Synthesis of Novel Double-Layer Nanostructures of SiC–WO x by a Two Step Thermal Evaporation Process
A novel double-layer nanostructure of silicon carbide and tungsten oxide is synthesized by a two-step thermal evaporation process using NiO as the catalyst. First, SiC nanowires are grown on Si substrate and then high density W18O49nanorods are grown on these SiC nanowires to form a double-layer nanostructure. XRD and TEM analysis revealed that the synthesized nanostructures are well crystalline. The growth of W18O49nanorods on SiC nanowires is explained on the basis of vapor–solid (VS) mechanism. The reasonably better turn-on field (5.4 V/μm) measured from the field emission measurements suggest that the synthesized nanostructures could be used as potential field emitters.
KeywordsSilicon carbide Tungsten oxide Nanowires Nanorods Vapor–solid mechanism Field emission
The one-dimensional (1D) semiconductor nanostructures have attracted considerable research activities not only because of their interesting electronic and optical properties intrinsically associated with their low dimensionality and the quantum confinement effect but also because of their potential applications in electronic and optoelectronic nanoscale devices [1–4]. Recently, heteronanostructures of various functional materials have attracted increasing attention in materials chemistry and nanoscience because of their many desirable properties, which can be tailored by fine-tuning the composition, morphology, size and self-assembly of nanosized building blocks for the fabrication of functional electronic and photonic devices [5–9]. These heteronanostructured materials provide the opportunity to study the properties of material combinations that are difficult or impossible to fabricate in the bulk. Considerable effort has been made in recent years to synthesize various types of heteronanostructures such as superlattice structures [9, 10], core-shell structures [11–14], coaxial or biaxial nanostructures [15–17], hierarchical heterostructures [18–22], and 1D heteronanostructures [23–25]. Various growth techniques have been employed including laser-assisted catalytic growth, chemical vapor deposition (CVD), metal–organic chemical vapor deposition (MOCVD), and thermal evaporation to fabricate various 1D semiconductor heteronanostructures [9–25]. Although significant advances have been made in the fabrication of simple binary semiconducting nanostructures, direct fabrication of complex heteronanostructures with controlled morphology, size, and composition remains still challenging.
Tungsten oxide is an n-type wide band gap (3.25 eV) semiconductor with a work function in the range of 5.59–5.70 eV which makes it attractive for the field emission applications. One-dimensional nanomaterials of tungsten oxide (WO3) and its sub-oxides (WO x ) have been intensively studied due to their excellent physical and chemical properties for various potential applications as field emitters, electro-chromic devices, semiconductor gas sensors, catalysts, information displays, and smart windows [26–30]. Silicon carbide is a wide band gap (2.3 eV) semiconductor with many interesting properties, such as high hardness, large thermal conductivity, a low coefficient of thermal expansion, and excellent resistance to erosion and corrosion. Various SiC nanostructures have attracted much attention in recent years due to their potential application in nanocomposite materials and microelectronic devices [31–33]. Because of their promising physical and electrical properties, nanostructures of tungsten oxide and silicon carbide might play a crucial role as the building blocks in the fabrication of functional heteronanostructures. Although the growth of different types of WO3 and SiC nanostructures have been reported in recent years, there are only few reports available on the heteronanostructures of WO3 and SiC with other materials. Chen and Ye  have reported the synthesis and photocatalytic properties of novel 3D hierarchical WO3 hollow shells, including hollow dendrites, spheres, and dumbbells, self organized from tiny WO3 nanoplatelets. Hierarchical heteronanostructure of W nanothorns on WO3 nanowhiskers (WWOs) was fabricated by Baek et al.  by a simple two-step evaporation process and the hierarchical WWOs were found to exhibit promising field emission properties. Tak et al.  synthesized heteronanojunction of ZnO nanorods on SiC nanowires by a combination of thermal evaporation and MOCVD process. Bae et al.  have fabricated heterostructures of ZnO nanorods with various 1D nanostructures (CNTs, GaN, GaP, and SiC nanowires) by thermal chemical vapor deposition of Zn at a low temperature. Shen et al.  have synthesized hierarchical SiC nanoarchitectures by a simple chemical vapor deposition process and reported their field emission properties. Since there are no reports available on the heteronanostructures of WO3 with SiC up to our knowledge, in this article, we report for the first time, the synthesis of SiC–WO x nanostructures by a simple two-step thermal evaporation process. We synthesized a novel double-layer SiC–WO x nanostructure with W18O49 nanorods on SiC nanowires.
Synthesis of SiC–WO x Double-Layer Nanostructures
The growth of 1D SiC–W18O49double-layer nanostructure was achieved by a simple two step evaporation process. The first step was the growth of SiC nanowires on Si(100) substrates to serve as the substrate for the growth of WO x nanostructures. The second step was to grow W18O49nanorods on the SiC nanowires to obtain SiC–WO x double-layer nanostructures.
Synthesis of SiC Nanowires (1st step)
First, core-shell SiC–SiO2 nanowires were grown on Si(100) substrates by carbothermal reaction of tungsten oxide (WO3) with graphite (C) using NiO catalyst . The substrates used in our experiment were highly doped (0.003 Ω-cm) n-type Si(100) wafers. The Si substrates were dipped in the Ni(NO3)2/ethanol solution (0.06 M) after being cleaned in an ultrasonic acetone bath for 20 min and then dried in the oven at 60 °C for 15 min. WO3 and C mixed powders were placed in an alumina boat and Ni(NO3)2-coated Si substrate was kept on the top of the boat. Then the source–substrate containing alumina boat was kept at the uniform temperature zone of the furnace. After the residual air in the furnace quartz tube was eliminated with Ar gas flow for 30 min, the furnace temperature was increased to about 1100 °C under a constant Ar flow of 500 sccm. Then the furnace temperature was maintained at 1100 °C for 3 h to grow core-shell SiO2–SiC nanowires. After cooling down to room temperature, the surface of the Si substrate was covered with a white colored deposit. The substrates with core-shell SiO2–SiC nanowires were etched in HF aqueous solution (49% HF:H2O = 1:4) for 3 min to remove the SiO2 shell layer.
Synthesis of SiC–WOxNanostructures (2nd step)
The synthesized HF-etched SiC nanowire samples were dipped in the Ni(NO3)2/ethanol solution (0.06 M) twice and then dried in the oven. High purity (Aldrich, 99.99%) WO3powder, deposited on the edge of an alumina boat, acted as the source material for the tungsten oxide nanorod growth. Then the SiC nanowire sample was placed on the top of the alumina boat with the SiC deposited side facing the source material. After evacuating the furnace to a vacuum of 100 mTorr, the temperature of the furnace was slowly increased from room temperature to the growth temperature of 1050 °C and the temperature was maintained constant for 1 h. After the growth process, the furnace was allowed to cool normally to room temperature. The surface of the substrate with white colored deposit became blue after tungsten oxide deposition and the obtained SiC–WO x double-layer nanostructures were characterized by using various techniques.
Characterization of SiC–WO x Double-Layer Nanostructures
The synthesized SiC–WO x double-layer nanostructures were characterized by using field-emission scanning electron microscopy (FE-SEM; JEOL JSM 330F), X-ray diffraction (XRD; Rigaku D-Max1400, CuK α radiation λ = 1.5406 Å), high-resolution transmission electron microscopy (HR-TEM; JEOL 2100F, accelerating voltage 200 kV, resolution 0.14 nm lattice), high-resolution scanning transmission electron microscope (HR-STEM), energy-dispersive X-ray spectroscopy (EDX), and field emission measurements.
Results and Discussion
We report, for the first time, the synthesis of new type of nanostructures comprising silicon carbide and tungsten oxide by a simple two step thermal evaporation process. The synthesized nanostructures are double-layer SiC–W18O49nanostructure. Based on TEM and EDX analysis, a possible VS growth mechanism was proposed for the grown double-layer nanostructure. At some certain conditions, we observed that W18O49nanorods having different density (density gradient) can be grown on the SiC nanowires and this is attributed to the mass transport effect of tungsten oxide source material. This simple method of fabricating a new type of double-layer nanostructures with one of the nanostructures acting as substrate for the growth of other nanostructure could be applied to other materials to create heteronanostructures for device applications. Field emission measurements showed that the fabricated double-layer nanostructures are good field emitters.
Hyeyoung Kim and Karuppanan Senthil contributed equally to this article.
This work was supported by grant No. RT104-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE), and the Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008-005-J00501).