Synthesis of Novel Double-Layer Nanostructures of SiC–WOxby 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.

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 (WO 3 ) 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][27][28][29][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][32][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 WO 3 and SiC nanostructures have been reported in recent years, there are only few reports available on the heteronanostructures of WO 3 and SiC with other materials. Chen and Ye [18] have reported the synthesis and photocatalytic properties of novel 3D hierarchical WO 3 hollow shells, including hollow dendrites, spheres, and dumbbells, self organized from tiny WO 3 nanoplatelets. Hierarchical heteronanostructure of W nanothorns on WO 3 nanowhiskers (WWOs) was fabricated by Baek et al. [20] by a simple two-step evaporation process and the hierarchical WWOs were found to exhibit promising field emission properties. Tak et al. [34] synthesized heteronanojunction of ZnO nanorods on SiC nanowires by a combination of thermal evaporation and MOCVD process. Bae et al. [25] 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. [35] 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 WO 3 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 W 18 O 49 nanorods on SiC nanowires.

Synthesis of SiC-WO x Double-Layer Nanostructures
The growth of 1D SiC-W 18 O 49 double-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 W 18 O 49 nanorods on the SiC nanowires to obtain SiC-WO x doublelayer nanostructures.

Synthesis of SiC Nanowires (1st step)
First, core-shell SiC-SiO 2 nanowires were grown on Si(100) substrates by carbothermal reaction of tungsten oxide (WO 3 ) with graphite (C) using NiO catalyst [36]. The substrates used in our experiment were highly doped (0.003 X-cm) n-type Si(100) wafers. The Si substrates were dipped in the Ni(NO 3 ) 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. WO 3 and C mixed powders were placed in an alumina boat and Ni(NO 3 ) 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 SiO 2 -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 SiO 2 -SiC nanowires were etched in HF aqueous solution (49% HF:H 2 O = 1:4) for 3 min to remove the SiO 2 shell layer.

Synthesis of SiC-WO x Nanostructures (2nd step)
The synthesized HF-etched SiC nanowire samples were dipped in the Ni(NO 3 ) 2 /ethanol solution (0.06 M) twice and then dried in the oven. High purity (Aldrich, 99.99%) WO 3 powder, 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, CuKa radiation k = 1.5406 Å ), high-resolution transmission electron Nanoscale Res Lett (2009) 4:802-808 803 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. The HRTEM lattice images from the WO x and 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 WO x is found to be 0.379 nm and this is in excellent agreement with the standard d-value of (010) plane of a monoclinic W 18 Figure 2 shows the XRD pattern obtained from the SiC-WO x double-layer nanostructures, indicating that both the WO x (marked green) and SiC (marked red) nanostructures  Figure 3c shows the high angle annular dark field (HAADF) STEM image from the SiC-WO x nanostructures. 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 W 18 O 49 nanorods 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, W 18 O 49 nanorods 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 W 18 O 49 nanorods 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.

Results and Discussion
Thus, high density W 18 O 49 nanorods are grown uniformly on the SiC nanowires, which acted as the substrate. The observation of SiC and W 18 O 49 nanostructures 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 doublelayer 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.
During the synthesis of WO x nanostructures, 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 WO x nanostructures under this condition. Figure 4a shows the digital photograph image of the SiC-W 18 (Fig. 4d) showing the high density W 18 O 49 nanorods 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.
The field emission measurements were performed inside a vacuum chamber of pressure below 1 9 10 -6 Torr. The Si substrate with SiC-W 18 O 49 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 lm 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 lA/cm 2 . It is found that apparent turn-on field was 5.4 V/lm. The field emission performance is compared with our previously reported results for WO x and SiC nanostructures. The obtained turn-on field is lower than that of our earlier reported values for W 18 O 49 nanowires (9.5 V/lm) [37], W/WO 3 heteronanostructures (6.2 V/lm) [20], and slightly higher than that of WO 3 nanowires (4.8 V/lm) [38], SiC nanowires (2-5 V/lm) [36,39]. The turn-on field value is comparable with many other types of nanostructures such as BN nanosheets aligned Si 3 N 4 nanowires (4.2 V/lm) [40], hierarchical AlN nanostructures (2.5-3.8 V/lm) [21], BN coated SiC nanowires (6 V/lm) [41], ZnS-In core-shell heteronanostructures (5.4-5.6 V/lm) [42], and hierarchical SiC Fig. 3 a,  The linearity of this curve shows that a conventional F-N mechanism was responsible for the field emission from our samples.

Conclusions
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.