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.
Keywords:Silicon 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 (WOx) 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–WOx nanostructures by a simple two-step thermal evaporation process. We synthesized a novel double-layer SiC–WOx nanostructure with W18O49 nanorods on SiC nanowires.
Synthesis of SiC–WOxDouble-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 WOxnanostructures. The second step was to grow W18O49nanorods on the SiC nanowires to obtain SiC–WOxdouble-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–WOxdouble-layer nanostructures were characterized by using various techniques.
Characterization of SiC–WOxDouble-Layer Nanostructures
The synthesized SiC–WOxdouble-layer nanostructures were characterized by using field-emission scanning electron microscopy (FE-SEM; JEOL JSM 330F), X-ray diffraction (XRD; Rigaku D-Max1400, CuKα 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
Figure 1a shows the surface morphology of the SiC–WOxnanostructures examined by scanning electron microscopy (SEM). The SEM image clearly shows that the synthesized SiC–WOxnanostructures are of double layer structures. The top of the SiC nanowires are covered uniformly by the high density WOxnanorods. The magnified images shown in Fig. 1b and c correspond to the SEM images from WOxand SiC nanostructures, respectively. The morphology of the WOxnanostructures is found to have rod-like structures with 100–400 nm in diameter and several micrometers in length. The SEM image of SiC nanowires shows that there are large amount of straight, curved, and randomly oriented and freestanding nanowires. SiC nanowires are of several tens of micrometers in length and 20–50 nm in diameter. The HRTEM lattice images from the WOxand SiC nanowires are shown in Fig. 1d and e, respectively. The clear stripes of lattice planes indicate that the grown nanostructures are highly crystalline. The spacing of the lattice fringes measured from the HRTEM lattice image of WOxis found to be 0.379 nm and this is in excellent agreement with the standardd-value of (010) plane of a monoclinic W18O49crystal, according to the JCPDS card No. 71-2450. The HRTEM lattice image of SiC shows a lattice fringe spacing of 0.248 nm, which can be indexed to the (111) plane of cubic SiC, according to the JCPDS card No. 29-1129.
Figure 1. aSEM image obtained from SiC–W18O49nanostructures (cross-sectional view);b,cmagnified image of (a) corresponding to W18O49nanorod layer and SiC nanowire layer, respectively;dHR-TEM lattice image from W18O49nanorod; andeHR-TEM lattice image from SiC nanowire
Figure 2shows the XRD pattern obtained from the SiC–WOxdouble-layer nanostructures, indicating that both the WOx(marked green) and SiC (marked red) nanostructures are highly crystallized. All the diffraction peaks can be indexed to monoclinic W18O49(JCPDS card No: 71-2450) and cubic SiC nanowires (29-1129).
Figure 2. X-ray diffraction pattern from SiC–W18O49double-layer nanostructures
Figures 3a and b shows the low magnification TEM images of the SiC–WOxnanostructures. TEM images did not show any junction between the W18O49and SiC nanostructures. The TEM images show that W18O49nanorod is smooth and straight without any particles. SiC nanowires are curved and there are few particles at some portion of the nanowires. The EDX analysis on these SiC nanowires showed that the particles are composed of W and O. The microscopic structure and chemical composition of SiC nanowires with few particles are investigated using a high-resolution scanning transmission electron microscope (HR-STEM). Figure 3c shows the high angle annular dark field (HAADF) STEM image from the SiC–WOxnanostructures. It was observed that there are some particles on the surface of the SiC nanowires. Figure 3d–f show corresponding EELS elemental mapping of Si, W, and O, respectively. The signal from C is not shown here since C signals come from the TEM grid also. The presence of W and O on the SiC nanowire surface suggests that the W18O49nanorods start to grow on the SiC nanowire surface with NiO as the catalyst. In a typical vapor–liquid–solid (VLS) mechanism, the catalyst particles are usually found at the top or bottom of the nanostructures. However, W18O49nanorods synthesized in this study do not have any catalyst particles (NiO or Ni) on its surface. Instead, the vapor–solid (VS) mechanism might be responsible for the growth of W18O49nanorods on SiC nanowire surfaces. When the temperature of the furnace is increased to high temperature, the tungsten oxide vapor will be continuously generated from the source. The generated vapor source becomes supersaturated for nucleation of small clusters and tungsten oxide is nucleated on the top of the SiC nanowire surface by VS mechanism. Thus, high density W18O49nanorods are grown uniformly on the SiC nanowires, which acted as the substrate. The observation of SiC and W18O49nanostructures separately in the TEM image indicates that the bonding between these two nanostructures might be weak and so they might have been detached during the sample preparation for TEM measurements. We could not observe uniform and high density tungsten oxide nanorods when NiO catalyst was not used before the growth of tungsten oxide. This might be due to the fact that NiO coated surface enhances the nucleation of tungsten oxide when compared with the uncoated surface. We have successfully fabricated a new type of double-layer nanostructures by a two-step thermal evaporation process. We believe that the similar kind of growth method can be applied for other materials to grow double-layer nanostructures.
Figure 3. a,bLow-magnification TEM images of SiC–W18O49double-layer nanostructures showing W18O49and SiC separately;chigh angle annular dark field (HAADF) STEM image of SiC nanowires with some particles; and corresponding EELS elemental mapping ofdSi,eW, andfO, respectively
During the synthesis of WOxnanostructures, some of the SiC nanowire samples we used are little longer than the width of the alumina boat. For these samples, end parts of the SiC nanowire surface do not face the tungsten oxide source material. The center part of the sample is very close to the source material and the end part is away from the source material. Interestingly, we observed a mass transport effect during the growth of WOxnanostructures under this condition. Figure 4a shows the digital photograph image of the SiC–W18O49double-layer nanostructure sample showing the mass transport effect. The image clearly shows the three different regions having different densities of W18O49nanorods. Figure 4b–d shows the SEM images from the three regions of the sample showing variation in the density of W18O49nanorods. The sample part (Fig. 4d) showing the high density W18O49nanorods is placed very close to the source whereas the sample part (Fig. 4b) showing only SiC nanowires is away from the source material and so there is no tungsten oxide growth. Thus the density of nanorods decreased gradually from the center to the end of the sample, owing to the mass transport of tungsten oxide source material. These kinds of nanostructures showing density gradient within the sample might be useful for some specific applications because of their different optical and electrical properties.
Figure 4. aDigital camera image of a SiC–W18O49double-layer nanostructure sample showing density gradient owing to mass transport of tungsten oxide source material; andb–dSEM image from different regions of the sample having only SiC nanowires, low density W18O49and high density W18O49nanorods, respectively
The field emission measurements were performed inside a vacuum chamber of pressure below 1 × 10−6 Torr. The Si substrate with SiC–W18O49 double-layer nanostructures was used as the cathode and indium tin oxide (ITO) coated glass plate was used as the anode. The cathode to anode distance was maintained at 100 μm for all the measurements. The emission current was measured as a function of applying voltage (voltage range of 100–750 V in steps of 10 V) after sweeping the voltage several times. During sweeping voltages, the adsorbates from the emitter surface are desorbed and the field emission becomes stable after several cycles. Figure 5, shows the emission current density (J) versus applied field (E). Here, we define the turn-on field as the electric field required to produce a current density of 10 μA/cm2. It is found that apparent turn-on field was 5.4 V/μm. The field emission performance is compared with our previously reported results for WOx and SiC nanostructures. The obtained turn-on field is lower than that of our earlier reported values for W18O49 nanowires (9.5 V/μm) , W/WO3 heteronanostructures (6.2 V/μm) , and slightly higher than that of WO3 nanowires (4.8 V/μm) , SiC nanowires (2–5 V/μm) [36,39]. The turn-on field value is comparable with many other types of nanostructures such as BN nanosheets aligned Si3N4 nanowires (4.2 V/μm) , hierarchical AlN nanostructures (2.5–3.8 V/μm) , BN coated SiC nanowires (6 V/μm) , ZnS-In core-shell heteronanostructures (5.4–5.6 V/μm) , and hierarchical SiC nanostructures (12 V/μm) . The reasonably better turn-on field for our nanostructures indicates that both the SiC and W18O49 nanostructures contribute to the field emission process and also it shows that both the nanostructures have good electrical bonding. The inset of Fig. 5 shows a typical Fowler–Nordheim (F–N) plot for our SiC–W18O49 nanostructures. The linearity of this curve shows that a conventional F–N mechanism was responsible for the field emission from our samples.
Figure 5. Field emission characteristics (current density–electric field) of SiC–W18O49double-layer nanostructures. The inset is the corresponding Fowler–Nordheim (F–N) plot
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).
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