Transitions In Si Research Articles

The impact of carbon material microstructure on li-ion storage behaviors of Si in Si/C anodes

Due to the demand for high mass loading in industrial application, micron-sized Si/C composites are preferred compared to nanoparticles. Utilizing Al-Si alloy as the raw material, an ant's nest-like porous Si/C composite material was successfully constructed through carbon coating and acid etching techniques, leading to an improvement in the cyclic stability of micron-sized Si/C composites under high mass loading. Furthermore, notable observations were made regarding the influence of carbon material type on the independent charge–discharge behavior of Si within the Si/C composite material, especially during the plateau region of Si-Li conversion processes. In-situ XRD test revealed transitions of Si between crystalline and amorphous states in the Si/C composite material during charge and discharge processes. Additionally, in Si/C composite materials fabricated using hard carbon, the increased lithium-ion transport rate attributed to the highly disordered structure of hard carbon promotes the acquisition of charges during delithiation process. As a result, this decelerates the lithium de-alloying process in Si-Li alloys, leading to an improved conversion depth of amorphous Si-Li alloys and a reduction in electrode capacity decay. This study provides valuable insights into Si conversion processes within Si/C composite materials and offers rational strategies for future material optimization.

Multiple structural phase transitions in single crystal silicon subjected to dynamic loading

Single crystal Silicon (SC Si), as one of the most extensively used elements, has a series of complex phase changes, however, it is not fully understood under dynamic loading conditions. In this letter, both the shock induced multiple solid-solid phase transformations and solid-liquid transition are systematically investigated using large-scale molecular dynamics (MD) simulations. With the increase of particle velocities up to 2.8 km/s, the SC Si is revealed to undergo a continuous structural phase transitions from cubic diamond (cd) to β-tin, bct5, near-simple cubic (sc), and body centered cubic (bcc) structure. Furthermore, the liquid Si is distinguished and shows solid-liquid coexistence under intensive shocks. Our work dissects the continues multiple phase transitions in shocked Si for the first time in MD simulations, which provides new understandings of phase transitions of Si under dynamic loadings, and strongly supports and complements the relevant reported experimental researches.

CLASSY. VI. The Density, Structure, and Size of Absorption-line Outflows in Starburst Galaxies

Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide information integrated along the line of sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (n e ), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (r n ) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine-structure excited transitions of Si ii (i.e., Si ii*). We determine n e from relative column densities of Si ii and Si ii*, given Si ii* originates from collisional excitation by free electrons. We find that the derived n e correlates well with the galaxy’s star formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we get r n ∼ 1–2r * or ∼5r *, respectively, where r * is the starburst radius. Based on comparisons to theoretical models of multiphase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds.

Time and space resolved operando synchrotron X-ray and Neutron diffraction study of NMC811/Si–Gr 5 Ah pouch cells

Silicon–Graphite blended electrodes in Li-ion batteries have been proposed as a way to harness the high capacity of Si as an anode material, while minimising the negative effects of their large volume expansion. NMC 811 is the current state-of-the-art layered oxide cathode material, where the cobalt content of the cathode has been minimised. These are the two of the most promising materials for achieving electric vehicle targets in terms of performance, cyclability and price, however their degradation mechanism is not fully understood. Here these two materials have been used to manufacture 5 Ah prototype multi-layer pouch cells, which are aged and then studied using two complimentary diffraction techniques. Neutron diffraction has enabled a quantitative analysis of phase transitions in Si–Gr anodes in a pristine and degraded cell, and the alloying behaviour of Si and Li has been inferred by comparison of identical cells with either graphite or Si–Gr anodes. Synchrotron X-ray Diffraction has been used to make an operando 2D map of the cathode and anode lithiation in the pouch cell, as well as to map the volume expansion across the cell. This approach has revealed that degradation entails significant inhomogeneities across both electrodes, linked to the inhomogeneous volume expansion of the Si–Gr anodes.

Fine-structure transitions of Si and S induced by collisions with atomic hydrogen

ABSTRACT Using a quantum-mechanical close-coupling method, we calculate cross-sections for fine-structure excitation and relaxation of Si and S atoms in collisions with atomic hydrogen. Rate coefficients are calculated over a range of temperatures for astrophysical applications. We determine the temperature-dependent critical densities for the relaxation of Si and S in collisions with H and compare these to the critical densities for collisions with electrons. The present calculations should be useful in modelling environments exhibiting the [S i] 25 μm and [S i] 57 μm far-infrared emission lines or where cooling of S and Si by collisions with H is of interest.

Atomic Transition Probabilities for Transitions of Si i and Si ii and the Silicon Abundances of Several Very Metal-poor Stars ∗ ∗ Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Other data

We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si i) originating in the 3s 23p4s 3Po 1,2,1Po 1, and 3s3p 3 1Do 1,2 upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the 4P1/2–2Po 1/2,3/2 intercombination lines in the spectrum of singly ionized silicon (Si ii). The weak spin-forbidden lines of Si i and Si ii provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.

The Silicon Lines in the Far Ultraviolet Spectrum of HD 172883

The comparison of the mean high resolution spectrum of the chemically peculiar star HD 172883 recorded by the SWP camera on board the International Ultraviolet Explorer with the mean spectrum of HD 38899, a normal late B-type star having similar fundamental parameters, reveals the large flux deficiency of HD 172883 shortwards of 1700 Å. This flux deficiency is due to the strong lines, photoionization edges and autoionization transitions of Si ii. The three strongest autoionization lines of Si ii are present The strong resonance line of Al ii conspicuous in HD 38899 is very ewak in HD 172883 highlighting a strong aluminum deficiency in HD 172883. Conversely, the Hg ii resonance line at 1649.94 Å is much stronger in HD 172883, confirming the large overabundance of mercury in this star.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front.

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

Oxide‐Based Optoelectronics

Integrated Si photonics has the potential to revolutionize the processing of information between different integrated chips, as well as within a single chip itself. By performing at least a part of the task with photons rather than electrons, new opportunities for broad‐band low‐power communication and computing are created. Herein, the theoretical description of the linear electro‐optic (EO), or Pockels, effect and a newly elucidated design rule for materials evaluation is summarized. Possible applications of Si‐integrated optical elements based on perovskite oxides and their heterostructures are also discussed. In particular, the Pockels effect in BaTiO3 films grown on Si and intersubband transitions in Si‐integrated perovskite quantum wells (QWs) is described.

Energy levels and radiative transitions of Mg VII and Si IX

Term energies, line strengths, transition probabilities, and transition wavelengths among the (1s2)2s22p2, 2s22p3p, 2s2p3, 2s22p3s and 2s22p3d 1,3,5L (L = S, P, D, F) configuration states in Mg VII and Si IX are calculated by using the multiconfiguration Rayleigh-Ritz variation method and restricted variation method. The transition line strengths and transition probabilities of the electric dipole transitions are both given in length and velocity gauges. Differences between these two gauge values are discussed. The calculated atomic parameters show good agreement with the observed experimental results and other theoretical data. Furthermore, the uncertainty of each electric dipole transition line strength is estimated. For several transition parameters, uncertainties are decreased compared to the values from the National Institute of Standards and Technology Atomic Spectra Database.

Pressure-Induced Phase Transitions in Nanostructured Silicon

Pressure-induced phase transitions in various nanostructured Si have been studied extensively but still remain not well-understood. In this study, we systematically investigated the phase transitio...

Plasma Diagnostics of the Supernova Remnant N132D using Deep XMM–Newton Observations with the Reflection Grating Spectrometer

Abstract We present XMM–Newton observations of N132D, the X-ray brightest supernova remnant in the Large Magellanic Cloud, using the Reflection Grating Spectrometer (RGS), which enables high-resolution spectroscopy in the soft X-ray band. A dozen emission lines from L-shell transitions of Si, S, Ar, Ca, and Fe at intermediate charge states are newly detected in the RGS data integrating the ∼200 ks on-axis observations. This enables accurate abundance measurements of these elements, whose K-shell emission is out of the RGS bandpass. The 0.3–2.0-keV spectra require at least three components of thermal plasmas with different electron temperatures and indicate clear evidence of non-equilibrium ionization (NEI). Our detailed spectral diagnostics further reveal that the forbidden-to-resonance line ratios of O vii and Ne ix are both higher than expected for typical NEI plasmas. This enhancement could be attributed to either resonance scattering or emission induced by charge exchange in addition to a possible contribution from the superposition of multiple-temperature components, although the lack of spatial information prevents us from concluding which is most likely.

Absorption of Si, Ge, and SiGe alloy nanocrystals embedded in SiO2 matrix

Using the atomistic sp3d5s∗ tight-binding method, we calculate the optical absorption spectra due to phononless optical transitions in Si, Ge, and SiGe alloy nanocrystals embedded in an amorphous dielectric SiO2 matrix. For the SiO2 matrix, we use a virtual crystal approximation assuming the cubic crystalline structure, similar to β crystobalite, in the proximity of the nanocrystal surface. The optical absorption is analyzed in detail: its dependency on the Ge content and nanocrystal size and the role of the SiO2 matrix are revealed. Our recent experimental measurements of the optical absorption in co-sputtered thin SiO2 films with arrays of Si, Ge, and SiGe alloy nanocrystals are discussed and compared with the simulations.

High-pressure nano-seismology: Use of micro-ring resonators for characterizing acoustic emissions

Microring resonator (MRR) ultrasound detectors provide orders of magnitude greater sensitivity and frequency range (to < 10 Pa, from DC to 100 s of MHz) than previously achieved in recording acoustic emissions from materials at high pressures. We characterize acoustic emissions from crystal-structural phase transitions in Si to pressures of 50 GPa, well beyond the brittle-ductile transition at room temperature, and find that the number of events increases nearly tenfold for each decade reduction in the duration of recorded events. The shortest-duration events arrive in clusters, suggestive of a self-propagating, transformation-catalyzed process.

The Star-grazing Bodies in the HD 172555 System

Abstract Kiefer et al. reported the detection of infalling Ca ii absorption in HD 172555, a member of the β Pictoris Moving Group (βPMG). We obtained HST Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph spectroscopy of this star at 2 epochs separated by a week, and we report the discovery of infalling gas in resonant transitions of Si iii and iv, C ii and iv, and neutral atomic oxygen. Variable absorption is seen in the C ii transitions and is optically thick, with covering factors which range between 58% and 68%, similar to features seen in β Pictoris. The O i spectral profile resembles that of C ii, showing a strong low-velocity absorption to +50 km s−1 in the single spectral segment obtained during orbital night, as well as what may be higher-velocity absorption. Studies of the mid-IR spectrum of this system have suggested the presence of silica. The O i absorption differs from that seen in Si iii, suggesting that the neutral atomic oxygen does not originate in SiO dissociation products but in a more volatile parent molecule such as CO.

Competition between homogeneous and inhomogeneous broadening of orbital transitions in Si:Bi

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime (T1) of the states range from the low 10’s to 100’s of ps, and we find that there is little extra dephasing (so that T1 is of the order of T2/2) in each case.

Energy levels, radiative rates and electron impact excitation rates for transitions in Si III

Energy levels and radiative rates (A-values) for four types of transitions (E1, E2, M1, and M2) are reported for an astrophysically important Mg-like ion Si III, whose emission lines have been observed in a variety of plasmas. For the calculations, well-known and widely-used grasp code has been adopted, and results are listed for transitions among the 141 levels of the 3ℓ3ℓ′ and 3ℓ4ℓ configurations. Experimental energies are available for only the lowest 58 levels but there is no major discrepancy with theoretical results. Similarly, the A-values and lifetimes show a satisfactory agreement with other available results, particularly for strong E1 transitions. Collision strengths are also calculated, with the darc code, and listed for resonance transitions over a wide energy range, up to 30 Ryd. No similar results are available in the literature for comparisons. However, comparisons are made with the more important parameter, effective collision strength (Υ), for which recent R-matrix results are available for a wide range of transitions, and over a large range of temperatures. To determine Υ, resonances have been resolved in a narrow energy mesh, although these are not observed to be as important as for other ions. Unfortunately, large discrepancies in Υ values are noted for about half the transitions. The differences increase with increasing temperature and worsen as the upper level J increases. In most cases the earlier results are overestimated, by up to (almost) two orders of magnitude, and this conclusion is consistent with the one observed earlier for Be-like ions.

Lattice deformation in silicon and germanium nanocrystals

AbstractWe present a theoretical study of lattice deformation and its influence on the electronic states and the optical transitions in Si and Ge nanocrystals covered by hydrogen. The lattice of Si and Ge nanocrystals has been optimized by Merck molecular force field method. The semiempirical tight‐binding computer simulation has shown that the lattice optimization reduces effective energy band gap in silicon nanocrystals and increases it in germanium nanocrystals. This is similar to the effect of the hydrostatic compression of bulk Si and Ge. The sketch of the state density distribution in k‐space is presented. It demonstrates the orbital‐valley mixing, that appears due to space‐quantization and lattice deformation influence.

Core-level attosecond transient absorption spectroscopy of laser-dressed solid films of Si and Zr

We investigated experimentally as well as theoretically the ultrafast response of the wave function of the conduction band (CB) of Si and Zr to a near-infrared laser field using extreme ultraviolet (XUV) absorption spectroscopy in the spectral range of 80--220 eV. The measured dynamics of the XUV transmission demonstrates that the wave function of the CB follows the electric field of the dressing laser pulse. In these terms, laser dressing was earlier mainly studied on gases. Measurements with two-femtosecond and 200-attosecond temporal steps were performed in the vicinity of the $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edge near 100 eV, the $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}{L}_{1}$ edge near 150 eV, and the $\mathrm{Zr}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edge near 180 eV. The observed changes were dependent on the core states being excited by the XUV probe pulse. At the $2p$ to CB transitions of Si, the XUV transmission increased via the effect of the dressing laser pulse, while at the $2s$ to CB transition of Si and the $3d$ to CB transition of Zr, the XUV transmission decreased. Furthermore, beats between the transition from $2{p}_{1/2}$ and $2{p}_{3/2}$ levels of Si and from $3{d}_{3/2}$ and $3{d}_{5/2}$ levels of Zr were observed with 20.7 fs and 3.6 fs periods.

Distribution of 28Si, 29Si and 30Si isotopes in subsurface layers of Si:B single crystals under plastic deformation

Broadening of the 28Si, 29Si isotope distribution in the subsurface layers of Si:B single crystals under plastic deformation has been observed by secondary ion mass spectroscopy. The 28Si, 29Si profiles broaden with increasing dislocation density. Accompanying changes in the 29Si16O profile caused by plastic deformation have been found. The shift of the 29Si16O profile to the crystal bulk correlates well with the 29Si+ isotope redistribution. The uphill diffusion or dislocation movement is a possible driving force of Si and SiO redistribution, whereas selectivity of the isotopes in oxidation reaction is provided by singlet–triplet transitions in Si⋯O short living pairs.