Synthesis of tungsten oxide comblike nanostructures
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Tungsten oxide comblike nanostructures were synthesized using a two-step thermal evaporation method. The first step involving high reactor pressure and temperature was to synthesize the cores of the comb structures, upon which the teeth of the comb were grown in the second step using low operation pressures and temperatures. The teeth of the comb structure are well aligned and vertical to the side surfaces of the cores. The effects of growth parameters were examined, and the growth mechanism was investigated.
I. INTRODUCTION
II. EXPERIMENTAL
Novel nanostructures continue to receive people’s attention for their unique growth characteristics and the potentials for some special applications. Materials in the form of nanoparticles and nanowires exhibit different electrical, magnetic, optical, and chemical properties, while other novel nanostructures such as nanobelts,1–4 nanotubes,5,6 nanoneedles,7 nanohelixes, and nanosprings8,9 are also discovered with specific physical and mechanical properties. Hierarchical nanostructures are of interest because of their specific geometry and possible way to manipulate nanowires, such as nanotrees,10,11 nanothorn arrays,12 etc. More importantly, they may lead to some special and promising applications for nanodevices. Among them, comblike nanostructures are important for their well-aligned branches. The comblike structure has been found for ZnO, which may act as the highly ordered nanowire ultraviolet laser arrays.13 Tungsten oxide represents an interesting and promising material for optical and electrochromic devices.14–16 Recently, attention has been focused on the synthesis and properties of nanostructured tungsten oxide. Several fabrication methods have been developed where different morphologies of tungsten oxide nanostructures were reported.4–7,17–20 Here, we shall report a two-step thermal evaporation method for fabricating tungsten oxide comblike nanostructures with controllable sizes. The comblike nanostructures are characterized by various techniques, while the effects of growth parameters have also been followed. The possible growth mechanism is discussed.
The growth experiments were conducted in a conventional horizontal tube furnace,21 where tungsten trioxide (WO3) powder (0.2 g, Fluka, 99.9% in purity; SigmaAldrich Chemic, Steinheim, Germany) was loaded in a quartz boat and placed in the uniform-temperature zone of the furnace. In the first step, a Si(100) substrate wafer was put in the low-temperature zone 11 to 12 cm downstream of the source. After the quartz tube was pumped to the desired vacuum of 5 × 10−3 Torr with the air flow rate of 0.18 sccm (cubic centimeter per minute at STP), the temperature of the furnace (TS) was raised from room temperature (RT) to 925 °C at a ramping rate of 30 °C/ min. The temperature of the substrate (Tsub) increased concurrently to 650 °C. After maintaining the reactor pressure, air flow, and temperature for 3 h, a layer of yellowish material was shown deposited on the Si(100) substrate. The sample was then reloaded into the fur
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