- Nano Express
- Open Access
Intense ultraviolet emission from needle-like WO3 nanostructures synthesized by noncatalytic thermal evaporation
© Park et al; licensee Springer. 2011
- Received: 17 May 2011
- Accepted: 13 July 2011
- Published: 13 July 2011
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 near-ultraviolet (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.
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.  suggested that the blue emission from WO3 nanoparticles in an ethanol solution was due to a band-to-band transition. Luo et al.  also reported that the NUV and blue emissions from the WO3 - 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.  and Feng et al.  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.  also suggested that the blue emission from nitrogen-doped 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 . 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 evaporation technique to obtain tungsten oxide nanostructures with a range of morphologies and sizes using a single apparatus and a single process and an intense ultraviolet emission from the needle-like tungsten oxide nanostructures grown in the temperature zone from 590 to 750°C by thermal evaporation.
The collected nanostructure samples were characterized by scanning electron microscopy (SEM, Hitachi S-4200, Hitachi Ltd., Tokyo, Japan), transmission electron microscopy (TEM, Philips CM-200, Koninklijke Philips Electronics N.V., Amsterdam, Netherlands) equipped with an energy-dispersive X-ray spectrometer, and X-ray diffraction (XRD, Philips X'pert MRD diffractometer, Koninklijke Philips Electronics N.V., Amsterdam, Netherlands). The samples used for characterization were dispersed in absolute ethanol and ultrasonicated before the SEM and TEM observations. Glancing angle (0.5°) XRD was performed to examine the phases of the products obtained. Photoluminescence (PL) measurements were conducted at room temperature by using a SPEC-1403 PL spectrometer (HORIBA Ltd., Tokyo, Japan) with a He-Cd laser (325 nm) as the excitation source. The power of the He-Cd laser was 55 mW, and the diameter of the focal spot was 1 mm. Thus, the power density at the surface of the sample surface was approximately 7 W/cm2.
Results and discussion
Oxygen vacancies: The NUV emission is attributed to the localized states of oxygen vacancies in the conduction band of the needle-like tungsten oxide nanostructures. Luo et al.  reported an NUV emission band centered at 395 nm from WO3 - x nanowire networks, even if the emission band was not as sharp and strong as the one from the needle-like tungsten oxide nanostructures synthesized in this work. They attributed the NUV emission to the states of oxygen vacancies in the conduction band of WO3 - x nanowire networks. They also demonstrated using SEM and X-ray photoemission spectroscopy analyses that oxygen vacancies existed in the WO3 - x nanowire network but not in the WO3 nanowire network. Needle-like tungsten oxide nanostructures were grown in zone 2 (590°C to 750°C), i.e., in quite a low-temperature range. The W/O atomic ratio (8.01/2.80) in the needle-like tungsten nanostructures is approximately 2.86 as shown in the energy-dispersive X-ray spectroscopy (EDS) line scanning profile (Figure 4), so that the nanostructures do not have a molecular formula of WO3 but of WO3 - x . This may be due to the relatively low process temperature for the tungsten nanowire synthesis. The tungsten oxide nanostructures grown at low temperatures have been reported to commonly possess more defects such as oxygen vacancies . Therefore, the NUV emission from the needle-like tungsten nanostructures was attributed to the localized states of oxygen vacancies, as Luo et al.  suggested.
Surface states: The needle-like nanostructures obviously have a far higher surface state density than other nanostructures, such as thicker nanorods and thin films synthesized in zones 1, 3, 4, and 5. Therefore, the far stronger NUV emission from the needle-like nanostructures than that from the WO3 - x nanowire networks in Luo et al's report  may be due partially to the higher density of surface states at the surfaces of the needle-like nanostructures.
The PL spectra showed that the NUV emission intensity tends to decrease with increasing substrate temperature, but the blue emission intensity tends to increase. This tendency appears to depend on the morphology of the tungsten oxide nanostructures because the morphology of the nanostructures also changes from whiskers to nanoneedles and nanorods to thin films. In other words, the surface-to-volume ratio of the nanostructures decreases with increasing substrate temperature. In addition, the oxide nanostructures synthesized at low temperatures commonly possess more oxygen vacancies. Therefore, the blue luminescence is predominant in tungsten oxide nanostructures with a low oxygen vacancy concentration and low surface-to-volume ratios synthesized at higher temperatures. This suggests that the blue emission does not originate from deep level defects but from a band-to-band transition. The strong blue emission obtained in the lowest temperature zone (zone 1) is presumably due to the low surface-to-volume ratio of the pad tungsten oxide layer with a thin film morphology synthesized in such a low-temperature range.
In summary, intense NUV emission was obtained from the needle-like WO3 nanostructures synthesized in the temperature range of 590°C to 750°C by the thermal evaporation of WO3 powders. The NUV emission might be due to localized states associated with oxygen vacancies and surface states.
This study was supported by the Korea Science and Engineering Foundation through "the 2010 Core Research Program."
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