Si-H Bonds Research Articles

Heterolytic Si-H Bond Cleavage at a Molybdenum-Oxido-Based Lewis Pair.

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6F5)3 gave access to the Lewis adduct [Mo{OB(C6F5)3}(NtBu)L2] featuring reversible B−O bonding in solution. The resulting frustrated Lewis pair (FLP)‐like reactivity is reflected by the compound's ability to heterolytically cleave Si−H bonds, leading to a clean formation of the novel cationic MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3)(NtBu)L2][HB(C6F5)3]. These compounds possess properties highly unusual for molybdenum d0 species such as an intensive, charge‐transfer‐based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single‐crystal X‐ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C6F5)3]− anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B−H bond of the borohydride anion.

UV Assisted Densification of Perhydropolysilazane (PHPS) Based Spin-on Glass in High Aspect Ratio Gap Fill Structure

As device dimensions scale, defect-free, robust gap filling in high aspect ratio structures such as STI and inter-gate spaces becomes increasingly challenging. This is due to very high aspect ratio (AR) of gap fill structure (AR > 7:1), shrinking CDs (<20nm) and nearly vertical sidewalls gate profiles (>88deg). The Spin-On Dielectric such as (SOD) PerHydroPolySilazane (PHPS) is preferred choice as it provides defect-free and structure agnostic gap fill (1). As coated, the PHPS film has a long polymeric chain of hydrogen terminated -Si-N-Si- network with high % of N and terminal H. The film as-coated has very high etch rate in HF based chemistry. The film structure needs to be fully converted into a dense, cross-linked network of SiO2 via silanol condensation reaction during steam anneal carried out at ≤600C. However, as the critical dimension reaches below 20nm, the inability of steam to diffuse to the bottom of high aspect ratio features results in oxide density gradient leaving very weak oxide near the bottom of trench. High temperature anneal (> 600⁰C) is not feasible for inter-gate gap fill due to its impact on junction. Non-thermal approaches are needed to improve the oxide quality inside high aspect ratio features. The use of monochromatic UV radiation for low temperature conversion of PHPS film is well documented (2). Here, broadband UV (200nm-450nm) was explored to assess its effectiveness in improving In-Trench oxide density post steam anneal. The effect of three UV conditions (UV-A, UV-B, UV-C) and modified steam anneal condition on structure and composition of as-coated PHPS film was assessed using FTIR (Fig. 1) and XPS Depth Profiling (Fig. 2). In blanket experiments, the densification effect by UV of as-coated PHPS films was assessed post-steam anneal by measuring through-thickness wet etch rate (Fig. 3). This was done by sequential etching of annealed films in 100:1 dHF chemistry (400s) followed by thickness measurement. The average relative (to thermal oxide) wet etch rate of blanket oxide films exposed to various UV conditions and modified anneal conditions is listed in Table 1. FTIR and XPS depth profiles (Fig. 1) of UV treated PHPS films show significant modulation with UV conditions. The more aggressive the UV conditions, the lower the overall %N. Data also suggests that UV treated films have progressively higher O2 and reduced N at more aggressive UV conditions. This is because the effective breaking of Si-H, N-H and SI-N bonds in UV treated films which loosen the backbone structure, thereby facilitating oxygen/moisture diffusion relative to as-coated film. Low-temperature O2 soak step prior to steam anneal was found to be very effective in achieving denser oxide post-anneal for UV cured films. However, due to limited O2 diffusivity of oxygen in as-coated PHPS film due to denser network for non-UV treated PHPS films, O2 soak prior to steam anneal is more effective only at higher temperature. Optimized UV and anneal conditions from the blanket study were further explored on integrated wafers consiting of 7nm dimension inter-gate gap fill. These structures were filled with PHPS and treated with different UV conditions prior to annealing and planarization. In-Trench wet etch rate was obtained by sequentially measuring In-trench wet etch rate of oxide similar to the blanket film etch study. In each cycle, the wafer was exposed for 60s in 100:1 HF chemistry. Optical Scatterometry was used to measure In Trench oxide recess amount per etch cycle as well as cumulative oxide recess after 5 sequential etch cycles (5X 60s HF (100:1)) (fig. 4). Table 2 displays average In Trench wet etch rate comparison between the UV treated films and control cell post-steam anneal. The blanket wet etch rate data in Table 1 suggest 18% lowering just by modifying anneal for non-UV cell. Insertion of UV between spin coating and anneal helps further reduction in blanket wet etch rates (UV-A > UV-B > UV-C). UV-C condition had 34% reduction in blanket etch rate. However, blanket etch rate trend with UV condition did not quite translate to the In-Trench wet etch rate. While the overall in-trench wet etch performance for UV treated cells was better than control, the UV-A and UV-B conditions had lower in-trench WERR than UV-C condition. These results will be explained in the light of moisture sensitivity of UV exposed PHPS films as a function of UV conditions and its impact on overall wet etch rate post-anneal. References Trivedi et al., J. Vac. Sci. Technol. B, Vol 27, No. 6, pp 3145-3148Prager et al., Chem. Eur. J., 2009, vol. 15, pp 675-683 Figure 1

Directional Building Blocks Determine Linear and Cyclic Silicon Architectures

Silicon nanomaterials combine earth abundance and biodegradability with exceptional electronic properties. Strategic synthesis promises access to novel architectures with well-defined surface structure, size, and shape. Herein, we describe a five-step synthesis of functional macrocyclic polysilanes. Comparison of the materials isolated from isomeric building blocks provides evidence that building block directionality controls the shape of the resulting nanomaterial. Infrared (IR) and 1H and 29Si NMR spectroscopies, coupled to computational data, provide evidence of a well-defined Si-H and Si-Me terminated structure. The intrinsic porosity and the polarization arising from the hydridic character of the Si-H bond suggest applications in lithium-ion batteries, which are supported by quantum chemical calculations.

Understanding the Mechanism of SiC Plasma-Enhanced Chemical Vapor Deposition (PECVD) and Developing Routes toward SiC Atomic Layer Deposition (ALD) with Density Functional Theory.

Understanding the mechanism of SiC chemical vapor deposition (CVD) is an important step in investigating the routes toward future atomic layer deposition (ALD) of SiC. The energetics of various silicon and carbon precursors reacting with bare and H-terminated 3C-SiC (011) are analyzed using ab initio density functional theory (DFT). Bare SiC is found to be reactive to silicon and carbon precursors, while H-terminated SiC is found to be not reactive with these precursors at 0 K. Furthermore, the reaction pathways of silane plasma fragments SiH3 and SiH2 are calculated along with the energetics for the methane plasma fragments CH3 and CH2. SiH3 and SiH2 fragments follow different mechanisms toward Si growth, of which the SiH3 mechanism is found to be more thermodynamically favorable. Moreover, both of the fragments were found to show selectivity toward the Si-H bond and not C-H bond of the surface. On the basis of this, a selective Si deposition process is suggested for silicon versus carbon-doped silicon oxide surfaces.

Hydrogen intercalation of epitaxial graphene and buffer layer probed by mid-infrared absorption and Raman spectroscopy

We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated single layer epitaxial graphene and buffer layer grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The Raman spectroscopy is used to show presence of buffer layer and single layer graphene prior to intercalation. We also present Raman spectra of quasi free standing monolayer and bilayer graphene after hydrogen intercalation at temperatures between 790 and 1510°C. We have found that although the Si-H bonds form at as low temperatures as 790°C, the well developed bond order has been reached only for samples intercalated at temperatures exceeding 1000°C. We also study temporal stability of hydrogen intercalated samples stored in ambient air. The optical spectroscopy shows on a formation of silyl and silylene groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process.

Pt nanoparticle/Si nanowire composites as an excellent catalytic counter electrode for dye-sensitized solar cells

Recently, much attention has focused on less platinum alternative materials serving as the counter electrode material for dye-sensitized solar cells, while platinum itself has a lot of room for improvement. Herein, we introduce the earth-abundant silicon and combine it with Pt nanoparticles as the counter electrode in dye-sensitized solar cells to enlarge surface area and enhance the activity of Pt. Si-H bonds can reduce metal ions and enable small metal nanoparticles grown on the surface of silicon nanowires, which can effectively prevent metal nanoparticles from agglomeration in the catalysis. The density functional theory (DFT) calculation shows that the adsorption energy of I atom on the Pt/Si interface is −0.8 eV, which is the optimal adsorption energy for triiodide reduction, indicating that Pt/Si is a perfect material for the counter electrode. The electrochemical characterizations and the photocurrent-voltage performance experimentally confirm that Pt/SiNW is a better counter electrode material than other metal/SiNW composites and Pt, which is in accordance with DFT calculations. The power conversion efficiency of device based on the Pt/SiNW electrode is higher than that of the device based on Pt (8.23% vs.7.93%).

Hollow Cathode Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using Pentachlorodisilane.

In this work, a novel chlorodisilane precursor, pentachlorodisilane (PCDS, HSi2Cl5), was investigated for the growth of silicon nitride (SiN x) via hollow cathode plasma-enhanced atomic layer deposition (PEALD). A well-defined self-limiting growth behavior was successfully demonstrated over the growth temperature range of 270-360 °C. At identical process conditions, PCDS not only demonstrated approximately >20% higher growth per cycle than that of a commercially available chlorodisilane precursor, hexachlorodisilane (Si2Cl6), but also delivered a better or at least comparable film quality determined by characterizing the refractive index, wet etch rate, and density of the films. The composition of the SiN x films grown at 360 °C using PCDS, as determined by X-ray photoelectron spectroscopy, showed low O content (∼2 at. %) and Cl content (<1 at. %; below the detection limit). Fourier transform infrared spectroscopy spectra suggested that N-H bonds were the dominant hydrogen-containing bonds in the SiN x films without a significant amount of Si-H bonds originating from the precursor molecules. The possible surface reaction pathways of the PEALD SiN x using PCDS on the surface terminated with amine groups (-NH2 and -NH-) are proposed. The PEALD SiN x films grown using PCDS also exhibited a leakage current density as low as 1-2 nA/cm2 at 2 MV/cm and a breakdown electric field as high as ∼12 MV/cm.

Label-free glucose detection over a wide dynamic range by mesoporous Si nanowires based on anomalous photoluminescence enhancement

We report on the label-free glucose detection over a wide dynamic range by mesoporous Si nanowires (NWs) through its photoluminescence (PL) quenching and anomalous PL enhancement caused by glucose (GL) in presence of its conjugate enzyme glucose oxidase (GOx). The GL and GOx induced surface evolution of Si NWs is examined by several spectroscopic tools. At a low concentration of GL, the visible-NIR PL emission from Si NWs is partially quenched due to the electron removal by the H2O2 produced by the GL/GOx solution. At higher concentration of GL, the SiH bonds on the surface of the Si NWs are eliminated and different functional groups, e.g., CC, CC, COH, CO and COOH are attached. This makes the Si NWs highly photoluminescent as compared to the bare Si NWs. The linear response of the PL enhancement of Si NWs with the logarithm of GL concentration is utilized for the label free sensitive detection of GL. A high dynamic range (0.1–50 mM) for GL detection and a considerable lower limit of detection (1.06 μM) are achieved for the newly developed biosensor. This report opens up possibilities for exploiting the low cost mesoporous Si NW arrays for ultrasensitive biosensing and bioimaging applications.

Nontrigonal constraint enhances 1,2-addition reactivity of phosphazenes

The syntheses and 1,2-addition reactivities of nontrigonal phosphazenes supported by trianionic tricoordinating chelates of the type L3P[double bond, length as m-dash]Ndipp (3: L3 = N[CHC( t Bu)O]2 3-; 4: L3 = N(o-NMeC6H4)2 3-; dipp = 2,6-diisopropylphenyl) are reported. These compounds are characterized by multinuclear NMR and single-crystal X-ray diffraction experiments. Distorted phosphazenes 3 and 4 are shown to add B-H, B-O, and Si-H bonds across the formal P[double bond, length as m-dash]N double bond, and their reactivities are contrasted with acyclic analogues. Derivatives of phosphazene 3 bearing sterically unencumbered N-substitutents readily dimerize to form the corresponding cyclodiphosphazanes; compounds with sterically demanding N-substituents are interconvertible between their monomeric and dimeric forms. The enhanced electrophilicity of the phosphorus center in nontrigonal phosphazenes 3 and 4 is rationalized by DFT calculations. Gas phase fluoride ion affinities are computed to be markedly higher for distorted phosphazenes, while proton affinities are largely unaffected by geometric distortion. These results are interpreted to suggest that distortion from pseudotetrahedral geometry results in stabilization of the P-based LUMO, while HOMO energies are essentially unchanged.

Catalytic oxidative coupling promoted by bismuth TEMPOxide complexes.

Bismuth(iii) TEMPOxide compounds have been synthesized from the coupling of Bi(ii) species with the TEMPO˙ radical. The steric profile of the supporting bis(amido)disiloxane ligand promotes different fluxional behaviour in solution, and DFT calculations suggest variation in the Bi-O bond character. These compounds are active catalysts for oxidative coupling of TEMPO and silane substrates, believed to proceed via metathesis of Bi-O and Si-H bonds followed by decomposition of bismuth-hydride intermediate species.

Study on the Mechanism of Micro-Defect Reduction during Si Final Polishing with Water-Soluble Polymers

The effect of water-soluble polymer in alkaline slurry on the micro-defect during Si final polishing was investigated. With the concentration of hydroxyethyl cellulose (polymer A) 0.15wt% and polyoxyethylene ether (polymer B) 0.05wt%, Haze value and the number of micro-defects (larger than 120nm) were reduced to 0.01 ppm and 7, respectively, on a 300mm silicon wafer surface. Also, the surface roughness was reduced from 17.2 to 0.153nm. Through the comprehensive analysis of slurry viscosity and Zeta Potential measurement, the mechanism of water-soluble polymers for micro-defect reduction during Si final polishing was discussed. Polymer B coated on the surface of SiO2 particle forming a “soft shell”, which functions as a “soft abrasive” and thus reduces friction between wafer surface and slurry abrasives. As for polymer A, the O atomic of -OH group in polymer A molecular could combine with Si-H bond on the surface of silicon wafer to form the hydrogen bond, like Si-H…O bond. As such, polymer A can be adsorbed on the surface of the silicon wafer to form a hydration film, which also can reduce friction between wafer surface and slurry abrasives/pad asperities. With the joint action of polymer A and polymer B, as experimental results showed, the number of micro-defects and Haze value in Si final polishing process can be significantly reduced.

Silylation reactions on nanoporous gold via homolytic Si-H activation of silanes.

Si-H bond activation is an important process implicated in many useful synthetic applications including silylation and transfer hydrogenation reactions. Herein we discovered homolytic activation of Si-H bonds on the surface of nanoporous gold (NPG), forming hydrogen radicals and [Au]-[Si] intermediates. By virtue of this new reactivity, we achieved highly selective mono and sequential alcoholysis of dihydrosilane. In addition, the amphiphilic nature of the [Au]-[Si] intermediate allows for a new bis-silylation reaction of cyclic ethers. The present work showcased that the surface reactivity of nanocatalysts may provide exciting opportunity for new reaction discovery.

An intramolecular ortho-assisted activation of the silicon-hydrogen bond in arylsilanes: an experimental and theoretical study.

An intramolecular activation of the Si-H bond in arylsilanes by selected ortho-assisting functional groups based on boron, carbon and phosphorus was investigated experimentally and by means of theoretical calculations. The major conclusion drawn is that the presence of a negatively charged oxygen atom in the functional group is essential for providing effective chelation to the silicon atom which in turn results in the increased hydridic character of a resulting five-coordinated species. In contrast, an intermolecular attack of hydroxide on the silicon atom in aryldimethylsilane results in the activation of the silicon-aryl bond. This increased reactivity of the Si-H bond in intramolecularly coordinated arylsilanes can be ascribed to a significant trans effect which operates in the preferred configuration. Hydrolytic cleavage of the Si-H bond results in dihydrogen elimination and the formation of various silicon heterocyclic systems such as benzosiloxaboroles, spiro-bis(siloxa)borinate, benzosilalactone and benzophosphoxasilole. In addition, intermolecular reduction of benzaldehydes with ortho-boronated arylsilane was observed whereas compounds bearing other reducible functional groups (COMe, COOEt, CN and NO2) were inert under comparable conditions. Specifically, an intramolecular reduction of the CN group in an ortho-silylated benzonitrile derivative was observed. The mechanism of Si-H bond activation was investigated by the DFT theoretical calculations. The calculations showed that the intramolecular coordination of the silicon atom effectively prevents the cleavage of the Si-aryl bond. Furthermore, the reaction is favored in anionic systems bearing COO-, B(OH)3- or CH2O- groups, while in the case of neutral functional groups such as PO(OEt)2 the process is much slower.

Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si-H bond activation via metal-ligand cooperation.

The double aminolysis reaction of [Co{N(SiMe3)2}2] by the salt 1-(6-((dicyclohexylphosphaneyl)methyl)pyridin-2-yl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide, which contains one phosphane, one pyridine and one imidazolium groups, of formula [o-Cy2PCH2(C5H3N)(o-C3H3N2DiPP)]Br and abbreviated as (CyPNpyrCim)Br, was previously shown to afford the Co(ii) complex [Co(CyP*NaCNHC)Br] (1) containing a dearomatised picolyl moiety in the tridentate, anionic donor ligand CyP*NaCNHC (Na = anionic amido, P* = vinylic P donor). We now report that formation of 1 is preceded by an intermediate tentatively assignable to the 5-coordinate Co(ii) complex [Co(CyPNpyrCNHC){N(SiMe3)2}Br] (2). The reaction of 1 with LiCH2SiMe3 afforded the dark purple, paramagnetic [Co(CyP*NaCNHC)CH2SiMe3] (3) with a low spin d7 CoII; the electronic configurations of 1 and 3 were corroborated by EPR spectroscopy. Addition of excess (≥4 equiv.) H2SiPh2 to a solution of 1 gave the diamagnetic [Co(CyP(SiHPh2)NCNHC)Br] (4) following Si-H activation and silylation of the ligand backbone at the α-CHP. Reduction of CoII to CoI by silanes is uncommon.

Influence of LDD Spacers and H+Transport on the Total-Ionizing-Dose Response of 65-nm MOSFETs Irradiated to Ultrahigh Doses

The degradation induced by ultrahigh total ionizing dose in 65-nm MOS transistors is strongly gate-length dependent. The current drive decreases during irradiation, and the threshold voltage often shifts significantly during irradiation and/or high-temperature annealing, depending on transistor polarity, applied field, and irradiation/annealing temperature. Ionization in the spacer oxide and overlying silicon nitride layers above the lightly doped drain extensions leads to charge buildup as well as the ionization and/or release of hydrogen. Charge trapped in the spacer oxide or at its interface modifies the parasitic series resistance, reducing the drive current. The released hydrogen transports as H+ with an activation energy of ~0.92 eV. If the direction of the electric field is suitable, the H+ can reach the gate oxide interface and depassivate Si-H bonds, leading to threshold voltage shifts. Newly created interface traps are most prominent near the source or drain. The resulting transistor responses and defect-energy distributions often vary strongly in space and energy as a result, as demonstrated through current–voltage, charge-pumping, and low-frequency noise measurements.

Flexible multi-wavelength photodetector based on porous silicon nanowires.

Porous silicon nanowires (PSi NWs) responding to red, green and blue lights simultaneously are reported in this study. NWs with a diameter of 120 nm were prepared from p-type Si wafer by metal-catalyst-assisted etching. The NWs produced in this manner had an outer porous region consisting of Si nanocrystals small enough to ensure the quantum confinement effect. Hydrazine treatment passivated the porous region by reduction of the surface oxides and the formation of Si-H bonds. The energy band structure of the porous region was a direct transition type, while the emission/absorption energies corresponded to red (1.94 eV), green (2.24 and 2.44 eV) and blue (2.98 eV) light photons. A device structure prepared by suspending 10 PSi NWs between two Al electrodes by dielectrophoresis generated photocurrents upon exposure to red, green and blue lights. The rising and decay times were both 30 μs and the ratio of photocurrent to dark current was 92.0, 59.8 and 24.4 for the blue, green and red lights, respectively. The device could be bent down to the bending radius of 5 mm without any significant deterioration of the photoresponse performances. Further, the device survived repeated bending and unbending over 10 000 cycles with this bending radius, demonstrating its application potential as a flexible device with multiple-wavelength photosensing capabilities.

Nitrido complex of high-valent Ru(VI) -catalyzed reduction of imines and alkynes with hydrosilanes: A theoretical study of the reaction mechanism

The reaction mechanism for hydrosilylation of imines and alkynes catalyzed by a nitrido complex of high-valent Ru(VI) has been studied using density functional theory. Four different reaction routes for RuVIN complex catalyzed hydrosilylation have been probed in detail, spanning those involving N, RuVI, or RuIII as active center in activating SiH bonds. The activation of SiH bond at the nitrido ligand can be discounted because they would lead to large activation barrier for reduction of imines and alkynes. Our computational results support an ionic outer-sphere mechanism involving silane initially binding with ruthenium metal. This proposed ionic outer-sphere mechanism is characterized by a rate-determining SiH bond cleavage upon imines nitrogen or alkynes carbon atoms backside attacking the silane-ruthenium complex, featuring a SN2@Si transition state. In addition, we found organic substrate is crucial in controlling the catalytic cycle. With imines, the ionic outer-sphere cycle occurs at the high-valent RuVI metal center is preferable. The highest energy barrier must overcome is 21.9 kcal/mol. With alkynes, the low-valent RuIII complex formed by NN coupling of RuVIN complex is the real catalytic species. The rate-determining energy barrier is calculated to be 12.7 kcal/mol.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds.

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R1R2PH, R2 = H or R1) with hydrosilanes (R3R4R5SiH, R4, R5 = H or R3) to produce synthetically useful silylphosphines (R1R2P-SiR3R4R5) has been achieved using B(C6F5)3 as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C6F5)3 but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R1R2HP·B(C6F5)3 adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C6F5)3 catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R1R2P-SiR3R4R5). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C6F5)3 at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H2-acceptor.

Aging- and thermal-annealing effects on the vibrational- and microstructural-properties of PECVD grown hydrogenated amorphous silicon carbon nitride thin films

In this work, the hydrogenated amorphous silicon carbon nitride (a-SiCN:H) thin films were grown by PECVD from the discharge of silane (SiH4), methane (CH4) and nitrogen (N2) gases. Thereafter, the stability of the bonding and microstructural properties with respect to the effects of aging (under atmospheric environment) and thermal annealing (under low and high N2 flow-rates) were investigated by using Fourier transform infrared (FTIR) and micro Raman scattering techniques. Analyses on the as-prepared films showed that the SiH, Si-OH, CH and NH vibrational bonds were stable even through prolong aging while that of SiCN and CN bonds increased and SiN bonds decreased after aging. Meanwhile, low temperature annealing were found to improve the film structure due to the out-diffusion process of N atoms from weak SiN and CN bonds and migration of these N atoms to form strong SiN and CN bonds as well as restructuring of dangling bonds to form new SiC bonds. Additionally, the graphitic phase was more prominent in the film grown at low N2 flow-rate upon annealing, especially when annealed at low temperature. The CN bonds in the graphitic phase were found to be weak and released N atoms even when annealed at low temperature. However, high temperature annealing was shown to remove the graphitic phases in the film structure.

Synthesis and characterization of bissilyl cobalt and iron hydrides bearing disilazane ligands via Si-H bond activation

Abstract The reaction between 2-pyridinetetramethyldisilazane 1 and CoMe(PMe3)4 led to the formation of the novel bissilyl cobalt(III) hydride CoH((C5H4N)-N(SiMe2)2)(PMe3)3 (2) via the cleavage of two Si-H bonds with the elimination of methane. The reaction of phenyltetramethyldisilazane 3 with CoMe(PMe3)4 could directly provide the hydrido cobalt(III) complex CoH((C6H5)-N(SiMe2)2)(PMe3)3 4. Complex 4 is an analogue to complex 2. This result indicates that a pre-coordination of the nitrogen atom in pyridine to the cobalt center before Si-H bond activation can be excluded in the formation mechanism of 2. This exclusion is further supported by the in situ 1H NMR spectra of the reaction mixtures between 2-pyridinetetramethyldisilazane 1 with CoMe(PMe3)4 in C6D6 in different reaction times. A new bissilyl iron hydride FeH((C5H4N)-N(SiMe2)2)(PMe3)3 (5) was obtained through the reaction of Fe(PMe3)4 with 1. Complexes 2, 4 and 5 with two metal-Si bonds are very stable and did not react with MeI, EtBr, acetylacetone, CO or CO2. The molecular structures of complexes 2, 4 and 5 were determined by single crystal X-ray diffraction.