The Synthesis And Character Of Silicide One-dimensional Nanomaterials

One-dimensional (1D) inorganic structures with nanometer scale diameters, suchas nanotubes and nanowires, have great potential for testing and understandingfundamental concepts about the effects of dimensionality and size on, for example,mechanical, optical and electrical properties.In particular, silicon and silicide nanowires have received considerable attentionowing to their potential ease of integration into conventional silicon-based electronics.SiC and Si3N4nanowires have potential applications in electronic and opticnanodevices in harsh conditions (high temperature, high frequency or high power)owing to their fine mechanical properties, low density, superplasticity, good resistanceto thermal shock and oxidation as also high fracture toughness. SiO2nanowire is ofgreat significance in the fields of photoluminescence, localization of light, near-fieldoptical microscopy, low-dimensional waveguide, etc.Over the past several years, inorganic nanowires and nanorods have beensynthesized by a variety of methods, including physical evaporation, laser ablation,chemical vapor deposition and solvothermal reactions. Here, we describe a simplemethod to synthesize a series of silicide nanowires by carbothermal route.The methodis based on the conventional direct current arc discharge process. In these reactions,carbon helps to form oxidic species, usually containing sub-oxides, in the vapor phase,which then transform to the final crystalline product. Because no metal catalysts areneeded, the nanowires prepared here may have high purity in compositions. Thismethod has also the advantages of simplicity, rapidity and low cost.Bulk quantities of long and straightβ-SiC nanowires with high aspect ratio have been prepared via the direct current arc discharge method by using a mixture ofgraphite, silicon and silicon dioxide as the precursor. XRD and Raman scatteringstudies show that the products are a well crystallized cubic SiC phase. SEM imagesshow that the conglomerated nanowires are randomly oriented, with clean and smoothsurfaces. TEM and SAED analyses confirm that the nanowire is composed of asingle-crystalline core with anamorphous SiOxshell. First, the nucleation andformation of the Si-rich droplets occur via the widely accepted oxygen-assistedgrowth (OAG) mechanism. Second, the nucleation and growth of the SiC nanowiresproceed via a plasma-assisted self-catalytic vapor-liquid-solid (VLS) mechanism withthe Si-rich droplets formed in the previous step as a catalytic agent. Compared withthe conventional metal-catalytic VLS mechanism, a unique characteristic is that themetal catalyst is no longer needed and thus metal contamination is eliminated.Furthermore, taking advantage of the OAG mechanism in the first step, 1D SiCnanowires with not only a fast growth rate but also high quality can be acquired usingsimple direct current arc evaporation and readily available precursors.Bulk quantities ofα-Si3N4have been produced from a mixture of carbon, siliconand silicon dioxide with a molar ratio of 1:1:1 in an N2atmosphere. The products arecomposed of nanowires of tens of micrometers long and 100-300 nm in diameter witha high yield. The surfaces of the nanowires are coated with amorphous layers, about2-5 nm in thickness, which may be amorphous SiOx. Because there are not anydroplets observed on the ends of the nanowires, the growth mechanism of theα-Si3N4nanowires might well be a vapor-solid (VS) process.α-Si3N4nanocombs weregenerated by a noncatalytic growth following a VS mode. The nanocombs arecomposed of Si3N4nanowires, which grow perpendicular to the stems in a closepacking manner. The growth directions of the stems and the branches are <1 1 0> and<0 0 1>, respectively. The growth mechanism is supposed to be dominated by acombination of VS mechanism and the secondary epitaxial nucleation process. Thepine shaped nanodendrites were obtained through a catalytic growth in a VLS process.The thin branches, whose diameters were only 20-50 nm, unsystematically grew onthe nanocone-like stems and demonstrated complex 3D nanostructures. The growth mechanism is supposed to be dominated by a combination of VLS and VS mechanismfor the stem, and followed by VLS for the branches. This well-controlled synthesisstrategy is expected to be applicable of abrication of other stem-branch featurednanostructures. Room-temperature PL spectra of these nanostructures show broadvisible emission around 400-750 nm, which can be attributed to defects in the Si3N4structures.Direct current arc discharge method is developed for the synthesis of amorphoussilicon oxycarbide nanowires. The typical size of the amorphous SiCO nanowires is20-100μm in length and 50-100 nm in diameter. The XPS analyses confirm that thenanowires are composed exclusively of C, Si and O elements. The proposed growthprocess of the amorphous SiCO nanowires is based on a VS growth mechanism. Twobroad PL emission peaks with its maximum located at about 454 and 531 nm areobserved and attributed to oxygen deficient-associated defect.Bulk quantities of ultrathin Si/SiOxnanowires with only 10 nm diameter havebeen prepared via the direct current arc discharge method by using a mixture ofsilicon and silicon dioxide as the precursor. At the high temperature region (arc zone),the SiO gases form (SiO)xclusters. The oxygen atoms in the larger clusters formedprefer to migrate from the centers to the exterior surfaces, leading to the formation ofSi cores. When the (SiO)xclusters transfer to the low temperature region (cathode),the Si droplets are formed, being coated with silicon sub-oxide shells. With theincreasing of SiO partial pressures, Si with SiOxnanowires are formed. The growthmodel is OAG mechanism.Bulk quantities of twisted Zn nanowires with a high aspect ratio have beenprepared via the DC arc discharge method by using a mixture of C and ZnO as theprecursors. XRD, HRTEM and electron diffraction studies show that the products arewell crystallized hexagonal Zn phase. SEM and TEM morphologies show that thenanowires usually have serpentine geometries with sharp turning angles. In thissynthesis process, reoxidation of Zn nanowires was avoided, because temperature ofthe arc zone is higher than 3000 C and only Zn (v) and CO vapors can be formed.The growth mechanism is supposed to follow a plasma-assisted VS growth model. In summary,β-SiC nanowires,α-Si3N4nanowires and SiCO amorphousnanowires are synthesized via the direct current arc discharge method through thecarbothermal route. There are two common characteristics for the growth of theseSi-related nanowires. First, the similar precursors, which are mixtures of silicon,carbon and silicon dioxide, are used in the synthesizing of these nanowires. Second,the growth processes of these nanowires can be attributed to the self-catalytic VS orVLS mechanism, without the designed addition of metal catalysts. The nanowiresprepared herein may have high purities. The method reported here might be exploitedto fabricate other Si-related nanowires.