Ultrasoft X-ray emission and the electronic structure of the acetaldehyde molecule

The electronic structure of the acetaldehyde molecule was studied by the ultrasoft X-ray emission method with the use of quantum-chemical calculations. The OK _?? and CK _?? spectra of the compound in the gas phase were obtained. Quantum-chemical calculations were performed at the RHF/6-311++G** level. The calculation results were used to construct theoretical X-ray spectra. The experimental spectra are interpreted.     

Si X-ray absorption near edge structure (XANES) of Si,SiC, SiO2, and Si3N4 measured by an electron probe X-ray microanalyzer (EPMA)

Silicon K X-ray emission spectra of Si, SiC, Si₃N₄, and SiO₂ are measured using a wavelength dispersive electron probe X-ray microanalyzer. It is shown that the fine structures in the line shape of the low energy tail of the Kα characteristic X-ray emission spectra resemble those of the K X-ray absorption near edge structure (XANES). XANES spectra of 1 μm² area can be obtained by this method.     

X-ray emission spectroscopy and theoretical study of the electronic structure of hexamethyldisiloxane and octamethylcyclotetrasiloxane

Electronic structure of hexamethyldisiloxane and octamethylcyclotetrasiloxane has been studied by means of X-ray emission spectroscopy and quantum-chemical simulation at the density functional theory level. From the analysis of the fine structure of X-ray emission SiKβ₁-spectra and simulated densities of electronic states, the special features of chemical interactions of Si, O, and C atoms in these molecules are determined.     

An X-ray spectral and theoretical study of the electronic structure and features of interatomic interactions in phenoxysilanes

The electronic structure and features of interatomic interactions providing the Si–O–C₆H₅ bonds in H_4-nSi(OC₆H₅)_n (n = 1–4) were studied by a combined analysis of the X-ray emission and photoelectron spectroscopic data and the results of quantum-chemical calculations. Theoretical calculations were carried out using the density functional theory. The distributions of the density of states were constructed, the correlation energy diagrams were presented, and the main types of interatomic interactions in phenoxysilanes were revealed.     

Electronic structure of silanes in the semi-empirical equivalent orbital method

The problem of the prediction of the valence IPs for silanes is considered. It is shown that the data on the silicon band structure combined with the photoelectron spectra of SiH₄, and Si₂H₆ permit to obtain the parameter scale, which includes all the nearest neighbour, second neighbour and the main third neighbour interaction parameters. Using the derived parameter scale the vertical ionization potentials of Si₃H₈, SiH(SiH₃)₃, Si(SiH₃)₄, the infinite polysilane valence band structure and the inner a _1g level for disilane are calculated. All the calculated levels are located above ? 20 eV and are expected to be measurable by the He (I) photoelectron spectroscopy.     

Copper position in type-I Ba8Cu4Si42 clathrate

Local structure of Cu in a type-I Ba₈Cu₄Si₄₂ clathrate has been investigated by synchrotron X-ray powder diffraction, Cu K-edge extended X-ray absorption fine spectroscopy, X-ray absorption near edge spectroscopy (XANES) and theoretical calculation. It is found that XANES spectra cannot be explained by the substitution of Cu atoms at Si16i, and Si24k positions. Our calculations show that the binding energies of the Si atom in Si16i, Si24k and Si6c positions are 9.000, 9.495 and 8.911 eV, respectively. Both experimental and theoretical results support that Cu atoms in the type-I Ba₈Cu₄Si₄₂ clathrate, as a doped element, prefer to occupy the least-binding Si, i.e., the Si6c sites. No structural change between 112 and 300 K was observed and the (100)-faceted cubic crystal has negligible distortion/ordering according to transmission electron microscopy.     

Infrared Spectrum of the Si3H8+ Cation: Evidence for a Bridged Isomer with an Asymmetric Three‐Center Two‐Electron Si?H?Si Bond

The IR spectrum of Si₃H₈⁺ ions produced in a supersonic plasma molecular beam expansion of SiH₄, He, and Ar is inferred from photodissociation of cold Si₃H₈⁺–Ar complexes. Vibrational analysis of the spectrum is consistent with a Si₃H₈⁺ structure ( 2⁺ ) obtained by a barrierless addition reaction of SiH₄ to the disilene ion (H₂Si?SiH₂⁺) in the silane plasma. In this structure, one of the electronegative H atoms of SiH₄ donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric Si? H? Si bridge of the 2⁺ isomer with a bond energy of approximately 60 kJ mol^?1 is characteristic for a weak three‐center two‐electron bond, which is identified by its strongly IR active asymmetric Si? H? Si stretching fundamental at about 1765 cm^?1. The observed 2⁺ isomer is calculated to be only a few kJ mol^?1 less stable than the global minimum structure of Si₃H₈⁺ ( 1⁺ ), which is derived from vertical ionization of trisilane. Although more stable, 1⁺ is not detected in the measured IR spectrum of Si₃H₈⁺–Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si₃H₈⁺ in the silane plasma, in which a high barrier between 2⁺ and 1⁺ prevents the efficient formation of 1⁺ . The potential energy surface of Si₃H₈⁺ is characterized in some detail by quantum chemical calculations. The structural, vibrational, electronic and energetic properties as well as the chemical bonding mechanism are investigated for a variety of low‐energy Si₃H₈⁺ isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si₃H₈⁺–Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si₃H₈⁺ ions. Comparison with the potential energy surface of C₃H₈⁺ reveals the differences between the silicon and carbon species.     

Facial dissociations of water molecules on the outside and inside of armchair single-walled silicon nanotubes: theoretical predictions from multilayer quantum chemical calculations

The possible reaction pathways of dissociative adsorption of a single water molecule on the sidewalls of armchair (n, n) (n?=?4?C10) single-walled silicon nanotubes (SWSiNT) have been investigated by the multilayer models. Both the simplified fragment embedding and ONIOM calculations were carried out to study the diameter dependence of reactivity for the dissociation of water on SWSiNTs. The active fragments with different cluster sizes, such as Si₁₆H₁₀, Si₃₀H₁₆, and Si_10mH_4m (m?=?4?C10), were used for the multilayer calculations. The employment of the medium-sized Si₃₀H₁₆ cluster is able to reach a good balance between the computational efficiency and accuracy for the large-sized reaction system. In comparison with those full B3LYP/LANL2DZ calculations for Si(4,4) and Si(5,5) nanotubes, the approximate multilayered models can give reasonable predictions on the optimized geometries, activation energies, and exothermic energies with significant reduction in computational cost. The external complexes of the dissociative adsorption of H₂O on SWSiNTs were predicted to be more stable than those internal complexes. The convex surfaces of SWSiNTs were also more reactive to H₂O with the smaller activation barrier energies (10?C13?kcal/mol) than those (15?C22?kcal/mol) on the concave side. Both the activation barriers and exothermic energies of dissociative adsorptions of H₂O on the internal (external) sidewalls of armchair SWSiNT were found to be insensitive to the tube curvature. The passivation of the outer surface and the removal of water molecules may be crucial for the experimental preparation of the single-walled silicon nanotubes.     

Core-hole photoionization study of polysilane compounds

We present the results obtained with a new experimental set-up designed for the study of free semi-conductor clusters. This set-up is aimed to study the mass distribution of particles and the evolution of electronic properties as a function of the size, using the technique of core hole photoionization. The cluster production is based on the technique of radio-frequency discharge decomposition of a gas. We study the gaseous particles (Si _n H _x , 0<x<2n+2) generated by pure silane (SiH₄) discharges at low pressures (<10 millitorr) under continuous RF (Radio-Frequency) excitation conditions. We have studied the neutral species present in the post discharge zone and the positive ions present in the discharge. We identify the neutral species as polysilane compounds. We have also compared the ionization spectra obtained near Si-2p edge for the particles containing few silicon atoms with the spectra of SiH₄ and Si₂H₆ molecules. For these molecules, the experimental observations are in agreement with theoretical calculations.     

The electronic structure of iron carbonyl complexes as probed by X-ray emission spectroscopy and quantum-chemical calculations

The electronic structure of the polynuclear iron carbonyl complexes [Et₂N][Fe₄N(CO)₁₂], [Et₄N]₂[Fe₅C(CO)₁₄], and [Et₄N]₂[Fe₆C(CO)₁₆] has been studied by X-ray emission spectroscopy and quantum-chemical calculations. The fine structure of the FeKβ₅ X-ray emission spectra characterizes the distribution of iron valence p electrons over the molecular orbitals of the compounds. Comparison of the fine structure of the FeKβ₅ X-ray emission spectra with the densities of states of all atoms in the molecules has made it possible to determine in detail the character and specific features of chemical bonding in the complexes.     

Effect of charge on bond strength in hydrogenated amorphous silicon

We have studied the effect of excess charge on the bond strength in the silanes SiH₄ and Si₂H₆ to assess whether charge trapping in a solid-state lattice might promote the technologically important photodegradation of amorphous silicon alloys (the Staebler-Wronski effect). The calculations indicate that both positive and negative charges reduce the strength of Si? H and Si? Si bonds considerably, to the point where they may be broken easily by visible or even infrared light. © 1994 by John Wiley & Sons, Inc.     

Density functional study of XH4 (XSi and Ge) reactivity upon dissociative adsorption onto the Si(100) surface

Total energy calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) and ultrasoft pseudopotential approximation and an analysis tool of atom‐resolved density of states (ADOS) have been used to investigate (1) the energetic profiles for the possible initial dissociative adsorption of XH₄ (X?Si and Ge) onto the Si(100)? (2 × 2) surface to evaluate their reactivity and (2) the effect of surface electronic states of Si(100)? (2 × 2) on gaseous molecular precursors XH₄ (X?Si and Ge) during initial dissociative adsorption to understand the factors governing their reactivity. Our calculated lower‐energy barrier for initial dissociative adsorption of GeH₄ is due to the forming of stronger bond of Si? H between H within GeH₄ and buckled‐down Si atom on the Si(100)? (2 × 2) surface accompanying the larger extent of unbuckling of the buckled Si?Si dimer on the Si(100)? (2 × 2) surface at the transition state. Our evaluated better reactivity for GeH₄ than SiH₄ (a factor of around 14.6) is slightly larger than observed higher reactivity for GeH₄ than SiH₄ (a factor of between 2 and 5 depending on the incident kinetic energy) employed supersonic molecular bean techniques. Finally, our calculated ADOS indicate that the surface electronic states of buckled Si?Si dimer on the Si(100)? (2 × 2) surface energetically favorably participate in the transition state during GeH₄ initial dissociative adsorption to reduce the energy barrier, i.e., enhance its reactivity, in comparison with SiH₄ initial dissociative adsorption onto the Si(100)? (2 × 2) surface under the same reaction conditions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Ab Initio Chemical Kinetics for SiHx Reactions with Si2Hy (x = 1,2,3,4; y = 6,5,4,3; x + y = 7) under a‐Si:H CVD Condition

Gas‐phase reactions of SiH_x with Si₂H_y (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6–311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n‐Si₃H₇ and i‐Si₃H₇. For example, SiH₂ can associate with Si₂H₅ producing n‐H₂SiSiH₂SiH₃ exothermically by 55.8 kcal/mol; SiH₃ can undergo addition to H₂SiSiH₂ to produce n‐Si₃H₇ or associate with H₃SiSiH barrierlessly forming i‐Si₃H₇; whereas SiH can insert into one of the Si─H bonds of Si₂H₆ to give excited n‐Si₃H₇. Similarly, H₂SiSiH and SiSiH₃ can undergo insertion reactions with SiH₄ producing n /i‐Si₃H₇ intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low‐energy channel products have been calculated for the temperature range 300–1000 K by variational RRKM (Rice–Ramsperger–Kassel–Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma‐enhanced chemical vapor deposition growth of a‐Si:H thin films under practical conditions.     

Structure and stability of SiH+4

Ab initio calculations at the 4-31G level are carried out on the species SiH_n (n = 0 to 4) and the corresponding ions. SiH⁺₄ is found to distort from T_d to D_2d. C_2v, and C_3v, with the latter structure being the lowest in energy by 11 kcal/mole. Consistent with experimental mass spectroscopy, SiH⁺₄ is found to be much less stable to dissociation than CH⁺₄.     

Bildung siliciumorganischer Verbindungen. 111. Die Hydrierung Si-chlorierter,C-spiroverbrückter 2,4-Disilacyclobutane mit LiAlH4 und iBu2AlH. Der Zugang zum Si8C3H20

Formation of Organosilicon Compounds. 111. The Hydrogenation of Si-chlorinated, C-spiro-linked 2,4-Disilacyclobutanes with LiAlH₄ or iBu₂AlH. The Access to Si₈C₃H₂₀ The hydrogenation of Si-chlorinated, C-spiro-linked 2,4-disilacyclobutanes containing C(SiCl₃)₂ terminal groups with LiAlH₄ in Et₂O proceeds under complete cleavage of the fourmembered rings and under elimination of one SiH₃ group. Such, Si₈C₃Cl₂₀ 4 forms (H₃Si)₂CH? SiH₂? CH(SiH₃)? SiH₂? CH(SiH₃)₂ 4 α, and even Si₈C₃H₂₀ 4a with LiAlH₄ forms 4 α. The hydrogenation of related compounds containing however CH(SiCl₃) terminal groups similarly proceeds under ring cleavage but no SiH₃ groups are eliminated. Such, (Cl₃Si)CH(SiCl₂)₂CH(SiCl₃) 41 forms (H₃Si)₂CH? SiH₂? CH₂(SiH₃) 41 α. However, in reactions with iBu₂AlH in pentane neither the disilacyclobutane rings are cleaved nor are SiH₃ groups eliminated. Only by this method Si₈C₃H₂₀ is accessible from 4 , Si₆C₂H₁₆ 3a from Si₆C₂Cl₁₆ 3 and Si₄C₂H₁₂ 41a from 41 . C(SiCl₃)₄ cleanly produces C(SiH₃)₄. Based on the knowledge about the different properties of LiAlH₄ and iBu₂AlH in hydrogenation reactions of disilacyclo-butanes it was possible to elucidate the composition and the structures of the hydrogenated derivatives of the product mixture from the reaction of MeCl₂Si? CCl₂? SiCl₃ with Si(Cu) [1] and to trace them back to the initially formed Si chlorinated disilacyclobutanes Si₆C₂Cl₁₅Me 34 , Si₆C₂Cl₁₄Me₂ 35 , Si₈C₃Cl₁₉Me 36 and Si₈C₃Cl₁₈Me₂ 37 . Compound 4a forms colourless crystals of space group P1 with a = 799.7(6), b = 1263.6(12), c = 1758.7(14) pm, α = 103.33(7)°, β = 95.28(6)°, γ = 105.57(7)° and Z = 4.     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

Synthese und Eigenschaften einigerp-tolyl-phenylsubstituierter Di- und Trisilane und von 1,1,1,3,3,3-Hexaphenyltrisilan

The synthesis and properties of [Ph ₂ p-TolSi]₂, [Ph ₂ p-TolSi]₂SiPh ₂, [Ph ₃ Si]₂Si(p-Tol)₂, [Ph ₂ p-TolSi]₂Si(p-Tol)₂ and (SiPh ₃)₂SiH₂ are described. The silanes are identified using IR,Ra- and²⁹Si-NMR-spectroscopy.

     

Ion—molecule reactions in GeH4/SiH4 systems studied by ion trap mass spectrometry

Gas phase ion—molecule reactions occurring in GeH₄/SiH₄ systems under different partial pressures and their mechanisms have been investigated by ion trap mass spectrometry (ITMS). SiH⁺_n (n=0–3) and GeH⁺_n (n = 0–3) are the main ionic species at zero reaction time when the GeH₄: SiH₄ ratio is in the range 1:1 to 1:12. Self-condensation sequences are observed at increasing reaction times. Moreover, formation of ions containing GeSi bonds, such as GeSiH⁺_n (in = 2–5) and GeSi₂H⁺_n (n = 4, 5), occurs by reactions of Si₂H⁺_n (n = 2–5) and Si₃H⁺_n (n = 4, 5) with GeH₄. At longer reaction times, further substitution of silicon with germanium in GeSiH⁺_n (n = 2–5) ions has been observed, to give Ge₂H⁺_n (n = 2–5).     

Bildung siliciumorganischer Verbindungen. 102. Zur Umsetzung von Chlormethanen mit elementarem Silicium (Bildung und Aufklärung linearer Carbosilane)

Formation of Organosilicon Compounds. 102. Reaction of Chlormethanes with Elemental Silicon. (Formation and Investigation of Linear Carbosilanes) Reactions of CH₂Cl₂, HCCl₃ and CCl₄ with silicon (Cu catalyst) in a fluid bed at about 320°C were carried out to investigate especially the Si-rich compounds. In the reactions of CH₂Cl₂ and CHCl₃, but not of CCl₄, in addition to already published compounds Si-rich viscous products are formed. The SiCl-containing mixtures were reacted with LiAlH₄, and the SiH-containing derivatives were separated by means of HPLC. CH₂Cl₂/Si forms unbranched chains of carbosilanes as Si_nC_n–1H_4n (n = 4—12,2 terminal SiH₃ groups) and Si_nC_nH_4n+2 (n = 4—9, 1 terminal SiH₃ and 1 CH₃ group) as well as 1,3,5-trisilacyclohexanes with carbosilane chains of various length attached either to a Si atom or to a C atom. CHCl₃/Si yields in addition to unbranched chains with terminal silyl group chains with one or two C-branches and 1,3,5-trisilacyclohexanes with 1, 2, or 3 silyl substituents attached to C atoms. The structure of the isolated compounds was investigated by nmr and mass spectrometry.     

Electronic energy structure of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, AsSI,AgAsS<Subscript>2</Subscript>, and TiS<Subscript>2</Subscript> semiconductors

Quantum-mechanical calculations with the FEFF8 code were used to study the electronic energy structure of 200-atomic clusters of As₂S₃, AsSI, AgAsS₂, and TiS₂ semiconductor compounds. The calculated local partial densities of electronic states are compared with the sulfur K and L X-ray emission spectra and sulfur K absorption spectra for fine powders of these compounds. Good agreement between theory and experiment has been obtained.