Structural studies of nanoporous carbon produced from silicon carbide

X-ray diffraction and small-angle scattering study of nanoporous carbon samples prepared from polycrystalline α SiC and single-crystal 6H SiC is reported. The distribution function of carbon nanoclusters in size was found. In α SiC samples, the small size (10–12 Å) of nanoclusters is combined with their high size uniformity. Graphite-like nanoclusters 30–60 Å in size were found in samples of both types. In 6H SiC samples, such clusters make up a notable fraction of the volume. The experimentally observed structural anisotropy of the samples is discussed.     

Electrophysical studies of nanoporous carbon materials prepared of silicon carbide powders

Measurements of the electrophysical properties of nanoporous carbon (NPC) samples (conductivity and Hall effect), as well as studies of the same samples using electron spin resonance (ESR), are summarized. Free holes are shown to play the major part in charge transport in such materials, thus permitting identification of the ESR signal with free holes. An analysis of the shape of the resonance lines, as well as of their dependence on temperature and the technology employed in sample preparation, established the ESR signals to consist of two superimposed resonance lines associated with carriers, free or localized to various extents, whose magnetic susceptibility obeys Pauli and Curie-Weiss laws, respectively. The temperature dependences of the ESR signal parameters were studied, and the experimental relations were compared with theory to determine the model-parameters. An analysis of the temperature behavior of these parameters suggests the conclusion that NPC samples are heterophase porous systems whose properties are dominated by structural characteristics.     

Structural study of nanoporous carbon produced from polycrystalline carbide materials: Small-angle x-ray scattering

An x-ray small-angle scattering study is reported of the structure of nanoporous carbon prepared by chlorinating carbide compounds having different crystal structures (SiC, TiC, Mo₂C). The measurements were carried out both in reflection and transmission. The angular dependences of the scattering intensity obtained are treated as a result of scattering from nanoparticles of different size. By unfolding the experimental curves into components corresponding to particles with different gyration radii R _g, scatterer distribution functions in gyration radius m(R _g) were found. It is shown that, irrespective of the type of the starting carbide, particles with R _g～5 Å make up the largest fraction in porous carbon. Samples prepared from different carbides differ in the degree of nanoparticle uniformity in size. The most uniform in size are nanoparticles in the samples prepared from SiC, in which the average value R _g ^av <6 Å. Nanoparticles in the porous carbon produced from Mo₂C are about twice larger.     

Synthesis of nanoporous silicon carbide ceramics by thermal evaporation process

New nanoporous β-SiC ceramics were synthesized by a simple thermal evaporation method with commercial silicon powder and activated carbon fragments. The results of scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicated that the microstructure of the β-SiC nanoporous ceramics was uniform and consistent with the pore size of 50-100 nm. The β-SiC nanocrystal grains of 50-200 nm were accumulated together to form a nanopore network. The formation mechanism was attributed to a template synthesis process, in which activated carbon fragments were employed as the template and they reacted with vaporized silicon through a vapor-solid way.     

SiC/C nanocable structure produced in silicon carbide by arc plasma heating

Defect-free and long SiC/C nanocables have been produced by heating SiC powder at 3000°C by employing dc arc plasma (Ar) in a specially designed configuration of graphite arc. Microstructural characterizations of the heat-treated powder carried out by TEM, HRTEM, SAED, EDS, and micro Raman spectroscopy showed the nanocables to consist of a SiC shell/sheath stuffed with wire type solid C core. A possible mechanism is discussed to explain the cable-type growth.     

Roentgenoluminescence from silicon carbide

It is found that single crystals of silicon carbide exposed to soft X rays exhibit luminescence in the visible spectral range. The luminescence intensity from the single crystals produced by different methods differs by three-to fivefold. Also, the emission intensity is nonuniformly distributed over the single crystal surface, which may be related to the nonuniform distribution of impurities (activators).     

Small-angle x-ray scattering in a carbon-sulfur nanocomposite produced from bulk nanoporous carbon

A new nanocomposite material containing approximately 50 vol % S is prepared by filling pores of bulk nanoporous carbon samples with sulfur. The initial nanoporous carbon samples are synthesized from polycrystalline α-SiC through the chemical reaction. A comparative investigation of small-angle x-ray scattering (SAXS) is performed for the prepared nanocomposite and the initial material. The possible changes in the scattering power of the initial material upon filling of its pores with sulfur are considered in the framework of a simple model. The regularities revealed are used to interpret the experimentally observed changes in the scattering power. The size distribution functions of incorporated sulfur nanoclusters in the nanocomposite (or filled nanopores in the initial material) are determined within the Guinier approximation. It is demonstrated that the smallest sized pores (8–16 Å) remain unfilled, whereas the filling factor for larger sized pores can reach several ten percent by volume. The conditions favorable for small-angle x-ray scattering upon filling of the nanopores are analyzed.     

Raman light scattering in nanoporous carbon obtained from silicon and titanium carbides

A study is reported on the Raman spectra of nanoporous carbon prepared in a chemical reaction from polycrystalline α-SiC and TiC. The spectra are shown to be of a multicomponent nature, which distinguishes this group of materials from graphites and disordered carbon structures. A series of low-frequency modes are detected. Anisotropy and dispersion effects are revealed. The results obtained argue in favor of a cluster structure of nanoporous carbon and size quantization of the electronic and vibrational spectra in carbon nanoclusters. The main structural elements of the nanoclusters in the porous framework are small fragments of bent or broken graphene sheets. The presence of fragments close in structure to strained cubic or hexagonal diamond is observed in a number of cases.     

Classification and structure of silicon carbide phases

Molecular-mechanical and semiempirical quantum-mechanical methods have been applied to simulate and calculate a geometrically optimized structure of clusters of polymorphic types of silicon carbide, and their structural parameters and some properties (densities, sublimation energies) have been determined. A classification of silicon carbide phases has been proposed, which shows the possible existence of twenty one SiC phases whose atoms are at crystallographically equivalent sites. The structures of seventeen proposed silicon carbide phases have been described and studied for silicon carbide for the first time.     

Cluster modeling of three types of double-walled armchair silicon carbide nanotubes

A systematic study of three types of armchair double-walled SiC nanotubes (DWNT) (n,n)@(m,m) (3 ≤ n ≤ 6; 7 ≤ m ≤ 12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized using the hybrid functional B3LYP (Becke’s three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibility of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent singlewalled nanotubes and their types. A study of the binding energies, Mulliken charges, density of states and HOMO-LUMO gaps has been performed for all nanotubes from (n,n)@(n + 3,n + 3) to (n,n)@(n + 6,n + 6) (n = 3 ? 6). Type 2 DWNTs do not preserve the coaxial geometry when the difference in the chirality of the outer and inner tubes is 5 or less. For type 3, this occurs when the chirality difference is 4 or less. The gaps of types 2 and 3 DWNTs are lower than those of the corresponding single-walled nanotubes and are significantly less than those of type 1 DWNTs.     

On fractal nature of the structure of nanoporous carbon obtained from carbides

The curves describing small-angle x-ray scattering at npor-C nanoporous carbon samples obtained from polycrystalline α-SiC, TiC, and Mo₂C and a 6H-SiC single crystal have been analyzed. An algorithm is developed for taking into account the corrections to experimental curves for the intensity of the primary beam transmitted through the sample and the height of the inlet slit in these measurements. Two systems of nanoclusters observed in the npor-C structure differ in the type of stacking of structural elements: small-scale mass fractals of a dimension 1<D ₂<3 and a size L ₂=50–90 Å, which depend on the type of the initial carbide, and large-scale nanoclusters having a size L ₁>550 Å. In most samples, large-scale nanoclusters can be regarded as objects with a fractal surface and a dimension 2<D ₁<13, which also depends on the type of the initial carbide. Large-scale nanoclusters in npor-C obtained from Mo₂C prove to be mass fractals with a dimension D ₁>2. Peculiarities of the structure formation of nanoporous carbon obtained from various carbides are discussed.     

Electronic band structure and optical properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is fabricated by hydrothermally etching single crystal silicon (c-Si) wafers in hydrofluoric acid containing ferric nitrate. Microstructure studies disclosed that it is a typical micron/nanometer structural composite system with clear hierarchical structures. The optical parameters of Si-NPA were calculated by general light-absorption theory and Kramers–Kronig relations based on the experimental data of reflectance and the variations compared with the counterparts of c-Si were analyzed. The features of the electronic band structure deduced from the optical measurements strongly indicate that Si-NPA material is a direct-band-gap semiconductor and possesses separated conduction sub-bands which accords with conduction band splitting caused by silicon nanocrystallites several nanometers in size. All these electronic and optical results are due to the quantum confinement effect of the carriers in silicon nanocrystallites.     

Emissivity of powders prepared from nanoporous carbon

Powders prepared from nanoporous carbon are promising for creating cold emitters, which are essential to the development of reliable next-generation monitors. The results of an experimental study of the temperature and time dependences of the emission current from nanoporous carbon coatings are reported. It is shown that the stable emission may last at least 20 h under continuous operation if the emission current density does not exceed 0.6 mA/cm² at room temperature and an accelerating field strength of 800–1200 V/mm. The highest values of the unstable-in-time current density vary from 2.5 to 3.2 mA/cm².     

Influence of growth time on field emission properties from carbon nanotubes deposited on arrayed nanoporous silicon pillars

We investigated the influence of growth time on field emission properties of multi-walled carbon nanotubes deposited on silicon nanoporous pillar array (MWCNTs/Si-NPA), which were fabricated by thermal chemical vapour deposition at 800 °C for 5, 15 and 25 min respectively, to better understand the origins of good field emission properties. The results showed that the MWCNTs/Si-NPA grown for 15 min had the highest field emission efficiency of the three types of samples. Morphologies of the products were examined by field-emission scanning electron microscope, and the excellent field emission performance was attributed not only to the formation of a nest array of multi-walled carbon nanotubes, which would largely reduce the electrostatic shielding among the emitters and resulted in a great enhancement factor, but also to the medium MWCNTs density films, there was an ideal compromise between the emitter density and the intertube distance, which also could effectively avoid electrostatic shielding effects, along with a high emitter density.     

Photoluminescent properties of silicon carbide and porous silicon carbide after annealing

Photoluminescent (PL) p-type 6H porous silicon carbides (PSCs), which showed a strong blue-green photoluminescence band centered at approximately 490 nm, were annealed in Ar and vacuum conditions. The morphological, optical, and chemical states after annealing are reported on electrochemically etched SiC semiconductors.The thermal treatments in the Ar and vacuum environments showed different trends in the PL spectra of the PSC. In particular, in the case of annealing in a vacuum, the PL spectra showed both a weak red PL peak near 630 nm and a relatively intense PL peak at around 430 nm in the violet region. SEM images showed that the etched surface had spherical nanostructures, mesostructures, and islands. With increasing annealing temperature it changes all spherical nanostructures. The average pore size observed at the surface of the PSC before annealing was of the order of approximately 10 nm.In order to investigate the surface of a series of samples in detail, both the detection of a particular chemical species and the electronic environments at the surface are examined using X-ray photoelectron spectroscopy (XPS). The chemical states from each XPS spectrum depend differently before and after annealing the surface at various temperatures. From these results, the PL spectra could be attributed not only to the quantum size effects but also to the oxide state.     

氧化锌/硅纳米孔柱阵列的结构和光致发光特性研究

以硅纳米孔柱阵列(Si-NPA)为衬底、用化学气相沉积法制备了具有规则阵列结构特征的ZnO/Si-NPA纳米复合体系，并对其结构和光致发光性质进行了表征. 实验结果显示，组成ZnO/Si-NPA表面阵列的每个柱子均呈现层壳结构. 不同于衬底Si-NPA的红光和蓝光发射，ZnO/Si-NPA在紫外光区和蓝绿光区呈现出两个强的宽发光峰. 分析表明，紫外光发射应归因于ZnO晶体的带边激子跃迁；而蓝绿光发射则来自于ZnO晶体本征缺陷所形成的两类深能级复合中心上载流子的辐射跃迁.     

掺氮碳化硅纳米管电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对本征碳化硅纳米管和掺氮碳化硅纳米管的电子结构进行了计算.计算表明本征（8,0）碳化硅为直接带隙半导体,能带间隙为0.94 eV;掺氮浓度为1.56%和3.12％的碳化硅纳米管的能带间隙减小为0.83 eV和0.74 eV.从差分电荷密度可以看出,能带间隙的减小是氮硅键与碳硅键相比共价成键能力降低的结果. 关键词： 碳化硅纳米管 掺氮 第一性原理 电子结构     

Thermal carbonization of nanoporous silicon: Formation of carbon nanofibres without a metal catalyst

An interesting phenomenon is observed while carrying out thermal carbonization of porous silicon (PS) with an aim to arrest the natural surface degradation, and it is a burning issue for PS-based device applications. A tubular carbon structure has been observed on the PS surface. Raman, Fourier transform infrared spectroscopy (FTIR) and electron microscope studies, revealed that the tubular structure is nothing but amorphous carbon nanofibres sprouted within the pores in the absence of a metal catalyst, for which a suitable explanation is proposed.     

Composition and structure of nanoporous silicon layers with galvanically deposited Fe and Co

The morphology and phase composition of porous silicon with galvanically deposited Fe and Co particles were investigated by ultrasoft X-ray emission spectroscopy, X-ray absorption near-edge structure spectroscopy, and scanning electron microscopy. It is shown that iron uniformly covers the surface of porous silicon, whereas cobalt penetrates into pores in the form of nanoparticles. It is established that iron is present mainly in the Fe₂O₃ phase with an admixture of FeO and Fe₃O₄. Cobalt forms a mixture of phases of metallic Co and Co₂O₃.