Effect of growth temperature on photoluminescence and piezoelectric characteristics of ZnO nanowires

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

Contents lists available atScienceDirect

Materials Science and Engineering B

Effect of growth temperature on photoluminescence and piezoelectric

characteristics of ZnO nanowires

Walter Water

_{, Te-Hua Fang}

_{, Liang-Wen Ji}

_{, Ching-Chin Lee}

b_{Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan}

a r t i c l e i n f o

Received 13 August 2008

Received in revised form 13 January 2009 Accepted 21 January 2009 Keywords: ZnO Nanowires Vapour–liquid–solid Photoluminescence Nanogenerator Piezoelectric

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

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide (ZnO) is an n-type semiconductor material with a wide bandgap of 3.2 eV and a large exciton binding energy of 60 meV at 300 K[1,2]. ZnO has a variety of potential applications such as acoustic devices because of its strong piezoelectric effect[3,4]. Thus, research on the optoelectronics and mechanical characteristics of ZnO has become a tremendously hot topic in recent years[5–8]. In addition, it is well-known that one-dimensional ZnO nanowire arrays with larger length-to-diameter and surface-to-volume ratio than ZnO bulk and films have attracted much attentions due to their physical and optoelectrical properties and have promising poten-tials in applications for lasers[9], transistors[10], solar cell[11,12], and nanogenerator[13,14].

One-dimensional ZnO nanostructures can be synthesized by various techniques, such as metal-organic chemical vapour depo-sition (MOCVD) [15], molecular beam epitaxy (MBE) [16], and vapour–liquid–solid (VLS) process[17]. Among them, the VLS pro-cess is a cheap and familiar technique to grow ZnO nanowires with excellent quality. It should be noted that the growth temperature is a very important factor for more reliable growing the ZnO nanos-tructures during the VLS process.

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

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

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 scanning electron microscope, X-ray diffraction (XRD), Ultraviolet/Visible (UV/VIS) spectroscopy, and photoluminescence, respectively. Then a nanogenerator with ZnO nanowires was fabricated and analyzed.

2. Experimental details

The ZnO nanowires were synthesized on a p-type (1 0 0) silicon substrate by vapour–liquid–solid mechanism using a tube furnace. The substrates were thoroughly cleaned using organic solvents and deionized (DI) water. Then an Au thin film was deposited on the silicon substrate by sputtering system and it was used as cata-lyst for growing nanostructures. The thickness of the Au was about 20 nm. The growth processes were made in a furnace with a quartz tube (6.4 cm inner diameter and 120 cm in length inside). The fur-nace was pumped down to 70 cm-Hg using a mechanical pump before introducing the gases. An Ar flow rate of 1900 standard cubic centimeters per minute (sccm) was introduced in the quartz tube serving as a transport gas. An O2 flow rate of 950 sccm was

introduced 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 at the downstream side of the Ar flow, 1–2 cm away from the source materials. The temperatures of the source material and the sub-strates were raised from room temperature to 450–600◦C and kept at these temperatures until the end of the growth process. Then

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

vacancies in ZnO samples. Absorption peaks present a distinct blue shift of samples synthesized at low growth temperatures (450 and 500◦C). This may be due to the particle size decreased at lower temperatures. The carriers of electrons and holes are confined in a very small region, and the coupling interaction is enhanced with each other. Then the exciton bounded and probability of binding are increased[21].

ZnO nanowires were synthesized at 600◦C and the size of nano-generator is 25 mm2 _{in effective substrate surface area.Fig. 6(a)}

shows the current–voltage (I–V) characteristic of the nanogenera-tor device. The device generated output current and demonstrates a Schottky-like current–voltage characteristic. The output current of the nanogenerator is shown inFig. 6(b). The current of the device is estimated to be−33 pA when the ultrasonic wave had been turned off due to the perturbation of the measured environment, and it can be regard as a background value. The output current of 0.35 nA is created when the ultrasonic wave is turned on, and the current disappears when the ultrasonic wave is turned off.

4. Conclusions

In summary, effect of synthesized temperature on crys-talline, optical, and piezoelectric characteristics of VLS grown ZnO nanowires was investigated in this work. ZnO nanowires with the diameter of 80 nm were successfully synthesized when the growth temperature at 600◦C. The 383 and 492 nm excited wave-lengths of ZnO were examined by photoluminescence. The excited wavelength at 492 nm was due to the defect of oxygen vacan-cies. Excitionic absorption peaks revealed a distinct blue shift of nanowires synthesized at 450 and 500◦C because of the smaller particle size. The nanogenerator with ZnO nanowires synthesized

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

Acknowledgement

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

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