Adjacent Silicon Atoms Research Articles

Controllable Low-Bias Rectifying Behaviors Induced by AA-P2 Dopants in Armchair Silicene Nanoribbons with Different Widths

The electronic transport properties and rectifying behaviors of armchair silicene nanoribbons (ASiNRs) were investigated by using first-principles density functional theory, in which the left lead was pristine ASiNR and the right lead was doped ASiNR where two phosphorus (P) atoms replaced a pair of adjacent silicon atoms in the same sublattice A (AA-P2). Two types of AA-P2-doped models were considered for P dopant-substitute silicon atoms at the center or edges. The results showed that the rectification behavior of the system with a large rectifying ratio could be found, which could be modulated by changing the width of the silicene nanoribbons or the position of the AA-P2 dopant. Mechanisms were revealed to explain the rectifying behaviors and provide a theoretical basis for semiconductor rectifier devices.

Cycling-induced structural damage/degradation of electrode materials–microscopic viewpoint

Most analyses of the mechanical deformation of electrode materials of lithium-ion battery in the framework of continuum mechanics suggest the occurring of structural damage/degradation during the de-lithiation phase and cannot explain the lithiation-induced damage/degradation in electrode materials, as observed experimentally. In this work, we present first-principle analysis of the interaction between two adjacent silicon atoms from the Stillinger–Weber two-body potential and obtain the critical separation between the two silicon atoms for the rupture of Si–Si bonds. Simple calculation of the engineering-tensile strain for the formation of Li–Si intermetallic compounds from the lithiation of silicon reveals that cracking and cavitation in lithiated silicon can occur due to the formation of Li–Si intermetallic compounds. Assuming the proportionality between the net mass flux across the tip surface of a slit crack and the migration rate of the crack tip, we develop analytical formulas for the growth and healing of the slit crack controlled by lithiation and de-lithiation, respectively. It is the combinational effects of the state of charge, the radius of curvature of the crack tip and local electromotive force that determine the cycling-induced growth and healing of surface cracks in lithiated silicon.

Heteroepitaxial growth on the air-solid interface: A large matching tolerance between nonpolar organic molecules on polar inorganic substrates

Inspired by the crystallization/fibrillization of proteins/peptides on the liquid–solid interfaces, here we demonstrate a novel approach to create low dimensional crystals of nonpolar organic molecules on polar inorganic substrates by the heteroepitaxial growth means on the air–solid interface. Using physical evaporation deposition (PVD) on air–solid interfaces, paraffin as a model nonpolar molecule can epitaxially grow into patterned crystal nanostructures on a polar substrate mica (001). With the atomic force microscopy and the small angle diffraction of synchrotron radiation X-ray, we figure out that the hydrocarbon chain of paraffin is perpendicular to the atomic lattice of mica, and the heteroepitaxial growth is originated from a large tolerance of the distance matching between the adjacent silicon atoms on the surface of mica and the adjacent paraffin terminals inside its crystal nanostructure. This study cannot only fill up the gap for researching the heteroepitaxial crystallization of nonpolar organic molecules on polar inorganic substrates, but also demonstrates that the approach reported in this work holds potential for creating low-dimensional molecular crystals, nanopatterns and functional nanofilms.

Non-symmetrical bis-silylated precursor can (also) self-direct the assembly of Silsesquioxane films

Symmetric α,ω-bis-silylated precursors are the standard building blocks of self-assembled bridged silsesquioxanes, a unique class of robust ordered nanomaterials prepared by sol-gel process without external surfactant. We report an unprecedented approach based on the utilization of a dissymmetric bis-silylated precursor, 1,2-bis(trimethoxysilyl)decane (1), in which the two alkoxy groups are carried by adjacent methylene groups. Extensive characterization—based on X-ray diffraction, real-time fourier transform infrared, electron and optical microscopy, 29Si solid-state nuclear magnetic resonance, thermogravimetry, and molecular modeling—shows surprisingly that such non-symmetrical precursor is highly conducive to achieve highly ordered lamellar mesostructure, able to sustain temperature up to 120 °C. To emphasize the effect of the alkoxy group functionality and position, comparison is made systematically with analogous silsesquioxanes derived from bis-(2) and mono-silylated (3) precursors. The sol-gel polymerization of 1 is unique by its ability to produce a homogeneous film possessing structural characteristic on multiple scales: uniform microcrystallites consisting of nanolamellar organosilica hybrid material. The most likely mesostructure consists of bilayers of slightly interpenetrated trans C8H9 chains, with a single central siloxy-hydrocarbon chain (Si2O(OH)4-C2H3) n . To permit a good lateral chain packing, the hydrocarbon chain of two adjacent silicon atoms point in opposite directions.

First-Principles Study of Nonradiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol

The photophysical properties of emissive silicon nanomaterials depend strongly on the chemical composition and structure of their surfaces, and the development of a causal, microscopic understanding of this relationship is highly desirable. One surface-dependent property of interest is the propensity for nonradiative recombination (NRR). In this work, we apply ab initio theoretical methods to investigate the mechanism of NRR in a silicon nanocrystal with a single surface silanol group. Ab initio multiple spawning simulations of an electronically excited cluster model (Si7H11OH) indicate ultrafast nonradiative decay to the electronic ground state. A multireference electronic structure study demonstrates that this nonradiative decay occurs near conical intersections between the ground and first excited electronic states of the cluster. These intersections are accessed after stretching of the bond between the silanol silicon atom and an adjacent silicon atom. The presence of this intersection in a true nanomaterial is confirmed by optimization of a similar conical intersection in a silicon nanocrystal (oblate, major diameter 1.4 nm, minor diameter 1.0 nm) with a silanol group on the surface (Si44H45OH). This intersection was identified using a graphics processing unit accelerated implementation of the configuration interaction singles natural orbital complete active space configuration interaction method. All intersections identified in this work are predicted to be at least 4.3 eV above the ground state minimum energy. This confirms the widely held view that silanol groups do not introduce efficient pathways for nonradiative recombination of excitons created upon absorption of visible light. That such an assignment is made entirely from first-principles underscores the value of conical intersection optimization as a tool for elucidating semiconductor photophysics.

Experimental observation of two-dimensional charge polarization in unisized platinum cluster disk bonded to silicon(1 1 1) surface

A local density of electronic states and a local HOMO–LUMO gap of a monatomic-layered platinum cluster with a size of 30 (Pt 30 cluster disk) on a silicon(1 1 1) surface were measured by space-resolved tunneling spectroscopy. It has been found that (1) both occupied and unoccupied electronic orbitals are delocalized due to Pt–Pt metallic interaction in the cluster disk and (2) electrons in the occupied orbitals of the cluster disk are transferred to the adjacent silicon atoms so as to induce two-dimensional charge polarization of the cluster disk. The center of the cluster disk is charged positively, while the boundary between the cluster disk and the silicon surface is charged negatively.

Cooperative molecular dynamics in surface reactions

The controlled imprinting of surfaces with specified patterns is important in the development of nanoscale devices. Previously, such patterns were created using self-assembled physisorbed adsorbate molecules that can be stabilized on the surface by subsequent chemical bonding. Here we show a first step towards use of the bonding within a surface to propagate reactions for patterning, namely the cooperative reaction of adjacent silicon atoms. We exploit the double-bonded silicon dimer pairs present on the surface of Si(100)-2×1 and show that the halogenation of one silicon atom (induced by electrons or heat) results in cooperative halogenation of the neighbouring silicon atom with unit efficiency. The reactants used were two 1-halopentane molecules physisorbed over a pair of silicon atoms. This cooperative pair of halogenation reactions was shown by ab initio calculation to be sequential on a timescale of femtoseconds.

Thin Oxidation Study with Various Thermodynamic Parameters

The structure analysis of extremely thin thermally grown SiO2 (< 25 Aå) indicates that most silicon atoms share two of the four tetrahedrally-coordinated oxygen atoms with the adjacent silicon atoms. Molecular mechanics simulation of the single chain of SiO2 tetrahedrons is presented. The extremely thin oxide film is composed of the close-packed short chains of the tetrahedrons perpendicular to Si substrate. Diffusion experiment of an acid vapor through non-oxide polyhydroxystyrene films de-convolutes four prototropic states of fluoresceinamine monolayer to resemble the change of the intermediate states of Si atoms during the thermal oxygen diffusion processes at Si/SiO2 interface.

Evidence for a chromasiloxane ring size effect in Phillips (Cr/SiO 2) polymerization catalysts

The ambient temperature reaction of CrO 2Cl 2 with silica, followed by mild heating to induce formation of uniform grafted silylchromate diesters, was studied as a function of the silica pretreatment temperature. The reactivity of the resulting chromate sites toward ethylene is qualitatively different: those formed on the silica pretreated at 200 °C are incapable of initiating polymerization, while those formed on silicas pretreated at 450 and 800 °C spontaneously induce polymerization with kinetic profiles closely resembling that of the calcined Phillips catalyst (CrO x /SiO 2). Comparison of their X-ray absorption spectra suggests subtle differences in the chromate–support interactions, which can be interpreted in terms of changes in the chromasiloxane ring size distribution. The unstrained sites favored on the highly hydroxylated silica surface are consistent with 8-membered chromasiloxane rings formed by attachment of the CrO 2 fragment to non-vicinal hydroxyls, while the strained sites on highly dehydroxylated silica surfaces are suggested to be 6-membered chromasiloxane rings created from vicinal hydroxyls located on adjacent silicon atoms. Simple computational models for these sites predict changes in the vibrational spectra and the XANES that are consistent with experimental observations.

Syntheses, Structures, and Reactions of 2,2,3,3-Tetrakis(trifluoromethanesulfonato)tetrasilanes: Hexacoordination ([4 + 2] Coordination) of the Two Central Silicon Atoms

The 2,2,3,3-tetrakis(trifluoromethanesulfonato)tetrasilanes RMe2Si−Si(OTf)2−Si(OTf)2−SiMe2R (OTf = OSO2CF3; 4a, R = Me; 4b, R = Et; 4c, R = t-Bu), a series of novel 2,2,3,3-tetrafunctionalized tetrasilanes, were synthesized by reaction of the respective 2,2,3,3-tetraphenyltetrasilanes 3a−3c with trifluoromethanesulfonic acid. The structural characterization of 4a−4c by single-crystal X-ray diffraction revealed a distorted octahedral coordination (distorted tetrahedral coordination, with the capping of two of the tetrahedral faces by oxygen atoms opposite short Si−O bonds) of the two central silicon atoms due to Si···O interactions with one oxygen atom of each of the two trifluoromethanesulfonato groups on the adjacent silicon atom. As demonstrated by the reaction of 4a with methylmagnesium chloride, acetyl chloride, and 2-propanol, the title compounds can serve as versatile starting materials for the synthesis of other 2,2,3,3-functionalized tetrasilanes.

1,5-Anti Stereocontrol in the Boron-Mediated Aldol Reactions of β-Alkoxy Methyl Ketones: The Role of the Formyl Hydrogen Bond

The boron-mediated aldol reactions of certain types of beta-alkoxy methyl ketone show remarkably high levels of stereoinduction with achiral aldehydes, leading preferentially to 1,5-anti related stereocenters. Given the low levels of asymmetric induction usually observed in acetate aldol reactions, this is of great synthetic utility and has been used successfully in the total synthesis of a number of polyketide natural products. We have investigated the effects of the alkoxy protecting group (OMe, OPMB, PMP acetal, tetrahydropyran, and OTBS) present in the boron enolate on the level and sense of remote 1,5-stereoinduction, using density functional theory calculations (B3LYP/6-31G**). Our predictions of diastereoselectivity from comparison of the competing aldol transition structures are in excellent qualitative and quantitative agreement with experimentally reported values. We conclude that the boron aldol reactions of unsubstituted boron enolates proceed via boat-shaped transition structures in which a stabilizing formyl hydrogen bond exists between the alkoxy oxygen and the aldehyde proton. It is this interaction that leads to preferential formation of the 1,5-anti adduct, by minimizing steric interactions between the beta-alkyl group and one of the ligands on boron. In the case of silyl ethers, the preference for this internal hydrogen bond is not observed due to the size of the protecting group and the electron-poor oxygen atom that donates electron density into the adjacent silicon atom. We show that this stereochemical model is also applicable in rationalizing the 1,4-syn stereoselectivity of boron aldol reactions involving certain alpha-chiral methyl ketones. These detailed results may be summarized as a conformational diagram that can be used to predict the sense of stereoinduction.

Addition of POSS−T8to the Si(100) Surface

The interaction of the polyhedral oligomeric silsesquioxane (POSS) T8, (HSiO3/2)8 with the Si(100) surface has been studied using a cluster model, represented by the single dimer Si9H12 to represent the surface. Hartree−Fock (HF), density functional theory (DFT), and multi-configuration self-consistent field (MCSCF) wavefunctions have been used to optimize the geometries of stationary points on the potential energy surface. The HF and MCSCF wavefunctions are augmented by second-order perturbation theory for improved relative energies. The POSS reacts with the Si(100) surface saturating dangling bonds on the surface, yielding two possible addition products. One of these forms a Si−Si bond between the POSS and the dimer at the same time that a hydrogen atom is transferred from the POSS to the adjacent silicon atom of the same dimer, with no structural modification of the cage. The other product involves the opening of the POSS cage with the corresponding formation of a pair of Si−Si and Si−O bonds with the ...

Angle-resolved study of hydrogen abstraction on Si(1 0 0) and Si(1 1 1): Evidence for non-activated pathways

The abstraction of chemisorbed hydrogen on Si(1 0 0) and Si(1 1 1) induced by atomic hydrogen has been investigated by studying with a rotatable mass spectrometer the angle-resolved molecular hydrogen desorption from a Si surface exposed to a chopped beam of atomic hydrogen. The angular distributions of desorbing molecules can be fitted independent of the surface temperature and the surface reconstruction by a cos n θ function with n < 1 for Si(1 0 0) and Si(1 1 1). These results are interpreted by non-activated pathways involving site-specific hot-atom abstraction on two adjacent silicon atoms with one having a dangling bond. Possible mechanisms according to the surface reconstructions are discussed.

Synthesis, stereochemistry and chiroptical properties of naphthylphenyl-substituted optically active oligosilanes with α,ω-chiral silicon centers

Novel naphthylphenyl-substituted optically active oligosilanes with α,ω-chiral silicon centers were synthesized. The absolute configuration of (1 R,2 R)-1,2-dimethyl-1,2-di(1-naphthyl)-1,2-diphenyldisilane ( R, R)- 3, one of the oligosilanes, was determined by X-ray diffraction, which gave a direct evidence for the retention and inversion stereochemistry of attacking silylanion and attacked chlorosilane. The intense π–π interaction of aryl substituents on chiral silicon centers enhanced by σ–π conjugation with oligosilane unit made it possible to clearly assign the absolute configuration and stable conformation of the optically active oligosilanes. The disilane ( R, R)- 3 showed positive exciton chirality originated from π–π interactions of 1B b,Np transition bands of two naphthyl chromophores on the adjacent silicon atoms. Contrary to this, (1 R,3 R)-1,3-di(1-naphthyl)-1,3-diphenyl-2-trimethylsilyl-1,2,3-trimethyltrisilane ( R, R)- 5, having silylene spacer with bulky trimethylsilyl group as branched substituent between two chiral centers, showed positive exciton chirality by interaction between red-shifted 1L a,Ph and 1B b,Np on the same silicon atom. (1 R,3 R)-1,3-Di(1-naphthyl)-1,3-diphenyl-1,2,2,3-tetramethyltrisilane ( R, R)- 4, which has dimethylsilylene group between two chiral silicon centers, showed two exciton chiralities, namely, positive chirality by 1L a,Ph and 1B b,Np on the same silicon atom, and negative chirality by two 1B b,Np on the adjacent silicon atoms. The intensified negative Cotton effect at 234 nm was caused by overlapping of Cotton effects of two exciton chiralities.

Quadruplevoids in amorphous Si : H

The mechanism of the Staebler–Wronski effect is still in question. We assume that a defect precursor A is transformed into the actual, metastable defect B by excitation and subsequent electron trapping. Calculations on two large, distorted silicon clusters are presented which contain a bent and a pyramidal void, respectively, created by removal of four adjacent silicon atoms. A QM/QM embedding procedure is employed using a density functional (BP86) and a semiempirical method (AM1). Full geometry optimization results in novel and unprecedented structure information. Excitation energies and electron attachment energies are presented. These findings substantiate the new ideas regarding the nature of A and B.

Electrostatic effects on the catalytic cleavage of ammonia at the stepped Si(111)-2×1 surface

We performed semiempirical AM1 molecular orbital calculations to study the dissociation of ammonia near reconstructed clean and stepped Si(111)-2×1 surface models. We also investigated the NH3→NH2+H process in the gas phase in the absence and presence of external electrostatic fields by ab initio calculations including electron correlation. It was found that the reaction mechanism near the surface essentially differs from that of the dissociation in the gas phase because in the former case both the leaving hydrogen atom and the remaining NH2 fragment bind to adjacent silicon atoms of the surface thereby making the structure of the transition-state complex different. The presence of a step on the reconstructed surface leads to a reduction of the activation energy by 41kJ/mol. Comparative calculations indicate that this is due to the electrostatic effects, the enhanced field provided by the presence of the reconstructed step stabilizes the transition state of the reaction that is much more polar than the initial state. Our study provides evidence for electrostatic catalysis in this specific case which means that, like in case of enzymes and some other condensed systems, the strong electrostatic field of the catalyst stabilizes the transition-state complex that is much more polar than the initial state.

Dynamic model for the structure of bond-centered muonium in silicon.

Ab initio restricted Hartree-Fock calculations have been carried out to determine the most stable structures for bond-centered muonium in crystalline silicon, the model generally accepted to explain the anomalous muonium state or ${\mathrm{Mu}}^{\mathrm{*}}$. The silicon crystal was represented by either the ${\mathrm{Si}}_{20}$${\mathrm{H}}_{30}$ or the ${\mathrm{Si}}_{26}$${\mathrm{H}}_{30}$ cluster. In addition to the zero-point motion of the muon, the motion of the adjacent silicon atoms has also been considered.

Step relaxation and surface stress at H-terminated vicinal Si(111)

Atomic relaxations at a newly discovered step structure on H-terminated Si(111) surfaces, miscut in the < 1 1 2> direction and prepared by a new wet chemical technique, are quantified by first-principles calculations and infrared absorption measurements. A strong HH steric interaction within this step structure produces a 20° SiH bond rotation and a large (≈0.15 Å) displacement of the adjacent step-edge silicon atoms. The range of the perturbations,however, remains confined to ≈ 10 Å of the step edge.

Structures of small hydrocarbons adsorbed on Si(001) and Si terminated β-SiC(001)

In this paper a theoretical examination of the adsorption of hydrocarbons atop Si(001) and silicon terminated β-SiC(001) is presented. In each case the addition of a hydrocarbon species on top of each silicon dimer of the originally clean (2 × 1) silicon terminated substrate is examined. Both C 2H 2 and C 2H 4 species are considered. The total energies of the various potential candidate structures are determined to find the preferred structure. Each structure is obtained by varying the atomic positions in the first four layers of the appropriate unit cell to obtain the minimum total energy configuration for each proposed structure. The resulting structures are then compared to see whether one particular structure is strongly preferred or whether more than one structure may coexist. The results for the two substrates are compared and show that a c(2 × 2) arrangement of carbon dimers is strongly preferred for the SiC(001) substrate. However this structure is not determined for the Si(001) substrate, where the bridging of adjacent silicon atoms by carbon pairs is found to be more energetically favourable.

Synthesis and reactions of silanes containing two triflate groups

1,2-Bis(trifluoromethanesulfonyloxy)tetramethyldisilane ( 1) and dimethylsilylbis-(trifluoromethanesulfonate) ( 2) have been prepared via displacement of phenyl, chloro, and methyl groups in the corresponding mono- and di-silanes. Phenyl groups are displaced more rapidly than chloro and methyl groups. The unreacted groups are strongly deactivated by the presence of a triflate group at the same silicon atom. The deactivation is much weaker when a triflate group is present at the adjacent silicon atom. Alcohols and amines react more rapidly with ditriflates than with monotriflates.