Presence Of Silicon Atoms Research Articles

X-ray Absorption Near Edge Structure (XANES) Spectrum Interpretation for LiFePO4-Silicon Doped Cathode Material

LiFePO4-Silicon doped has been widely developed as a battery cathode material. Due to the doping of silicon atoms, this study aims to determine the peak energy (E0), oxidation state and percentage of component composition for Fe K-edge. The experimental sample used are LFP Si-0%, LFP Si-1%, LFP Si-3% and LFP Si-6%. These samples were characterized using XAS (X-ray Absorption Spectroscopy). XANES spectrum showed the peak energy of Pre-edge area is 7113.08 – 7114.49 eV. The oxidation state of Fe atoms were increased and tend to be Fe3+ than Fe2+. Moreover, Linear Combination Fitting (LCF) used to determine the ratio composition FeO/Fe2O3 in experimental samples. The ratio composition indicated Fe atom is multivalent, when the percentage of silicon-doped increases, the ratio of Fe3+ increased.It has been found that the presence of silicon atoms in the LiFePO4 structure has an effect in electronic structure.

Role of Chemical Etching in the Nucleation of Nanopores in 2D MoS2: Insights from First-Principles Calculations

Nanoscale devices made using two-dimensional (2D) materials, including transition-metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2), are currently being explored for various applications, e.g., optoelectronic devices, water desalination membranes, and DNA sequencing. In such applications, defects and nanopores play a key role in modulating the 2D material’s physicochemical properties, e.g., the band gap, chemical reactivity, catalytic activity, and molecular/ionic permeation rate. However, there exists a lack of a fundamental understanding of nanopore formation in TMDs, especially MoS2. In this work, we elucidate the mechanism of nanopore formation in 2D MoS2 using an extensive set of first-principles density functional theory (DFT) calculations. We calculate the energies of etching of atoms using DFT and use Marcus theory for atom-transfer reactions to convert the energies to activation barriers. We postulate the role of silicon as an etchant and show that the MoS6 vacancy can be a potential nucleation site for the growth of nanopores in MoS2. By fitting our kinetic predictions to experimental microscopy data on the formation time of a MoS6 vacancy, we show that the intrinsic barrier for the etching of atoms in MoS2, as described by Marcus theory, is 0.228 eV in the presence of silicon atoms. We find that the slowest step in the formation of the MoS6 vacancy is the step leading from a S5 vacancy to a S6 vacancy, with an activation barrier of ∼0.85 eV. This allows atoms to be etched at close to room-temperature conditions (27 °C) under the action of an electron beam that can move around the silicon etchant atoms. Using this new understanding of the nanopore formation process, researchers will be able to accurately model the controlled fabrication of nanopores in 2D MoS2.

Controlling Calcium Carbonate Particle Morphology, Size, and Molecular Order Using Silicate

Calcium carbonate (CaCO3) is one of the most abundant substances on earth and has a large array of industrial applications. Considerable research has been conducted in an effort to synthesize calcium carbonate microparticles with controllable and specific morphologies and sizes. CaCO3 produced by a precipitation reaction of calcium nitrate and sodium carbonate solution was found to have high polymorphism and batch to batch variability. In this study, we investigated the polymorphism of the precipitated material and analyzed the chemical composition, particle morphology, and crystalline state revealing that the presence of silicon atoms in the precipitant is a key factor effecting particle shape and crystal state. An elemental analysis of single particles within a polymorphic sample, using energy-dispersive X-ray spectroscopy (EDS) conjugated microscopy, showed that only spherical particles, but not irregular shaped one, contained traces of silicon atoms. In agreement, silicon-containing additives lead to homogenous, amorphous nanosphere particles, verified by X-ray powder diffraction (XRD). Our findings provide important insights into the mechanism of calcium carbonate synthesis, as well as introducing a method to control the precipitants at the micro-scale for many diverse applications.

Role of Silicon Carbide in Arc Discharge Synthesis of Higher Fullerenes

The effects that the presence of silicon atoms in the zone of electric arc used for evaporation of graphite electrodes has on the efficiency of formation of higher fullerenes C76, C78, C80, C82, C84, C86, etc., are studied. Optimization of the evaporation conditions and the use of composite graphite electrodes containing silicon carbide, the additive, in the range of 0 to 6.3 wt % enables us to produce soot with the fullerene content at up to 15.5 wt % in a soot extract and, at the time, achieve a high net fullerene yield of 10.5% of the soot weight.

The effect of heteroatoms in carbonaceous surfaces: computational analysis of H chemisorption on to a PANH and Si-doped PAH

ABSTRACT In this work, we studied the effect of a heteroatom (nitrogen and silicon) inside the main skeleton of the carbonaceous surface in the H chemisorption reaction. The process taking place on to an N-doped polycyclic aromatic hydrocarbon (PAH), known as PANHs, shows differences in the energetic parameters only when the process is carried out on to the N atom. When N is located in an external site of the surface, the process is barrierless, whereas if N is in an internal position of the surface the activation energy drastically increases. The aromaticity of these N-doped systems does not change much concerning pristine coronene. In a Si-doped PAHs, the chemisorption on to the Si atom takes place in the absence of activation energy, regardless the position of Si on the surface. Moreover, the adsorption on to their neighbour carbon atoms is carried out with lower activation energies than those found in the reaction on to pristine PAH, indicating that the presence of silicon atoms in the surface favours H chemisorption. This might be due to a loss of aromaticity on the surface. In both cases, the reactions become significantly more exoenergetic. Finally, the presence of heteroatoms favours kinetically the reaction, where the rate coefficient of H2 formation process, calculated considering all of the sites of every PAH studied in this work, reaches a close value to the reported for diffuse interstellar medium and photodissociation regions ($R_{_{\mathrm{ H}\mathrm{ }_2}} = 1 \times 10^{-17}$ cm3 s−1 at 40 K).

Why does the presence of silicon atoms improve the emission properties of biphenyl derivatives? - Verification of various hypotheses by experiment and theory.

In the course of studying silicon modifications to improve emission properties of commonly used organic compounds, biphenyl with dimethylsilylvinyl groups in the para position (3-Si) was investigated. A comparative study was performed on the exact C-analogue (3-C) and expanded to biphenyl and dimethylbiphenyl to emphasize the general trend observed. Compound 3-Si displayed emission properties clearly different than all of the investigated hydrocarbon compounds, i.e. twice stronger fluorescence (Φf = 0.6) and a 3-times larger radiative rate constant as compared to 3-C in acetonitrile. Searching for the source of the unique emission of 3-Si, singlet and triplet processes were investigated for all of the compounds using steady-state and time-resolved methods, and their principal photophysical parameters are reported. Experimental work was supported by the theoretical predictions obtained using the EOM-CCSD method. The results led to the conclusion that the strong emission of 3-Si must be due to silicon's presence that enhanced intensity borrowing from the strongly allowed S0 → S2 transition and the larger S1 → S0 transition moment.

New Route for "Cold-Passivation" of Defects in Tin-Based Oxides.

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. For Sn-based TCOs, oxygen deficiencies and undercoordinated Sn atoms result in an extended density of states below the conduction band edge. Although shallow states provide free carriers necessary for electrical conductivity, deeper states inside the band gap are detrimental to transparency. In zinc tin oxide (ZTO), the overall optoelectronic properties can be improved by defect passivation via annealing at high temperatures. Yet, the high thermal budget associated with such treatment is incompatible with many applications. Here, we demonstrate an alternative, low-temperature passivation method, which relies on cosputtering Sn-based TCOs with silicon dioxide (SiO2). Using amorphous ZTO and amorphous/polycrystalline tin dioxide (SnO2) as representative cases, we demonstrate through optoelectronic characterization and density functional theory simulations that the SiO2 contribution is twofold. First, oxygen from SiO2 passivates the oxygen deficiencies that form deep defects in SnO2 and ZTO. Second, the ionization energy of the remaining deep defect centers is lowered by the presence of silicon atoms. Remarkably, we find that these ionized states do not contribute to sub-gap absorptance. This simple passivation scheme significantly improves the optical properties without affecting the electrical conductivity, hence overcoming the known transparency–conductivity trade-off in Sn-based TCOs.

Effect of Surface Structure on Transformation of 4H-SiC by High-Temperature Annealing

The transformation of SiC etching shapes by high-temperature annealing has been investigated. Without silicon atoms on the surface, transformation of the etching shapes hardly occurred even after annealing in pure Ar at 1700 °C, where transformation should occur without the loss of silicon atoms. When SiH4 was added to Ar, the surface tended to revert to SiC, and the transformation was enhanced with increasing SiH4 addition. Therefore, the presence of silicon atoms is necessary to transform the etching shapes on SiC surfaces.

Influence of Silicon Defects on the Adsorption of Thiophene-like Compounds on Polycyclic Aromatic Hydrocarbons: A Theoretical Study Using Thiophene + Coronene as the Simplest Model

Physisorption and chemisorption processes of thiophene on coronene and 2Si-coronene have been studied using density functional theory and MP2 methods. These systems have been chosen as the simplest models to describe the adsorption of thiophene-like compounds on polycyclic aromatic hydrocarbons (PAHs). The calculated data suggest that the presence of silicon atoms in PAHs could favor their interaction with thiophene and similar compounds. Small stabilization energies have been found for several physisorbed complexes. The thiophene chemisorption on coronene seems very unlikely to occur, while that on 2Si-coronene leads to addition products which are very stable, with respect to the isolated reactants. These chemisorption processes were found to be exoergic (DeltaG < 0) in the gas phase and in the nonpolar liquid phase. The results reported in this work suggest that silicon defects on extended polycyclic aromatic hydrocarbons, such as graphite, soot, and large-diameter carbon nanotubes, could make them useful in the removal processes of aromatic sulfur compounds from oil hydrocarbons.

Silicon-Atom Induced Fermi-Level Pinning of Fully Silicided Platinum Gates on HfO2 Dielectrics

Fully silicided platinum gates have been proposed as metal gate electrodes for scaled complementary metal oxide semiconductor field-effect transistors (CMOSFETs). The Pt monosilicide (PtSi) phase was formed on gate dielectrics by a full silicidation reaction at 400°C. PtSi gate electrodes on SiO2 possess a high effective work function of 4.9 eV, which could be used as a possible metal gate electrode for p-MOSFETs, while at the PtSi/HfO2 interface, Fermi-level pinning is observed independent of the PtSi fabrication process. The Fermi-level pinning at the PtSi/HfO2 interface is comparable to that at the poly-Si/HfO2 interface, and Fermi-level pinning does not occur at the Pt/HfO2 interface. These results show that Fermi-level pinning is induced by the presence of silicon atoms at the HfO2 upper interface.

Description in a Local Model of the Magnetic Field Distributions of Mechanically Alloyed Fe-Si (3.5 wt%)

A local model based on the idea that the atoms are distributed within a crystalline lattice, following a binomial probability method for describing the hyperfine field distribution (HFD) obtained from room temperature M�ssbauer spectra is presented. It is applied to disordered, 3.5 wt%, Fe–Si mechanically alloyed during 15, 30, 50 and 75 h. This model also permits, through a fitting parameter, to evaluate the influence of the alloy milling time. The theoretical curves have fitted quite well with the experimental ones and the fitting parameters show that, in agreement with the experimental data, when each silicon atom substitutes an iron atom in the nearest neighborhood, an approximately 3 T diminishment in the hyperfine field is presented. This model also allows us to conclude that the presence of silicon atoms in the next nearest neighborhood have practically no influence in the hyperfine field, because its representative parameter fits for these alloys with a zero value.