Intense ultraviolet emission from needle-like WO 3 nanostructures synthesized by noncatalytic thermal evaporation
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NANO EXPRESS
Open Access
Intense ultraviolet emission from needle-like WO3 nanostructures synthesized by noncatalytic thermal evaporation Sunghoon Park, Hyunsu Kim, Changhyun Jin and Chongmu Lee*
Abstract Photoluminescence measurements showed that needle-like tungsten oxide nanostructures synthesized at 590°C to 750°C by the thermal evaporation of WO3 nanopowders without the use of a catalyst had an intense nearultraviolet (NUV) emission band that was different from that of the tungsten oxide nanostructures obtained in other temperature ranges. The intense NUV emission might be due to the localized states associated with oxygen vacancies and surface states. Background Tungsten oxide is of particular interest owing to its outstanding electrochromic, optochromic, and gas chromic properties [1-3], which make it a promising candidate for applications in smart windows, wide-angle high-contrast displays, gas, and temperature sensors [4-6]. Tungsten oxide in bulk form has been studied extensively over the past few decades. Nevertheless, there are relatively few reports on tungsten oxide nanostructures. In particular, little is known about the luminescence properties of tungsten oxide nanostructures possibly because tungsten oxide is an indirect band gap semiconductor with low-emission efficiency. Two strong emissions from tungsten oxide nanostructures, near-ultraviolet (NUV) emission and blue emission, have been reported [7-12]. Nevertheless, there is still some controversy regarding the origins of the two emissions. Niederberger et al. [7] suggested that the blue emission from WO3 nanoparticles in an ethanol solution was due to a bandto-band transition. Luo et al. [8] also reported that the NUV and blue emissions from the WO 3 - x nanowire network were due to the state of oxygen vacancies and a band-to-band transition, respectively. On the other hand, several reports have suggested the opposite. Lee et al. [9] and Feng et al. [10] reported that the NUV emission was attributed to a band-to-band transition;
whereas, the blue emission was due to the localized states of oxygen vacancies or defects. Chang et al. [11] also suggested that the blue emission from nitrogendoped tungsten oxide nanowires was due to oxygen vacancies. In recent years, one-dimensional (1D) nanostructures have been investigated extensively owing to their interesting properties and potential applications in electronics and optoelectronics. A range of methods have been used to synthesize tungsten oxide 1D nanostructures, such as thermal oxidation, thermal evaporation, chemical vapor deposition, hydrothermal reaction, electrochemical techniques, aid of intercalated polyaniline, solution-based colloidal approach, and a combination of electrospinning and sol-gel techniques [13]. Of these, thermal evaporation might be the most attractive technique with the advantage of synthesizing a range of tungsten oxide nanostructures depending on the substrate temperature at lower temperatures than other techniques. This paper reports a simple novel thermal evaporati
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