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Materials Science and Engineering B 158 (2009) 75–78

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

ffect of growth temperature on photoluminescence and piezoelectricharacteristics of ZnO nanowires

alter Watera, Te-Hua Fanga,b,∗, Liang-Wen Ji a, Ching-Chin Leea

Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, TaiwanInstitute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan

r t i c l e i n f o

rticle history:eceived 13 August 2008eceived in revised form 13 January 2009ccepted 21 January 2009

a b s t r a c t

ZnO nanowire arrays were synthesized on Au-coated silicon (1 0 0) substrates by using vapour–liquid–solid process in this work. The effect of growth temperatures on the crystal structure and the surfacemorphology of ZnO nanowires were investigated by X-ray diffraction and scanning electron microscope.The absorption and optical characteristics of the nanowires were examined by Ultraviolet/Visible spec-

eywords:nOanowiresapour–liquid–solidhotoluminescence

troscopy, and photoluminescence, respectively. The photoluminescence results exhibited ZnO nanowireshad an ultraviolet and blue emission at 383 and 492 nm. Then a nanogenerator with ZnO nanowire arrayswas fabricated and demonstrated Schottky-like current–voltage characteristics.

© 2009 Elsevier B.V. All rights reserved.

anogeneratoriezoelectric

. Introduction

Zinc oxide (ZnO) is an n-type semiconductor material with aide bandgap of 3.2 eV and a large exciton binding energy of 60 meV

t 300 K [1,2]. ZnO has a variety of potential applications such ascoustic devices because of its strong piezoelectric effect [3,4]. Thus,esearch on the optoelectronics and mechanical characteristics ofnO has become a tremendously hot topic in recent years [5–8].n addition, it is well-known that one-dimensional ZnO nanowirerrays with larger length-to-diameter and surface-to-volume ratiohan ZnO bulk and films have attracted much attentions due to theirhysical and optoelectrical properties and have promising poten-ials in applications for lasers [9], transistors [10], solar cell [11,12],nd nanogenerator [13,14].

One-dimensional ZnO nanostructures can be synthesized byarious techniques, such as metal-organic chemical vapour depo-ition (MOCVD) [15], molecular beam epitaxy (MBE) [16], andapour–liquid–solid (VLS) process [17]. Among them, the VLS pro-ess is a cheap and familiar technique to grow ZnO nanowires with

xcellent quality. It should be noted that the growth temperature isvery important factor for more reliable growing the ZnO nanos-

ructures during the VLS process.

∗ Corresponding author at: Institute of Mechanical and Electromechanical Engi-eering, National Formosa University, Yunlin 632, Taiwan. Tel.: +886 5 6315395.

E-mail address: fang.tehua@msa.hinet.net (T.-H. Fang).

921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2009.01.021

In this study, the effect of growth temperature in vapour–liquid–solid mechanism on ZnO nanowire arrays has been inves-tigated. The ZnO nanowires were characterized by using scanningelectron microscope, X-ray diffraction (XRD), Ultraviolet/Visible(UV/VIS) spectroscopy, and photoluminescence, respectively. Thena nanogenerator with ZnO nanowires was fabricated and analyzed.

2. Experimental details

The ZnO nanowires were synthesized on a p-type (1 0 0) siliconsubstrate by vapour–liquid–solid mechanism using a tube furnace.The substrates were thoroughly cleaned using organic solvents anddeionized (DI) water. Then an Au thin film was deposited on thesilicon substrate by sputtering system and it was used as cata-lyst for growing nanostructures. The thickness of the Au was about20 nm. The growth processes were made in a furnace with a quartztube (6.4 cm inner diameter and 120 cm in length inside). The fur-nace was pumped down to 70 cm-Hg using a mechanical pumpbefore introducing the gases. An Ar flow rate of 1900 standardcubic centimeters per minute (sccm) was introduced in the quartztube serving as a transport gas. An O2 flow rate of 950 sccm wasintroduced in the quartz tube for reactive gas after raising the sub-strate temperature. The source material was Zn powder (99.9995%,

Johnson Matthey GmbH, Germany). Si substrates were placed atthe downstream side of the Ar flow, 1–2 cm away from the sourcematerials. The temperatures of the source material and the sub-strates were raised from room temperature to 450–600 ◦C and keptat these temperatures until the end of the growth process. Then

76 W. Water et al. / Materials Science and

Fig. 1. (a) SEM image of the nanogenerator electrode; (b) I–V measurement system.

Fig. 2. SEM images of ZnO nanostructured films grown at (a)

Engineering B 158 (2009) 75–78

the temperatures were cooled from growth temperatures to roomtemperatures after shut off the heater without any control.

The surface morphology and the crystalline structure of theZnO nanowires were investigated by scanning electron micro-scope (FE-SEM, JEOL JSM-7401F) and X-ray diffraction (MO3XHF22,MAC-Science), respectively. The optical absorption spectra of theZnO nanostructured films were measured by Ultraviolet/Visiblespectrophotometer (U-2800, Hitachi). The photoluminescencespectra were examined by fluorescence spectrophotometer (F-4500, Hitachi) using an excited wavelength with 300 nm.

Based on previous studies [13–14,18], a nanogenerator with ZnOnanowires was then fabricated and characterized. Making on topof electrode were conducted by the standard photolithographyand etching technique for fabricate nanogenerator device with theZnO nanowires. The pitch, depth, and width of the electrode areabout 2, 5, and 10 �m, respectively. The groove of the electrodehad about a taper angle of 54◦ as shown in Fig. 1(a). The deviceis packaged by thermal assembling process using a thermoplas-tic polyethyleneterephthalate (PET) membrane with a thicknessof 80 �m. The current–voltage characteristics were measured byKeithley 2400 as shown in Fig. 1(b). The micro-current of nano-generator device was tested under DI water situation by drive ofultrasonic waves with a frequency of 43 kHz and a power of 50 W.

3. Results and discussion

Fig. 2(a)–(d) show the SEM images of the morphology for ZnOnanostructure at different synthesized temperatures. It can be seenthat the grain sizes of ZnO films were increased with increasinggrowth temperatures. ZnO nanowires were not found as the growth

temperature less than 600 ◦C, however, we found the nanowireswith 80-nm-diameter at 600 ◦C due to the higher reactive ratebetween zinc and oxygen at high temperatures.

Fig. 3 shows the XRD of the as-grown ZnO films with varioussynthesized temperatures. The (1 0 0), (0 0 2), (1 0 1), (1 0 2), and

450 ◦C, (b) 500 ◦C, (c) 550 ◦C, and (d) 600 ◦C.

W. Water et al. / Materials Science and Engineering B 158 (2009) 75–78 77

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nanowires synthesized at high temperatures (550 and 600 ◦C), theabsorption peaks of all ZnO samples are inspected and the peakclose to bulk ZnO crystals (380 nm). The wide absorption peakoccurred in wavelength range of 600–700 nm due to the oxygen

ig. 3. XRD patterns of ZnO nanostructured films grown at different temperatures.

1 1 0) orientations of ZnO films were examined and the better crys-al structures were obtained when synthesized temperature was

ore than 550 ◦C. The intensities of the peaks were not very distin-uished between 550 and 600 ◦C, which meant the crystallographicroperties of the as-grown ZnO were similar. The crystal structuresf SiO2 can be seen while the growth temperature of the VLS pro-ess is >500 ◦C, it can be attributed that the thermal natural siliconxide took place on the silicon substrate at higher temperatures.n this study, the ZnO nanostructured films revealed a deviation ofhe position for the XRD peak of its bulk-type. The (0 0 2) peak ofhe ZnO samples grown at 450, 500, 550 and 600 ◦C were about4.76◦, 34.68◦, 34.64◦ and 34.64◦, respectively. However, the (0 0 2)eak position of ZnO bulk is located at 2� = 34.4◦. The angular peak0 0 2) of the deposited film is more than that of the bulk-type value.

hen the growing temperature increased, the intrinsic stress wasecreased. It can be attributed to the thermal annealing and pre-ipitating within the nanostructured film.

Fig. 4 presents the photoluminescence spectra of ZnO filmsrown at various synthesized temperatures. The intensity ofxcited wavelength at 379 nm of ZnO nanowires was increasedith increasing synthesized temperatures. At 600 ◦C, the 383

nd 492 nm excited wavelengths are determined by ZnO photo-uminescence spectrum. The excited wavelength at 383 nm is aear-band-edge while the 492-nm emission is due to the defect ofxygen vacancies [19]. The oxygen vacancies generated may be due

ig. 4. Photoluminescence of ZnO nanostructured films grown at different temper-tures.

Fig. 5. Absorption spectra of ZnO nanostructured films grown at different temper-atures.

to the oxygen out-diffused from ZnO at high growth temperature[20].

Fig. 5 shows the UV–VIS optical absorption spectra of ZnOnanowires grown at different temperatures. When the ZnO

Fig. 6. (a) I–V curve of nanogenerator and (b) output current of nanogenerator.

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8 W. Water et al. / Materials Scienc

acancies in ZnO samples. Absorption peaks present a distinct bluehift of samples synthesized at low growth temperatures (450 and00 ◦C). This may be due to the particle size decreased at loweremperatures. The carriers of electrons and holes are confined in aery small region, and the coupling interaction is enhanced withach other. Then the exciton bounded and probability of bindingre increased [21].

ZnO nanowires were synthesized at 600 ◦C and the size of nano-enerator is 25 mm2 in effective substrate surface area. Fig. 6(a)hows the current–voltage (I–V) characteristic of the nanogenera-or device. The device generated output current and demonstrates achottky-like current–voltage characteristic. The output current ofhe nanogenerator is shown in Fig. 6(b). The current of the device isstimated to be −33 pA when the ultrasonic wave had been turnedff due to the perturbation of the measured environment, and itan be regard as a background value. The output current of 0.35 nAs created when the ultrasonic wave is turned on, and the currentisappears when the ultrasonic wave is turned off.

. Conclusions

In summary, effect of synthesized temperature on crys-alline, optical, and piezoelectric characteristics of VLS grown ZnOanowires was investigated in this work. ZnO nanowires withhe diameter of 80 nm were successfully synthesized when therowth temperature at 600 ◦C. The 383 and 492 nm excited wave-

engths of ZnO were examined by photoluminescence. The excitedavelength at 492 nm was due to the defect of oxygen vacan-

ies. Excitionic absorption peaks revealed a distinct blue shift ofanowires synthesized at 450 and 500 ◦C because of the smallerarticle size. The nanogenerator with ZnO nanowires synthesized

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Engineering B 158 (2009) 75–78

at 600 ◦C exhibits the Schottky-like current–voltage characteristicand a good performance of piezoelectricity.

Acknowledgement

This work was supported by the National Science Council ofTaiwan under Grant No. NSC 96-2628-E150-005-MY3.

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