Formation of gold-capped silicon nanocolumns on silicon substrate

Gold-capped silicon nanocolumns regularly distributed over silicon substrate were obtained. The columns length was roughly 100?nm; their deviation from perpendicular axis was less than 2°. The diameter of the columns was of the order of 10?nm or below of that. The proposed procedure of nanostructuring included the following main steps: deposition of aluminum thin layer (100?C500?nm) by magnetron sputtering on (100) oriented Si wafers; formation of porous self-ordered alumina structures by electrochemical anodizing of the Al film in oxalic acid; electroless inversion of Au in alumina pores; and reactive ion etching. The obtained Si?CAu structures are of importance as the platforms for biosensing applications, while the gold-free structures are of interest in photovoltaics.     

Porous Silicon Carrier Matrices in Micro Enzyme Reactors-Influence of Matrix Depth

The influence of the carrier matrix depth was investigated for porous silicon enzyme reactors. For the experiments, <110> oriented silicon, p-type (20–70?Ω?cm), was used. Porous silicon was generated on planar surfaces and on anisotropically pre-etched high aspect-ratio parallel channel reactors. For each type of sample the porous silicon layer was generated for three depths, controlled by the anodisation time, and two current densities, to yield different morphologies. Glucose oxidase (GOx) was immobilised on the porous matrix by standard procedures for immobilisation of enzymes on silica. The enzyme activity of the samples was monitored by a colorimetric assay. The results clearly display the influence of the matrix depth for both the planar and the reactor structures. A 170-fold increase in catalytic turn-over, in comparison to an identical non-porous reference, was recorded for a reactor with an average pore depth of 10?μm. At depths above 10?μm the increase in catalytic efficiency levelled off. For the planar samples the levelling off occurred at an average pore depth of 20?μm.     

Tight‐binding molecular dynamics simulation of Si?H bond dissociation in silicon clusters

New model of Si? H bond dissociation is proposed and tested in the cluster Si₁₀H₁₆ by the simulation approach that combines classic molecular dynamics method and the self‐consistent tight‐binding electronic and total energy calculation one. It is shown that the monohydride Si? H bond is unstable with respect to silicon dangling bond and bend‐bridge Si? H? Si bond formation when this cluster traps the single positive charge and that hydrogen migrates through a path involving rather rotation around the Si? Si bond than the center of this bond (the bond‐centered position). These results can be useful for understanding hydrogen‐related phenomena at surfaces, interfaces, and internal voids of various hydrogenated silicon systems: electronic devices, silicon solar cells, and nanocrystalline and porous silicon. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 351–359, 2003     

PO → Si intramolecular coordination in the derivatives of 1,4‐phosphasilacyclohexane 1‐oxides

The chair and boat conformers for a series of derivatives of 1,4‐phosphasilacyclohexane 1‐oxides have been calculated at the B3LYP/6‐311+G** level of theory in the gas phase and taking into account the effect of solvent polarity using the IEF‐PCM model. The stability of the boat conformers containing pentacoordinate silicon due to formation of the P?O→Si intramolecular coordination bond depends on the environment of the phosphorus atom and polarity of the solvent, and the strength of the transannular bond depends also on the nature of the substituents at the silicon atom. The highly polar boat conformers are strongly stabilized in the DMSO solution. NBO analysis showed the importance of the σ(C? Si) → σ*(H₃C? N) hyperconjugative interaction in the two H₃C? N? C? Si fragments of the ring favoring the formation of the pentacoordinate silicon atom. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Reactivity of the Donor‐Stabilized Silylenes [iPrNC(Ph)NiPr]2Si and [iPrNC(NiPr2)NiPr]2Si: Activation of CO2 and CS2

Activation of CO₂ by the bis(amidinato)silylene 1 and the analogous bis(guanidinato)silylene 2 leads to the structurally analogous six‐coordinate silicon(IV ) complexes 4 (previous work) and 8 , respectively, the first silicon compounds with a chelating carbonato ligand. Likewise, CS₂ activation by silylene 1 affords the analogous six‐coordinate silicon(IV ) complex 10 , the first silicon compound with a chelating trithiocarbonato ligand. CS₂ activation by silylene 2 , however, yields the five‐coordinate silicon(IV ) complex 13 with a carbon‐bound CS₂^2? ligand, which also represents an unprecedented coordination mode in silicon coordination chemistry. Treatment of the dinuclear silicon(IV ) complexes 5 and 6 with CO₂ also affords the six‐coordinate carbonatosilicon(IV ) complexes 4 and 8 , respectively.     

Sol–gel derived ZnO/porous silicon composites for tunable photoluminescence

ZnO/porous silicon nanocomposites were fabricated by spin-coating the sol?Cgels of zinc acetate onto the top surface of porous silicon films. The photoluminescent properties of ZnO/porous silicon nanocomposites were investigated as a function of the concentration of zinc cations in the sol?Cgels. Characterizations with scanning electron microscopy, X-ray diffractometry and photospectroscopy indicated that ZnO nanocrystals were embedded into the spongy nanostructures of porous silicon after heat treatment at 245?°C for 20?min in air. The recorded photoluminescence exhibited that orange to green?Cblue emissions were achieved for the ZnO/porous silicon nanocomposites as the concentration of zinc cations in the sol?Cgels increased from 4 to 260?mM. The mechanisms on the tunability of the photoluminescence were discussed for the ZnO/porous silicon nanocomposites. Our results have demonstrated that the incorporation of green?Cblue phosphors into the porous matrix of porous silicon represents one endeavor to tune the photoluminescence of porous silicon across the visible spectral region.     

Effect of polyethylene glycol on corrosion of fresh macroporous silicon in 1.0 M NaOH and application in determination of porosity and thickness by gravimetric measurement

Abstract

This work presents on improvement in gravimetric measurement for determining the porosity and thickness of microporous silicon. Herein, the corrosion of fresh macroporous silicon (f-MPSi) in 1.0?M NaOH with different concentrations of polyethylene glycol (PEG 200/400/600) was studied by weight loss measurement and scanning electron microscopy (SEM). The results showed that the corrosion rate decreased with increasing polyethylene glycol concentration, and increased with an increase in temperature. Polyethylene glycol can inhibit the corrosion of f-MPSi in NaOH solution. Moreover, 1.0?M NaOH/PEG 600 (10%) can be used as the optimized solution to remove f-MPSi for measuring its porosity and thickness by gravimetric measurement.     

Illuminating the Mechanism of the Borane‐Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane

The reduction of C?O groups with silanes catalyzed by electron‐deficient boranes follows a counterintuitive mechanism in which the Si? H bond is activated by the boron Lewis acid prior to nucleophilic attack of the carbonyl oxygen atom at the silicon atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a silicon‐stereogenic silane, but applying the same technique to the related reduction of C?N groups was inconclusive due to racemization of the silicon atom. The present investigation now proves by the deliberate combination of our axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity‐determining hydride‐transfer step. These experiments became possible after the discovery that our axially chiral C₆F₅‐substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.     

Asymmetric Desymmetrization via Metal‐Free C−F Bond Activation: Synthesis of 3,5‐Diaryl‐5‐fluoromethyloxazolidin‐2‐ones with Quaternary Carbon Centers

We disclose the first asymmetric activation of a non‐activated aliphatic C?F bond in which a conceptually new desymmetrization of 1,3‐difluorides by silicon‐induced selective C?F bond scission is a key step. The combination of a cinchona alkaloid based chiral ammonium bifluoride catalyst and N,O‐bis(trimethylsilyl)acetoamide (BSA) as the silicon reagent enabled the efficient catalytic cycle of asymmetric C_sp3?F bond cleavage under mild conditions with high enantioselectivities. The ortho effect of the aryl group at the prostereogenic center is remarkable. This concept was applied for the asymmetric synthesis of promising agrochemical compounds, 3,5‐diaryl‐5‐fluoromethyloxazolidin‐2‐ones bearing a quaternary carbon center.     

Study of the process of the formation of Mg(Fe0.8Ga0.2)2O4 − δ films on Si

Conditions of the synthesis of magnetic semiconductor films of composition Mg(Fe_0.8Ga_0.2)₂O_4?δ on silicon have been optimized. As the barrier layer, which prevents the interaction between the film and the substrate during high-temperature crystallization of the films, a film of nanosized silicon dioxide was used. This, together with a high homogeneity of the composition of the target, allowed us to obtain Mg(Fe_0.8Ga_0.2)₂O_4?δ films on silicon characterized by the saturation magnetization (37 A m²/kg) that noticeably exceeds that for the volumetric analog (28 A m²/kg).     

Indirect atomic absorption spectroscopic determination of SiO2 in copper composite coatings

A fast, sensitive and reliable method for the indirect atomic absorption determination of SiO₂ in copper composite coatings after extraction of silicomolybdic acid in a mixture of isobutyl methyl ketone and butanol (volume ratio 5?:?1) and measurement of the molybdenum absorbance in an air/acetylene flame is described. The experimental conditions are optimized for (i) prevention of the silicon polymerization during sample preparation and storage; (ii) for overcoming of the Cu(SiF₆) – complex formation and (iii) for quantitative extraction of the yellow complex into the organic phase for a wide silicon concentration range. The method permits the determination of 0.5–10 mg/g Si in copper. The analytical performance of the proposed method is compared with direct Si determination using ICP-AES.     

Silicon‐ and Tin‐Based Cuprates: Now Catalytic in Copper!

Silicon‐ and tin‐containing molecules are versatile building blocks in organic synthesis. A stalwart method for their preparation relies on the stoichiometric use of silicon‐ and tin‐based cuprates, although a few copper(I)‐catalyzed or even copper‐free protocols have been known for decades. In this Concept, we describe our efforts towards copper(I)‐catalyzed carbon? silicon and also carbon? tin bond formations using soft bis(triorganosilyl) and bis(triorganostannyl) zinc reagents as powerful sources of nucleophilic silicon and tin. Conjugate addition, allylic substitution, and carbon? carbon multiple bond functionalization is now catalytic in copper!     

Crystallographic report: tert‐Butyldifluoro(2,4,6‐tris‐iso‐propylphenyl)silane,t‐Bu(2,4,6‐i‐PrC6H2)SiF2

Owing to steric congestion within the title compound, the geometry at the silicon atom deviates slightly from ideal tetrahedral geometry with an increased C? Si? C angle of 115.97(12)° and an elongated Si? C_aryl bond distance. Copyright © 2004 John Wiley & Sons, Ltd.     

The synthesis and reactivity of polysilylacetylenes: Diels—Alder reactions to give bis (silyl) benzenes

Six bis(silyl)acetylenes (XMe²Si? C?C? SiMe₂X) with the following varied silicon substituents X were prepared: 1 (Me, Me); 2 (H, H); 3 (C1, H); 4 (CI, CI); 5 (MeO, H); 6 (MeO, MeO). While 1 and 2 may be prepared by the reaction of dilithio- or bis(bromomagnesium)-acetylide with the appropriate chlorosilane, similar reactions designed to give 3–6 yielded oligomers, XMe₂Si? (? C?C? SiMe₂)_n? X, 7, X=CI, MeO, as the major products, indicating that the acetylenic functionality on silicon activates the chlorosilane towards nucleophilic substitution. Compounds 3 and 4 were prepared by free radical chlorination of 2. Methanolysis of 3 and 4 gave quantitative yields of 5 and 6 respectively. Compounds 1–6 undergo a Diels–Alder reaction with α-pyrone to produce, after loss of carbon dioxide, bis(silyl)-substituted benzene derivatives. The order of reactivity has been determined to be: 4=6>3=5>1>2, indicating that chloro or alkoxy substituents favor the cycloaddition with 2- pyrone. The adducts formed by compounds 3–6 undergo an unusually facile hydrolysis or elimination to give 1,1,3,3-tetramethyl-1,3-disila-2-oxaindane.     

High photocurrent in silicon photoanodes catalyzed by iron oxide thin films for water oxidation

Silicon splits: The application of silicon to water oxidation is limited due to unfavorable interface properties. However, these can be circumvented by using a high-performance silicon photoanode with a catalytically active iron oxide thin film (see picture). This approach results in photocurrents as high as 17?mA?cm(-2) under 1?sun and zero overpotential conditions.     

Platinum Nanoflowers on Scratched Silicon by Galvanic Displacement for an Effective SALDI Substrate

We report a new and facile method for synthesizing 3D platinum nanoflowers (Pt Nfs) on a scratched silicon substrate by electroless galvanic displacement and discuss the applications of the Pt Nfs in surface‐assisted laser desorption/ionization‐mass spectrometry (SALDI‐MS). Surface scratching of n‐type silicon is essential to induce Pt Nf growth on a silicon substrate (to obtain a Pt Nf silicon hybrid plate) by the galvanic displacement reaction. The Pt Nf silicon hybrid plate showed excellent SALDI activity in terms of the efficient generation of protonated molecular ions in the absence of a citrate buffer. We propose that the acidity of the Si? OH moieties on silicon increases because of the electron‐withdrawing nature of the Pt Nfs; hence, proton transfer from the Si? OH groups to the analyte molecules is enhanced, and finally, thermal desorption of the analyte ions from the surface occurs. Signal enhancement was observed for protonated molecular ions produced from a titania nanotube array (TNA) substrate on which Pt nanoparticles had been photochemically deposited. Moreover, surface modification of the Pt Nf silicon hybrid plate by perfluorodecyltrichlorosilane (FDTS) (to obtain an FDTS‐Pt Nf silicon hybrid plate) was found to facilitate soft SALDI of labile compounds. More interestingly, the FDTS‐Pt Nf silicon hybrid plate acts 1) as a high‐affinity substrate for phosphopeptides and 2) as a SALDI substrate. The feasibility of using the FDTS‐Pt Nf silicon hybrid plate for SALDI‐MS has been demonstrated by using a β‐casein digest and various analytes, including small molecules, peptides, phosphopeptides, phospholipids, carbohydrates, and synthetic polymers. The hybridization of Pt Nfs with a scratched silicon substrate has been found to be important for achieving excellent SALDI activity.     

Synthesis and characterisation of a silicon oxide film solid-phase extraction system for lead traces determination: an all the way green analytical method

In this work, an all the way green analytical procedure based on a silicon oxide film-solid phase extraction system is proposed for lead traces determination. From the synthesis of a solid phase extraction (SPE) system and throughout the metal preconcentration and determination only aqueous media were employed. Characterisation of the film was carried out by Scanning Electron Microscopy and Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy. Preconcentration conditions, prior to Pb(II) Electrothermal Atomic Absorption Spectrometry (ETAAS) determination, such as adsorption and desorption time, pH and temperature, were optimised. Langmuir, Freundlich and Dubinin-Radushkevich isotherm models were analysed along with the evaluation of adsorption energy and standard free energy (ΔG ⁰). The greatest adsorption was obtained with incubation at pH 7 and 37°C. By using a small volume of 0.5?mol?L^?1 HNO₃ (0.5?mL) lead was desorbed from the silicon oxide film after 2?h incubation, generating low amount of waste. The films showed better adjustment for the Langmuir model (R^{2 ?}=?0.989). The detection limit (3.29σ) for Pb(II) was 0.228?µg?L^?1. The developed procedure is 10-fold more sensitive in comparison to direct ETAAS determination. Recovery values from soft tap-water and soft well-water were above 95%. When hard water was analysed, Pb(II) adsorption was found to be interfered by Mg²⁺ and Ca²⁺. After five preconcentration cycles relative recovery was found not to decay below 90%, indicating that the silicon oxide film could be used for multiple lead determinations.     

Fourier transform NMR studies of organometallic compounds. II—synthesis and NMR spectroscopy of propenyl and isopropenyl derivatives of silicon and lead

Isomeric mixtures of compounds Me_nM(CH?CHMe)_4?n (M=Si, Pb; n=0?3) have been prepared and studied, as well as pure Me₃M(CMe?CH₂) and mixtures containing propenyl isopropenyl residues bonded to silicon and lead. ¹H, ¹³C, ²⁹Si and ²⁰⁷Pb NMR data are presented; as previously observed for the corresponding tin compounds, the ²⁹Si and ²⁰⁷Pb shifts for the Me₃MC₃H₅ isomers can be used to calculate the shifts expected for the other isomers; while for lead the agreement is good, calculated and observed values for silicon diverge with decreasing n due, at least in part, to steric factors.     

Synthesis and properties of nanosilicon prepared by homogeneous and heterogeneous reduction of tetraethyl orthosilicate

Nanocrystalline silicon was synthesized by reduction of tetraethyl orthosilicate both in homogeneous and heterogeneous processes with surface stabilization by nitrogen heterocyclic carbene (NHC) ligands. The presence of NHC ligands on the surface of silicon nanocrystals was demonstrated by IR spectroscopy. According to transmission electron microscopy data, the size of silicon nanocrystals is in the range of 2.2–8.4 nm. Photoluminescence of the samples is clearly recorded in the blue spectral region.     

Novel Transition‐Metal (M=Cr,Mo, W,Fe) Carbonyl Complexes with Bis(guanidinato)silicon(II) Ligands

The donor‐stabilized silylene 2 (the first bis(guanidinato)silicon(II ) complex) reacts with the transition‐metal carbonyl complexes [M(CO)₆] (M=Cr, Mo, W) to form the respective silylene complexes 7 – 10 . In the reactions with [M(CO)₆] (M=Cr, Mo, W), the bis(guanidinato)silicon(II ) complex 2 behaves totally different compared with the analogous bis(amidinato)silicon(II ) complex 1 , which reacts with [M(CO)₆] as a nucleophile to replace only one of the six carbonyl groups. In contrast, the reaction of 2 leads to the novel spirocyclic compounds 7 – 9 that contain a four‐membered SiN₂C ring and a five‐membered MSiN₂C ring with a M?Si and M?N bond (nucleophilic substitution of two carbonyl groups). Compounds 7 – 10 were characterized by elemental analyses (C, H, N), crystal structure analyses, and NMR spectroscopic studies in the solid state and in solution.