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 2 3 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 6 7 8 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 12 13 14, coaxial or biaxial nanostructures 15 16 17, hierarchical heterostructures 18 19 20 21 22, and 1D heteronanostructures 23 24 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 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 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 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 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 18 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. 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 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.

Figure 1a shows the surface morphology of the SiC–WO _x nanostructures examined by scanning electron microscopy (SEM). The SEM image clearly shows that the synthesized SiC–WO _x nanostructures are of double layer structures. The top of the SiC nanowires are covered uniformly by the high density WO _x nanorods. The magnified images shown in Fig. 1b and c correspond to the SEM images from WO _x and SiC nanostructures, respectively. The morphology of the WO _x nanostructures 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 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 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 2shows 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 are highly crystallized. All the diffraction peaks can be indexed to monoclinic W₁₈O₄₉(JCPDS card No: 71-2450) and cubic SiC nanowires (29-1129).

Figures 3a and b shows the low magnification TEM images of the SiC–WO _x nanostructures. 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–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 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.

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–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.

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 WO _x and SiC nanostructures. The obtained turn-on field is lower than that of our earlier reported values for W₁₈O₄₉ nanowires (9.5 V/μm) 37 , W/WO₃ heteronanostructures (6.2 V/μm) 20 , and slightly higher than that of WO₃ nanowires (4.8 V/μm) 38 , 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) 40 , hierarchical AlN nanostructures (2.5–3.8 V/μm) 21 , BN coated SiC nanowires (6 V/μm) 41 , ZnS-In core-shell heteronanostructures (5.4–5.6 V/μm) 42 , and hierarchical SiC nanostructures (12 V/μm) 35 . 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.

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.