin films. Centres of enhanced photoelectron yield, so-called hot spots, were observed on the surfaces of various samples. The multi-photon electron yield of the metallic films (permalloy and lead) depends strongly on the sample topography and the photon polarisation.
Thermostimulated luminescence and electron spin resonance in X-ray- and photon-irradiated oxygen-deficient silica
2007
Abstract Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused silica were studied. It is found that thermostimulated luminescence (TSL) peaks are different for photo (7.7 eV) and X-ray excitation at 77 K. X-ray excitation produces TSL peaks at 125 and 170 K corresponding to the anneal temperatures of two types of self-trapped holes centers STH2 and STH1, respectively, detected by electron spin resonance (ESR). Oxygen-deficiency apparently increases the number of electron traps, stabilizing a larger number of STHs in the continuous defect-free silica network than is observed in similarly X-irradiated stoichiometric silica glasses. Photoexcitation of oxygen-def…
Investigation of optical and radiation properties of oxygen deficient silica glasses
1999
The deficiency of oxygen in pure silica manifests an absorption band at 5 eV as well as an absorption band of higher intensity at 7.6 eV. The band at 5 eV is associated with lone twofold-coordinated silicon centers. The nature of the main band at 7.6 eV has been studied using silica samples with different levels of oxygen deficiency. The excitation via the 7.6 eV band produces a photoelectric response as well as inner center and recombination type luminescence. Two main luminescence bands of the twofold-coordinated silicon center appear: a blue band (2.7 eV) and a UV band (4.4 eV). Induced absorption with several bands as well as thermally stimulated luminescence with complex peak structure…
MS-CASPT2 analysis of the UV thermochromism of octamethyltrisilane
2006
We interpret the reversal of the direction of the thermochromic shift of the first absorption band of peralkylated oligosilanes as the silicon chain is extended, based on multistate complete active space second-order perturbation theory (MS-CASPT2) calculations for octamethyltrisilane, Si3Me8. The observed shift is attributed to the effect of b1 distortions from ground state equilibrium geometry on vertical excited state energies and intensities. A generally contracted basis set of atomic natural orbitals (ANOs) at a ground state geometry optimized in the second-order Moller–Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-zeta basis set (cc-pVTZ)…
Photoelectron lifetime determination of Ag(1 1 1) films at the Fermi surface
2001
The electronic properties of 10 monolayers Ag(111) films deposited onto Si(111)-7 x 7 substrates at room temperature have been studied by scanning the photoelectron intensity at the Fermi level in different symmetry directions. The main features observed in these profiles correspond to Lorentzian-like peaks produced by the pass of the sp band through the Fermi level. A simple model has been developed, which connects the photoemission peak linewidth with the lifetime of photoelectrons excited from the Fermi level. The obtained inverse photoelectron lifetime values have been found to be in excellent agreement with the typical values of the Ag single crystals. These results support the fact th…
Structural and in situ vibrational study of luminescent cluster assembled silicon thin films
2006
A Low Energy Cluster Beam Deposition apparatus is employed to produce cluster assembled silicon thin films (1-500 nm thick) by using a laser vaporization source. The generated clusters are studied since their formation through time of flight mass spectra and the calculated size in the gas phase are compared with those of the deposited aggregates obtained through Dynamic Scanning Force Microscopy. The deposited material is also studied "in situ" by Raman and infrared spectroscopy. The spectra reveal that the as deposited clusters are hydrogenated with negligible amount of oxide. A comparison of the film properties before and after their air exposure shows that the exposition induces a consis…
Physical modelling of the melt flow during large-diameter silicon single crystal growth
2003
Abstract The reported investigations concern physical modelling of Czochralski growth of silicon large-diameter single crystals. InGaSn eutectic was used as a modelling liquid, employing actual criteria of the real process (Prandtl, Reynolds, Grashof numbers, etc.) and geometric similarity. A multi-channel measuring system was used to collect and process the temperature and flow velocity data. The investigations were focused on the study of heat transfer, in particular, the instability of the “cold zone” of the melt at the crystallization front.