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Copyright © 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 12, 3430–3433, 2012

Synthesis of Ga-Doped ZnO Nanorods Using an AqueousSolution Method for a Piezoelectric Nanogenerator

Ju-Hyuck Lee1, Keun Young Lee2, Brijesh Kumar2, and Sang-Woo Kim1�2�∗1SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT),

Sungkyunkwan University, Suwon 440-746, Republic of Korea2School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea

Mechanical energy is a potential energy source for self-powered electronic devices. Due to theirunique semiconducting and piezoelectric properties, wurtzite-structured nanomaterials have beenconsidered as potential candidates for piezoelectric nanogenerators that convert mechanical energyinto electricity. In the present work, we report on the growth of Ga-doped ZnO (GZO) nanorods andinvestigate the performance of nanogenerators fabricated from undoped ZnO (UZO) nanorods, lowGa-doped ZnO (LGZO) nanorods, and high Ga-doped ZnO (HGZO) nanorods. A nanogeneratorintegrated with LGZO nanorods exhibited a current density of 1.2 �A/cm2, an enhancement over the0.4 �A/cm2 and 0.7 �A/cm2 current densities of nanogenerators integrated with UZO and HGZOnanorods, respectively.

Keywords: ZnO, Gallium-Doping, Nanogenerator, Current Density.

1. INTRODUCTION

Energy shortage has long been a concern of the inter-national community. In addition, fossil fuels reserves arelimited, and their usage increases the amounts of CO2

and other gases in the atmosphere, which in turn causesglobal warming. As such, there is a need for the devel-opment of clean and environmentally-friendly alterna-tive energy sources.1�2 In recent years, researchers havedevoted a great deal of effort to the study of solarcells,3�4 fuel cells,5�6 thermoelectric,7 and piezoelectrictechnologies8�9 because of their potentials for convertingphoton, chemical, thermal and mechanical energies intoelectrical energy. Among these technologies, piezoelectricenergy harvesting is an eco-friendly energy-converting sys-tem in which waste mechanical energy, such as tiny vibra-tions or the movement of the human body, is convertedinto electricity.10 These small and lightweight energy-converting devices have unique advantages such as a sim-ple structure and a high energy conversion efficiency.9�10

Recently, the piezoelectric properties of several materi-als such as zinc oxide (ZnO),8–12 lead zirconate titanate,13

cadmium sulfide,14 barium titanate,15 and gallium nitride16

have been successfully demonstrated.Researchers have attempted to improve the piezoelectric

output performances of materials for use in the operation

∗Author to whom correspondence should be addressed.

of self-powered nanodevices. There are several significantfactors that must be considered to improve the perfor-mance of piezoelectric nanogenerators. One of the mostimportant of these is increased carrier density. It has beenfound that the carrier density arising from ultraviolet irra-diation effects the output currents of nanogenerators.17�18

However, the n-type doping effect on the performances ofdirect current (DC)-mode nanogenerators has rarely beenstudied. ZnO is usually doped with group III elementssuch as B,19 In,20 Al,21 and Ga.22–24 Among these ele-ments, Ga is an especially promising n-type dopant forlow resistivity ZnO films with high transmittance in thevisible range.22–24 In Ga doping, Zn ions in the ZnO lat-tice are replaced by Ga atoms, which act as a substitu-tion impurity. In this process, a free electron is releasedin the conduction band at room temperature. The ionicand covalent radii (0.62 and 1.26 Å, respectively) of Gaare similar to those of Zn (0.74 and 1.34 Å). As a result,only a small lattice deformation occurs at high Ga-dopingconcentrations.23�24

In this paper, we report on the growth of Ga-doped ZnO(GZO) nanorods and investigate the performances of nano-generators fabricated from undoped ZnO (UZO) nanorodsand GZO nanorods with various Ga dopant concentra-tions. We were ultimately able to determine the properamount of Ga dopant in the ZnO nanorods for maximumperformance.

3430 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 4 1533-4880/2012/12/3430/004 doi:10.1166/jnn.2012.5632

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Lee et al. Synthesis of Ga-Doped ZnO Nanorods Using an Aqueous Solution Method for a Piezoelectric Nanogenerator

2. EXPERIMENTAL DETAILS

In order to grow UZO and GZO nanorods, a ZnO bufferlayer with a thickness of about 50 nm was initiallydeposited onto indium tin oxide (ITO)/polyethersulfone(PES) substrates via radio frequency sputtering. The ZnOnanorods were then grown using the hydrothermal method.In this process, an aqueous solution was first preparedwith 25 mM zinc nitrate hexahydrate (Zn(NO3�2 · 6H2O),25 mM hexamethylenetetramine (C6H12N4�, and differentamounts of gallium nitrate (0 wt%, 1 wt%, and 10 wt%of the zinc nitrate hexahydrate). The substrates with aZnO buffer layer were then placed into this solution at95 �C for 3 h. A PES substrate onto which gold (Au),deposited via thermal evaporation, was used as the topelectrode of the structure in order to ensure Schottky con-tact with the ZnO nanorods. The size and morphology ofthe ZnO nanorods were characterized using field emissionscanning electron microscopy (FE-SEM). X-ray diffrac-tion (XRD) measurements were used to confirm the crystalgrowth quality and the Ga doping of the ZnO nanorods.X-ray photoelectron spectroscopy (XPS) was employed toconfirm the relative Ga doping concentration in the ZnOnanorods. A Keithley 6485 picoammeter was used for low-noise current measurements in order to detect the currentgenerated by the piezoelectric nanodevices.

3. RESULTS AND DISCUSSION

FE-SEM images of the typical surface morphologies ofUZO nanorods, low (gallium nitrate at 1 wt% of thezinc nitrate hexahydrate concentration) Ga-doped ZnO(LGZO) nanorods, and high (gallium nitrate at 10 wt%of the zinc nitrate hexahydrate concentration) Ga-dopedZnO (HGZO) nanorods grown on ITO/PES substratesare shown in Figure 1. As the Ga dopant concentrationincreased, the nanorod density remained fairly constant,while the average diameter of the nanorods decreasedslightly. The diameters of the UZO, LGZO, and HGZOnanorods were 110 nm (Fig. 1(a)), 85 nm (Fig. 1(b)), and75 nm (Fig. 1(c)), respectively. Due to the difference inthe radii of Zn and Ga, rapid growth occurred in the (002)direction. Doping with Ga can support the growth of ZnOin this direction.22�25

The diffraction patterns of the UZO and GZO nanorodsfrom a synchrotron X-ray source are shown in Figure 2(a).The scans were taken along the surface normal direction,Qz [Q = 4� sin�2�/2�/�], in reciprocal space. The peakpositions at Qz values of 2.413 Å−1 and 2.229 Å−1 wereassigned to the (002) and (100) planes, respectively, ofthe ZnO nanorods. The XRD spectra revealed that GZOwas formed in the c-axis direction.26 Since Ga doping wasemployed, the intensity of the (002) peak decreased, andthe full width at half maximum (FWHM) remained con-stant for the LGZO nanorods and slightly increased from0.005 Å−1 to 0.006 Å−1 for the HGZO nanorods. It was

Fig. 1. FE-SEM images of ZnO nanorods prepared via an aqueous solu-tion method with different gallium nitrate concentrations: (a) 0 wt%,(b) 1 wt%, and (c) 10 wt%.

found that a low amount of Ga doping did not deteri-orate the crystallinity of the GZO. In the XRD spectra,no Ga2O3 peak was detected. The absence of this peakindicates that Ga atoms successfully replaced Zn sites inthe lattice. The XPS data confirmed the presence of Gadopant in the GZO nanorods, as shown in Figure 2(b). Thepeaks at 1116.94 eV and 1143.95 eV are attributed to theelectronic states of Ga 2p3/2 and Ga 2p1/2, respectively.The energy gap of 27.01 eV is consistent with the value ofGa in LGZO and HGZO nanorods,22 while no peak relatedto Ga was detected in the UZO nanorods. The higher Gadoping of 10 wt% was confirmed by the relatively highermagnitudes of the Ga 2p3/2 and Ga 2p1/2 peaks comparedto those obtained at the lower Ga doping concentration.

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Synthesis of Ga-Doped ZnO Nanorods Using an Aqueous Solution Method for a Piezoelectric Nanogenerator Lee et al.

Fig. 2. (a) XRD data for UZO, LGZO, and HGZO nanorods using asynchrotron X-ray source. (b) XPS results showing Ga 2p3/2 and Ga 2p1/2peaks obtained from LGZO and HGZO nanorods.

To investigate the effect of Ga doping on the current-generating performance of piezoelectric nanogeneratorswith external force applied, we fabricated a series of nano-generator devices from UZO nanorods and GZO nanorodswith different amounts of Ga. A schematic of the nano-generators integrated with piezoelectric UZO nanorodsand GZO nanorods grown on ITO/PES substrates with anAu/PES top electrode is shown in Figure 3(a).To confirm that the measured signal originated from

the device, a switching polarity test of the induced outputcurrent density was performed, as shown in Figure 3(b).When the current meter was forward connected to aUZO- or GZO-based nanogenerator, we measured a posi-tive signal of the current pulse during pushing. When the

Fig. 3. (a) Schematic of an integrated nanogenerator. (b) Current den-sities from the UZO nanorod-based piezoelectric nanodevice (switchingpolarity test).

current meter was reversely connected, the negative signalof the current pulses was recorded. The measured signalsof the current density for both connection conditions werealmost the same.27 Typically, this signal looks like the DCmode of a generator. However, we repeatedly detected avery small signal in the opposite direction as the referencepoint. As shown in Figure 1, this is due to the existenceof a few nanorods that were vertically grown on the sub-strate. These nanorods acted as a potential gate to repulseelectrons from the top electrode to the bottom electrode.The electrons then returned to the top electrode.28

We compared the induced current density performanceof different piezoelectric nanogenerators by applying thesame pushing force to the top electrode of the nano-devices in the vertical direction. Compared to the nano-generators integrated with UZO nanorods (Fig. 3(b)) orHGZO nanorods (Fig. 4(b)), the nanogenerator integratedwith LGZO nanorods showed enhanced performance(Fig. 4(a)). At a load of 0.5 kgf, the nanogenerators withUZO, LGZO, and HGZO nanorods had average currentdensities of 0.4 �A/cm2, 1.2 �A/cm2, and 0.7 �A/cm2,respectively.29 The extent of Ga doping and the freeelectron density were also found to affect the current-generating performance. With a large number of free elec-tron carriers or defects in the ZnO nanorods, the chargeof the piezoelectric would be screened, and the amplitudeof the piezoelectric potential would be decreased. This

Fig. 4. Output current density from (a) the LGZO-based nanogeneratorand (b) the HGZO-based nanogenerator.

3432 J. Nanosci. Nanotechnol. 12, 3430–3433, 2012

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Lee et al. Synthesis of Ga-Doped ZnO Nanorods Using an Aqueous Solution Method for a Piezoelectric Nanogenerator

would result in a reduction in the output current density.However, if the free electron carrier density was very lowin the ZnO nanorods, even though the local piezoelectricpotential could increase, the output current would decreasebecause a lower number of free carriers transferred fromthe ZnO to the top Au electrode during Schottky junctionformation. Hence, an optimum free electron carrier den-sity in ZnO is required to optimize the generator for thehighest output.17�18

4. CONCLUSION

The obtained experimental results reveal that a smallamount of Ga dopant in ZnO supplies an optimum con-centration of free electron carriers to enhance the piezo-electric output current. However, a larger amount of Gadopant reduces the output current of the nanogenerator dueto a slight deterioration in the crystallinity of the HGZOnanorods, where defects act as trapping sites for carriersand the piezoelectric potential. Such a scenario is evi-dent in the XRD patterns, where the FWHM increased forincreased Ga doping in ZnO. The present work suggeststhat the amount of gallium dopant is an important issue toconsider when maximizing the performance of a nanogen-erator. Through variation of the Ga dopant concentration,we optimized the performances of nanogenerators fabri-cated with Ga-doped ZnO nanorods.

Acknowledgment: This work was financially supportedby the International Research and Development Program ofthe National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(MEST) (2010-00297) and by Basic Science ResearchProgram through the NRF funded by the MEST (2009-0077682 and 2010-0015035). Synchrotron X-ray scatteringexperiments at Pohang Light Source (PLS) were supportedin part by the MEST and POSTECH, Republic of Korea.

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Received: 31 October 2010. Accepted: 19 March 2011.

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