Thin Solid Films 519 (2010) 1668–1672
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Growth and optical properties of uniform tungsten oxide nanowire bundles via a two-step heating process by thermal evaporation Yun-Tsung Hsieh a, Meng-Wen Huang b, Chen-Chuan Chang a, Uei-Shin Chen a, Han-C. Shih a,c,⁎ a b c
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan
a r t i c l e
i n f o
Available online 7 September 2010 Keywords: Tungsten oxide nanowires WO3 Chemical vapor deposition UV–visible Optical properties Cathodoluminescence
a b s t r a c t Tungsten oxide (WO3) nanowires with diameters of 15–40 nm and lengths of hundreds of nanometers were synthesized by thermal chemical vapor deposition (CVD) without using any catalyst in a low-temperature zone (200–300 °C) of a tube furnace via a two-step heating process. The morphology, composition, and crystal structure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), Raman, ultraviolet UV–visible, and cathodoluminescence (CL) spectroscopy. XRD and TEM confirmed that the nanowires were triclinic WO3 with growth direction along [001]. Blue emission was observed in both the UV–visible and CL spectrum, indicating that the WO3 nanowires exhibited a red-shift at an optical absorption wavelength due to oxygen deficiencies. The crystallinity and size distribution of the nanowires influenced the bandgap. In the CL spectrum, the blue emission was at shorter wavelengths than reported previously, which can be attributed to the nanoscale size effect. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the last few decades, research interest in the study of nanomaterials and their applications has been increasing because these materials often demonstrate very different properties at the nanoscale level as compared to those at the macro level, such as new optical, magnetic, and electronic characteristics [1–4]. Furthermore, nanomaterials with high aspect-ratio structures and large surface areas offer exciting research possibilities because of such novel physical or chemical properties. As a result, the synthesis and characterization of one-dimensional metal oxide nanostructures have attracted considerable attention from researchers. Among the metal oxide materials, tungsten oxide (WO3), which is an important semiconductor material with a wide bandgap ranging from 2.5 to 3.6 eV, is of great interest because of its potential applications to optical devices, photocatalysts, electrochemical devices, field-emission devices, and gas sensors [5,6]. Therefore, the synthesis of WO3 nanostructures with various morphologies and phases using either physical or chemical routes has been the subject of considerable research. Recently, many novel methods of fabrication metal oxide nanomaterials have been reported including pulsed laser irradiation of iron metal in pure oxygen gas, unique precursors used in furnace and
⁎ Corresponding author. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. E-mail address:
[email protected] (H.-C. Shih). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.162
magnetron sputtering [7–10]. Further, WO3 nanowires have been extensively investigated in the previous decades, and various reaction methods have been developed to synthesize nanostructures, such as electrochemical techniques, template-directed synthesis, physical vapor deposition process, sol–gel process, pulsed electrodeposition, solvothermal and hydrothermal reaction, solution-based colloidal approaches, and the electrospinning method [11,12]. For practical purposes, it is desirable to develop a single efficient method for the fabrication of WO3 nanowires. As far as we know, few literature report that the chemical vapor deposition (CVD) method offers significant advantages such as total control over shapes and sizes, high homogeneity, considerably short processing time, cost effectiveness, high efficiency, easy synthesis, and simple experimental equipment; moreover high quality can be easily obtained by controlling CVD parameters with good reproducibility [13,14]. However, to the best of our knowledge, few studies have been conducted on the fabrication of WO3 nanowires by CVD because of the lack of practical preparation methods for such materials. The most common white light-emitting diode (LED) structure comprises a blue InGaN chip coated with yellow cerium-doped yttrium aluminum garnet phosphor [15]. This type of LED has a high luminous efficiency but limited color rendering capabilities due to the gaps in the red– and blue–green regions. Furthermore, the white light preferred for home illumination requires a correlated color temperature (CCT) of less than 4000 K, which is very difficult to achieve with this structure. Hence, novel phosphors with high efficiency, stability, and environmental compatibility in service should be developed. Furthermore, blue
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luminescence emission has attracted considerable attention recently due to its applications to white LEDs [16]. In this context, the potential of not only direct bandgap semiconductors, such as GaAs, ZnSe, and GaN, but also indirect bandgap semiconductor-tungsten oxides have been extensively studied [17,18]. However, there have been few observations of the luminescence properties of WO3 because of its low emission efficiency in conventional indirect bandgap semiconductors. Nevertheless, researches on the luminescence of WO3 have seen considerable progress. Niederberger et al. reported room-temperature blue emission from WO3 nanoparticles in an ethanol solution [19]. Feng et al. realized strong room-temperature photoluminescence (PL) from WO3 nanoparticles and W18O49 nanowires [6]. It is believed that the particle size, morphology, and quantum-confinement effect play important roles in the room-temperature luminescence emission [20]. In this letter, we report a more economical and simple thermal CVD method for the large-scale fabrication of WO3 nanowires on silicon (100) substrates without using any catalysts. By heating tungsten powder to 1000 °C in vacuum (4.6 × 10−2 Torr ) using a twostep process, WO3 nanowires with circular or polygonal cross sections were produced in high yield in a low-temperature zone (200–300 °C) of a furnace. Furthermore, in a series of experiments, we successfully achieved room-temperature blue emission from the as-synthesized WO3 nanowires; these emissions can be attributed to the band–band indirect transitions of the WO3 nanowires. 2. Experimental WO3 nanostructures were synthesized in a conventional horizontal tube furnace using a quartz working tube. Tungsten powder (0.05 g, Alfa AESAR; particle size, 12 μm; purity, 99.99%), which acted as the source material, was deposited on a ceramic boat and placed in the constanttemperature zone of the horizontal tube furnace. A pure silicon (100) wafer was subjected to ultrasonic cleaning in ethanol for 30 min. Then, it was cleaned consequentially in dilute HF (1 wt.%) for 45 s and deionized wafer followed by blow drying with nitrogen. Afterward it was placed in the low-temperature zone (ranging from 100 °C to 500 °C), 10 cm downstream from the source. This wafer acted as the substrate. After the quartz tube was pumped to the required vacuum of 5–6 × 10−5 Torr, the temperature of the furnace was raised from room temperature to 800 °C at a ramping rate of 30 °C/min (first heating stage). Meanwhile, the flow rates of the mixture gases of Ar and oxygen were controlled by using a mass flow meter; typical flow rates of Ar and oxygen were 10 sccm and 1 sccm, respectively. The pressure was maintained at 4.6 × 10−2 Torr, and the temperature of the furnace was increased from 800 °C to 1000 °C (second heating stage). After maintaining at this temperature for 1 h, the furnace was allowed to cool naturally to room temperature before the sample was removed for characterization.
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A scanning electron microscope (JEOL JSM-6500F) was used to perform morphological analysis. X-ray analysis was performed using a Shimadzu Lab XRD-6000 diffractometer with a graphite monochromator. The copper Kα radiation had a wavelength of λ = 1.54056 Å, and it was operated at 43 kV and 30 mA. High-resolution transmission electron microscopy (HRTEM), TEM, and energy-dispersive X-ray spectroscopy (EDS) were conducted on a JEOL 2010 transmission electron microscope operated at 200 kV. Raman spectroscopy was performed using a micro-Raman setup (LabRAM; Dilor) with a He–Ne laser emitting radiation at 632.8 nm. The optical properties of the as-prepared nanowires were measured on a UV–visible absorption spectrometer (Hitachi, U3010) and by cathodoluminescence (CL) spectrometry on a JEOL-JSM-7001F fieldemission scanning electron microscopy (FESEM) at room temperature. A 15-keV electron beam was used to excite the sample. The CL light was dispersed by a 1200 nm grating spectrometer and detected by a liquid nitrogen-cooled charge coupled device. 3. Result and discussion Fig. 1 shows SEM images of a typical sample at two different magnifications. The morphology of the obtained WO3 nanowires on the silicon substrate can be clearly seen. The nanowires have a unitary one-dimensional morphology with a high density and large scale over a large field of view. The diameters of the nanowires are also uniform and their values range from 15 to 40 nm, while their lengths have values of up to hundreds of nanometers. Phase identification for the as-prepared WO3 nanowires was performed by XRD. As shown in Fig. 2, all the peaks were indexed well to the triclinic phase of WO3 in accordance with the JCPDS Card No. 71-0305 (lattice constants, a = 0.7309 nm, b = 0.7522 nm, c = 0.7678 nm, and β = 90.92°). No diffraction peaks corresponding to any tungsten oxide other than WO3 could be detected in the pattern. The morphology and structure of the nanowires were further studied using TEM. Fig. 3(a) shows a low-magnification TEM image of straight nanowires with a uniform diameter (20 nm), and the circular cross section of the nanowire. The HRTEM image in Fig. 3(b) is a magnified view of the region outlined in Fig. 3(a). It shows the crystal structure and growth direction of individual nanowires. The lattice SPACINGS are measured to be 0.382 and 0.374 nm along the two orthonormal directions, corresponding to the (002) and (020) planes of triclinic WO3, respectively. It is shown that [001] is the primary growth direction of the nanowire. The selected-area diffraction (SAD) pattern is presented in the inset of Fig. 3(b) with the [100] zone axis, which confirms the major growth direction [001] and also shows that the nanowires had a single-crystal structure. Fig. 3(c) shows the corresponding elemental line-scan mapping of the nanowire obtained
Fig. 1. SEM images of tungsten oxide nanowires at different magnifications.
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(004)
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2 -Theta (Degrees) Fig. 2. XRD pattern of the tungsten oxide nanowires.
in the scanning TEM (STEM) mode. The map confirms the purity of the nanowires. Fig. 3(d) shows the oxygen and tungsten atomic content determined by energy-dispersive X-ray spectroscopy (EDS); the nanowires contain only O and W atoms and C and Cu signals can be attributed to the Cu grids used for our TEM measurements. Further, from the corresponding single nanowire analysis, the atomic ratio of
elements W and O is 25.14 and 74.86% [21,22], which is consistent with the XRD and TEM results for WO3. Raman scattering spectroscopy was used to characterize the nanowires, and the Raman spectrum of the as-synthesized nanowires is shown in Fig. 4. Three broad bands were clearly detected: 900– 1000 cm−1, 600–800 cm−1, and 200–400 cm−1. The most intense peaks, at 803 and 718 cm−1, are assigned to the symmetric and asymmetric vibration of W6+–O bonds (O–W–O stretching modes), respectively. The peaks at 272 and 323 cm−1 correspond to the W–O–W bending modes of the bridging oxygen. The peaks around 909 and 965 cm−1 are attributed to the –W=O bonds; this has been referred to in the literature as indicating the presence of a nanocrystalline structure [23]. In this study, the vapor-solid (VS) mechanism is responsible for the growth of WO3 nanowires since no catalysts were used [24]. Tungsten powder begins to sublimate from the quartz boat when the temperature is increased to 800 °C, and this process is greatly enhanced at a temperature of 1000 °C. The sublimated tungsten vapor reacts with adequate oxygen, and subsequently, the tungsten trioxide vapor flows to the lower temperature zone where the silicon substrate is placed and becomes supersaturated with nucleation of small clusters; this leads to the subsequent growth of nanowires. To our knowledge, it has been previously reported that when the vacuum is not sufficiently high, the mean free path of the vapor in the furnace is small; this affects the degree of supersaturation over the substrate and thereby hinders the nucleation process [25]. In this experiment, where the furnace has
Fig. 3. (a) TEM image of a WO3 nanowire. (b) High-resolution TEM (HRTEM) image of a nanowire. Inset shows the corresponding selected-area diffraction (SAD) pattern. (c) The corresponding EDS elemental line profile (d) EDS results show that the nanowires contain only O and W atoms and C and Cu signals can be attributed to the Cu grids used for the TEM measurements. Further, from the corresponding single nanowire analysis, the atomic ratio of elements W and O is 25.14 and 74.86%.
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Wavenumber (cm-1) Fig. 4. Raman spectrum of as-prepared WO3 nanowires.
intermediate vacuum, nucleation on the silicon substrate is rare. After increasing the temperature and selecting a suitable gas flux, nucleation is promoted by modifying the adsorption and diffusion characteristics of the surface. This increases the efficiency of nucleation, thereby promoting the nanowire growth. Because of its extremely high melting temperature of 3422 °C, a high heating temperature over 1000 °C has been normally employed for the tungsten source to produce enough vapors for growth of oxide nanostructures [26–28], and it was reported that WO3 vapor was the major vapor species for the nanowire growth [28]. With the low heating temperature of 800 °C used in our experiments, there was also few tungsten vapor produced. As examined after the growth, the top surface of the tungsten powders turned from silver gray into dark blue indicating that an oxidation reaction occurred at the surface of the powders during the growth and the majority of the powders were still kept as tungsten. Hence, the tungsten-contained vapors in our experiments are expected to be dominated by WO3. It is extremely interesting to compare these results with some literature in which WO3 nanostructures with different types of catalysts are fabricated on the silicon substrate [29]. Our study suggests that the O2 flow rate plays a very important role in the growth of WO3 nanowires under these growth conditions. Fig. 5(a) and (b) shows the SEM images of a sample that has been synthesized using the same methods and conditions as those for growing the sample shown in Fig. 1; however, the O2 flow rates were 0 and 0.5 sccm, respectively. In Fig. 5(a), where the O2 flow rate is zero, no nanowire can be seen but many unknown WOx nanoparticles existed on the substrate.
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Wavelength (nm) Fig. 6. UV–visible spectrum of the as-prepared WO3 nanowires; the inset shows the corresponding (αhν)1/2 vs. hν plot.
For an O2 flow rate of 0.5 sccm, although WO3 nanowire growth can be observed, the yield is still low, and many WOx clusters remain on the surface of the silicon wafer as shown in Fig. 5(b). This shows that it is difficult to obtain bundled WO3 nanowires without sufficient O2 flow rate in this growth process. Fig. 6 shows the UV–visible spectrum of the WO3 nanowires. It is remarkable that the absorption wavelength is approximately ~415 nm and the bandgap value of WO3 is ~2.99 eV, as defined as the intercept of the plot of (αhν)1/2 vs. hν, where α and hν denote the absorption coefficient and photon energy, (Fig. 6 inset) compared with the absorption wavelength value of ~ 360 nm reported for WO3 of nanostructures [30]. Hence, this implies that our as-synthesized nanowires of WO3 exhibited a red-shift in the optical absorption wavelength. It has been reported that both the size distribution and crystallinity of the WO3 nanostructures influence the bandgap values; further, the oxygen deficiencies at defects also play important roles [31]. The effects of oxygen concentration on the optical properties of WO3 nanostructures have been investigated in the literature [32], and it appears that the synthesis temperature and time will influence the oxygen ion deficiencies in the WO3 nanostructures. This will make the bandgap value change from 3.30 to 2.0 eV. Therefore, it can be concluded that the oxygen deficiencies within the nanomaterials are responsible for the decrease in the bandgap value of the WO3 nanowires in the present research.
Fig. 5. SEM image of the WO3 nanowires grown at different O2 flow rates: (a) 0 sccm; (b) 0.5 sccm.
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successfully exhibited blue luminescence emission. The red-shift phenomenon and blue emission could be explained by the oxygen deficiencies, crystallinity, and size distribution of WO3 nanostructures. In particular, the size effect resulted in blue emission (415 nm) at shorter wavelengths than reported previously. These results indicate that the WO3 nanowires have significant potential applications in LEDs, especially in solid state lighting for home illumination that comprises a major portion of our energy consumption.
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Acknowledgment This research was supported by the National Science Council through Grant No. 96-2221-E-034-006-MY2 and No. 98-2221-E-034-007-MY2. 325
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Wavelength (nm) Fig. 7. Cathodoluminescence (CL) spectrum of the as-prepared WO3 nanowires.
As for as we know, few literature reports on the CL properties of tungsten oxide nanomaterials exist. Fig. 7 shows the room-temperature CL spectrum of the as-prepared WO3 nanowires. The two strongest CL emission peaks are at 370 nm (3.35 eV) and 415 nm (2.99 eV). Lee et al. [33] and Feng et al. [6] also reported the similar PL properties of tungsten oxide nanostructures. The emission peak at 370 nm has been inferred that this is due to the intrinsic band–band transition emission induced by quantum-confinement effects in nanostructures with an ultrafine diameter (b5 nm) of individual nanowires within each bundle are considered [6,33]. However, the strong-intensity blue-emission peak at 415 nm has been previously attributed to oxygen vacancies, which are often implicit in the preparation of oxide semiconductors. The strong blue emission is indicative of the existence of a large amount degree of oxygen vacancies in the WO3 nanowires [33,34]. It is very interesting that the blue emission (415 nm ) was at shorter wavelengths than reported previously [6,33]. The emission wavelength ranges from 423 to 437 nm [35], which indicates that the nanoscale size effect must have occurred. Further, oxygen vacancies in the nanowires caused the intensity of the blue-emission peaks to be considerably higher than that of a band-to-band emission peak in the CL spectrum [36]. The nanowires absorb strongly in the near-UV to blue light region, matching the emission wavelength of a near-UV or blue LED. Hence, they may be appropriate for use in white LED [37]. 4. Conclusion In summary, WO3 nanowires were generated by thermal CVD via a two-step heating process without using any catalyst. The WO3 nanowires were deposited in a low-temperature zone (200–300 °C) of a tube furnace. The nanowires were 15–40 nm in diameter and up to hundreds of nanometers in length. The XRD and TEM results confirmed that the nanowires were triclinic WO3, with the growth direction along [001]. The Raman spectrum suggested that the grown WO3 nanowires were highly nanocrystalline. The as-synthesized WO3 nanowires
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