A simple route to nanocrystalline silicon carbide

Nanocrystalline silicon carbide has been prepared via reacting magnesium silicide (Mg₂Si) with carbon tetrachloride (CCl₄) in an autoclave at 450-600°C. X-ray diffraction patterns of the products can be indexed as the cubic cell of SiC with the lattice constant, a=4.352 Å, in good agreement with a=4.349 Å (JCPDS card No. 75-0254). The transmission electron microscopy images show that the sample mainly consists of nanoparticles with an average size from 30 to 80 nm co-existing with a small fraction of nanorods and nanowires. Typically the nanorods range from 20 to 40 nm in diameter and the nanowires have diameters of 20 nm and lengths up to 10 μm. The Raman spectrum shows a characteristic sharp peak at 790 cm⁻¹. X-ray photoelectron spectra (XPS) gives an atomic ratio of Si to C as 1.08:1.00 from the quantification of the peak intensities. Photoluminescence spectrum reveals that the SiC sample emits ultraviolet light of 328 nm. A possible mechanism and the influence of temperature on the formation of crystalline SiC are proposed.     

Synthesis of nanocrystalline silicon carbide using the sol-gel technique

Highly disperse silicon carbide is synthesized using a hybrid method comprising the sol-gel step to provide the SiO₂-C starting mixture involving the formation of a transparent gel and carbothermal synthesis under relatively soft conditions, namely, at temperatures of 1200–1500°C under a dynamic vacuum. The elemental and phase compositions of the products and their thermal behavior in air are studied. A relationship is found to exist between the microstructure of the product, on the one hand, and the temperature and time of heat treatment, on the other.     

Physical and chemical properties of silicon carbonitride nanocrystalline films

Nanocrystalline transparent films SiC_xN_y were obtained by plasma-enhanced chemical deposition within the temperature range 473–1173 K from low pressure gas phase from a mixture of hexamethyldisilazane vapor, ammonia, and helium. Physical chemical properties of the films obtained were studied by IR and Raman spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron spectroscopy and synchrotron radiation powder diffaction. Voltage-capacity and voltage-current measurements were also made. The dependence of chemical and phase composition of the films on deposition conditions was determined, and the formation of approximately 2 nm sized spherical nanocrystals within the films was established. The nanocrystals are formed by a phase similar to usual α-Si₃N₄, with silicon atoms partially substituted by carbon ones.     

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.     

Dielectric loss of neutron-irradiated nanocrystalline silicon carbide (3C-SiC) as a function of frequency and temperature

At the present work, nanocrystalline 3C-SiC has been irradiated by neutron flux (2 × 10¹³ n·cm⁻²s⁻¹) up to 20 h in a TRIGA Mark II type research reactor. The dielectric loss of nanocrystalline 3C-SiC was studied comparatively before and after neutron irradiation. The increased dielectric loss was clearly observed after neutron irradiation in both f(tanδ) ∼ f(f) and f(tanδ) ∼ f(T) plots. Furthermore, slope observed on the f(tanδ) ∼ f(f) plots at certain values of the frequency. Dielectric loss increasing and shifted slope explained by the neutron transmutation, dangling bonds, the formation of the defects or additional charge carriers. Moreover, the mechanism of all effects obtained from the experiments was explained by the polarization approach.     

Atmosphere effects in the processing of silicon carbide and silicon oxycarbide thin films and coatings

Silicon carbide and silicon oxycarbide films were prepared from solutions of polycarbosilane and methyldimethoxysilane + tetraethoxysilane, respectively, and deposited on different substrates (Si wafers, stainless steel plates, sapphire and SiC fibers). The coatings were heated at different temperatures and in different atmospheres, such as regular grade argon, ultra high purity and argon vacuum. The films were characterized using different techniques (FT-IR, XRD, SIMS, Ellipsometry).The influence of the processing parameters (heat treatment temperature and atmosphere) on the final microstructure of the coatings is discussed in this article.     

Electrochemistry of carboxylated nanodiamond films

The electrochemical behavior of nanodiamond (ND) film functionalized with carboxylic acid groups was studied systematically on a glassy carbon (GC) electrode. One stable redox couple corresponding to the carboxylic acid group was observed. At the scan rate of 0.1 V/s, the cathodic and anodic peak potentials were 0.093 V and 0.088 V (vs. Ag/AgCl), respectively. The carboxylic acid groups on the ND surface were reduced to CH 2 OH via a four electron redox process. The ND film modified electrode showed favorable electrocatalytic behavior toward the oxidation as well as the reduction of biomolecules, such as tryptophan and nicotinamide adenine dinucleotide.     

Effect of negative substrate bias on HWCVD deposited nanocrystalline silicon (nc-Si) films

The influence of the negative substrate bias on the interfacial and microstructural characteristics of nanocrystalline silicon (nc-Si) thin films was deposited by hot wire chemical vapor deposition (HWCVD). Structural characterization of nc-Si films was performed by small angle X-ray diffraction (SAXRD), Raman spectroscopy, X-ray reflectivity (XRR) and field emission scanning electron microscopy (FESEM). Crystalline fraction and crystallite size increases from 61.31 to 74.13% and 13.3 to 21.6 nm, respectively, with an increasing negative bias from 0 to ?200 V. Furthermore, the deposition rate of nc-Si films increases from 25 to 68 nm/min by increase of negative substrate bias from 0 to ?200 V.     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999     

Microstructure and composition of silicon carbide films deposited on carbon fibers by chemical vapor deposition

The microstructure and the composition of CVD silicon carbide films used as fiber coatings in composite materials were investigated by photoelectron spectroscopy and transmission electron microscopy. The films with a uniform thickness of 50 nm consisted of small SiC grains with a mean diameter of 15 nm and showed a stripe contrast in bright field images. Large grains with diameters in the dimension of the film thickness were used for imaging the lattice structure by high-resolution electron microscopy. The results are discussed as a polytype of cubic lamellae of a few nanometers and intermediate random stacking sequences of hexagonal structure. Received: 30 July 1997 / Accepted: 16 December 1997     

Electrochemistry of cytochrome P450 BM3 in sodium dodecyl sulfate films

Direct electrochemistry of the cytochrome P450 BM3 heme domain (BM3) was achieved by confining the protein within sodium dodecyl sulfate (SDS) films on the surface of basal-plane graphite (BPG) electrodes. Cyclic voltammetry revealed the heme FeIII/II redox couple at -330 mV (vs Ag/AgCl, pH 7.4). Up to 10 V/s, the peak current was linear with the scan rate, allowing us to treat the system as surface-confined within this regime. The standard heterogeneous rate constant determined at 10 V/s was estimated to be 10 s-1. Voltammograms obtained for the BM3-SDS-BPG system in the presence of dioxygen exhibited catalytic waves at the onset of FeIII reduction. The altered heme reduction potential of the BM3-SDS-graphite system indicates that SDS is likely bound in the enzyme active-site region. Compared to other P450-surfactant systems, we find redox potentials and electron-transfer rates that differ by approximately 100 mV and >10-fold, respectively, indicating that the nature of the surfactant environment has a significant effect on the observed heme redox properties.     

NH3 gas sensing properties of nanocrystalline ZnO based thick films

Zinc acetate derived precursor used in the present sol-gel synthesis of zinc oxide nanoparticles is described. The reaction product obtained before and after reflux of propanolic zinc acetate solution have been studied by UV-vis, photoluminescence and FT-IR studies which confirm the formation of oligomeric precursor Zn4O(Ac)6 (Ac=CH3COO). The formation of approximately 7 nm zinc oxide nanoparticles were confirmed by X-ray diffraction (XRD) and Transmission electron microscopic studies (TEM). The gaseous ammonia gas sensing characteristics of the nano-zinc oxide sensor showed high sensitivity compared to sensor fabricated with commercial zinc oxide powder.     

Fe-N/C catalysts for oxygen reduction based on silicon carbide derived carbon

Two different Fe-N/C(SiC) catalysts (Fe + Bipyr/C(SiC) and Fe + Phen/C(SiC)) for oxygen reduction based on silicon carbide derived carbon were synthesized and investigated in 0.1 M KOH aqueous solution by rotating disc electrode method. It was found that the electrocatalytic activity and stability are significantly influenced by the change of the nitrogen ligand in the catalyst. Comparable current density values obtained for 20%Pt-Vulcan electrode could be achieved for Fe + Bipyr/C(SiC) and Fe + Phen/C(SiC) catalysts in alkaline media. The durability tests (~ 150 h) showed that the decrease of the activity for Fe + Bipyr/C(SiC) and Fe + Phen/C(SiC) is only 0.5 mV h^− 1 and 0.17 mV h^− 1, respectively. The Fe + Bipyr/C(SiC) catalyst demonstrated higher activity in the RDE measurements, but during the long-term test the Fe + Phen/C(SiC) catalyst prove to be more stable than Fe + Bipyr/C(SiC).     

Helium-3 activation analysis of oxygen in silicon nitride films on silicon wafers

Oxygen in silicon nitride films on silicon wafers was analyzed by activation with the¹⁶O(³He, p)¹⁸F reaction. By³He bombardment of samples propertly arranged under consideration of the¹⁸F recoil effect, total oxygen was reliably determined and its predominant part was estimated to be located whether on film surface, in film interior, or on film-substrate interface. Sample films with 0.1 to 2 μm thicknesses were found to contain 0.2 to 2 μg/cm² of oxygen in locations varying with preparation conditions. This method has been compared with ESCA and other methods for surface analysis.     

Exciton-mediated hydrosilylation on photoluminescent nanocrystalline silicon

A novel white light-promoted reaction using photoluminescent nanocrystalline silicon enables the hydrosilylation of alkenes and alkynes, providing stabilization of the porous silicon without significant loss of the photoemissive qualities of the material. Photopatterning and lithographic fabrication of isolated porous silicon structures are made possible. Experiments and observations are presented which indicate that the light promoted hydrosilylation reaction is unique to photoluminescent silicon, and does not function on nonemissive material. Hydrosilylation using a reactive center generated from a surface-localized exciton is proposed based upon experimental evidence, explaining the photoluminescence requirement. Indirect excitons formed by light absorption mediate the formation of localized electrophilic surface states which are attacked by incoming alkene or alkyne nucleophiles. Supra-band gap charge carriers have sufficient energy to react with nucleophilic alkenes and alkynes, thereupon causing Si-C bond formation, an irreversible event. The light-promoted hydrosilylation reaction is quenched by reagents that quench the light emission from porous silicon, via both charge transfer and energy transfer pathways.     

Porous ceramics based on silicon carbide

A method was described for producing porous ceramics based on ultradispersed fibrous silicon carbide in an autonomous protective atmosphere. The protective atmosphere is formed by carbon dioxide being released in the synthesis.