Si Sites Research Articles

Investigating the Al/Si mixed site occupancy in the β-AlFeSi phase.

This work investigates the mixed site occupancy of aluminium and silicon atoms in the β-AlFeSi phase. For this purpose, the six mixed Al/Si sites of the β-AlFeSi structure were considered independent and alternatively substituted by Al or Si, thus generating 64 ordered structures or end-members. The enthalpy of formation of each end-member was calculated by DFT. These calculations allowed us to derive the enthalpy of mixing of the solid solution at 0 K, over a wide range of chemical compositions, from the Al-Fe binary system to the Si-Fe binary system. In addition, the heat capacities of the solid solution were determined using a Debye model based on the calculation of the elastic constants and the equation of state of each end-member. These heat capacity values were used along with the enthalpy of formation we calculated to determine the Gibbs free energies of all the end-members of the β-AlFeSi structure. Finally, the configurational entropy of mixing from the Compound Energy Formalism (CEF) for the configurational entropy of mixing was subsequently used to calculate the occupation fractions of the Si sites on the Al sites of the β-AlFeSi structure, at 300 K and 938 K, the latter being the thermal decomposition temperature of this compound. These original site occupancy data were used to quantify the chemical ordering of the solid solution and to compare different sublattice (SL) models. We thus highlight that the SL model of the β-AlFeSi solution most commonly accepted in the literature generates considerable errors in its thermodynamic description, contrary to the model proposed in this paper, which is both simple and particularly accurate, consisting in merging the sites Al(1)-Al(6), the sites Al(2)-Al(3) as well as the sites Al(4)-Al(5).

Adsorbate-driven dynamic active sites in stannosilicate zeolites

Elucidating the nature of the active sites in heterogeneous catalysts is fundamental for understanding their reactivity and catalytic performances. Although stannosilicate zeolites have tremendous application potential for catalyzing biomass-related compounds in aqueous media, the detailed local structures and features of the real active sites and their possible structural variations under reaction conditions are poorly understood to date. In this study, a dynamic transformation of framework Sn−O−Si sites to Sn−OH/Si−OH pairs and subsequently a pseudo-Brønsted acid in stannosilicate zeolites upon molecular adsorption, which is analogous to various adsorbates/reactants under working conditions, was identified by solid-state nuclear magnetic resonance (NMR) spectroscopy for the first time, which challenges the widespread assumption that the active center structures remain rigid/stable during the catalytic process. These results provide new comprehensive insights for the fundamental understanding of the dynamic and flexible active centers and involved reaction mechanisms of novel zeolite catalysts with heterometal atoms, such as Sn, Ti, and Zr.

Ammonia Electrosynthesis with a Stable Metal-Free 2D Silicon Phosphide Catalyst.

Metal-free 2D phosphorus-based materials are emerging catalysts for ammonia (NH3 ) production through a sustainable electrochemical nitrogen reduction reaction route under ambient conditions. However, their efficiency and stability remain challenging due to the surface oxidization. Herein, a stable phosphorus-based electrocatalyst, silicon phosphide (SiP), is explored. Density functional theory calculations certify that the N2 activation can be realized on the zigzag Si sites with a dimeric end-on coordinated mode. Such sites also allow the subsequent protonation process via the alternating associative mechanism. As the proof-of-concept demonstration, both the crystalline and amorphous SiP nanosheets (denoted as C-SiP NSs and A-SiP NSs, respectively) are obtained through ultrasonic exfoliation processes, but only the crystalline one enables effective and stable electrocatalytic nitrogen reduction reaction, in terms of an NH3 yield rate of 16.12µgh-1 mgcat. -1 and a Faradaic efficiency of 22.48% at -0.3V versus reversible hydrogen electrode. The resistance to oxidization plays the decisive role in guaranteeing the NH3 electrosynthesis activity for C-SiP NSs. This surface stability endows C-SiP NSs with the capability to serve as appealing electrocatalysts for nitrogen reduction reactions and other promising applications.

Surface Immobilization of a Re(I) Tricarbonyl Phenanthroline Complex to Si(111) through Sonochemical Hydrosilylation

A sonochemical-based hydrosilylation method was employed to covalently attach a rhenium tricarbonyl phenanthroline complex to silicon(111). fac-Re(5-(p-Styrene)-phen)(CO)3Cl (5-(p-styrene)-phen = 5-(4-vinylphenyl)-1,10-phenanthroline) was reacted with hydrogen-terminated silicon(111) in an ultrasonic bath to generate a hybrid photoelectrode. Subsequent reaction with 1-hexene enabled functionalization of remaining atop Si sites. Attenuated total reflectance-Fourier transform infrared spectroscopy confirms attachment of the organometallic complex to silicon without degradation of the organometallic core, supporting hydrosilylation as a strategy for installing coordination complexes that retain their molecular integrity. Detection of Re(I) and nitrogen by X-ray photoelectron spectroscopy (XPS) further support immobilization of fac-Re(5-(p-styrene)-phen)(CO)3Cl. Cyclic voltammetry and electrochemical impedance spectroscopy under white light illumination indicate that fac-Re(5-(p-styrene)-phen)(CO)3Cl undergoes two electron reductions. Mott-Schottky analysis indicates that the flat band potential is 239 mV more positive for p-Si(111) co-functionalized with both fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene than when functionalized with 1-hexene alone. XPS, ultraviolet photoelectron spectroscopy, and Mott-Schottky analysis show that functionalization with fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene introduces a negative interfacial dipole, facilitating reductive photoelectrochemistry.

Effect of B substitution for Si on the electronic structure and magnetic properties of Heusler alloys Fe2CoSi1-xBx (x = 0–0.25)

A series of Fe2CoSi1-xBx (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) Heusler alloys has been synthesized using melt-spinning method, which can help to reduce the precipitation of the secondary phase and form a highly ordered structure. The XRD patterns indicate that, when x = 0–0.1 the Fe2CoSi1-xBx melt-spun ribbons are single Heusler phases, but when x = 0.2 and 0.25 a small amount of secondary phase appears. The lattice constant decreases with increasing B content, suggesting that B mainly enters the substitutional sites of Si. The Curie temperatures TC of Fe2CoSi1-xBx alloys are all higher than 1000 K, and a 21 K increase of TC is observed as x varies from 0 to 0.25. The hysteresis loops of Fe2CoSi1-xBx alloys show soft magnetic character. Their saturation magnetizations Ms at 5 K and 300 K are similar and close to 5μB / f.u. The zero-gap half-metallic character reported for Fe2CoSi has been confirmed in present work. But with the substitution of B for Si, the Fermi level moves from the center in Fe2CoSi DOS to the left edge of the minority energy gap in Fe2CoSi0.75B0.25, which may disturb the half-metallic character when B content is high. When × = 0–0.1, the Ms at 5 K of Fe2CoSi1-xBx agree well with the calculated total spin moments, but when the B content is high, especially when x = 0.2 and 0.25, a deviation between the experimental and theoretical results is observed, which is related to the secondary phase in these samples.

Effects of iron impurities on the photoelectric response characteristics of silicon.

Since Fe contamination is easily mixed into Si-based photoelectric materials during the fabrication process, the relative changes of energy levels and photoelectric properties of Si material mixed with Fe impurities in different occupancy sites are studied. Based on the first principle and photoelectric response theory, an analysis model of the response characteristics of Si material mixed with Fe impurities is established. The changes of the material's electronic structure are calculated, and the effects of different occupancy sites of Fe impurities on the photoelectric response characteristics of materials are comparatively analyzed. Results show that when the Fe atom occupies the tetrahedral interstitial site in Si, the energy band structure and response characteristics of the material are relatively obviously affected. In this case, the impurity energy band introduced by the Fe impurity passes through the Fermi level, leading to the disappearance of the band gap. The absorption of Si material outside the response band is significantly enhanced. And a new absorption peak is generated at about 1530nm, with an absorption coefficient of about 25513 cm-1. Thus, the Si material can produce a relatively strong response to the light beam outside the response band. In the meantime, the saturation threshold of the Si-based photoreceptor is significantly lower than that of the other two position types. For the irradiation light at the wavelength of 1319nm, the saturation power is only 0.0035 W∙cm-2. The analysis results provide a reference for the application and development of photoelectric devices.

Defect structures and dopant solution states of Hf‐doped Si3N4 ceramics

AbstractTransition‐metal‐doped silicon nitride ceramics have attracted much attention as gate materials for semiconductors because of their electrical properties as well as chemical and thermal stability. The present study aims to clarify the defect structures of Hf‐doped β‐Si3N4 by theoretical calculations and scanning transmission electron microscopy (STEM). First‐principles calculations based on a hybrid functional method were performed. It was found that Hf dopants are mainly substituted for the Si sites and can be occasionally located at interstitial sites in the lattice of β‐Si3N4. The substitution sites of Hf dopants predicted by the first‐principles calculations were also confirmed by the high‐resolution STEM images.

Electronic structure of Humble defects in Ge and Ge0.8Si0.2

The group-IV diamond-structure elements are known to host a variety of planar defects, including {001} planar defects in C and {001}, {111} and {113} planar defects in Si and Ge. Among the {001} planar defects, the Humble defect, known for some time to occur in Ge, has recently also been observed in Si/Ge alloys, but the details of its electronic structure remain poorly understood. Here we perform first-principles density functional calculations to study Humble defects in both Ge and Ge$_{0.8}$Si$_{0.2}$. We also measure the Si L$_{2,3}$-edge electron energy loss spectra both at the defect and in a bulk-like region far from the defect, and compare with theoretical calculations on corresponding Si sites in our first-principles calculations. We find that inclusion of core-hole effects in the theory is essential for reproducing the observed L$_{2,3}$ edge spectra, and that once they are included, the results provide a set of fingerprints for different types of local atomic bonding environments in Ge$_{0.8}$Si$_{0.2}$. Our first-principles calculations reveal that the Humble defects have a tendency to enlarge the electronic band gap, which may have potential uses in band engineering. The use of hybrid functionals for an improved description of the band gap in these systems is also discussed.

The S solubility of Cr and Al containing simulated low-activity waste glass

The presence of sulfur-containing molten salt phases during vitrification is a particular problem which can be detrimental to the melter. Borosilicate compositions were designed to explore both the effects of a fixed amount of Cr, as Cr2O3 or Na2CrO4, and the impact of Al2O3, on sulfur solubility as determined by progressively adding elemental S. The NABS series (with Al2O3) has a higher SO3 solubility of ∼3.5 mol% while the NBS series (without Al2O3) has a SO3 solubility of ∼2 mol%. Crystalline Cr2O3 formed in the NABS series upon the addition of S but is not present in the NBS series. At the highest S addition, a separate salt phase always forms, and was identified as Na2(S,Cr)O4 via electron dispersive spectroscopy, ultraviolet-visible spectroscopy, x-ray diffraction, and thermal analysis. Magic angle spinning nuclear magnetic resonance identified no change in the B coordination in either series as S is added, suggesting that the Na is scavenged from other locations, such as the Si sites, to charge compensate sulfate, whether in the glass or in the salt. The Cr precursor identity did not affect the observed overall behavior regarding sulfur, but did impact Cr redox as identified with Raman spectroscopy.

DFT Modelling of Li6SiO4Cl2 Electrolyte Material for Li-Ion Batteries

There is significant interest in finding a promising lithium-containing oxide that can act as a solid electrolyte in a rechargeable lithium-ion battery. Li6SiO4Cl2 is a candidate electrolyte material which was recently characterized using both experimental and computational techniques. In this study, density functional theory simulation was used to examine the intrinsic defects, solution of promising isovalent and aliovalent dopants, possible reaction routes for the formation of Li6SiO4Cl2, and the feasibility of incorporating additional Li in this material. The results revealed that the O–Cl anti-site cluster was the lowest energy defect in this material. The LiCl Schottky was the second lowest energy defect process, and the Li Frenkel was higher—only by 0.06 eV—than the LiCl Schottky. The candidate dopants on the Li, Si and Cl were Na, Ge and F, respectively. Substituting Al on the Si site was an efficient way of increasing the amount of Li in this material. Incorporation of extra Li (up to three) was considered and this process was endothermic. Different chemical reaction routes were constructed and their reaction energies were calculated to predict the feasibility of the formation of Li6SiO4Cl2. The formation of Li6SiO4Cl2 from constituent elements (Li, Si O2 and Cl2) is thermodynamically feasible.

Effect of Sb/Bi doping sites on electronic structure and transport properties of Mg2Si0.375Sn0.625 alloy from first-principles calculations

Defect formation energy, electronic structures, and transport properties of Sb/Bi-doped Mg2Si0.375Sn0.625 alloys were successfully determined by first-principles calculations. Si sites in these alloys were preferentially substituted by Sb and Bi atoms. At the same doping content, Sb atoms provided larger carrier effective mass and lower electron concentration compared with Bi atoms. Correspondingly, the maximum Seebeck coefficient and power factor value of Mg2Si0.365Sn0.625Sb0.01 alloy were −260 μVK−1 and 5.53 mWm−1 K−1, respectively. The highest electrical conductivity value of 1620 Scm−1 was achieved with Mg2Si0.375Sn0.615Bi0.01 alloy, due to higher electron concentration supplied by its Bi atoms. Experimental results were also obtained, verifying the reliability of calculations. These results provide theoretical reference for optimizing the performance of Sb/Bi-doped Mg2(Si, Sn)-based alloys.

Synthesis, Characterization and Reactivity of a σ‐Donating Ni0‐Stabilized Silyliumylidene Ion

AbstractThe synthesis of a silyliumylidene cation complex 2 stabilized by a Ni0‐based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic SiII center is stabilized by a dative Ni→Si σ‐interaction and π‐donations from the amino‐ and Ni‐moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

Synthesis, Characterization and Reactivity of a σ-Donating Ni0 -Stabilized Silyliumylidene Ion.

The synthesis of a silyliumylidene cation complex 2 stabilized by a Ni0‐based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic SiII center is stabilized by a dative Ni→Si σ‐interaction and π‐donations from the amino‐ and Ni‐moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

The investigation into the dehydroaromatization of ethane over cobalt-modified ZSM-5 catalyst

Ethane nonoxidative dehydroaromatization (EDA) to aromatics has been an effective way of upgrading shale gas. In this work, cobalt-modified zeolite catalysts are synthesized and applied to catalyze ethane to aromatics. Reaction conditions, zeolite topology, Co content, Al content and catalyst synthesis strategies are optimized and investigated systemically, in which the catalyst with 2.0 wt%Co over ZSM-5, SiO2/Al2O3 ratio of 60 presents the highest aromatization performance (0.0169 mmol aromatics·s−1· gCo−1) and the lowest catalyst deactivation rate (0.052 h−1). According to systematic characterizations, it is found that the tetrahedral Co2+ species is not only the active center of ethane dehydrogenation to ethylene, but also is beneficial to promote the subsequent ethylene aromatization simultaneously. And the generation rate of ethylene is the rate-limiting step of EDA. The existence of high proportion of Co2+ species and the appropriate Brønsted acid sites of Si–O(H)–Al species over 2.0Co/ZSM-5-60 catalyst are responsible for the high-performance of EDA. Finally, it is demonstrated that Co/ZSM-5-IE via ion exchange synthesis presents superior EDA catalytic performance (0.0395 mmol aromatics·s−1· gCo−1) compared with incipient wetness impregnation method, which comes from more highly dispersed and anti-reduction Co2+ species over Co/ZSM-5-IE.

Substitutional Tungsten Doping in Silicon Carbide Introducing Magnetic Properties: A Computational DFT Approach

Background: Small concentration of magnetic material, in general, the transition metal atoms (TM), when doped into a semiconductor, behaves as a diluted magnetic semiconductor (DMS). It has an application to Quantum computing & spintronic devices. DMS silicon carbide has strong coupling and high Curie temperature. The magnetic and electronic properties of SiC with TMs impurities have been in focus for theoretical and experimental researchers. Objective: The objective of this work is to study the electrical and magnetic properties of tungsten doped cubic SiC. Comparing the density of states plot with and without impurity, the change in property happening due to the presence of tungsten is observed. Partial density of states is also plotted and interpreted. Self-consistent spin polarized calculations are done to study the magnetic properties. Magnetic Moment is also calculated for substitutional doping of SiC at different sites. Methods: Tungsten doped 3C-SiC is investigated by using the first-principle energy code, Quantum Espresso, which uses pseudopotential within Density Functional Theory (DFT). The calculations are done by density functional pseudopotential energy calculations in periodic systems by solving iteratively the Kohn Sham equation in a plane wave basis set. Both norm conserving and Vanderbilt USPP are used. Self-consistent iterations were performed until convergence of total energy and total charge was obtained. We used different k-point meshes for different supercells with 16, 54 and 128 atoms giving results for carious impurity percentages. Results: The formation energy values obtained indicate that the W impurity prefers the Si site to the C site in cubic SiC. The presence of a narrow band towards the conduction band minimum is due to the W-d states for Si site substitution. Both spin-up and spin-down states contribute towards the valence band, and a small contribution goes towards the conduction band. The magnetic moment values for C site substitution are lower than Si site substitution. Conclusion: It is observed W doped with Si site of cubic silicon carbide shows ferromagnetic behavior. Hence, there is a possibility of 3C SiC doped with W at the C site to behave as a semi-insulating material.

First-principles study on optical properties of group-III doped SiCNRs

Based on first-principles calculation, the effects of replacing C or Si sites with group-III elements X (X = B, Al, Ga and In) on optical properties of SiCNRs were studied. The results have shown that, except for the intrinsic impurity like level, XSi-SiCNRs appears an acceptor level and a deep level. XC-SiCNRs appears only one deep level. In addition, SiCNRs has a significant photoelectric response in 170–180 nm, InC-ZSiCNRs also has a significant photoelectric response in visible region. SiCNRs of group-III to replace C have different dielectric responses in the near UV–visible. InC-SiCNRs presents a new dielectric response near 280 nm.

First-Principles Study of Metal Impurities in Silicon Carbide: Structural, Magnetic, and Electronic Properties

The configurations of 10 types of metal-doped silicon carbide (SiC) systems were investigated by the first-principles calculations. The dopants include eight types of 3d-series transition metal atoms, one semi-metal Ge atom, and one other metal Al atom. For all the metal-doped SiC systems, the steadiest doping sites are fixed at the substituted Si site, while the Ti-SiC system exhibits the most potent binding activity. The properties of these new systems vary with the doping atoms. The SiC- and Al-SiC systems convert to magnetic metals. The Ti- and Ge-SiC systems remain non-magnetic semiconductors, while the V-, Cr-, Mn-, Fe-, Co-, and Zn-SiC systems turn into magnetic semiconductors with magnetic moments related to the valence electron number of dopants. Partial charge transfers from the metal atoms to the adjacent C atoms accompanied the change in the electron-emitting capacity of the new systems. The work function achieves the minimum of 3.439 eV in the Co-SiC system, just 71.6% of the original SiC system. Our analysis indicates that the potent binding energy of the Ti-SiC system is due to the complete bonding states between the transition metal Ti and the adjacent C atoms. The magnetism evolution in semiconducting metal-doped SiC is attributed to the occupation mode of the hybridization orbitals nearby the Fermi level, which are determined by the coupling of the 3d orbital of transition metal atoms and the defect states of the vacancy atoms. The adjustable magnetic and electronic properties of the metal-doped SiC systems provide a flexible method in designing more suitable SiC-based spintronics and field electron-emitting devices.

Strong-coupling superconductivity with Tc ∼ 10.8 K induced by P doping in the topological semimetal Mo5Si3

By performing P doping on the Si sites in the topological semimetal Mo5Si3, we discover strong-coupling superconductivity in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0). Mo5Si3 crystallizes in the W5Si3-type structure with space group of I4/mcm (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter a decreases while c increases monotonously. Bulk superconductivity is revealed in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0) from resistivity, magnetization, and heat capacity measurements. Tc in Mo5Si1.5P1.5 reaches as high as 10.8 K, setting a new record among the W5Si3-type superconductors. The upper and lower critical fields for Mo5Si1.5P1.5 are 14.56 T and 105 mT, respectively. Moreover, Mo5Si1.5P1.5 is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo5Si3. The Tc and upper critical field in Mo5Si3−xPx are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W5Si3-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

Highly Active Si Sites Enabled by Negative Valent Ru for Electrocatalytic Hydrogen Evolution in LaRuSi.

The discovery and identification of novel active sites are paramount for deepening the understanding of the catalytic mechanism and driving the development of remarkable electrocatalysts. Here, we reveal that the genuine active sites for the hydrogen evolution reaction (HER) in LaRuSi are Si sites, not the usually assumed Ru sites. Ru in LaRuSi has a peculiar negative valence state, which leads to strong hydrogen binding to Ru sites. Surprisingly, the Si sites have a Gibbs free energy of hydrogen adsorption that is near zero (0.063 eV). The moderate adsorption of hydrogen on Si sites during the HER process is also validated by in situ Raman analysis. Based on it, LaRuSi exhibits an overpotential of 72 mV at 10 mA cm-2 in alkaline media, which is close to the benchmark of Pt/C. This work sheds light on the recognition of real active sites and the exploration of innovative silicide HER electrocatalysts.

Highly Active Si Sites Enabled by Negative Valent Ru for Electrocatalytic Hydrogen Evolution in LaRuSi

AbstractThe discovery and identification of novel active sites are paramount for deepening the understanding of the catalytic mechanism and driving the development of remarkable electrocatalysts. Here, we reveal that the genuine active sites for the hydrogen evolution reaction (HER) in LaRuSi are Si sites, not the usually assumed Ru sites. Ru in LaRuSi has a peculiar negative valence state, which leads to strong hydrogen binding to Ru sites. Surprisingly, the Si sites have a Gibbs free energy of hydrogen adsorption that is near zero (0.063 eV). The moderate adsorption of hydrogen on Si sites during the HER process is also validated by in situ Raman analysis. Based on it, LaRuSi exhibits an overpotential of 72 mV at 10 mA cm−2 in alkaline media, which is close to the benchmark of Pt/C. This work sheds light on the recognition of real active sites and the exploration of innovative silicide HER electrocatalysts.