Thin Solid Films 519 (2010) 1668–1672

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Growth and optical properties of uniform tungsten oxide nanowire bundles via atwo-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 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwanb Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwanc Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan

⁎ Corresponding author. Department of Materials ScieTsing Hua University, Hsinchu 300, Taiwan.

E-mail address: hcshih@mx.nthu.edu.tw (H.-C. Shih

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.08.162

a b s t r a c t

a r t i c l e i n f o

Available online 7 September 2010

Keywords:Tungsten oxide nanowiresWO3Chemical vapor depositionUV–visibleOptical propertiesCathodoluminescence

Tungsten oxide (WO3) nanowires with diameters of 15–40 nm and lengths of hundreds of nanometers weresynthesized by thermal chemical vapor deposition (CVD) without using any catalyst in a low-temperaturezone (200–300 °C) of a tube furnace via a two-step heating process. Themorphology, composition, and crystalstructure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmissionelectron 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 oxygendeficiencies. The crystallinity and size distribution of the nanowires influenced the bandgap. In the CLspectrum, the blue emission was at shorter wavelengths than reported previously, which can be attributed tothe nanoscale size effect.

nce and Engineering, National

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1. Introduction

Over the last few decades, research interest in the study ofnanomaterials and their applications has been increasing becausethese materials often demonstrate very different properties at thenanoscale level as compared to those at the macro level, such as newoptical, magnetic, and electronic characteristics [1–4]. Furthermore,nanomaterials with high aspect-ratio structures and large surface areasoffer exciting research possibilities because of such novel physical orchemical properties. As a result, the synthesis and characterization ofone-dimensional metal oxide nanostructures have attracted consider-able attention from researchers. Among the metal oxide materials,tungsten oxide (WO3), which is an important semiconductor materialwith a wide bandgap ranging from 2.5 to 3.6 eV, is of great interestbecause 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 morphol-ogies and phases using either physical or chemical routes has been thesubject of considerable research.

Recently, many novel methods of fabrication metal oxide nano-materials have been reported including pulsed laser irradiation of ironmetal in pure oxygen gas, unique precursors used in furnace and

magnetron sputtering [7–10]. Further, WO3 nanowires have beenextensively investigated in the previous decades, and various reactionmethods have been developed to synthesize nanostructures, such aselectrochemical techniques, template-directed synthesis, physicalvapor deposition process, sol–gel process, pulsed electrodeposition,solvothermal and hydrothermal reaction, solution-based colloidalapproaches, and the electrospinning method [11,12]. For practicalpurposes, it is desirable to develop a single efficient method for thefabrication ofWO3 nanowires. As far as we know, few literature reportthat the chemical vapor deposition (CVD) method offers significantadvantages such as total control over shapes and sizes, highhomogeneity, considerably short processing time, cost effectiveness,high efficiency, easy synthesis, and simple experimental equipment;moreover high quality can be easily obtained by controlling CVDparameters with good reproducibility [13,14]. However, to the best ofour knowledge, few studies have been conducted on the fabrication ofWO3 nanowires by CVD because of the lack of practical preparationmethods for such materials.

The most common white light-emitting diode (LED) structurecomprises a blue InGaN chip coated with yellow cerium-doped yttriumaluminum garnet phosphor [15]. This type of LED has a high luminousefficiency but limited color rendering capabilities due to the gaps in thered– and blue–green regions. Furthermore, thewhite light preferred forhome illumination requires a correlated color temperature (CCT) of lessthan 4000 K,which is very difficult to achievewith this structure.Hence,novel phosphors with high efficiency, stability, and environmentalcompatibility in service should be developed. Furthermore, blue

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luminescence emission has attracted considerable attention recentlydue to its applications towhite LEDs [16]. In this context, thepotential ofnot only direct bandgap semiconductors, such as GaAs, ZnSe, and GaN,but also indirect bandgap semiconductor-tungsten oxides have beenextensively studied [17,18]. However, there have been fewobservationsof the luminescence properties of WO3 because of its low emissionefficiency in conventional indirect bandgap semiconductors. Neverthe-less, researches on the luminescence of WO3 have seen considerableprogress. Niederberger et al. reported room-temperature blue emissionfromWO3 nanoparticles in an ethanol solution [19]. Feng et al. realizedstrong room-temperature photoluminescence (PL) from WO3 nano-particles and W18O49 nanowires [6]. It is believed that the particle size,morphology, and quantum-confinement effect play important roles inthe room-temperature luminescence emission [20].

In this letter, we report a more economical and simple thermalCVD method for the large-scale fabrication of WO3 nanowires onsilicon (100) substrates without using any catalysts. By heatingtungsten powder to 1000 °C in vacuum (4.6×10−2 Torr ) using a two-step process, WO3 nanowires with circular or polygonal cross sectionswere produced in high yield in a low-temperature zone (200–300 °C)of a furnace. Furthermore, in a series of experiments, we successfullyachieved room-temperature blue emission from the as-synthesizedWO3 nanowires; these emissions can be attributed to the band–bandindirect transitions of the WO3 nanowires.

2. Experimental

WO3 nanostructures were synthesized in a conventional horizontaltube furnaceusing a quartzworking tube. Tungstenpowder (0.05 g, AlfaAESAR; particle size, 12 μm; purity, 99.99%), which acted as the sourcematerial, was deposited on a ceramic boat and placed in the constant-temperature zone of the horizontal tube furnace. A pure silicon (100)waferwas subjected toultrasonic cleaning in ethanol for 30 min. Then, itwas cleaned consequentially in diluteHF (1 wt.%) for 45 s and deionizedwafer followed byblowdryingwith nitrogen. Afterward itwasplaced inthe low-temperature zone (ranging from 100 °C to 500 °C), 10 cmdownstreamfromthe source. Thiswafer acted as the substrate. After thequartz tubewaspumped to the required vacuumof 5–6×10−5 Torr, thetemperature of the furnacewas raised from room temperature to 800 °Cat a ramping rate of 30 °C/min (first heating stage).Meanwhile, theflowrates of the mixture gases of Ar and oxygen were controlled by using amass flowmeter; typical flow rates of Ar and oxygenwere 10 sccm and1 sccm, respectively. The pressure was maintained at 4.6×10−2 Torr,and the temperature of the furnace was increased from 800 °C to1000 °C (second heating stage). After maintaining at this temperaturefor 1 h, the furnace was allowed to cool naturally to room temperaturebefore the sample was removed for characterization.

Fig. 1. SEM images of tungsten oxide na

A scanning electron microscope (JEOL JSM-6500F) was used toperform morphological analysis. X-ray analysis was performed using aShimadzu Lab XRD-6000 diffractometer with a graphite monochroma-tor. The copper Kα radiation had a wavelength of λ=1.54056 Å, and itwas operated at 43 kV and 30 mA. High-resolution transmissionelectron microscopy (HRTEM), TEM, and energy-dispersive X-rayspectroscopy (EDS) were conducted on a JEOL 2010 transmissionelectron microscope operated at 200 kV. Raman spectroscopy wasperformed using a micro-Raman setup (LabRAM; Dilor) with a He–Nelaser emitting radiation at 632.8 nm.

The optical properties of the as-prepared nanowires weremeasuredon a UV–visible absorption spectrometer (Hitachi, U3010) and bycathodoluminescence (CL) spectrometry on a JEOL-JSM-7001F field-emission scanning electron microscopy (FESEM) at room temperature.A 15-keV electron beamwas used to excite the sample. The CL lightwasdispersed by a 1200 nm grating spectrometer and detected by a liquidnitrogen-cooled charge coupled device.

3. Result and discussion

Fig. 1 shows SEM images of a typical sample at two differentmagnifications. The morphology of the obtained WO3 nanowires onthe silicon substrate can be clearly seen. The nanowires have a unitaryone-dimensional morphology with a high density and large scale overa large field of view. The diameters of the nanowires are also uniformand their values range from 15 to 40 nm, while their lengths havevalues of up to hundreds of nanometers.

Phase identification for the as-prepared WO3 nanowires wasperformed by XRD. As shown in Fig. 2, all the peaks were indexedwell to the triclinic phase ofWO3 in accordancewith 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 toany tungstenoxideother thanWO3 could bedetected in thepattern. Themorphology and structure of the nanowires were further studied usingTEM. Fig. 3(a) shows a low-magnification TEM image of straightnanowires with a uniform diameter (20 nm), and the circular crosssection of the nanowire. The HRTEM image in Fig. 3(b) is a magnifiedviewof the region outlined in Fig. 3(a). It shows the crystal structure andgrowth direction of individual nanowires. The lattice SPACINGS aremeasured to be 0.382 and 0.374 nm along the two orthonormaldirections, corresponding to the (002) and (020) planes of triclinicWO3, respectively. It is shown that [001] is theprimary growthdirectionof the nanowire. The selected-area diffraction (SAD) pattern ispresented in the inset of Fig. 3(b) with the [100] zone axis, whichconfirms the major growth direction [001] and also shows that thenanowires had a single-crystal structure. Fig. 3(c) shows thecorresponding elemental line-scan mapping of the nanowire obtained

nowires at different magnifications.

20 30 40 50

(123)

(004)

(040)

(020)

(002)

(311)

Inte

nsi

ty (

a.u

.)

2 -Theta (Degrees)

(200)

Fig. 2. XRD pattern of the tungsten oxide nanowires.

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in the scanning TEM (STEM) mode. The map confirms the purity of thenanowires. Fig. 3(d) shows the oxygen and tungsten atomic contentdetermined by energy-dispersive X-ray spectroscopy (EDS); thenanowires contain only O and W atoms and C and Cu signals can beattributed to theCu grids used for our TEMmeasurements. Further, fromthe corresponding single nanowire analysis, the atomic ratio of

Fig. 3. (a) TEM image of a WO3 nanowire. (b) High-resolution TEM (HRTEM) image of a nacorresponding EDS elemental line profile (d) EDS results show that the nanowires contain onmeasurements. Further, from the corresponding single nanowire analysis, the atomic ratio

elements W and O is 25.14 and 74.86% [21,22], which is consistentwith the XRD and TEM results for WO3.

Raman scattering spectroscopy was used to characterize thenanowires, and the Raman spectrum of the as-synthesized nanowiresis shown in Fig. 4. Three broad bands were clearly detected: 900–1000 cm−1, 600–800 cm−1, and200–400 cm−1. Themost intensepeaks,at 803 and 718 cm−1, are assigned to the symmetric and asymmetricvibration ofW6+–Obonds (O–W–Ostretchingmodes), respectively. Thepeaks at 272 and 323 cm−1 correspond to the W–O–W bending modesof the bridging oxygen. The peaks around 909 and 965 cm−1 areattributed to the –W=Obonds; this has been referred to in the literatureas indicating the presence of a nanocrystalline structure [23].

In this study, the vapor-solid (VS) mechanism is responsible for thegrowth of WO3 nanowires since no catalysts were used [24]. Tungstenpowder begins to sublimate from thequartz boatwhen the temperatureis increased to 800 °C, and this process is greatly enhanced at atemperature of 1000 °C. The sublimated tungsten vapor reacts withadequate oxygen, and subsequently, the tungsten trioxide vapor flowsto the lower temperature zonewhere the silicon substrate is placed andbecomes supersaturated with nucleation of small clusters; this leads tothe subsequent growth of nanowires. To our knowledge, it has beenpreviously reported that when the vacuum is not sufficiently high, themean free path of the vapor in the furnace is small; this affects thedegree of supersaturation over the substrate and thereby hinders thenucleation process [25]. In this experiment, where the furnace has

nowire. Inset shows the corresponding selected-area diffraction (SAD) pattern. (c) Thely O andW atoms and C and Cu signals can be attributed to the Cu grids used for the TEMof elements W and O is 25.14 and 74.86%.

300 400 500 600 700 800 900 1000

803

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

718323272

909 965

Fig. 4. Raman spectrum of as-prepared WO3 nanowires.

(αhυ

)1/2 (e

V/c

m)1/

2

2.5 3.0 3.5 4.0

Photo Energy (eV)

2.99 eV

400 500 600 700

Wavelength (nm)

Ab

sorb

ance

(a.

u.)

Fig. 6. UV–visible spectrum of the as-prepared WO3 nanowires; the inset shows thecorresponding (αhν)1/2 vs. hν plot.

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intermediate vacuum, nucleation on the silicon substrate is rare. Afterincreasing the temperature and selecting a suitable gas flux, nucleationis promotedbymodifying the adsorption anddiffusion characteristics ofthe surface. This increases the efficiency of nucleation, therebypromoting the nanowire growth. Because of its extremely high meltingtemperature of 3422 °C, a high heating temperature over 1000 °C hasbeen normally employed for the tungsten source to produce enoughvapors for growth of oxide nanostructures [26–28], and it was reportedthat WO3 vapor was the major vapor species for the nanowire growth[28]. With the low heating temperature of 800 °C used in ourexperiments, therewas also few tungsten vapor produced. As examinedafter the growth, the top surface of the tungsten powders turned fromsilver gray into dark blue indicating that an oxidation reaction occurredat the surface of the powders during the growth and themajority of thepowders were still kept as tungsten. Hence, the tungsten-containedvapors in our experiments are expected to be dominated by WO3. It isextremely interesting to compare these results with some literature inwhich WO3 nanostructures with different types of catalysts arefabricated on the silicon substrate [29].

Our study suggests that the O2 flow rate plays a very important role inthe growth of WO3 nanowires under these growth conditions. Fig. 5(a)and (b) shows the SEM images of a sample that has been synthesizedusing the same methods and conditions as those for growing the sampleshown 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 canbe seen but many unknownWOx nanoparticles existed on the substrate.

Fig. 5. SEM image of the WO3 nanowires grown at d

For an O2 flow rate of 0.5 sccm, although WO3 nanowire growth can beobserved, the yield is still low, and many WOx clusters remain on thesurface of the silicon wafer as shown in Fig. 5(b). This shows that it isdifficult to obtain bundledWO3 nanowireswithout sufficient O2 flow ratein this growth process.

Fig. 6 shows the UV–visible spectrum of the WO3 nanowires. It isremarkable that the absorption wavelength is approximately~415 nm and the bandgap value of WO3 is ~2.99 eV, as defined asthe intercept of the plot of (αhν)1/2 vs. hν, where α and hν denote theabsorption coefficient and photon energy, (Fig. 6 inset) comparedwith the absorption wavelength value of ~360 nm reported for WO3

of nanostructures [30]. Hence, this implies that our as-synthesizednanowires of WO3 exhibited a red-shift in the optical absorptionwavelength. It has been reported that both the size distribution andcrystallinity of theWO3 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 ofWO3 nanostructures have been investigated in the literature [32], andit appears that the synthesis temperature and time will influence theoxygen ion deficiencies in theWO3 nanostructures. This will make thebandgap value change from 3.30 to 2.0 eV. Therefore, it can beconcluded that the oxygen deficiencies within the nanomaterials areresponsible for the decrease in the bandgap value of the WO3

nanowires in the present research.

ifferent O2 flow rates: (a) 0 sccm; (b) 0.5 sccm.

325 350 375

370

415

400 425 450 475 500

Wavelength (nm)

Inte

nsi

ty (

a.u

)

Fig. 7. Cathodoluminescence (CL) spectrum of the as-prepared WO3 nanowires.

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As for as we know, few literature reports on the CL properties oftungsten oxide nanomaterials exist. Fig. 7 shows the room-temperatureCL spectrum of the as-prepared WO3 nanowires. The two strongest CLemission 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 tungstenoxide nanostructures. The emission peak at 370 nm has been inferredthat this is due to the intrinsic band–band transition emission inducedby quantum-confinement effects in nanostructures with an ultrafinediameter (b5 nm) of individual nanowires within each bundle areconsidered [6,33]. However, the strong-intensity blue-emission peak at415 nm has been previously attributed to oxygen vacancies, which areoften implicit in the preparation of oxide semiconductors. The strongblue emission is indicative of the existence of a large amount degree ofoxygen vacancies in the WO3 nanowires [33,34]. It is very interestingthat the blue emission (415 nm ) was at shorter wavelengths thanreported previously [6,33]. The emissionwavelength ranges from423 to437 nm [35], which indicates that the nanoscale size effect must haveoccurred. Further, oxygen vacancies in the nanowires caused theintensity of the blue-emission peaks to be considerably higher thanthat of a band-to-band emission peak in the CL spectrum [36]. Thenanowires absorb strongly in the near-UV to blue light region,matchingthe emission wavelength of a near-UV or blue LED. Hence, they may beappropriate for use in white LED [37].

4. Conclusion

In summary, WO3 nanowires were generated by thermal CVD via atwo-step heating process without using any catalyst. The WO3

nanowires were deposited in a low-temperature zone (200–300 °C) ofa tube furnace. The nanowires were 15–40 nm in diameter and up tohundreds of nanometers in length. The XRD and TEM results confirmedthat the nanowires were triclinic WO3, with the growth direction along[001]. The Raman spectrum suggested that the grown WO3 nanowireswere highly nanocrystalline. The as-synthesized WO3 nanowires

successfully exhibited blue luminescence emission. The red-shiftphenomenon and blue emission could be explained by the oxygendeficiencies, crystallinity, and size distribution of WO3 nanostructures.In particular, the size effect resulted in blue emission (415 nm) atshorter wavelengths than reported previously. These results indicatethat theWO3 nanowires have significant potential applications in LEDs,especially in solid state lighting for home illumination that comprises amajor portion of our energy consumption.

Acknowledgment

This researchwas supportedby theNational ScienceCouncil throughGrant No. 96-2221-E-034-006-MY2 and No. 98-2221-E-034-007-MY2.

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