Bond Angle Deviation Research Articles

A new look at an explanation of band gap of PECVD grown a-Si:H thin films using absorption spectra, spectroscopic ellipsometry, Raman, and FTIR spectrosopy

Amorphous silicon is seen to be among the most promising materials for solar cell applications because of its higher temperature coefficient of resistance, adjustable forbidden-band gap, enhanced optical absorption, and capacity to reduce solar module costs. Changes in deposition rate, absorption coefficient and optical band gap of a-Si:H films with varying silane flow rate show that SiH4 molecules density influence the plasma growth process. Silane flow rate affects the percentage of silicon atoms during processing. The deposition rate of a-Si: H films increased from 14.36 nm/min to 15.97 nm/min by increasing silane flow rate from 25 sccm to 45 sccm. Broadening of optical band is observed. Films deposited at 35 sccm of silane flow rate shows the higher absorption coefficient. All films are characterized by FTIR, Raman spectroscopy, spectroscopic ellipsometry and optical absorption to understand the changes in band gap. The band gap Eg of all films is correlated with microstructure factor, Q factor, bond angle deviation Δθ and width of Si–H2 mode to analyse the effect of defects, disorders and microvoids.

Unsymmetrical Bis‐Coumarin Based on o‐Hydroxy Aldehydes and 4‐Hydroxy Coumarin: Expedient Synthesis, Single Crystal X‐rays and DFT Studies

AbstractBis‐coumarins are distinguished derivatives of coumarin which bear numerous pharmaceutical and other applications. In continuation of our ongoing research towards the development of benign protocols for various heterocyclic systems, herein we report an Amberlyst 15 mediated synthesis of unsymmetrical bis‐coumarins via in situ generation and cyclization of symmetrical bis‐coumarinyl methanes derived from o‐hydroxy aromatic aldehydes and 4‐hydroxy coumarin. Aldehydes of both electron‐donating and electron‐withdrawing units underwent smooth conversion to their respective unsymmetrical bis‐coumarins (76–98 % yield) without leaving any side products or intermediates. The catalyst used in the present protocol is cost‐effective, eco‐friendly, and readily available which could be recovered and reused several times. This methodology offers operational simplicity, low catalyst loading, and avoids the use of toxic solvents/reagents and the product can be isolated without chromatography. DFT study indicated that the driving force for the cyclisation of symmetrical bis‐coumarin is the minimization of the tetrahedral bond angle deviations along with other small non‐covalent interactions. Some DFT data was also verified with the data obtained from the X‐rays crystallographic study.

Sensing cyclosarin (a chemical warfare agent) by Cucurbit[n]urils: A DFT/TD-DFT study

The detection of toxic chemical warfare agents (CWAs) has been a prime aim of the scientific community. The non-toxic nature and capability to mimic the biological systems of the Cucurbit[n]uril (CB[n]), a macrocyclic molecular family has made them good candidates for capturing and sensing the CWAs. Here density functional theory (DFT) and time-dependent density functional theory (TD-DFT) has been exploited to study the sensing of an important nerve agent i.e., cyclosarin molecule with CB[n] (n = 5–8) host molecules. M06-2X (out of twelve functionals tested) offers good agreement with the experiment in terms of mean absolute deviations (MAD) of bond lengths and bond angles. The cyclosarin binding with CB[6] is thermodynamically favoured compared to other CB[n]s indicated by vander Waals interactions through Bader's quantum theory of “Atoms in Molecules” (QTAIM) calculations. This binding stability fizzles out on further addition of cyclosarin guest (total five) with the host molecules. The inclusion complexes having cyclosarin within the CB[6-8] host cavity are thermodynamically favoured over lateral or exterior host-guest interactions. The Infra-red (IR) spectroscopic vibrational signatures indicate the charge transfer which provide stability to the inclusion complexes. The smaller Egap values in CB[6-8]_NBD (where NBD is 7-nitrobenzo-2-oxa-1,3-diazole) compared to the simple CB[6-8] indicate kinetic control and reversible nature of inclusion complex formation in the former case. The sensing of cyclosarin is not possible through naked eye because the absorption and fluorescence spectra of CB[6-8]_cyclosarin complexes fall outside the visible region of wavelengths (380–750 nm). The NBD (7-nitrobenzo-2-oxa-1,3-diazole) fluorophore attached hosts CB[n]_NBD absorb and fluoresce in visible range (380–750 nm) on cyclosarin binding, thus making them important fluorescent sensors for cyclosarin and other CWAs.

Microstructure and phonon behavior in W/Si periodic multilayer structures

The crystallinity of the tungsten (W) phase was improved with an increase in the thickness of this layer in the periodic W/Si multilayer structure. Both the α- and β-W phases were grown simultaneously and the contribution of these phases was modified by a change in the thickness of the W layers. For thinner W layers, the thermodynamically metastable β-W phase dominated, and with an increase in thickness, this phase was suppressed and the stable α-W phase became prominent. The crystallite size of these phases was almost linearly proportional to the thickness of the W layers in the multilayers. With the increase in thickness of Si layers in multilayers, Raman scattering showed a decrease in the bond-angle deviation of Si–Si bonding in the amorphous Si phase. The study revealed ordering of Si–Si bonding in the amorphous phase of Si with an increase in thickness of these layers in periodic W/Si multilayers.

Simplified quality assessment for small-molecule ligands in the Protein Data Bank

More than 70% of the experimentally determined macromolecular structures in the Protein Data Bank (PDB) contain small-molecule ligands. Quality indicators of ∼643,000 ligands present in ∼106,000 PDB X-ray crystal structures have been analyzed. Ligand quality varies greatly with regard to goodness of fit between ligand structure and experimental data, deviations in bond lengths and angles from known chemical structures, and inappropriate interatomic clashes between the ligand and its surroundings. Based on principal component analysis, correlated quality indicators of ligand structure have been aggregated into two largely orthogonal composite indicators measuring goodness of fit to experimental data and deviation from ideal chemical structure. Ranking of the composite quality indicators across the PDB archive enabled construction of uniformly distributed composite ranking score. This score is implemented at RCSB.org to compare chemically identical ligands in distinct PDB structures with easy-to-interpret two-dimensional ligand quality plots, allowing PDB users to quickly assess ligand structure quality and select the best exemplars.

Mono-, di-, tri- and tetra-silacyclobutenes: Strain energy, hyperconjugation and ring-opening reaction

The ring strain, π-σ hyperconjugation and ring-opening reaction of silicon-substituted cyclobutenes (monosilacyclobutenes, disilacyclobutadienes, trisilacyclobutenes and tetrasilacyclobutene) are systemically studied at the level of B3LYP/6–311 + G(d,p). The strengths of the ring strains are characterized by the bond angle deviation from the normal angle of hybrid orbitals of the silicon and carbon atoms. A reasonable correlation is found between the strain energy and the departure of the bond path length from a linear geometrical bond. The positions of strongly-bent σ bonds are regarded as an important factor that causes different strains in silacyclobutenes. The π-σ hyperconjugation is directly estimated using the second order perturbation energy from the NBO analysis and we found that hyperconjugation is responsible for relaxing the ring strain in silacyclobutenes. The thermal ring opening reactions of silacyclobutenes are predicted to possess a symmetry-allowed conrotatory mechanism. The contribution of the bond energy is the main factor that dictates the ring-opening process to be either endothermic or exothermic.

Density Functional Theory Calculation and Raman Scattering of the Antihistamine Ebastine

The vibrational properties of ebastine have been investigated by density functional theory (DFT) calculations and Raman scattering experiments. The DFT calculation was performed with geometry optimization and harmonic vibration using the B3LYP function with the 6-31G(d) basis set. The DFT calculated spectrum of ebastine is in good agreement with Raman scattering experiments. A good linear correlation between calculated and experimental wavenumbers has been obtained in the spectral range of 500–3250 cm–1. Also, the calculated bond lengths and bond angles of the ebastine molecule are consistent with reported X-ray diffraction results. The deviations of bond lengths and bond angles are smaller than 0.034 A and 3.1°, respectively. In addition, the experimentally observed vibrational modes have been assigned and the characteristic modes of the three parts benzhydryloxy, piperidine-butanone, and tert-butylphenyl of the ebastine molecule have been discussed, which would be helpful for future degradation and activity studies of ebastine.

Phase-microstructure of Mo/Si nanoscale multilayer and intermetallic compound formation in interfaces

Microstructural investigation of the Mo/Si nanoscale multilayers (MLs) is essential to design high-reflective nanomirrors. Raman spectroscopy showed existence of silicon layer in the amorphous structure. Small fraction of the nanocrystalline silicon (nc-Si) phase was found to be embedded in the amorphous silicon (a-Si) matrix. The disorder in a-Si is increased with the decrease in the thickness of silicon layer in the Mo/Si MLs, determined by the increased bond-angle deviation (ΔΘ) of the Si–Si network. The MLs exposed to high-temperature (HT) showed a decrease in the ΔΘ value, which signifies the relaxation of both molecular disorder and residual stress. X-ray diffraction analysis confirmed the polycrystalline bcc phase of Mo and the formation of intermetallic t-Mo5Si3 phase in the interfaces. This phase is more prominent for higher value of β (thickness ratio of high atomic weight (z) layer to ML period d, i.e. β = dMo/d). Intensity of the Mo (110) plane is decreased for the sample annealed at HT, which is directly related to the degree of amorphization and the formation of intermetallic phases. However, HT annealed Mo/Si showed the growth of Mo (220) planes for higher β value of MLs. Current findings revealed that the disorder in the a-Si and microstructure of the Mo and intermetallic phase depend on the annealing temperature and the β value. The analysis of the intermetallic phase is critical for the development of high reflective mirrors.

Computational generation of voids in a -Si and a -Si:H by cavitation at low density

Use of amorphous silicon ($a$-Si) and hydrogenated amorphous silicon ($a$-Si:H) in photovoltaics has been limited by light-induced degradation (the Staebler-Wronski effect) and low hole mobilities, and voids have been implicated in both problems. Accurately modeling the void microstructure is critical to theoretically understanding the cause of these issues. Previous methods of modeling voids have involved removing atoms according to an {\it a priori} idea of void structure and/or using computationally expensive molecular dynamics. We propose a new fast and unbiased approach based on the established and efficient Wooten-Winer-Weaire (WWW) Monte Carlo method, by using a range of fixed densities to generate equilibrium structures of $a$-Si and $a$-Si:H that maintain 4-coordination. We find a smooth evolution in bond lengths, bond angles, and bond angle deviations $\Delta \theta$ as the density is changed around the equilibrium value of $4.9\times10^{22}\ $atoms/cm$^3$. However, a significant change occurs at densities below $4.3\times10^{22}\ $atoms/cm$^3$, where voids begin to form to relieve tensile stress, akin to a cavitation process in liquids. We find both small voids (radius $\sim$3 \AA) and larger ones (up to 7 \AA), which compare well with available experimental data. The voids have an influence on atomic structure up to 4 \AA beyond the void surface and are associated with decreasing structural order, measured by $\Delta\theta$. We also observe an increasing medium-range dihedral order with increasing density. Our method allows fast generation of statistical ensembles, resembles a physical process during experimental deposition, and provides a set of void structures for further studies of their effects on degradation, hole mobility, two-level systems, thermal transport, and elastic properties.

Evolution mechanism of surface hydroxyl groups of silica during heat treatment

The surface hydroxyl groups of nano-silica have a great influence on its application properties. In this paper, the changes of the number of various hydroxyl groups after dehydroxylation and rehydration progress of silica were quantitatively analyzed by thermogravimetric (TG) analysis and 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. According to the different stability of Si-O-Si bond formed after dehydroxylation, we propose two types of hydroxyl groups depending on the relative positions of them: chain adjacent hydroxyl groups (C-OHs) and spatially adjacent hydroxyl groups (S-OHs). DFT calculations indicate that two C-OHs dehydroxylation form a four-atoms ring structure, and the resulting OSiO bond angle deviation from optimal value is up to 19.56°, and the average bond length of the SiO bond increase more than 4.66%. Further more, its rehydration reaction activation energy (Ear) is very low, even only 0.21 kJ/mol, so it is easy to re-open and return to hydroxyl groups under environmental conditions. Conversely, the S-OHs dehydroxylation form a stable multi-atoms ring. The DFT calculations are a good illustration of the phenomenon in which some unstable siloxane bonds in a dehydroxylated sample are restored to hydroxyl groups in TG analysis and 29Si MAS NMR measurements.

Pressure-induced tuning of lattice distortion in a high-entropy oxide

As a new class of multi-principal component oxides with high chemical disorder, high-entropy oxides (HEOs) have attracted much attention. The stability and tunability of their structure and properties are of great interest and importance, but remain unclear. By using in situ synchrotron radiation X-ray diffraction, Raman spectroscopy, ultraviolet–visible absorption spectroscopy, and ex situ high-resolution transmission electron microscopy, here we show the existence of lattice distortion in the crystalline (Ce0.2La0.2Pr0.2Sm0.2Y0.2)O2−δ HEO according to the deviation of bond angles from the ideal values, and discover a pressure-induced continuous tuning of lattice distortion (bond angles) and band gap. As continuous bending of bond angles, pressure eventually induces breakdown of the long-range connectivity of lattice and causes amorphization. The amorphous state can be partially recovered upon decompression, forming glass–nanoceramic composite HEO. These results reveal the unexpected flexibility of the structure and properties of HEOs, which could promote the fundamental understanding and applications of HEOs.

Hot-wire chemical vapor deposition low-loss hydrogenated amorphous silicon waveguides for silicon photonic devices

We demonstrate low-loss hydrogenated amorphous silicon (a-Si:H) waveguides by hot-wire chemical vapor deposition (HWCVD). The effect of hydrogenation in a-Si at different deposition temperatures has been investigated and analyzed by Raman spectroscopy. We obtained an optical quality a-Si:H waveguide deposited at 230°C that has a strong Raman peak shift at 480 cm−1, peak width (full width at half-maximum) of 68.9 cm−1, and bond angle deviation of 8.98°. Optical transmission measurement shows a low propagation loss of 0.8 dB/cm at the 1550 nm wavelength, which is the first, to our knowledge, report for a HWCVD a-Si:H waveguide.

Tuning charge transfer in the LaTiO3/RO/LaNiO3 (R = rare-earth) superlattices by the rare-earth oxides interfaces from a first-principles calculation

We investigate the internal charge transfer at the isopolar interfaces in LaTiO3/RO/LaNiO3 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) superlattices by means of density functional theory calculations. The charge transfer from Ti sites to Ni sites in all superlattices is induced by the electronegativity difference between the elements Ti and Ni, and the lanthanide oxides interfaces can modulate the amount of charge transfer. Comparison of the perovskite heterostructures with the different rare-earth interfaces shows that increasing the deviations of bond angles from 180.0° and the oxygen motions near the interfaces enhance charge transfer. The 4f electrons themselves of rare-earth elements have faint influences on charge transfer. In addition, the reasons why our calculated 4f states of Sm and Tm elements disagree with the experimental systems have been provided. It is hoped that all the calculated results could be used to design new functional nanoelectronic devices in perovskite oxides.

Computational molecular spectroscopy of [formula omitted] NCS: Electronic properties and ro-vibrationally averaged structure

For NCS in the X̃2Π electronic ground state, three-dimensional potential energy surfaces (3D PESs) have been calculated ab initio at the core-valence, full-valence MR-SDCI+Q/[aug-cc-pCVQZ (N, C, S)] level of theory. The ab initio 3D PESs are employed in second-order-perturbation-theory and DVR3D calculations to obtain various molecular constants and ro-vibrationally averaged structures. The 3D PESs show that the X̃2Π NCS has its potential minimum at a linear configuration, and hence it is a “linear molecule.” The equilibrium structure has re(N–C) = 1.1778 Å, re(C–S) = 1.6335 Å, and ∠e(N–C–S) = 180°. The ro-vibrationally averaged structure, determined as expectation values over DVR3D wavefunctions, has 〈r(N–C)〉0 = 1.1836 Å, 〈r(C–S)〉0 = 1.6356 Å, and 〈∠(N–C–S)〉0 = 172.5°. Using these expectation values as the initial guess, a bent r0 structure having an 〈∠(N–C–S)〉0 of 172.2° is deduced from the experimentally reported B0 values for NC32S and NC34S. Our previous prediction that a linear molecule, in any ro-vibrational state including the ro-vibrational ground state, is to be “observed” as being bent on ro-vibrational average, has been confirmed here theoretically through the expectation value for the bond-angle deviation from linearity, 〈ρ¯〉, and experimentally through the interpretation of the experimentally derived rotational-constant values.

Tunability of dielectric, optical and magnetic property by simultaneous co-substitution in LaFeO3

We report room temperature tunability of structural, dielectric, optical and magnetic properties by Ca/Mn co-substitution in LaFeO3. XRD reveals single phase formation. The co-substitution driven lattice strains are noticed in terms of changes in Fe/Mn–O–Fe/Mn bond angle and bond lengths. The XRD results on structural parameters are corroborated with Fourier transform infrared spectrum. The change in magnetic response appear to be well correlated with structural distortion in terms of Fe/Mn–O–Fe/Mn bond angle deviation and possible indirect exchange interactions between Fe and Mn via intervening oxide ion. Substantial improvement in dielectric response (εr∼103) at RT was noted on co-substitution in LaFeO3. The dielectric spectrum exhibits non Debye type behavior and the same is confirmed in modulus spectrum. Modulus spectrum confirmed the presence of concentration dependent relaxation. Dielectric and optical property seems consistent with electrical conductivity changes as a function of concentration. Evidence of thermally activated Arrhenius type conduction is noticed.

Effect of co-substitution on structural, optical, dielectric and magnetic behavior of LaFeO3

Effect of co-substitution by Na at La site and Mn at Fe site of perovskite type LaFeO3 on its structural, optical, dielectric and magnetic properties at room temperature have been studied. Single phase nanocrystalline La1−xNaxFe1−yMnyO3 (x = y = 0–0.2) have been prepared by modified Pechini route. Substantial effect of substitution on structural property has been observed, which is clearly visible in view of changes in Fe/MnOFe/Mn bond angle and lattice strain. The substitution concentration dependent structural changes have also been confirmed by Fourier transform infrared (FTIR) spectrum in terms of upward shift of stretching mode (Fe/MnO) and downward shift of bending mode of vibrations (Fe/MnOFe/Mn). Typical nanocrystalline surface morphology has been observed with visible change in grain size on changes in concentration, which is also confirmed by X-ray diffraction (XRD). These changes have profound effect on magnetic property tailoring and appear to be correlated with Fe/MnOFe/Mn bond angle deviation and indirect exchange interactions between Fe and Mn ions via intervening Oxygen ion. Dielectric response (εr ∼ 103) at room temperature was observed on Na and Mn co-substituted LaFeO3. The dielectric spectrum shows typical non Debye type behavior. Modulus spectrum reveals shift of peak position toward higher frequency side with increasing concentration i.e. concentration dependent relaxation time. Electrical conductivity shows thermally activated Arrhenius type response with a maximum of ∼2.5 × 10−4 S cm−1 at 450 K for La0.85Na0.15Fe0.85Mn0.15O3. Both grain and grain boundary contribution to conduction mechanism has been observed.

Molprobity's ultimate rotamer-library distributions for model validation.

Here we describe the updated MolProbity rotamer-library distributions derived from an order-of-magnitude larger and more stringently quality-filtered dataset of about 8000 (vs. 500) protein chains, and we explain the resulting changes and improvements to model validation as seen by users. To include only side-chains with satisfactory justification for their given conformation, we added residue-specific filters for electron-density value and model-to-density fit. The combined new protocol retains a million residues of data, while cleaning up false-positive noise in the multi- χ datapoint distributions. It enables unambiguous characterization of conformational clusters nearly 1000-fold less frequent than the most common ones. We describe examples of local interactions that favor these rare conformations, including the role of authentic covalent bond-angle deviations in enabling presumably strained side-chain conformations. Further, along with favored and outlier, an allowed category (0.3-2.0% occurrence in reference data) has been added, analogous to Ramachandran validation categories. The new rotamer distributions are used for current rotamer validation in MolProbity and PHENIX, and for rotamer choice in PHENIX model-building and refinement. The multi-dimensional χ distributions and Top8000 reference dataset are freely available on GitHub. These rotamers are termed "ultimate" because data sampling and quality are now fully adequate for this task, and also because we believe the future of conformational validation should integrate side-chain with backbone criteria. Proteins 2016; 84:1177-1189. © 2016 Wiley Periodicals, Inc.

Dependence of short and intermediate-range order on preparation in experimental and modeled pure a-Si

Variability in the short-intermediate range order of pure amorphous Si synthesized by different experimental and computational techniques is probed by measuring mass density, atomic coordination, bond-angle deviation, and dihedral angle deviation. It is found that there is significant variability in order parameters at these length scales in this archetypal covalently bonded, monoatomic system. This diversity strongly reflects preparation method and thermal history in both experimental and simulated systems. Where experiment and simulation do not quantitatively agree, this is partly due to inherent differences in analysis and time scales. Relaxed forms of amorphous Si quantitatively match continuous random networks generated by a hybrid method of bond-switching Monte Carlo and molecular dynamics simulation. Qualitative trends were identified in other experimental and computed forms of a-Si. Ion-implanted a-Si′s are less ordered than the relaxed forms. Preparation methods which narrowly avoid crystallization such as experimental pressure-induced amorphization or simulated melt-quenching result in the most disordered structures. As no unique form of amorphous Si exists, there can be no single model for the material.

Studies on the MLn(M=Na+/K+; L=NMA, H2O;n=1-6) in terms of quantum chemical method and ABEEMσπ/MM

本文使用从头计算MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)和ABEEM sp 极化力场(ABEEM sp /MM)方法, 对ML n (M=Na+/K+; L= N -methlylacetamide, H2O; n =1~6)体系的结构、结合能和电荷分布进行系统研究. 研究结果表明, ABEEM sp /MM所获得的各种物理量与从头算结果符合很好, 其中键长的平均绝对偏差(AAD)接近0.03 A, 键角的AAD小于0.69°, 结合能的AAD小于5%, ABEEM sp 的电荷分布与从头算的电荷分布的线性相关系数均在0.97以上. The structures, binding energies and charge distributions of MLn (M=Na+/K+; L=N-methylacetamide (NMA), H2O; n=1~6) were studied by high-level quantum chemical methods. In the spirit of atom-bond electronegativity equalization fluctuating charge force field model (ABEEMσπ/MM) and based on high-level quantum chemistry calculations, the interaction potential function for the above systems was constructed. The parameters were optimized and then the molecular potential functions were applied to studies of MLn (M=Na+/K+; L=N,N-methylacetamide (NMA), H2O; n=3–6). Results show that the performance of ABEEMσπ/MM potential is quite good. Compared with the results from MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p), the average absolute deviation (AAD) of bond lengths, bond angles, and binding energies from our potential are less than 0.03 A, 6.9° and 5%, respectively. The linear correlation coefficients of charge distributions from ABEEM and quantum chemical calculations are above 0.97.

Micro-Raman spectroscopy characterization of silicon with different structures irradiated with energetic Bi-ions

Researches of irradiation effects on silicon possess not only fundamental interests but also potential application prospects. Comparison studies about structural modification of silicon materials with different structures under identical irradiation conditions can reveal the irradiation mechanisms for amorphous and crystalline phases of silicon. For this purpose, amorphous silicon (a-Si) and nano-crystalline silicon (nc-Si) films as well as mono-crystalline silicon (c-Si) samples were irradiated with 6.0MeV Bi-ions at room temperature. The ion fluences are 1.0×1013, 5.0×1013 and 1.0×1014ions/cm2. All samples were analyzed by using a Raman spectrometer. The obtained results show that the crystalline fraction of c-Si and nc-Si decrease with increasing fluence, which indicates that the irradiation induces the amorphization of nc-Si and c-Si samples. In addition, the variation in Raman frequency of crystalline peak after irradiation reveals that the irradiation also results in the increased stress in crystalline phase of c-Si and nc-Si samples. As the fluence increases, the bond angle deviation and the ratio of TA to TO mode of amorphous network of a-Si and nc-Si films initially increase and then decrease by a diminishing degree, while the bond angle deviation and the ratio of TA to TO mode of amorphous network of c-Si samples increase continuously. This gives the dependence of short-range structural order of amorphous network of a-Si, nc-Si and c-Si samples on the ion fluence, which is related with the irradiation induced variation of local free energy. It is considered that the irradiation induced structural modification of silicon samples is mainly attributed to the nuclear energy loss. The irradiation effects of energetic heavy-ions on crystalline and amorphous phases of silicon have been discussed, respectively.