Silicon Chemistry Research Articles

Infrared Intensities of ν(Si−H) on H/Si(100)-2×1: Effect of O Incorporation and Agglomeration

Hybrid density functional calculations have been carried out on cluster models of the pristine and oxidized hydrogen-terminated Si(100)-2×1 surface to investigate the changes in the surface infrared absorption spectrum upon incorporation of oxygen. Due to a combination of geometric and electronic effects, a dramatic increase in the ν(Si−H) band intensity is found for the oxidized surface. An increase in infrared intensity of up to 62% is predicted for multiply oxidized sites. The results presented here may prompt the reinterpretation of infrared studies examining silicon surface chemistry using the ν(Si−H) region to probe changes in surface structure and coverage.

Engineering the Chemistry and Nanostructure of Porous Silicon Fabry-Pérot Films for Loading and Release of a Steroid

A method for engineering the surface chemistry and pore dimensions in porous Si films for the purpose of controlling the loading and release of a hydrophobic drug is described. Loading of the steroid dexamethasone is confirmed by Fourier transform infrared spectroscopy, and the release rates are characterized by observation of the appearance of the drug in solution (UV-vis absorption spectroscopy) and by measurement of the Fabry-Perot fringes in the optical reflectivity spectrum of the porous Si film. Optical reflectivity changes provide a measure of the release rate of the drug that is amenable to in-vivo diagnostic applications. Fresh porous Si films are prepared by electrochemical etch and subsequently modified by hydrosilylation with 1-dodecene. The dodecene-modified samples are more robust in aqueous environments and exhibit slower release rates of the drug relative to freshly etched porous Si. Whereas the relatively large dexamethasone molecule is found to infiltrate the freshly etched samples, it does not enter the chemically modified films, because of steric crowding from the dodecyl species. To achieve a high degree of loading into these modified films, the pores are enlarged before hydrosilylation by treatment with an aqueous solution containing HF and dimethyl sulfoxide. The pore expanded, chemically modified samples admit approximately 70% of the dexamethasone that can be admitted into an unmodified (freshly etched) sample. Diffusion of the steroid from the modified, pore expanded films into phosphate-buffered saline solution is slower than from the unmodified sample by a factor of approximately 20, with 90% of the drug delivered in 3 days for the chemically modified films compared to 3 h for the unmodified films.

Structure, Chemistry, and Electrical Performance of Silicon Oxide-Nitride-Oxide Stacks on Silicon

The structure and chemistry of silicon oxide-nitride-oxide (ONO) stacks on silicon with differently processed top oxide layers were analyzed using high-resolution transmission electron microscopy, electron energy loss spectroscopy, secondary ion mass spectroscopy, and X-ray specular reflectometry. The changes observed in the structure and chemistry of the ONO stacks were correlated with the electrical performance of these stacks in flash-memory devices. The results demonstrated that using larger thermal budgets to form the top oxide layer yields (i) broader N distribution across the nitride/oxide interfaces, reduced H content at the interfaces, increased density of the top oxide layer, and ultimately, improved electrical performance of ONO-based memory devices. © 2004 The Electrochemical Society. All rights reserved.

A study of the surface chemistry, morphology and wear of silicon based MEMS

AbstractPolysilicon microelectromechanical systems (MEMS) are the subject of intense research activity. This paper reports on the surface chemistry, topography and nanowear properties of a MEMS test structure fabricated at Sandia National Laboratories, studied using XPS and atomic force microscopy (AFM). XPS C 1s and Si 2p spectra from the polysilicon components, silicon nitride substrate and a reference Si wafer are compared. The results confirm the presence of a self‐assembled monolayer (SAM) on the MEMS surface. An island‐like morphology is found on both polysilicon and silicon nitride surfaces of the MEMS. The islands take the form of caps, being up to 0.5 µm in diameter and 20 nm in height. It is concluded that a dual columnar and equiaxed microstructure develops during growth of these low pressure chemical vapour deposition (LPCVD) layers and the islands are caps to the columnar structures. A 35 µN load applied to the AFM diamond tip leads to a wear depth of 1.4 ± 0.13 nm and 2.1 ± 0.06 nm on the polysilicon and silicon nitride MEMS equiaxed surfaces, respectively. Under the same load, greater wear of the columnar caps on both surfaces is observed. The results suggest that the morphology present on the polysilicon surface will be worn flat during operation and will not adversely affect the wear properties of the polysilicon components. Copyright © 2004 John Wiley & Sons, Ltd.

Silicon Chemistry. From the Atom to Extended Systems. Edited by Peter Jutzi and Ulrich Schubert.

Silicon Chemistry. From the Atom to Extended Systems. Edited by Peter Jutzi and Ulrich Schubert.

A theoretical investigation of the Chalk–Harrod and modified Chalk–Harrod mechanisms involved in hybrid integrated circuit building

Nowadays the relevance of silicon chemistry is spreading due to the applications of Si, in particular in semiconductors, in order to obtain an increasing number of performing materials. A theoretical investigation based on the Density Functional Theory has been done on the reaction paths implied in the Pt-catalysed alkene hydrosilylation, a process through which the selective grafting of organic molecules to silicon can be obtained. We studied the SiH oxidative addition of SiH 4 to a diphosphine molecule, the C 2H 4 insertion on PtH and PtSiH 3 bonds, the isomerisation of the product of the C 2H 4 insertion, and the two following SiC and CH reductive eliminations. The set of these processes are known as the Chalk–Harrod mechanism and the modified Chalk–Harrod mechanism, respectively. The main goal of this work has been to identify the rate determining step of both mechanisms. Moreover, by employing dynamics simulations, we have reached a possible alternative product in the oxidative addition step.

3 Synthetic methods : Part (iii) Heteroatom methods

This chapter concentrates on the organic chemistry of phosphorus, sulfur, selenium, tellurium, and silicon, particularly the formation of carbon–heteroatom bonds, and heteroatom facilitated carbon–carbon bond formation; the corresponding heterocyclic, free radical and transition metal chemistry is covered more thoroughly elsewhere. The review was compiled using a combination of on-line searching and selective reading of the literature. A particular focus this year is allylation chemistry that relies upon activation of allylsilanes, discussed in Section 4. Interesting recent developments in this strategy rely upon the capability of silicon to form a hypercoordinate species upon coordination of a Lewis base, with organophosphorus and organosulfur species, among others, being suitable. Kobayashi and co-workers have defined these Lewis bases as “neutral coordinate-organocatalysts” to distinguish them from species such as alkoxides and halides and to emphasise the fact that these reactions fall into the category of metal-free organic molecule catalysed reactions. Particularly attractive are chiral Lewis bases which turn-over in the reactions, resulting in protocols for metal-free asymmetric synthesis. This strategy is applicable to metals or semi-metals other than silicon, but there are few, if any, such examples yet reported; silicon has the major advantage over tin of lack of toxicity—thus allylstannanes are becoming side-lined in modern synthetic methodology—but allyl boron compounds may be amenable to this approach. Other modes of activation are also successful and these, and the Lewis base approach, are being applied to reactions other than allylations, see Section 4.

Silicon Biotechnology: New Opportunities for Carbohydrate Science.

Silicon is the second most abundant element in the Earth’s crust after oxygen, with a chemistry second only in complexity to that of carbon.[1] In addition, the involvement of silicon in many life processes has been acknowledged for some time, although the exact mechanisms underlying this association are still poorly defined.[2,3] Recent discoveries of molecules responsible for the biomineralization of silica, coupled with the growing influence of biotechnology, have reinvigorated interest in the biochemistry of silicon.[4] This has led to the emergence of silicon biotechnology, an area of interdisciplinary research that seeks to combine silicon chemistry with the life sciences.[4,5] Commercial interests are also aggressively pursuing the merger of silicon chemistry and biotechnology, most notably the Silicon Biotechnology platform initiated in late 2001 as a strategic alliance between Genencor International and the Dow Corning Corporation.[5] The current scope of this effort is broad and includes the study of how silicon is processed in the biosphere, biocatalytic production of siliconcontaining polymers and their precursors, delivery systems for enzymes, development of robust biosensor systems, and the enzymatic formation of silicon–carbon bonds. Many of these areas present significant opportunities for carbohydrate chemists and biologists alike, promising to reinforce the long association between silicon and carbohydrate chemistry.

Recent Advances in the Chemistry of Silicon—Silicon Multiple Bonds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Silicon Biotechnology: New Opportunities for Carbohydrate Science

Silicon is the second most abundant element in the Earth’s crust after oxygen, with a chemistry second only in complexity to that of carbon.[1] In addition, the involvement of silicon in many life processes has been acknowledged for some time, although the exact mechanisms underlying this association are still poorly defined.[2,3] Recent discoveries of molecules responsible for the biomineralization of silica, coupled with the growing influence of biotechnology, have reinvigorated interest in the biochemistry of silicon.[4] This has led to the emergence of silicon biotechnology, an area of interdisciplinary research that seeks to combine silicon chemistry with the life sciences.[4,5] Commercial interests are also aggressively pursuing the merger of silicon chemistry and biotechnology, most notably the Silicon Biotechnology platform initiated in late 2001 as a strategic alliance between Genencor International and the Dow Corning Corporation.[5] The current scope of this effort is broad and includes the study of how silicon is processed in the biosphere, biocatalytic production of siliconcontaining polymers and their precursors, delivery systems for enzymes, development of robust biosensor systems, and the enzymatic formation of silicon–carbon bonds. Many of these areas present significant opportunities for carbohydrate chemists and biologists alike, promising to reinforce the long association between silicon and carbohydrate chemistry.

New Approaches Toward the Formation of Silicon—Carbon Bonds on Porous Silicon

AbstractFor Abstract see ChemInform Abstract in Full Text.

Multiple Bonds to Silicon: 20 Years Later

Abstract The discovery of stable compounds containing SiC and SiSi bonds in 1981 led to the overturning of the ‘double bond rule’, and paved the way for the recent explosive growth in multiple bond chemistry of the heavier elements. This paper traces the events leading up to the 1981 discoveries, summarizes some of the developments in multiply-bonded silicon chemistry since that date, and outlines challenges for further research in this area.

Die Packungsdichte der Atome in Modifikationen von Kohlenstoff und Silicium

Abstract The crystal chemistry of carbon and silicon is discussed with respect to the packing density of atoms in their crystalline modifications. The reduced atomic volumes of metallic carbon as calculated from diamond, hexagonal diamond and buckminsterfullerene C60 agree within less than 0.2, indicating the correctness of the reducing procedure. A much lower value calculated from graphite, rhombohedral graphite and a low-temperature modification of buckminsterfullerene remains unexplained. Two proposed crystal structures of a cubic body-centered form of carbon appear unacceptable. The reduced atomic volumes of diamond-type silicon and metastable γ-Si are in good agreement. Their values, calculated from six allotropic modifications (Si I, II, XI, V, VI, VII, and Si X) at pressures up to ≈200 GPa, are represented by an empirical exponential function of pressure. From this function, an isothermal equation of state is derived including the packing density of the atoms.

Investigations into the small-bite N-functionalised amido chemistry of silicon and tin(IV)

Treatment of [(APyTMS)Li(OEt2)], (APyTMSH=6-methyl-2-(trimethylsilylamino)-pyridine) with one equivalent of SiCl 4 affords [SiCl 3(APyTMS)], while similar treatment of [(APyTMS)Li(OEt 2)] with 0.5 equivs of SiCl 4 or SnCl 4 yields [SiCl 2(APyTMS) 2] and [SnCl 2(APyTMS) 2], respectively, in good yield. [SiMe 2(APyH) 2] is obtained by treating [(APyH)Li(OEt 2)] with 0.5 equivs of Cl 2SiMe 2. All complexes were characterised by NMR and IR spectroscopy, and in the case of [SiCl 3(APyTMS)] and [SnCl 2(APyTMS) 2] by X-ray crystallographic methods. The X-ray structure of [SiCl 3(APyTMS)] reveals a mononuclear compound with a hypervalent five-coordinate silicon centre, while [SnCl 2(APyTMS) 2] is mononuclear with a distorted octahedral tin(IV) centre.

New Approaches Toward the Formation of Silicon-Carbon Bonds on Porous Silicon

Porous silicon is a material of intense technological and fundamental interest, due to its high surface area, quantum confinement effects, light emission, and complex nanoscale architecture. There is a strong desire to be able to control the surface properties of this versatile material at will through functionalization with monolayers bound through the stable silicon-carbon bond. Incredibly, the surface chemistry of porous silicon is often very different from that of a silicon-based molecule, or a flat surface, due to interesting electronic effects resulting from both the semiconducting and nanoscale characteristics of the material. Recent results outlining the rapid progress of silicon-carbon bond formation on porous silicon surfaces will be described.

Some recent advances in the chemistry of silicon and its homologues in low coordination states

This review summarises some recent results on synthesis, structures, and reactivity of the heavier analogues of carbenes, alkenes, and dienes. With the isolation of the first diplumbenes, the series R 2EER 2, E=CPb, has been completed. These double bond systems show enormous variations in the EE bond lengths, E=SiPb, and the trans-bent angles. Furthermore, new families of multiple-bonded species have been discovered, such as a spiropentasiladiene, molecules with conjugated SiSi, GeGe, and GeC double bonds, and a diplumbylene RPbPbR. However, no stable compounds featuring homonuclear triple bonding between heavier Group 14 elements are known. Theoretical insights on the nature of such triple bonds as well as first attempts to achieve this target are discussed.

Multiple bonds to silicon: 20 years later

The discovery of stable compounds containing SiC and SiSi bonds in 1981 led to the overturning of the ‘double bond rule’, and paved the way for the recent explosive growth in multiple bond chemistry of the heavier elements. This paper traces the events leading up to the 1981 discoveries, summarizes some of the developments in multiply-bonded silicon chemistry since that date, and outlines challenges for further research in this area.

Rate coefficients for the reactions of Si(3PJ) with C2H2 and C2H4: Experimental results down to 15 K

Rate coefficients for the reaction of ground-state silicon atoms Si(3PJ) with acetylene and ethylene have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of Si(CH3)4 to generate Si(3PJ) atoms and laser-induced fluorescence to observe the kinetic decay of the atoms and hence determine the rate coefficients. Both reactions are found to be fast, and the reaction rates show a very mild dependence on temperature. The rate coefficients match the expressions k(Si+C2H2)=(2.6±0.6)10−10(T/300)−(0.71±0.24) exp(−(29±10)/T) cm3 molecule−1 s−1 and k(Si+C2H4)=(3.7±0.3)10−10(T/300)−(0.34±0.10) exp(−(16±4)/T) cm3 molecule−1 s−1 in the temperature range 15–300 K. The nature of the products and the similarities of the carbon and silicon chemistry are discussed.

Contributions to the chemistry of silicon–sulfur compounds. Part 75. Cobalt(II) tri- tert-butoxysilanethiolates. Synthesis, properties, crystal and molecular structures of [Co{SSi(O tBu) 3} 2(L)] and [Co{SSi(O tBu) 3} 2(L) 2] type complexes with monodentate nitrogen ligands

The title heteroleptic neutral cobalt(II) tri- tert-butoxysilanethiolate complexes with monodentate nitrogen bases (L) as additional ligands have been prepared by the reactions of [Co{μ-SSi(O t Bu) 3}{SSi(O t Bu) 3}(NH 3)] 2 ( 1) with respective bases. For pyridine both types have been prepared — with two ( 2) or one ( 3) nitrogen ligand bonded to cobalt(II). [Co{SSi(O t Bu) 3} 2(L)] complexes have been obtained also with 2-picoline ( 5), 2,4-lutidine ( 6), 3,5-lutidine ( 7), and [Co{SSi(O t Bu) 3} 2(L) 2] also with N-methylimidazole ( 8) and morpholine ( 9). Molecular and crystal structures of the six compounds have been determined by single-crystal X-ray structural analysis. In 3, 5 and 7 three-coordinated cobalt(II) seems to interact very weakly with two oxygen atoms from two Si(O t Bu) 3 moieties approaching highly distorted trigonal bipyramidal geometry. Compounds 2, 8 and 9 have distorted tetrahedral structures. Both types of complexes gave characteristic electronic spectra, similar within each type.

The Si($^\mathsf3$PJ) + O$_\mathsf2$ reaction: A fast source of SiO at very low temperature; CRESU measurements and interstellar consequences

The rate coecient of the reaction Si( 3 PJ )+O 2! SiO +Oh as been measured at temperatures down to 15 Ku sing aC RESU (Cinetique de R eaction en Ecoulement Supersonique Uniforme) apparatus coupled with the PLP-LIF (Pulsed Laser Photolysis { Laser Induced Fluorescence) technique. The temperature dependence of the rate coecient is well tted using the expression: 1:7210 10 (T /300 K) 0:53 exp( 17 K=T )c m 3 molecule 1 s 1 in the temperature range 15{300 K. The silicon chemistry in interstellar clouds is reviewed and possible consequences of our study are stressed.