Si Transition Research Articles

Mechanisms and energetics of calcium aluminosilicate glass dissolution through ab initio molecular dynamics-metadynamics simulations

The dissolution of silicate glasses has implications in diverse fields ranging from the immobilization of radioactive waste to the development of sustainable alternatives to Portland cement. Here, we used ab initio molecular dynamics simulations biased with well-tempered metadynamics to study Si-O-T bridge dissociation in calcium aluminosilicate glasses, crucial for understanding their dissolution. In a departure from the conventional Michalske-Freiman model, our findings reveal a nucleophilic substitution reaction mechanism characterized by a short-lived, 5-fold coordinated Si intermediate or transition state, depending on the Si bridge coordination, with a near-trigonal bipyramidal geometry. We find that the reorganization required for reaching this state causes the activation energy barriers to be dependent on the Si bridge coordination, with Si Q3 species serving as the rate-limiting step in the dissolution reaction. Our findings not only challenge long-standing theoretical models but also pave the way for more accurate and comprehensive frameworks for understanding the dissolution of silicate glasses in various applications.

Dynamic modulation of electric and magnetic toroidal dipole resonance and light trapping in Si-GSST hybrid metasurfaces.

The weak coupling of a toroidal dipole (TD) to an electromagnetic field offers great potential for the advanced design of photonic devices. However, simultaneous excitation of electric toroidal dipoles (ETDs) and magnetic toroidal dipoles (MTDs) is currently difficult to achieve. In this work, we propose a hybrid metasurface based on Si and phase transition material G e 2 S b 2 S e 4 T e 1 (GSST), which is formed by four Si columns surrounding a GSST column and can simultaneously excite two different TD (ETD and MTD) resonances. We also calculated the electric field distribution, magnetic field distribution, and multipole decomposition of the two resonances, and the results show that the two modes are ETD resonance and MTD resonance, respectively. The polarization characteristics of these two modes are also investigated, and the average field enhancement factor (EF) of the two modes is calculated. The dynamic modulation of the relative transmission and EF is also achieved based on the tunable properties of the phase change material GSST. Our work provides a way to realize actively tunable TD optical nanodevices.

Preparation of diamond on GaN using microwave plasma chemical vapor deposition with double-substrate structure

Combining diamond with GaN can significantly improve the heat dissipation performance of GaN-based devices. However, how to avoid the destructive damage to the GaN epi-layer caused by high-temperature hydrogen plasma during the diamond growth is still a problem. This study employed a Si transition layer and double-substrate structure microwave plasma chemical vapor deposition (MPCVD) to prepare diamond film on GaN epi-layer. The effects of double-substrate structure on the diamond growth were studied. The microwave plasma parameters of both single-substrate structure and double-substrate structure MPCVD diagnosed by emission spectra were comparatively investigated. It has been found that the microwave plasma energy of double-substrate structure MPCVD is relatively more concentrated and has higher radicals activity, which is beneficial to the diamond growth. The impacts of the Si transition layer on the diamond growth were also investigated. It demonstrates that the Si transition layer can effectively protect the GaN epi-layer from being etched by hydrogen plasma and improve the diamond growth. The relationship between the thickness of the Si transition layer and the diamond growth and the relationship between diamond film thickness and adhesion has been studied in detail.

Multicomponent (Hf0.25Zr0.25Ti0.25Cr0.25)B2 ceramic modified SiC–Si composite coatings: In-situ synthesis and high-temperature oxidation behavior

High-entropy ceramics, a novel type of multicomponent materials with broad application prospects, have stirred up world-wide interests for over a decade. In the current work, in-situ high-entropy (Hf0.25Zr0.25Ti0.25Cr0.25)B2 ceramic modified SiC–Si (HETMB2-SiC-Si) coating was deposited on carbon/carbon (C/C) composites via gaseous reactive infiltration of Si assisted slurry painting (GRSI-SP) method, to improve the oxidation protective ability of C/C composites at 1973 K. The formation and oxidation mechanisms of the coating was explored by first-principles simulation, experiment and thermodynamic analyses. The coating prepared at 2373 K shows dense mosaic structure filled with HETMB2-rich Si-based multiphase. This coating adheres well with the C/C substrate, which is ascribed to the formed zigzagged SiC–Si transition layer. This coating protected C/Cs from oxidation for more than 205 h at 1973 K. The enhanced oxidation protective ability is mostly ascribed to the subsequently generated compact and stable Hf-Zr-Ti-Cr-Si-O composite oxidation scale. This research will start up novel research ares of developing high-entropy materials modified coatings with improved protective ability under extreme environments.

Nano‐Twisted Double Helix Carbon Debris Improves the Wear Resistance of Ultra‐Thick Diamond‐Like Carbon Coatings

AbstractThick diamond‐like carbon (DLC) coatings with different silicon (Si) transition layer thickness are deposited by hollow cathode plasma immersion coating technology. The results show that the internal stress of the coating can be tuned by changing the deposition time of Si transition layer, thereby obtaining good adhesion strength. Friction and wear results show that all coatings have good tribological properties. The coating with 8 min Si transition layer (Si‐8) demonstrates the lowest friction coefficient (μ ≈ 0.03), even achieves ultralow friction (μ ≈ 0.008) for a short time. However, the wear rate of Si‐8 coating is found to be the largest (2.1 × 10−16 m3 (N m)−1. The low friction coefficient of Si‐8 coating is related to its high degree of graphitization, complete transfer film and fine wear debris. In contrast, the Si‐16 coating has the lowest wear rate of about 5.6 × 10−17 m3 (N m)−1 with a relatively high friction coefficient (μ ≈ 0.1). This is due to its high hardness and the formation of a nano‐twisted double helix carbon debris during the friction process. This nano‐twisted double helix structure can consume additional plastic deformation energy, while reducing the interface contact area and increasing the interface bearing capacity, thereby improving the wear resistance of the thick DLC coating.

Nucleophilic substitution reaction mechanisms: An atomic-molecular perspective on chemical speciation and transport properties in silicate melts

There has been no systematic attempt to determine if nucleophilic substitution (SN) reactions proceed in silicate melts, even though they commonly occur in gaseous and liquid phases containing C, Si, P, and Ge centered tetrahedra. The oversight is here rectified by providing such an analysis. Conditions required for nucleophilic substitution reactions to occur are: (1) the presence of nucleophiles (Lewis bases) which in silicate melts are bridging oxygen (BO), non-bridging oxygen (NBO−) and free oxygen (O2−); (2) the presence of tetrahedra (Q species) with strongly electrophilic centers (i.e., Si atoms); (3) the presence of Si transition species containing pentahedrally coordinated Si (i.e., VSi species); (4) rapid reaction rates among tetrahedral species. All conditions are met for silicate melts. For example, the strong nucleophile, NBO− exists at high temperatures in binary alkali and alkaline earths silicate melts due primarily to thermal agitation whereby some Si-NBO-M bonds are ruptured to produce the nucleophilic Si-NBO− moiety. This nucleophile attacks the Si center of an adjacent tetrahedron to form a SiO bond thereby producing a VSi transition species. The transition species decomposes by rupture of another SiO bond located on the polar opposite side of the transition species. Three types of SN reaction are recognized and all involve VSi transition species. They are NBO-BO exchange reactions (e.g., Q3 + Q4 → Q4 + Q3), disproportionation reactions (e.g., 2Q3 → Q4 + Q2) and polymerization reactions (e.g., Q3 → Q4 + 1/2O2−). H2O and OH− are also nucleophiles and their reaction with Q species proceeds via SN reaction mechanisms, and may cause depolymerization of melts. Hydrogen bonding of H2O to BO and NBO may also occur, as in ice and water, thereby enhancing H2O solubilities in melts. These latter reactions should neither depolymerize melts nor affect NBO/T values.The diffusivity of Si and O in melts, anionic conductivity and chemical speciation (e.g. Q species abundances) proceed via one or other SN reaction, with the transition species assuming a critical role in diffusivity, conductivity and viscosity. The SN reaction mechanism, coupled with transition state theory, provides explanations for: (1) the formation of pentahedrally coordinated Si (VSi) and its apparent restriction to highly siliceous glasses; (2) the remarkably similar diffusivities of Si and O in silicate melts; (3) the ‘jump distance’ (α) of the Eyring equation, which by the SN mechanism is ~3.5 Å (i.e., diameter of the VSi transition species); (4) the minimum number of monomeric units (i.e., Q species) involved in the ‘cooperative region’ of the Adam-Gibbs equation; and (5) the quantitative distribution of Q species in Na silicate glasses up to ~50 mol% Na2O.

Влияние слоя нанопористого кремния на практическую реализацию и особенности эпитаксиального роста слоев GaN на темплейтах SiC/por-Si/c-Si

Abstract A set of structural and spectroscopic methods of diagnostics is used to study the influence of a nanoporous silicon ( por -Si) transition layer on the practical implementation and specific features of growth of GaN layers on SiC/ por -Si/ c -Si templates by molecular-beam epitaxy with the plasma activation of nitrogen. It is shown that a por -Si transition layer introduced into a template, in which a 3 C -SiC layer is created by the method of atom substitution, offers unquestionable advantages over standard silicon substrates. Specifically, such an approach makes it possible to lower the level of stresses in the crystal lattice of the epitaxial GaN layer by about 90% and to reduce the fraction of vertical dislocations in the GaN layer. The GaN layer is grown on the surface of the SiC layer, which in turn is on the surface of the SiC/ por -Si/ c -Si template. It is found for the first time that the use of the SiC/ por -Si/ c -Si template brings about the formation of a qualitatively more uniform GaN layer free of visible extended defects.

Оптические свойства гибридных гетероструктур GaN/SiC/por-Si/Si(111)

Abstract Using a complex of structural and spectroscopic methods of diagnostics, the influence of a nanoporous-silicon ( por -Si) transition layer on the optical properties of GaN layers grown on SiC/ por -Si/ c -Si templates by molecular-beam epitaxy with the plasma activation (MBE PA) of nitrogen is studied. It is shown for the first time that the MBE PA technology of the synthesis of GaN on a virtual SiC/ por -Si/ c -Si substrate provides a means for producing a GaN film of much higher structural and optical quality at a much lower growth temperature compared to those in similar studies, in which growth on porous Si substrates is demonstrated. The use of a por -Si layer makes it possible to improve the structural and morphological properties of the GaN epitaxial layer and to attain unique optical and electrical characteristics of the layer. The data obtained in the study will serve as an important basis for understanding the foundations of the physics of GaN/SiC/ por -Si nanoheterostructures and for promoting their potential use in optoelectronics.

Understanding social innovation for the well-being of forest-dependent communities: A preliminary theoretical framework

•Presentation of an original definition of social innovations for marginalised rural areas. Key mechanisms that enable efforts for social innovations in forest-dependent communities to emerge and develop.•Transdisciplinary framework of theoretical-empirical expert understanding of SI in forest-dependent communities.•Identification of key factors and SI transition stages for forest- dependent communities in marginalised rural areas.•Determination of the key development trajectories that differ by type of actors and knowledge which enter the action arena.•Determine the adaptive or transformative character of social innovations in forest-dependent communities.

Atomistic and Optical Properties of Group IV Ultrathin Superlattices

Engineering atomically abrupt interfaces between multi-layered thin films of different material have yielded some significant fundamental and technological breakthroughs in the past. For example, the Nobel Prize winning integral and quantum Hall effect owes its discovery to abrupt semiconductor interfaces grown by molecular beam epitaxy. On the other hand, the functionalization of many modern-day semiconductor devices depend critically on the quality of interface of the various thin films, which constitute the device. Silicon (Si)/Germanium (Ge) and Si/SiGe based materials have emerged in the recent years as an integral part to the Si semiconductor industry, promising a wide range of devices ranging from Fin-field effect transistors, quantum cascade lasers, quantum dots, near and mid-infrared photodetectors, resonant tunnel diodes, to name a few.1–3 To this end, a continuous feedback from post-growth characterization techniques elucidating the intermixing of atoms at the interface, the interfacial width and roughness, the defect density at the interfaces as a function of the growth conditions and integration parameters is therefore crucial for optimizing the ultimate performance of these devices. Not surprising that the semiconductor industry is directing a significant amount of its resources and efforts towards enriching our current understanding of the buried interfaces of these materials. In this work, we use a combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) to present an atomistic understanding of the nature of interfaces in Si/Ge and Si/SiGe superlattices, grown on 300 mm Si wafers using chemical vapor deposition. The APT investigations were done using the state-of-the-art LEAP 5000 providing a supreme mass sensitivity and a detection efficiency of 80%, a 30% improvement compared to its predecessor. Figure 1(a) shows the STEM-APT of the last six bi-layers of a sixteen period Si/SiGe superlattice. Figure 1(b) shows the one-dimensional (1D) concentration profile obtained from the 3D APT reconstruction of the sample along the growth (analysis) direction. While the STEM data reveals the average Si and SiGe layer thickness to be about 1.3 nm and 2.2 nm respectively, the concentration profile from the APT data reveals the Ge content within the SiGe layers to be 25.0 +/- 0.5 at.%. The average interfacial width, evaluated as the separation between the 10.0 at.% and 90.0 at.% Ge concentration points, revealed the Si to SiGe transition to be about 1.4 +/- 0.3 nm wide while the width of the SiGe to Si transition to be narrower than 1.0 nm. The possible role of Ge diffusion on the observed interfacial thicknesses shall be discussed in details in this work. Figure 1(c) shows the Si/SiGe hetero-interfaces drawn as isoconcentration surfaces at 12.0 at.% Ge concentration. The work shall also highlight the results obtained from the STEM-APT measurements on superlattices of different period lengths, layer thicknesses and growth conditions from that shown in Figure 1. Alongside the structural aspects, the vibrational properties of these superlattices as obtained from Raman spectroscopic measurements and optical properties as obtained from the ellipsometry and absorption measurements shall be highlighted. Such superlattices holds tremendous prospect for Si semiconductor industry wherein a magic sequence of the number of monolayers within each period can render the whole superlattice to be direct gap4 for optoelectronic applications or an array of ultrathin free-standing nanowires could be fabricated by selective etching of the Ge layers in a Si/Ge superlattice for nanoelectronic applications. (1) Tan, K.-M.; Liow, T.-Y.; Lee, R. T. P.; Hoe, K. M.; Tung, C.-H.; Balasubramanian, N.; Samudra, G. S.; Yeo, Y.-C. IEEE Electron Device Lett. 2007, 28 (10), 905–908. (2) Dehlinger, G.; Diehl, L.; Gennser, U.; Sigg, H.; Faist, J.; Ensslin, K.; Grutzmacher, D.; Muller, E. Science 2000, 290 (5500), 2277–2280. (3) Maune, B. M.; Borselli, M. G.; Huang, B.; Ladd, T. D.; Deelman, P. W.; Holabird, K. S.; Kiselev, A. A.; Alvarado-Rodriguez, I.; Ross, R. S.; Schmitz, A. E.; Sokolich, M.; Watson, C. A.; Gyure, M. F.; Hunter, A. T. Nature 2012, 481 (7381), 344–347. (4) d’Avezac, M.; Luo, J.-W.; Chanier, T.; Zunger, A. Phys. Rev. Lett. 2012, 108 (2), 27401. Figure 1

Mechanical properties of Al2O3 ceramic coatings prepared by plasma spraying on magnesium alloy

An Al2O3 coating has been successfully prepared on an AZ91HP magnesium (Mg) alloy by plasma spraying technology. Al–Si eutectic alloy was used as the transition layer in order to ease the physical and chemical differences between the Al2O3 and the Mg alloy. The mechanical properties of the coating were studied. The results show that the coating consists mainly of stable α-Al2O3 and metastable γ-Al2O3 phases, with a lamellar microstructure. Nano-indentation results exhibit that the peak loads of the plasma-sprayed Al2O3 layer, the Al–Si transition layer, and the Mg alloy are 2.2, 0.6 and 0.25mN, respectively. The elastic modulus and microhardness of the plasma-sprayed Al2O3 layer are 250 and 7.45–8.90GPa, respectively, which are higher than those of the Mg alloy (50 and 0.8GPa). The wear volume of the plasma-sprayed Al2O3 coating is two orders of magnitude smaller than that of the Mg alloy.

Lithium Insertion in Nanostructured Si1-xTix Alloys

Nanostructured Si1-xTix alloys (0 ≤ x ≤ 0.3) prepared by ball milling were studied as negative electrode materials in Li cells. The alloys comprised a nanocrystalline and amorphous Si phase and a nanocrystalline C49 TiSi2 phase. When x ≥ 0.15 the nanocrystalline Si phase was completely eliminated and such alloys consisted only of amorphous Si and the C49 TiSi2 phase. The alloys with x ≥ 0.15 also completely suppressed the formation of Li15Si4 during cycling, resulting in good capacity retention, and, unlike other Si transition metal alloys, without introducing noticeable cell polarization. The observed capacity of the Si1-xTix alloys suggested that the TiSi2 is an inactive phase toward Li. The Si0.85Ti0.15 composition had a high capacity of 1710 Ah/L with 96% capacity retention after 50 cycles, making it attractive for use in Li-ion batteries.

ChemInform Abstract: Recent Progress of Silicon Composites as Anode Materials for Secondary Batteries

AbstractReview: 117 refs.

Structure, micro-hardness and corrosion behaviour of the Al–Si/Al2O3 coatings prepared by laser plasma hybrid spraying on magnesium alloy

An Al–Si/Al2O3 coating was successfully prepared on the surface of the AZ91D magnesium alloy substrate by a new technology laser plasma hybrid spraying in this paper, and the influence of laser power on the structure and property of the coating was studied simultaneously. The performance of the coating, such as the morphology, porosity, phase composition, micro-hardness, was characterized. The results showed that the Al–Si transition layer significantly improved the adhesion between the coating and the substrate. With the increasing of laser power, the unfused particles in the coating decreased and the coating was more compact. The porosity of the coating decreased firstly and then increased with the increasing of laser power, and the lowest porosity was 3.6%. The bonding strength of the coating was 25.62 MPa and the highest hardness was up to HV0.05470 (4.61 GPa). The corrosion resistance of the coating can exceed 900 h under neutral salt spraying conditions.

Design and Deposition of Infrared Antireflection Protection Film on Magnesium Fluoride Substrate for 3~5 μm

The infrared antireflection protection film is designed and deposited on magnesium fluoride substrate for 3~5 μm. The multilayer antireflection film is designed and prepared by ion beam assisted electron-beam deposition. After repeated experiments, the absorption peak of SiO2 is found at 3000nm,using MgF2 and SiO2 as the low-index materials to reduce the absorption. The coating structure is Sub|1.46H1.37L2.19H1.45L2.49M|Air. DLC (diamond-like carbon) protection coating is deposited by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). Through contrast tests, the best deposition process is obtained (the butane flow rate is 5sccm/s, deposition temperature is 80°C) by setting deposition time as invariant, the reaction gas flow rate and deposition temperature as variables. Meanwhile the problem of associativity between antireflection film and DLC protection coating is solved by adding a Si transition layer. The result shows that the average transmittance of this film in 3~5μm is 95.5%, and this film can withstands harsh environment tests. Key words: Magnesium fluoride substrate; Antireflection protection film; 3~5 μm infrared band Document Code: A Article ID: DIO

Silicon and Oxygen in High-Al-Content AlGaN: Incorporation Kinetics and Electron Paramagnetic Resonance Study

The high-Al-content AlxGa1-xN alloys, x>0.70, and AlN is the fundamental wide-band-gap material system associated with the technology development of solid-state LEDs operating at the short wavelengths in the deep-UV (λ < 280 nm). Yet, their properties are insufficiently understood. The present study is intended to bring elucidation on the long-time debated and much speculated Si transition from shallow donor in GaN to a localized deep DX defect in AlxGa1-xN alloys with increasing Al content. For that purpose electron paramagnetic resonance is performed on a particular selection of high-Al-content epitaxial layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition.

Theoretical calculations on rhodium(III)-Cp* catalyzed asymmetric transfer hydrogenation of acetophenone using monosulfonamide ligands derived from (1R,2R)-diaminocyclohexane

Chiral secondary alcohols are valuable synthetic intermediates in the pharmaceutical industry in making complex biologically active molecules. Noyori’s ATH of aromatic ketones using Ru(II)-monotosylated diamine complexes are catalyst is one the useful methods to induce high yields and enantioselectivities in the formation of the secondary alcohol. Using Ru(II) as catalyst, the calculated energy difference between the Re and Si transition state were found to be 2.1kcal/mol. We found Rh(III)Cp*-monotosylated diamine complexes gave better results in aqueous conditions compared to the Ru(II) complexes in the ATH of aromatic ketones. Here we show a similar calculation for the transition state involving Rh(III), which gives the energy difference between the Re and Si to be 3.9kcal/mol.

Post deposition annealing temperature effect on silicon quantum dots embedded in silicon nitride dielectric multilayer prepared by hot-wire chemical vapor deposition

The preparations of the 20-period of a Si quantum dot (QD)/SiN x multilayer in a hot-wire chemical vapor deposition (HWCVD) chamber is presented in this paper. The changes in the properties of Si-QDs after the post deposition annealing treatment are studied in detail. Alternate a-Si:H and SiN x layers are grown in a single SiN x deposition chamber by cracking SiH 4, and SiH 4 + NH 3, respectively at 250 °C. The as-deposited samples are annealed in the temperature range of 800 °C to 950 °C to grow Si-QDs. All the samples are characterized by confocal micro Raman, transmission electron microscope (TEM), and photoluminescence (PL) to study the changes in the film structures after the annealing treatment. The micro Raman analysis of the samples shows the frequency line shifting from 482 cm − 1 to 500 cm − 1 indicating the Si transition from an amorphous to a crystalline phase. The TEM micrograph inspection indicates the formation of Si-QDs of size 3 to 5 nm and a density of 5 × 10 12/cm 2. The high resolution TEM micrographs show an agglomeration of Si-QDs with an increase in the annealing temperature. The PL spectra show a peak shifting from 459 nm to 532 nm with increasing the annealing temperature of the film.

Morphology and chemical composition analysis of inorganic nanosheets by the field-emission scanning electron microscope system

Nanosheets can be used as building blocks to fabricate versatile nanostructured materials. In this paper, morphology of the Cs(4)W(11)O(36) and Nb(3)O(8) and TaO(3) sheets with different layers are analyzed by different field-emission scanning electron microscopes (FE-SEMs). Chemical composition of the single-layered Cs(4)W(11)O(36) with thickness of about 2 nm, and multilayered Nb(3)O(8) nanosheets with thickness of less than 14 nm are analyzed by both the Si(Li) solid-state detector and transition edge sensor (TES) microcalorimeter, successfully. The effects of energy resolution, accelerating voltage and substrate on the quantitative analysis are discussed briefly.

Tribological performance of ultrathin diamond-like carbon films prepared by plasma-based ion implantation

Ultrathin diamond-like carbon (DLC) films with thicknesses of 5–60 nm have been prepared on Si by plasma-based ion implantation. Raman spectrum and x-ray photoelectron spectroscopy (XPS) show that these DLC films present high sp3/sp2 ratios. XPS also displays that each DLC film firmly adheres to the Si substrate owing to a C–Si transition layer. Atomic force microscopy shows that the DLC films are smooth and compact with average roughness (Ra) of about 0.25 nm. Sliding friction experiments reveal that these DLC films show significantly improved tribological performance. With increase of DLC film thickness, the sp3/sp2 ratio increases, the roughness decreases, the hardness increases, the adhesive wear lightens and thereby the tribological performance becomes enhanced. Also, the effects of the applied load and the reciprocating frequency on the tribological performance are discussed.