Silicon Sites Research Articles

F+ center exchange mechanism and magnetocrystalline anisotropy in Ni-doped 3C-SiC

• F + center exchange mechanism has been invoked in order to explain the long range magnetic order in the system. • The Curie temperature is found to be above 350 K for all the doped samples obtained from the modified Bloch’s T 3/2 law. • The dynamic nature of the BMPs sizes have been studied extensively using Hc vs T data. • The spontaneous spin polarization has been induced in the system due to strong hybridization between (C) 2p and (Ni) 3d orbital. • The decrease in magneto crystalline anisotropy constant value with increase in temperature essentially indicates the weakening of the magnetic interaction in the system. • The incomplete quenching of orbital angular momentum and partial removal of orbital degeneracy resulted in reduction of line width and enhancement of integrated intensity as the temperature rises. Towards the development of a magnetic semiconductor suitable for spintronic device applications in extreme environments, we explored the possibility of inducing magnetic interaction in SiC by doping Nickel. The X-ray diffraction and Raman Spectroscopy studies confirm the incorporation of Ni into the host lattice. The magnetic measurements and electron spin resonance studies indicate the presence of room temperature ferromagnetic interaction in the system. The Curie temperature of 1, 3, and 5% Ni-doped samples have been found to be 420 K, 520 K, and 540 K respectively. Electron spin resonance study reveals that the valence state of Ni is 2 + , which implies the creation of vacancies at both Silicon (V Si ) and Carbon (V C ) sites as they are tetravalent. The change in magnetization of the system with an increase in dopant concentration is consistent with the variation in the number of vacancies and free holes. The analysis of magnetization data using the Law of approach to saturation shows that the anisotropic constant decreases with an increase in temperature. The long-range magnetic interaction in the system is explained using the F + center exchange mechanism.

The Nature of Interface Interactions Leading to High Ionic Conductivity in LiBH4/SiO2 Nanocomposites.

Complex metal hydride/oxide nanocomposites are a promising class of solid-state electrolytes. They exhibit high ionic conductivities due to an interaction of the metal hydride with the surface of the oxide. The exact nature of this interaction and composition of the hydride/oxide interface is not yet known. Using 1H, 7Li, 11B, and 29Si NMR spectroscopy and lithium borohydride confined in nanoporous silica as a model system, we now elucidate the chemistry and dynamics occurring at the interface between the scaffold and the complex metal hydride. We observed that the structure of the oxide scaffold has a significant effect on the ionic conductivity. A previously unknown silicon site was observed in the nanocomposites and correlated to the LiBH4 at the interface with silica. We provide a model for the origin of this silicon site which reveals that siloxane bonds are broken and highly dynamic silicon–hydride–borohydride and silicon–oxide–lithium bonds are formed at the interface between LiBH4 and silica. Additionally, we discovered a strong correlation between the thickness of the silica pore walls and the fraction of the LiBH4 that displays fast dynamics. Our findings provide insights on the role of the local scaffold structure and the chemistry of the interaction at the interface between complex metal hydrides and oxide hosts. These findings are relevant for other complex hydride/metal oxide systems where interface effects leads to a high ionic conductivity.

Cobalt Silylenes as Platforms for Catalytic Nitrene‐Group Transfer by Metal–Ligand Cooperation.

A cobalt silylene (Co=Si) linkage enables a distinct metal/ligand cooperative activation of an organic azide, where nitrene transfer occurs to and from the Co⋅⋅⋅Si linkage without ligand dissociation from the 18-electron cobalt center. This process utilizes the orthogonal binding affinities of the silicon and cobalt sites to avoid CO poisoning that would otherwise inhibit reactivity, leading to significantly improved catalytic isocyanate generation compared with related systems. The dual-site approach demonstrates the potential of metal/main-group bonds to access new and efficient catalytic pathways.

Cobalt Silylenes as Platforms for Catalytic Nitrene‐Group Transfer by Metal–Ligand Cooperation.

A powerful approach to cooperative group-transfer catalysis is demonstrated using the Co=Si bond of a cobalt silylene to provide two distinct sites for substrate activation. The orthogonal selectivity of the Co and Si centers enables efficient nitrene-group transfer to carbon monoxide by avoiding poisoning that would result from substrates competing for a single reactive site.

Si–CN for the oxygen reduction reaction in alkaline media, the effect of synthesis temperature

The electrocatalytic properties of silicon-doped carbon nanotubes (Si–CN) for oxygen reduction reaction (ORR) in alkaline media are reported. The Si-CNs were obtained by a modified chemical vapor deposition method. The Si–CN showed good activity towards ORR since most of the process was carried out efficiently by the four-electron transfer pathway, and it exhibited a low Tafel slope of 84.28 mV dec−1 indicating that the material has a good kinetic activation response for ORR. This favorable response was attributed to 0.29 at. % silicon intercalated in the carbon lattice enhancing electrons transport at neighboring silicon sites. In addition, its mechanical robustness and thermodynamic stability influenced by the synthesis temperature contributed favorably to the energetic activation of Si–CN to the adsorption and reduction of oxygen. Therefore, Si–CN could be considered a potential metal-free and low-cost material for the ORR in alkaline fuel cells.

Theoretical Investigation on H-Abstraction Reactions of Silanes with H and CH3 Attacking: A Comparative Study with Alkane Counterparts.

Silicon-based organic precursors are widely applied in the vapor-fed flame synthesis of monocrystalline silicon, silicon dioxide, and silicon nitride. Due to the lack of kinetic investigations on reactions of silicon-based organic precursors, rate constants were usually analogized to those of their hydrocarbon counterparts. Investigations on the similarities and differences between the two types of compounds become necessary. This work reports a comparative theoretical investigation on H-abstraction reactions with H and CH3 attacking for silanes and their alkane counterparts, including silane and methane, disilane, methylsilane and ethane, dimethylsilane and propane, trimethylsilane and iso-butane, and tetramethylsilane and neo-pentane at the domain-based local pair natural orbital coupled cluster with perturbative triple excitations (DLPNO-CCSD(T))/cc-pVTZ//M06–2X/cc-pVTZ level. The rate constants were calculated using the conventional transition-state theory coupled with the asymmetric Eckart tunneling corrections over 600–2000 K. The calculated results show that dramatic discrepancies exist between H-abstraction from silicon sites in silanes and equivalent carbon sites in their alkane counterparts with H and CH3 attacking. The H-abstraction reactions from the primary carbon sites in silanes have generally lower barrier energies than the similar reactions in their alkane counterparts, while those in methylsilane and dimethylsilane with H attacking are the only two with higher barrier energies. Electrostatic potential mapped molecular van der Waals surfaces were adopted to provide insight into the calculated trends in barrier energies. The H-abstraction reactions from silicon sites in silanes have much higher rate constants than those from equivalent carbon sites in their alkane counterparts, especially under low-temperature conditions, while the rate constants of H-abstraction reactions from primary carbon sites in silanes and their alkane counterparts show relatively strong analogy.

Effect of Aluminum Incorporation on the Reaction Process and Reaction Products of Hydrated Magnesium Silicate

In this study, we investigated the impact of aluminium ion (Al3+) incorporation on the microstructure and the phase transformation of the magnesium silicate hydrate system. The magnesium silicate hydrate system with aluminium was prepared by mixing magnesium oxide and silica fume with different aluminium ion contents (the Al/Si molar ratios of 0.01, 0.02, 0.05, 0.1, 0.2) at room temperature. The high degree of polymerization of the magnesium silicate hydrate phases resulted in the limited incorporation of aluminium in the structure of magnesium silicate hydrate. The silicon-oxygen tetrahedra sites of magnesium silicate hydrate layers, however, were unable to substitute for silicon sites through inverted silicon-oxygen linkages. The increase in aluminium ion content raised the degree of polymerization of the magnesium silicate hydrate phases from 0.84 to 0.92. A solid solution was formed from residual aluminum-amorphous phases such as hydroxyl-aluminum and magnesium silicate hydrate phases. X-ray diffraction (XRD), field emission scanning electron microscope (F-SEM), and 29Si and 27Al MAS NMR data showed that the addition of Al3+ promotes the hydration process of MgO and has an obvious effect on the appearance of M-S-H gel. The gel with low aluminum content is fluffy, while the gel with high aluminum content has irregular flakes. The amount of Al3+ that enters the M-S-H gel increased with the increase of Al3+ content, but there was a threshold: the highest Al/Si molar ratio of M-S-H gel can be maintained at about 0.006.

First principles analysis of impurities in silicon carbide grain boundaries

Silicon carbide is an important structural and electronic ceramic material that finds many uses in a wide variety of applications that require stability at extreme conditions. In this study, we provide a detailed investigation of the formation energies of point defects and the stability of a wide variety of dopants in bulk cubic silicon carbide (3C-SiC) and in 3C-SiC grain boundaries (GBs) using first principles methods and a detailed charge distribution analysis. We also determine the driving force (segregation energies) of these dopants towards GBs. Our results show that smaller, more electronegative elements such as oxygen and nitrogen occupy carbon substitutional sites whereas larger transition metals such as molybdenum and rare earth elements such as cesium, substitute for silicon sites in SiC. Such dopants tend to migrate to more open spaces as provided in the Σ9 GB. This suggests that Σ9 GB is more effective in pinning the defect centers as compared to Σ3 GB. These findings provide the chemical landscape for defect engineering through which doped 3C-SiC can be designed for targeted materials development purposes.

Electronic and transport properties of nitrogen and boron doped zigzag silicon carbide nanoribbons: First principle study

We have performed a theoretical ab-initio calculation for the stability and electronic properties of pristine as well as Boron and Nitrogen doped zigzag silicon carbide Nanoribbons (ZSiCNR). We observed that the boron and nitrogen atoms energetically prefer to be localized at the edges of the nanoribbons. However, boron preferentially substitutes at silicon site and nitrogen prefers to occupy carbon site. The energy calculation reveals that the nitrogen substitution is the most preferred configuration in terms of stability. The substitution of boron and nitrogen impurity atom transforms narrow band gap semiconductor to metallic systems at most of doping sites.

T2 phase site occupancies in the Cr–Si–B system: a combined synchroton-XRD/first-principles study

Boron and Silicon site occupancies of the T2 phase in the Cr–Si–B system were investigated experimentally and by first-principles electronic-structure calculations. A sample with nominal composition Cr0.625B0.175Si0.2 was arc-melted under argon, encapsulated in a quartz-tube and heat-treated at 1400∘C for 96 hours. It was then analyzed using Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD) with synchrotron radiation. An excellent agreement was obtained between experiments and theoretical calculations, revealing that Si occupies preferably the 4a sublattice of the structure, as boron atoms in 8h nearest-neighbors form very stable dumbbell bonds. Site preferences are thus a key factor for the stabilization of the structure. The results of this work provide important information to support a better description of this phase in alloys with Si and B, since T2 phases are known to occur in many important Transition Metal–Si–B ternary systems, such as Nb/Mo/W/Ta/V–Si–B.

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

Investigating potential properties of (8,0) Silicon Carbide Nanotube for applications in band gap tuning

The changes in electronic and Magnetic properties of spin polarized silicon carbide nanotube (SiCNT), investigated using density functional theory and Quantum-Espresso computational package. In the present work, the electronic and magnetic properties of pristine, vacancy and transition metal(TM) doped (8,0) SWSiCNT are systematically investigated to give a detailed understanding of these materials. Magnetic moment were calculated, the electronic properties of zigzag (8,0) semiconductor silicon carbide nanotubes (SiCNTs) doped once at the time with Iron, Cobalt, Mnganse, Gadolinium and Hafinime. We have looked at the two possible scenarios where the guest atom X (Fe, Co, Mn, Gd, Hf), substituting in the Site of the Silicon atom X Si and carbon atom X C with different concentration, in the unit cell. We found that in the case of one atom Fe doped SiCNT, Co doped SiCNT, Mn doped SiCNT, Gd doped SiCNT and Hf doped SiCNT replacing a Silicon atom position annotated by Fe Si exhibits a magnetic moment of 1.99 μ B /cell and Mn Si exhibits a magnetic moment of 0.98 μ B /cell, Co Si exhibits a magnetic moment of 0.02 μ B /cell, whereas, in the case of two atom 2Fe doped SiCNT, 2Co doped SiCNT, 2Mn doped SiCNT, Gd doped SiCNT and Hf doped SiCNT replacing at two carbon atom position annotated by Fe Si exhibits a magnetic moment of 3.98 μ B /cell and Mn Si exhibits a magnetic moment of 1.99 μ B /cell, Also, two atom Cobalt replacing carbon, (CoC), induce a magnetic moment of 5.13 μ B and for three atom cobalt replacing Carbon 5.36. Substitution of three Cobalt in site of Silicon gives the maximum magnetic moment 9.00 μ B /cell but in contrast the calculated results show that the Co doping in Silicon carbide nanotube induces the ferromagnetism and metallic property for all concentrations.The calculated results show that the Mn doping in Silicon carbide nanotube induces the ferromagnetism and originates a half metallic (HM) gap at Fermi level (EF)in minority spin channel (MIC) for for 3.125 %, 6.25%, 9.37% and 12.75% concentrations. Moreover, the calculated results show that the Fe doping in Silicon carbide nanotube induces the ferromagnetism and Half Metalic property for 3.125%, 6.25% and 9.37% concentrations. Doping concentrations and dopant positions regulate the electronic structure and Magnetic property of the Silicon carbide nanotube, exhibiting both semiconducting and half-metallic behaviours as a response to the concentration of Transition Metals.

Discrimination of Aluminum from Silicon by Electron Crystallography with the JUNGFRAU Detector

The crystal structure of a chemical compound serves several purposes: its coordinates represent three-dimensional information about the connectivity between the atoms; it is the only technique that determines the absolute configuration of chiral molecules; it enables determining structure–function relations; and crystallographic data at atomic resolution distinguish between element types and serve as a confirmation of synthesis protocols. Here, we collected electron diffraction data from albite and from a Linde Type A (LTA) type zeolite. Both compounds are aluminosilicates with well-defined silicon and aluminum crystallographic sites. Data were recorded with the “adJUstiNg Gain detector FoR the Aramis User station” (JUNGFRAU detector) and we made use of its capability of energy discrimination to suppress noise. For both compounds, crystallographic refinement distinguishes correctly between silicon and aluminum, even though these elements have very similar electron scattering factors. These results highlight the quality of the electron diffraction data and the reliability of the models for chemical interpretation. Further development in this direction will provide enormous opportunities for structure–function studies by diffraction.

Nanoscale zero-valent iron supported by attapulgite produced at different acid modification: Synthesis mechanism and the role of silicon on Cr(VI) removal

The attapulgite of different morphologies and mineral compositions were successfully obtained following the treatment by HCl and HF with different concentrations. Variations of morphologies, elemental and mineral components of the pristine and modified attapulgite were investigated and assessed in detail by a series of characterization methods. The SEM-EDS results indicated significant variations on the contents and morphologies of silicon after acid modification. The Cr(VI) removal efficiencies under pristine and modified attapulgite-supported nZVI composites were evaluated with the removal rate in case of 0.5HAT-nZVI being 69.2% more superior than that of 6FAT-nZVI. The reaction kinetic is well fitted with pseudo second order kinetics model. The correlation analysis indicated that Cr(VI) removal efficiency was positively correlated with the content of active silicon in the attapulgite-nZVI composites (R2 = 0.979∗∗). Additionally, the reduction of Cr(VI) is more likely to occur in silicon-rich composites based on the analysis of XPS spectra and Cr concentration changes, which were mainly attributed to the enhanced Si–O–Fe coupling mediated by silicon. Attapulgite with more exposure sites of silicon enhanced the Cr(VI) reduction process and promoted crystallization of the reaction products. Simultaneously, the nZVI consumption caused by oxidation and aggregation were improved by silicon in attapulgite. It is concluded that silicon played a significant role on Cr(VI) removal through the reductive precipitation by Si–O–Fe coupling.

Alkali activation of mechanically activated low-grade clay

This article reports the results of an investigation into the efficiency of mechanical activation to increase the reactivity in alkali activated mortar synthesized from low-grade clay. Mechanical activation significantly changed the structure of clay, increasing the specific surface area, and decreasing the particle size. The 7-day compressive strength of mortar synthesized from untreated clay was 31.7 MPa, which increased to 35.3 after 4 h milling. A further increase of grinding time to 8 h did not result in any increase in compressive strength which is attributed to a decrease in the workability. However, the extent of reactivity did significantly increase as determined from solid-state MAS NMR, FTIR, and EDS analyses. Solid-state MAS NMR results revealed the increased formation of Q4(2Al) silicon sites, which is correlated with improved reaction. Furthermore, the EDS and FTIR analysis results indicated greater incorporation of aluminium into the matrix structure with increased grinding time.

High-resolution spectroscopy of individual erbium ions in strong magnetic fields

In this paper we use electrically detected optical excitation spectroscopy of individual erbium ions in silicon to determine their optical and paramagnetic properties simultaneously. We demonstrate that this high spectral resolution technique can be exploited to observe interactions typically unresolvable in silicon using conventional spectroscopy techniques due to inhomogeneous broadening. In particular, we resolve the Zeeman splitting of the 4I15/2 ground and 4I13/2 excited state separately and in strong magnetic fields we observe the anti-crossings between Zeeman components of different crystal field levels. We discuss the use of this electronic detection technique in identifying the symmetry and structure of erbium sites in silicon.

Structural and Electronic Properties of Zigzag Single Wall (8,0), (9,0), and (10,0) Silicon Carbide Nanotubes Sandwiched between Different Elements

Using first principles density functional theory calculations, the electronic and geometric structures of three different types of zigzag single wall silicon carbide nanotubes (ZSSiCNt), (8,0), (9,0), and (10,0), have been studied by sandwiching nanotubes between different semiconductors, group three elements, and some transition metal elements. When the (9,0) zSSiCNt is sandwiched between different semiconductors, the binding energy (B E ) varies from 2.05 to 5.68 eV/atom. When the (10,0) zSSiCNt is sandwiched between group III elements, the B E varies from 4.35 to 6.89 eV/atom. When the (10,0) zSSiCNt is sandwiched between some transition elements, the B E varies from 1.63 to 5.97 eV/atom. The binding energy variation of SWSiCNT by substituting different elements at the site of carbon and silicon at the end of the zigzag silicon carbide nanotubes is shown. Substitution of Fe and Ga at the site of Si increases the binding energy to 4.04 eV/atom & 6.32 eV/atom, respectively. The cohesive energy appears to saturate between 6.00 to 6.90 eV, and the stability of different (8,0) SWNt is compared.

ELVISort: Encoding Latent Variables for Instant Sorting, an Artificial Intelligence-Based End-to-End Solution

The growing number of recording sites of silicon based probes means that an increasing amount of neural cell activities can be recorded simultaneously, facilitating the investigation of underlying complex neural dynamics. In order to overcome the challenges generated by the increasing number of channels, highly automated signal processing tools are needed. In this paper we present ELVISort, a deep learning method which combines the detection and clustering of different action potentials in an end-to-end fashion. We show that the performance of ELVISort is comparable with other spike sorting methods which use manual or semi-manual techniques, while exceeding the methods which use an automatic approach. We also demonstrate that ELVISort can exploit the massively parallel processing capabilities of GPUs via deep learning frameworks, with the potential to be used on other cutting-edge AI specific hardware such as TPUs.

Effect of decomposition of chemically precipitated Al(OH)3 on nano silicon carbide experimental and reaxff molecular dynamic study

Chemically precipitated Al(OH)3 on SiC particles is effective and economical additives as sintering aids for silicon carbide materials. Nanocomposite SiC powder was produced by chemically precipitated Al(OH)3 via heterogeneous nucleation. Produced powder was calcined under controlled atmosphere at 1350 °C for 1 h. Effect of calcination of Al(OH)3 on the nano sized SiC particles was investigated experimentally by using XRD, Thermogravimetric Analysis and Transmission Electron Microscope. Molecular dynamic study using ReaxFF reactive force field was performed to understand the mechanism of Al2O3 and SiC interaction at elevated temperatures under oxygen free environment. The experimental analyses showed that polymorphic transition sequence of alumina, which was precipitated on the SiC nanoparticles, was retarded to the relatively higher temperatures. Molecular dynamic study revealed that silicon site was more prone to oxidation and silicon atoms preferred to react with Al-O species.

Structural and Electronic Properties of Zigzag Single Wall (8,0), (9,0), and (10,0) Silicon Carbide Nanotubes Sandwiched Between Different Semiconductor and Transition Metal Elements

Using first principles density functional theory calculations, the electronic and geometric structures of three different types of zigzag single wall silicon carbide nano-tubes (ZSSiCNt), (8,0), (9,0), and (10,0), have been studied by sandwiching nano-tubes between different semiconductors, group three elements, and some transition metal elements. When the (9,0) zSSiCNt is sandwiched between different semiconductors, the binding energy (B E ) varies from 2.05 to 5.68 eV/atom. When the (10,0) zSSiCNt is sandwiched between group III elements, the B E varies from 4.35 to 6.89 eV/atom. When the (10,0) zSSiCNt is sandwiched between some transition elements, the B E varies from 1.63 to 5.97 eV/atom. The binding energy variation of SWSiCNT by substituting different elements at the site of carbon and silicon at the end of the zigzag silicon carbide nano-tubes is shown. Substitution of Fe and Ga at the site of Si increases the binding energy to 4.04 eV/atom & 6.32 eV/atom, respectively. The cohesive energy appears to saturate between 6.00 to 6.90 eV, and the stability of different (8,0) SWNt is compared.