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Scripta Materialia 66 (2012) 101–104

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The stress-dependent piezoelectric coefficient of ZnO wire measuredby piezoresponse force microscopy

Hyunsoo Lee,a Joonkyu Park,a Sang A. Han,a Donghyeok Lee,a K.B. Kim,a N.S. Lee,a

Jun-Young Park,a Yongho Seo,a,⇑ SangWook Leeb,⇑ and Young Jin Choic

aFaculty of Nanotechnology and Advanced Material Engineering and Graphene Research Institute,

Sejong University, Seoul 143-747, South KoreabDivision of Quantum Phases & Devices, School of Physics, Konkuk University, Seoul 143-107, South Korea

cDepartment of Physics, and Department of Nano Science and Engineering, MyongJi University, Yongin 449-728, South Korea

Received 1 August 2011; revised 6 October 2011; accepted 9 October 2011Available online 13 October 2011

We studied a converse piezoelectric effect of individual ZnO wires by using a conductive cantilever tip attached to the end of eachwire. The piezoelectric coefficients d33 of individual wires were measured and found to be in the range of 1–45 pm V�1, some of thembeing much larger than that of bulk ZnO. A 20% increment of d33 was found when 0.32 GPa stress was applied, but the d33 wasdecreased when the wire was bent by excessive compression.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: ZnO; ZnO wire; PFM; Piezoelectricity; Energy harvesting

ZnO wire has become one of the most attractivematerials for use in energy harvesting systems [1–5].ZnO is a semiconducting and piezoelectric material, andcould be used for a minuscule power generating deviceby combining these two physical properties [3]. Zhaoet al. [6] measured the effective piezoelectric coefficient(d33) of an individual (0001) surface-dominated ZnOnanobelt using piezoresponse force microscopy (PFM).They found that the coefficient d33 of a ZnO nanobeltwas dependent on frequency and varied from 14.3 to26.7 pm V�1 in the frequency range of 30–150 kHz, whichis much larger than that of bulk (0001) ZnO(9.93 pm V�1). In contrast, Fan et al. [7] reported thatthe piezoelectric coefficient for a ZnO nanopillar with adiameter of about 300 nm is smaller than that of the bulksample. A theoretical study performed by Xiang et al. [8]showed that ZnO nanowires with diameters less than2.8 nm have a larger effective piezoelectric constant thanbulk ZnO due to their free boundary, while the piezoelec-tric constant is saturated for ZnO nanowires with diame-ters larger than 2.8 nm.

Song et al. [2] found that an electrical voltage wasgenerated at the end of a ZnO wire when it was bent by

1359-6462/$ - see front matter � 2011 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2011.10.013

⇑Corresponding author; e-mail addresses: yseo@sejong.ac.kr;leesw@konkuk.ac.kr

an atomic force microscope (AFM) tip. They used aZnO single crystal wire that was 1.5 lm in diameter and200 lm in length. A piezoelectric voltage of 3–4 mV wasdetected when a Pt-coated tip touched the compressedside of the bent wire. Liu et al. [5] analyzed the factors thatdetermine the power output of the piezoelectric nanowirearray. Instead of the AFM tip, a Pt-coated Si electrodewith V-shape grooves was placed on the top of the nano-wire array as the top electrode. By growing uniform ZnOnanowires with diameters of 100 nm and lengths of 5 lm,an output current density of 8.3 lA cm�2 and an outputvoltage of 10 mV were achieved, where mechanical vibra-tion was excited by an external ultrasonic device. Re-cently, Wen et al. [9] measured piezoelectric currentsfrom a high-order array of ZnO nanowires with a 70 nmdiameter by scanning them with a Pt-coated AFM tip.The average output peak signal was about 50 pA, andthe corresponding resistance of the AFM tip–ZnO nano-wire interface was approximately 4 GX. In this context,our experimental study was set up to measure the piezo-electric effect of the individual ZnO nanowire to confirmthe feasibility of the energy harvesting device. Specifi-cally, we used a Pt-coated tip for the piezoelectric gener-ation experiments because the interface between the Ptand ZnO forms a Schottky barrier and a rectified signalcan be obtained.

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Figure 1. (a) Schematic diagram of measurement. An AC voltage wasapplied to the sample through the conductive cantilever and silverpaste electrode. The cantilever is located in two different directions andthe longitudinal motion of the sample was detected via the A–B or C–D signal of a quadrature photodiode. (b and c) Optical microscopeimages of the sample with the silver paste electrode and cantilever inthe A–B and C–D configurations, respectively.

102 H. Lee et al. / Scripta Materialia 66 (2012) 101–104

In our experiment, ZnO wires were synthesized by thechemical vapor deposition method. A mixture of ZnOand graphite powder with equal mass ratio was preparedinside the tube furnace (type F21100, Thermolyne Co.).Once the temperature at the center of the quartz tubewas increased to 1000 �C in an Ar atmosphere, O2 gaswas inserted into the tube so that the ZnO wires weregrown on the Si substrate. During the synthesis, theAr and O2 flow rates were maintained at 140 and50 sccm, respectively. After 30 min of synthesis time,ZnO wires, 200 lm in length on average, were obtained.ZnO wires were placed on top of an SiO2/Si substrateand separated out by ultrasonic agitation in isopropylalcohol. An individual ZnO wire was then mechanicallyanchored and electrically connected to the electrodewith silver paste. Nine samples were prepared for mea-surements, the dimensions of which are listed in Table 1.

In order to measure the converse piezoelectric effectfor the individual wire, we used a commercial AFMset-up (NanoFocus Inc., n-Tracer) combined with alock-in amplifier (Stanford Research Systems, SR830).Figure 1 shows (a) a schematic diagram and (b and c)photographs taken by an optical microscope, showingthe sample and the cantilever. One end of the ZnO wireis electrically connected to a silver paste electrode andthe other end is in contact with the tip of the cantilever.The cantilever, including the tip, was coated withmetallic film to be conductive. AC voltages were appliedthrough the sample via the silver paste electrode and thecantilever. When the cantilever is located in a directionparallel to the wire, as shown in Figure 1(b), the cantile-ver will bend up and down as the wire elongates andshrinks. Then the laser beam reflected by the cantileverwill be shifted up and down, which is detected by thequadrature photodiode signal (labeled A–B). On theother hand, when the cantilever is positioned perpendic-ular to the wire, as shown in Figure 1(c), the cantileverwill be twisted by the longitudinal vibration of the wire[10]. Then the optical signal will be shifted in a lateraldirection, which is labeled as C–D. The experimentaldata were taken with these two configurations (A–Band C–D), and substantially the same results were ob-tained irrespective of configuration. Three different me-tal-coated cantilevers were used to apply the voltage.Most experimental data were taken with the Pt-coatedone (multi75E-G) with constant 75 kHz resonancefrequency and 3 N m�1 force, and similar results wereobtained with the other cantilevers.

Table 1. Dimensions, piezoelectric coefficients d33 measured at 30 kHzand expected resonance frequencies of the samples from thedimensions.

Samplenumber

Length(lm)

Diameter(lm)

d33 (pm V�1

@ 30 kHz)f0 (kHz)

#1 30.0 4.0 45.3 3107#2 115 1.52 1.3 80#3 117 1.20 – 61#4 121 2.43 2.28 116#5 72 1.16 2.22 156#6 124 1.42 – 64#7 120 1.81 24.39 87#8 34 2.0 1.968 1209#9 122 1.69 17.31 79

As shown in Figure 2(a), the converse piezoelectricsignals (amplitude and phase) were detected by thelock-in amplifier. An AC voltage of 4 V with a 32 kHzfrequency was applied to the sample, and the cantileverwas repeatedly moved to the sample (contact) and re-tracted 1 lm away from it (retracted). The x-axis inthe figure indicates time. We found clear evidence thata converse piezoelectric effect was indeed detected. Ifthe signal were induced via an electrical capacitivecoupling, no clear difference between the contact andnon-contact conditions would be expected. The signal-to-noise ratio of the amplitude is roughly more than 5,as shown in Figure 2(a), and the phase signal shows thatthe noisy signal in non-contact became stable in contact.

Similar measurements were performed with changingamplitude of applied voltage Vapp, as shown in Figure2(b). It shows that the A–B signal amplitude was pro-portional to the amplitude of Vapp. When the cantilevertip was located at the side of the wire, no noticeable A–Bsignal was found. It is well known that ZnO wire isgrown along the c-axis and has a piezoelectric effect(d33) only along this axis. Locating the tip at the endof the wire was an extremely delicate process. Whenthe tip slightly deviated from the target, no signal wasfound.

The A–B signal amplitude could be converted into alongitudinal vibration along the length scale. For thiscalibration, the sample is moved manually by the scan-ner and the A–B signal shift measured. The conversionfactor d was defined as

d ¼ DxV A�B

ð1Þ

where Dx is the displacement (or longitudinal vibrationamplitude) of the sample and VA–B is the voltage ofthe A–B signal. d was estimated to be in the range of10–70 nm V�1, which could be dependent on the

Figure 2. (a) Raw data of a converse piezoelectric signal. An ACvoltage with 32 kHz frequency and 4 Vapp was applied to the sample,and the cantilever was repeatedly moved to and retracted away fromthe sample. (b) Representative data show the applied voltage depen-dence of the converse piezoelectric motion. As the applied voltage waschanged from 4 to 9 Vpp, the A–B signal increased proportionally. (c)Representative data taken at different frequencies for sample #1. Theconverse piezoelectric signal amplitude was proportional to the appliedvoltage Vapp. The d33 values were estimated from the slope.

H. Lee et al. / Scripta Materialia 66 (2012) 101–104 103

cantilever dimension, the tip length and the tip–samplesliding conditions. Most Vapp vs. Dx data showed lineardependence, in general, as shown in Figure 2(c). Figure2(c) shows the linear dependence of the converse piezo-electric effect on Vapp for sample #1 with different fre-quencies. From the slopes in this plot, the piezoelectriccoefficient d33 was estimated by [6]

d33 ¼Dx

V appð2Þ

assuming the electric field inside the nanowire is uniform.This assumption is plausible considering that the ZnOwire is semiconducting and has uniform conductivity.

It is notable that the piezoelectric coefficients havesuch wide variation (from 1 to 45 pm V�1) for individualsamples. These values of d33 we measured fall within the

range of values for a ZnO nanobelt measured by others[6,11,12]. Some samples have quite different values ofd33, even though they were produced under the samegrowth conditions. No correlation between the sampledimensions and d33 was found, as shown in Table 1. Itis believed that the individual sample quality, includingthe intrinsic strain and the amount of defects, caused thevariation of d33. According to Taylor and Damjanovic[13], the piezoelectric effect d33 depends on the crystallo-graphic orientation and its quality, taking into accountthe displacement of the domain wall.

On the other hand, it is noted that the larger piezo-electric signals were detected in higher frequencies, asshown in Figure 2(c). This variation in d33 dependingon frequency is attributed to the resonance behavior ofthe cantilevers and the electrical bandwidth of the posi-tion sensitive photodiode (PSPD) signal amplifiers.

There are number of issues concerning the frequencydependence of the PFM signal. Harnagea et al. [14] re-ported that PFM measurements with soft cantileverswere strongly influenced by the experimental conditions,such as the electro-mechanical properties of the cantile-ver, tip, sample and adhesion layer. They suggested thatquantitative measurements of PFM should be per-formed with stiff cantilevers in a strong indentation re-gime, but only to test frequencies at least one order ofmagnitude below the resonance frequency. The use ofhigh frequencies (>100 kHz) generally improves the ver-tical PFM contrast, with a higher signal-to-noise ratio,and the nonlocal cantilever effects will be minimized athigh frequencies [15].

We also found that d33 of ZnO wire depends stronglyon the stress. Figure 3 shows representative data for thepiezoelectric signal intensity changes when the cantilevertip was moved to compress the wire in the longitudinaldirection. The x-axis in Figure 3(a) shows the displace-ment of the cantilever compressing the ZnO wire (sam-ple #1), with the contact point set as zero. It wasfound that the piezoelectric signal increased as the wirewas compressed. The stress r on ZnO wire is given by

r ¼ EDll0

ð3Þ

where E is the Young’s modulus, l0 is the original lengthand Dl is the length of the compressed wire. We considerthe axial stiffnesses of the wire and cantilever in thequantitative estimation of the stress. The axial stiffnessfor compression is given by k ¼ AE=L, where A is thecross-sectional area and L is the length of the wire.The stiffness of sample #1 (k1 = 2.40 � 105 N m�1)was much larger than that of the cantilever (k2 = 6.91� 104 N m�1). The effective compression length Dl* ofthe wire for a system with two springs coupled in seriesis Dl� ¼ k2=ðk1 þ k2ÞDl ¼ 0:22Dl. As a result, a 20%increment of d33 was found when a stress of 0.32 GPawas applied to the ZnO wire.

A theoretical study of the stress dependence was per-formed by Hill and Waghmare [16]. With an appliedstress in the range of -1 to 1 GPa, d33 was changed byabout 15–30%. The stress dependence was explained asa result of strong, bond-length-dependent hybridizationbetween the O2p and Zn3d electrons. Our experimental

Figure 3. (a) Representative data for the converse piezoelectric signalchanges, while the cantilever tip compresses the wire (sample #1). (b)The converse piezoelectric signal change was measured while the wire(sample #2) was bent by excessive compression. The optical micro-scopic images are shown for each data point.

104 H. Lee et al. / Scripta Materialia 66 (2012) 101–104

result is in qualitative agreement with their theoreticalprediction.

When the wire was compressed excessively, the stresscould not be endured, so the wire would bend to releasethe stress. A converse piezoelectric effect of a bent wirewas measured with high stress loading. Figure 3(b)shows a representative result measured from sample#2, where the change in amplitude of the piezoelectricsignal was measured as a function of compressive dis-placement. Each inset for the data points shows the opti-cal microscopic image of the sample deformationcorresponding to the compression. The amplitude ofVA–B increased initially up to 1 lm compression, thendecreased sharply down to 40% of the original value.The stiffness of the wire (k = 9.14 � 103 N m�1 for sam-ple #2) is much lower than that of the cantilever, and thedisplacement of the cantilever was transferred to thesample stress almost fully. More precisely, when thestress r > 1.1 GPa (Dl� ¼ 883 nm), the wire was bentand the d33 was decreased. This effect can be explainedby the excessive bending of the ZnO wire causing drasticchanges in the physical and crystallographic properties,causing the piezoelectric effect to deteriorate.

In summary, the converse piezoelectric coefficient d33

of ZnO wire was measured by the scanning probemicroscopy (SPM) technique. The measured d33 are in

the range of 1–75 pm V�1. This wide variation in d33 issuspected to be due to crystallographic defects anddifferent dimensions. As the frequency of the appliedAC voltage was increased in the range of 10–200 kHz,d33 increased monotonically. At low frequency, the elec-tric field reduction due to conductance of ZnO and con-tact resistance between the tip and wire caused anunderestimation of d33. A compressive stress on theZnO wire increases the effective piezoelectric coefficientd33 up to 20%, but wire bending due to excessive com-pression causes the piezoelectric effect to deteriorate.Our experimental study provides fundamental knowl-edge for ZnO wire-based energy harvesting applications[1–3].

This work was supported by National ResearchFoundation of Korea (NRF) grants funded by the Kor-ean Government Ministry of Education, Science andTechnology (MEST) (Grant Nos. R01-2008-000-20185-0, 331-2008-1-C00102, 2009-0070725 and 2010-0005393) and the Priority Research Centers Program(2010-0020207).

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.scriptamat.2011.10.013.

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