Binary Oxide p‑n Heterojunction Piezoelectric Nanogenerators withan Electrochemically Deposited High p‑Type Cu2O LayerSeung Ki Baek,† Sung Soo Kwak,‡ Joo Sung Kim,† Sang Woo Kim,†,‡ and Hyung Koun Cho*,†

†School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si,Gyeonggi-do 16419, Republic of Korea‡SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), SungkyunkwanUniversity (SKKU), Suwon-si, Gyeonggi-do 16419, Republic of Korea

*S Supporting Information

ABSTRACT: The high performance of ZnO-based piezo-electric nanogenerators (NGs) has been limited due to thepotential screening from intrinsic electron carriers in ZnO. Wehave demonstrated a novel approach to greatly improvepiezoelectric power generation by electrodepositing a high-quality p-type Cu2O layer between the piezoelectric semi-conducting film and the metal electrode. The p-n hetero-junction using only oxides suppresses the screening effect byforming an intrinsic depletion region, and thus sufficientlyenhances the piezoelectric potential, compared to the pristineZnO piezoelectric NG. Interestingly, a Sb-doped Cu2O layerhas high mobility and low surface trap states. Thus, this dopedlayer is an attractive p-type material to significantly improve piezoelectric performance. Our results revealed that p-n junctionNGs consisting of Au/ZnO/Cu2O/indium tin oxide with a Cu2O:Sb (cuprous oxide with a small amount of antimony) layer ofsufficient thickness (3 μm) exhibit an extraordinarily high piezoelectric potential of 0.9 V and a maximum output current densityof 3.1 μA/cm2.

KEYWORDS: cuprous oxide, zinc oxide, piezoelectric, nanogenerator, antimony

1. INTRODUCTION

The sudden development and use of various wireless sensorsand individual portable electric devices attests to theirattractiveness as power sources. Energy harvesters exploitingthermal gradients, solar irradiation, and mechanical vibrationsmake use of energy in the environment that otherwise goes towaste.1−3 Among these various sustainable energy harvesters,piezoelectric energy harvesters are potential devices, whichpower electronic devices by capturing diverse types of energy,such as the mechanical vibrations in human movement.4,5

Among the various materials available for high-performancepiezoelectric energy harvesters, the metal oxide ZnO hasattracted a great deal of attention due to the coupling of itsgood piezoelectric and semiconducting properties.6,7 Inparticular, ZnO has been considered as one of the mostpromising candidates for piezoelectric devices because of itsrelatively low toxicity and cost-effectiveness in comparison toother piezoelectric materials.8 Nevertheless, the realization ofZnO-based NGs with large output performance has unfortu-nately proven difficult due to the free carrier density of the ZnOlayer with intrinsically n-type characteristics, which oppositelyscreens the piezoelectric potential generated by mechanicaldeformation.9

To prevent this screening effect, several attempts have beenput forward to form a p-n junction structure with a p-type

polymer.10−12 With the typical p-n junction structure, it is verylikely that the output performance of the nanogenerator (NG)is improved by decreasing the screening effects. However, todate, this approach has been limited by the expense of p-typepolymers and by severe material degradation under humid andhot environmental conditions. Stable inorganic p-type oxidematerials, such as CuO and NiO, have been scarcely used toform p-n junction piezoelectric NGs.13−15 These materials areusually deposited using high-vacuum and high-temperatureprocesses. Such deposition conditions form a large technicalbarrier for substituting p-type polymers, regarding the cost-effectiveness and the use of flexible substrates. To enhance thecompetitiveness of p-n based piezoelectric NGs, a low-costsolution-based deposition process should be used. Among theseveral possible methods, electrochemical deposition using aliquid electrolyte provides excellent uniformity over a large area,and is a low-cost processing method.16

In this study, we produced a Cu2O p-type layer byelectrodeposition to enhance the performance of ZnO-basedpiezoelectric NGs. The Cu2O is intrinsically a p-type oxidesemiconductor with a band gap of 2.1 eV and a high hole

Received: March 30, 2016Accepted: August 11, 2016

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.6b03649ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

mobility.17 Thus, it has been actively studied as a hole transportlayer or absorber in solar cells and in photocathodes for water-splitting systems.18−20 Nevertheless, electrodeposited Cu2Ofilms exhibit low electrical conductivity, limiting the applicationof Cu2O. We confirmed a considerable enhancement inpiezopotential output and current in ZnO-based NGs byintroducing an electrodeposited p-type Cu2O layer in the p-njunction. Moreover, we used a Cu2O:Sb (cuprous oxide with asmall amount of antimony) layer, reported in our previousstudy, which has a higher hole mobility and a preferred grainorientation compared to the undoped Cu2O layer.21 Thepiezoelectric performances of ZnO, ZnO/Cu2O, and ZnO/Cu2O:Sb structures were compared. Piezoelectric measure-ments revealed that the Sb-doped Cu2O layer dramaticallyimproves the piezoelectric output performance, compared withpristine ZnO and undoped Cu2O, due to the formation of ahigh-quality p-n junction.

2. EXPERIMENTAL SECTIONThe p-type binary oxide Cu2O layer was synthesized on indium tinoxide (ITO) (thickness of 200 nm)-coated polyethylene naphthalate(PEN) (thickness of 200 μm) substrates by electrochemical depositionin an aqueous Cu bath consisting of 0.4 M copper(II) sulfate (Junsei,>98%, anhydrous), 3 M lactic acid (Sigma-Aldrich, 85%), and 4 Msodium hydroxide solution to adjust the pH to 11. For the Cu2O:Sbfilm, 2 mM antimony sulfate (Sigma-Aldrich, >98%) was added to theCu2O chemical bath. A Pt mesh and Ag/AgCl (saturated NaCl) wereused as the counter electrode and reference electrode, respectively.The electrochemical deposition was potentiostatically progressed at−0.4 V with a Princeton Applied Research Versatate 4. A ZnO thinfilm was deposited by radio frequency (RF) magnetron sputtering.During the deposition process, RF power, working pressure, and theargon:oxygen mixture ratio were maintained at 80 W, 9 mTorr, and20:2, respectively, at room temperature. An Au electrode of 200 nmwas coated on the surface of the ZnO layer to provide a Schottkycontact, as the top electrode of the piezoelectric NG.The morphology of the ZnO/Cu2O structure was examined with a

field-emission scanning electron microscope (FE-SEM, JSM-6700F, 10kV). The structural investigation of the samples was confirmed usingX-ray diffraction (XRD, Bruker AXS D8 discover). An oscilloscope(Tektronix DPO 3052 Digital Phosphor) and low-noise currentpreamplifier (SR570, Stanford Research Systems, Inc.) were used tocharacterize the output voltage and current of the piezoelectric NG. Abending tester was used to create the bending strain on the NG.

3. RESULTS AND DISCUSSIONA schematic diagram of the oxide p-n junction ZnO/Cu2O-based piezoelectric nanogenerator structure and its FE-SEMimage are shown in parts (a) and (b), respectively, of Figure 1.Here, the p-type Cu2O films were electrodeposited on the ITO-coated PEN substrate and slightly doped with Sb due to thepresence of 2 mM Sb2(SO4)3 in solution. In our previous study,the p-type Cu2O:Sb film exhibited well-aligned vertical grainboundaries and high conductivity compared to the undoped,randomly oriented, and large-grained Cu2O film because the Sbacted as an inorganic surfactant during electrodeposition. Afterthe electrodeposition of the p-type Cu2O and Cu2O:Sb layers,the semi-insulating intrinsic ZnO thin films were subsequentlydeposited using RF magnetron sputtering producing a layerwith 200 nm of thickness at room temperature. Then an Auelectrode was placed on the ZnO film to form a Schottkycontact with an energy barrier to charge transport. X-raydiffraction patterns revealed the crystallinity of the Cu2O/ZnOand Cu2O:Sb/ZnO on the PEN substrates, as shown in Figure1b. These results show that the electrodeposited Cu2O:Sb

layers have considerably improved crystallinity even on theITO/PEN substrate, based on the narrow and intense [111]Cu2O peak at 2θ = 36.5°. As explained in the previous study,the addition of Sb element led to vertically well-aligned grainswith low-angle tilted boundaries, by inducing fast nucleationand promoting lateral growth.21 As shown in Figure 1c, theelectrodeposition of the Cu2O:Sb clearly shows vertically well-aligned grain boundaries despite the use of a flexible PENsubstrate. On the other hand, the ZnO layers have a relativelylow degree of crystallinity with a weak ZnO (001) peak at 34.3°due to deposition at room temperature (Figure S1 of theSupporting Information). HRTEM images (Figure 1d andFigure S2) show Moire fringes arising from the overlap of thelattice fringes at the Cu2O:Sb/ZnO interface, and these fringesclearly indicate that the ZnO is crystalline phase and cangenerate the piezopotential.Considering the existence of a Schottky contact between

ZnO and Au, this structure is expected to have two resistancesin series consisting of p-n and Schottky junctions. Thus, thetotal voltage drop between the Au and ITO electrodes can besimply divided into two parts: a junction voltage appliedbetween p-Cu2O and n-ZnO and the voltage induced from theSchottky contact. Resultantly, the Au/ZnO/Cu2O/ITOstructure clearly exhibits good rectifying diode characteristicsregardless of the Sb doping in the Cu2O, as shown in Figure2a,b. The current density of the forward bias region in thesample with Cu2O:Sb is 2.8 mA/cm

2 at 3 V, which is over twicethat of the sample without Sb doping. In addition, the reversecurrent density of the ZnO/Cu2O:Sb is very low, 9.1 × 10−6

mA/cm2 under −2 V, indicating that the Sb incorporationproduces a sample with a high-quality p-n junction. The carrierrecombination and transport mechanism can also be comparedby a semilog plot of current versus bias (0−0.5 V), as shown inFigure 2b. The current−voltage characteristics of these curvescan be described by the sum of the p-n diode equation (Cu2O/ZnO) and the thermionic field-emission Schottky diodeequation (ITO/Cu2O):

22

= − =I I qV n kT I I qV n kT[exp( / ) 1], [exp( / )]1 S1 1 1 2 S2 2 2

Figure 1. (a) Schematic of the oxide p-n junction ZnO/Cu2O-basedpiezoelectric nanogenerator structure and (b) XRD pattern of Cu2Oand Cu2O:Sb films on an ITO-coated PEN substrate. (c) FE-SEMimage and (d) HRTEM image of the ZnO/Cu2O:Sb interface.

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where IS is the reverse saturation current, n is the junctionideality factor, q is the electron charge, k is the Boltzmannconstant, and T is the absolute temperature. Here, the idealityfactor can be calculated from the slope of the current−voltagesemilog plot. Since the carrier density of the Cu2O:Sb samplehas similar order of magnitude as that of the Cu2O sample, theeffect of the Schottky junction is minor (similar n2 values) andthe difference in n1 from the p-n diodes is significant, indicatingthat the ideality factor of the p-n diode in the Cu2O:Sb samplewas lower than that of the Cu2O sample. The relatively lowerideality factor implies that the Cu2O:Sb film had a higherconductivity and fewer interface trap states generating carrierrecombination, which thus justifies the good performance ofthe ZnO/Cu2O:Sb p-n heterojunction upon Sb doping.To demonstrate the influence of the p-n junction on NG

performance, the output performance of the ZnO NG withoutCu2O is first presented as a reference in Figure 3a,b. The NGconsisting of the simple Au/ZnO/ITO with only the n-typeoxide was slightly bent with a curvature of 0.5 cm on thebending stage. The bending direction was applied with a tensilestrain in the ZnO/ITO interface, and the bending process wascomposed of tensile strain (0.1 s) and a neutral state (1.2 s), asshown in Figure 3c. A positive piezoelectric potential (V+) anda negative piezoelectric potential (V−) are naturally created atthe ZnO−ITO interface and the ZnO−Au interface,respectively, resulting in an energy band bending bydeformation (Figure 3d).23 Figure 3a shows the variation inpiezopotential caused by the bending strain, starting with anegative voltage sign and current pulse under the forwardconnection between two electrodes. Here, piezoelectricallygenerated electrons are gradually driven from the Au to theITO electrode through an external circuit by a piezopotential,generating the negative current pulse. Subsequently, theseelectrons are accumulated at the ZnO/ITO interface until the

potential driven by these electrons is fully balanced with anequilibrium in piezopotential. In contrast, as soon as the strainis released, the piezoelectric potential immediately disappears.Accumulated electrons appear, resulting in a positive currentsignal, as shown in Figure 3b. The basic ZnO-based NGwithout any p-type material demonstrates a maximum piezo-electric voltage of 0.15 V and a piezoelectric current density of1.15 μA/cm2. Symmetric current density of a similar level isusually observed under stress and release conditions. However,the piezoelectric potential and current exhibit relatively lowvalues, arising from the native electrons in the ZnO havingintrinsically n-type characteristics, which screen the positivepiezoelectric potential of the ZnO/ITO interface.An effective strategy to enhance the piezoelectric output of

the ZnO NG is to suppress the screening effect. We suggestedp-n heterojunction-based NGs, where the intrinsically p-typeCu2O layers were electrochemically deposited to give a finalstructure of Au/ZnO/Cu2O/ITO (Figure 4a). Here, thebending direction of the NG applies strain on the ITO/ZnO.When the NG is under tensile strain, a positive piezoelectricpotential (V+) is created at the ZnO/Cu2O interface instead ofat the ZnO/ITO interface present in the Au/ZnO/ITO. Anegative piezoelectric potential (V−) is still generated at theZnO/Au interface with the deformation of the energy banddiagram, as shown in Figure 4b. The potential gradient in theinternal ZnO layer gradually develops a depolarization electricfield (Edep) which leads to the movement of carriers (theintrinsic electrons in the ZnO).24 This movement attenuatesthe piezoelectric-induced polarization, as mentioned in the Au/ZnO/ITO. However, unlike the Au/ZnO/ITO, it is very likelythat the p-n junction readily decreases the screening effect dueto two reasons: (i) according to the fundamental properties ofthe p-n junction, the p-type Cu2O layer forms a depletion layer

Figure 2. I−V characteristics from the Au/ZnO/Cu2O/ITO structure:(a) linear scale over a wide range and (b) semilog scale over a narrowrange (0−0.5 V).

Figure 3. NG performance of the ZnO NG without Cu2O (Au/ZnO/ITO): (a) piezopotential and (b) piezocurrent. The sample wasslightly bent with a curvature of 0.5 cm on the bending stage. (c) Bendcycle of ZnO NG and (d) a band diagram showing the energy bandbending under the deformed condition: positive piezoelectric potential(V+) at the ZnO/ITO interface and negative piezoelectric potential(V−) at the ZnO/Au interface.

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with built-in potential near the p-n junction interface. Here, thefree electrons existing in the ZnO layer are efficiently depletedby the p-n junction, thus preventing the screening effect, whichgenerates the low piezopotential. The built-in electric field (Ebi)in the p-n junction can reduce the strength of the negativepolarization-induced electric field (Edep), resulting in thesuppression of the screening effect. (ii) The piezopotential ofthe p-n junction NG can be increased by the additional p-njunction capacitance. Typically, the overall capacitance,connected in series, is less than the value of the individualcapacitances, according to the following equation:

= +‐C C C

1 1 1

total piezo p n

As mentioned previously, the Au/ZnO/Cu2O/ITO structurehas two resistances in series, consisting of p-n and Schottkyjunctions. Thus, the piezoelectric-induced capacitance in theZnO and the p-n junction capacitance together decrease theoverall capacitance. Finally, the increase in the piezopotential isexpected from the equation Q = CV under the same chargedensity. Also, the interface roughness may influence thepiezoelectric potential, but can be excluded due to the

Figure 4. Band diagrams showing the energy band bending (a) from the Au/ZnO/Cu2O/ITO (inducing Ebi) and (b) under the deformed condition(opposite direction between Edep and Ebi). The NG performance of the ZnO NG with undoped Cu2O (Au/ZnO/Cu2O/ITO): (c) piezopotentialand (d) piezocurrent.

Figure 5. Band diagrams showing the energy band bending (a) from the Au/ZnO/Cu2O:Sb/ITO (inducing enhanced Ebi in the ZnO) and (b)under the deformed condition. The NG performance of the ZnO NG with Cu2O:Sb (Au/ZnO/Cu2O:Sb/ITO): (c) piezopotential and (d)piezocurrent.

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extremely different structural properties between undoped andSb-doped Cu2O layers.25 In addition, the enhancement inpiezoelectric-induced current density is also due to the higherpiezopotential upon insertion of the p-type Cu2O layer, whichcan make more electrons flow from Au to ITO. Consequently,the piezopotential of the Au/ZnO/Cu2O/ITO under tensilestrain has an asymmetric voltage pulse with a significantenhancement from 0.15 to 0.45 V in negative voltage,compared to that of the reference ZnO NG (Figure 4c). Themaximum piezocurrent density is ∼2.35 μA/cm2, twice that ofthe NG without the Cu2O layer (Figure 4d). However, whenthe strain is released, the slight enhancement in the positivevoltage pulse is due to the existence of the built-in potentialinduced by the p-n junction. When the NG is bent with thetensile strain in the ZnO/Cu2O interface, potential gradientsgenerated from both the built-in potential and the piezoelectricpotential coexist. On the other hand, when the strain isreleased, only the piezoelectric potential vanishes, which causesan asymmetric piezoelectric potential. In contrast, under thecompressive strain of the ITO/ZnO layers, the ZnO region ofthe ZnO/Cu2O interface reveals a negative piezopotential,opposite to that of the ZnO/Au interface, which shows apositive piezopotential. In this case, the ITO electrode and thenegative piezopotential at the ZnO/Cu2O interface have aseparation corresponding to Cu2O thickness, and the Edep andEbi have the same direction. Thus, compressive strain leads toinsufficient suppression of the screening effect.It was demonstrated that the electrical resistivity of the

electrochemically deposited Cu2O film is reduced by 1 order ofmagnitude by adding 2 mM Sb2(SO4)3 to a cupric bath. The p-type Cu2O:Sb layer has a considerably high hole mobility of28.6 cm2/(V s), which is 10 times higher than that of theundoped Cu2O layer. Thus, we used a Cu2O:Sb layer (3 μm)with high conductivity in the p-n junction NG consisting ofAu/ZnO/Cu2O/ITO. Under compressive strain at the Au/ZnO interface, the piezoelectric potential is measured to beextraordinarily high, 0.9 V, which is approximately twice as highas the ZnO/Cu2O NG, as shown in Figure 5c. Similarly, themaximum output current density also increased to 3.1 μA/cm2,as a negative current pulse (Figure 5d). Generally, it is possibleto enhance the piezopotential of a p-n junction NG through theformation of a space charge region. If the built-in electric fieldexists due to the p-n junction, the effect of Edep on the actualpotential gradient can be reduced because of the opposingdirections of Ebi and Edep, as shown in Figure 4b and 5b. Freeelectrons become depleted in this region of the p-n junction,preventing the screening effect, which reduces the depolariza-tion electric field. Thus, the degree of the screening effect isstrongly correlated with the magnitude of a built-in electric fieldin the p-n junction, and the output voltage of p-n junctionpiezoelectric nanogenerator is determined by the actualpotential gradient which is influenced by the Ebi and Edep,and the origin of these factors is described in Figure S3. Toevaluate the carrier densities near the surface of theelectrodeposited Cu2O layer, Mott−Schottky plots of thesemiconductor electrolyte interface were obtained from acimpedance measurements. An ac amplitude of 10 mV and an acfrequency of 1 kHz were used in the potential range of 0.35−0.60 V, as shown in Figure 6. The calculated surface carrierdensity in the Cu2O:Sb layer is 4.7 × 1016 cm−2, a value 50%higher than that of the undoped Cu2O layer. Thus, the built-inpotential of the ZnO/Cu2O:Sb junction can be estimated to be∼0.5 V by the following equation,26

=⎛⎝⎜⎜

⎞⎠⎟⎟V

k Te

N Nn n

lnbiB d a

pi

ni

where Nd and Na are the donor and acceptor concentrations ofn- and p-type layers, respectively, and ni is the intrinsic carrierconcentration. The Cu2O layers electrodeposited with Sbdopants showed high crystallinity, high optical transmittance,and high electrical conductivity, and resulted in dense anddefect-reduced p-type layers.21 Because of the high mobility andhigh surface carrier concentration of the Cu2O:Sb layer, it isexpected that a high-quality ZnO/Cu2O:Sb p-n junction isformed with low interface trap states. As a result, we believe thiscontributes to the production of high piezopotential byincreasing the built-in potential, as shown in Figure 5c. If thep-n junction has a significantly high density of defect states atthe junction interface, the charge carriers might be partiallyremoved from the depletion layer, resulting in the reduction ofbuilt-in potential (Figure 4a).27 Consequently, the reduceddefects and high mobility/carrier concentration in the ZnO/Cu2O:Sb junction are preferable to induce a higher built-inpotential. Furthermore, additional effective carrier transport viathe Cu2O:Sb layer can contribute to the higher piezocurrent.Finally, we compared the output performance of ZnO/

Cu2O:Sb NGs with different Cu2O:Sb thicknesses to character-ize the effect of the p-type thickness on NG performance.Figure 7 shows the output piezopotential and piezocurrentfrom a ZnO/Cu2O:Sb NG with a 300 nm Cu2O:Sb layer. Theoutput potential of the sample with a 300 nm Cu2O:Sb layer isdecreased to 40% of that of the NG with 3 μm Cu2O:Sb, andpiezo-induced current density is similar to that of the ZnO/undoped Cu2O NG (60% of the value of the ZnO/Cu2O:Sb 3μm). These variations in potential and current are involvedwith the change of depletion width, which affects the junctioncapacitance (c = ϵ/W). Typically, the depletion width in the p-njunction can be estimated from the following equation,

=ϵ +⎡

⎣⎢⎤⎦⎥W

N N VeN N

2 ( )D a bi

D a

1/2

where ϵ is the dielectric constant of Cu2O. Here, the calculateddepletion width in the p-type Cu2O region is about 3 μm,which is much larger than the Cu2O:Sb thickness of 300 nm.Therefore, the real depletion layer (W) in the 300 nm Cu2Olayer is too narrow due to the incomplete generation of thebuilt-in potential. Subsequently the junction capacitance of thesample with a 300 nm Cu2O:Sb layer is expected to be higher

Figure 6. Mott−Schottky plot of the Cu2O and Cu2O:Sb films onITO substrate from electrochemical analysis in aqueous system with1.0 M sodium sulfate with 0.1 M potassium phosphate monobasic (pH5).

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than that of the thicker sample, Cu2O:Sb 3 μm. Thus, the p-type Cu2O with sufficient thickness is necessary to producehigh built-in potential and minimize total capacitance.

4. CONCLUSIONSWe developed binary oxide-based p-n heterojunction piezo-electric nanogenerators with electrochemically deposited highp-type Cu2O layers. Here, we selected an intrinsically p-typeoxide, Cu2O, as the p-type material layer. Cu2O was formed bya cost-effective electrodeposition process to enhance theperformance of ZnO-based piezoelectric NGs. The piezo-electric performance of these NGs was significantly improvedby introducing p-type Cu2O layers doped with Sb. The use ofthe p-type Cu2O:Sb layer distinctly reduced screening effectsfrom the intrinsic electrons in the ZnO. Resultantly, the ZnO/Cu2O:Sb heterojunction NGs showed a sixfold increase inpiezoelectric potential and 2.5 times higher current values,compared to the NG with only ZnO. Additionally, we believethat electrodeposition is a suitable process for flexiblepiezoelectric NGs because it is possible to deposit thin filmsat low temperature. Consequently, this work using high-qualityelectrodeposited Cu2O:Sb will provide a plausible platform tofabricate low-cost and high-performance flexible piezoelectricNGs.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b03649.

XRD result from the ITO/Cu2O:Sb/ZnO films;HRTEM image of Cu2O/ZnO interface; power

generation mechanism of ITO/Cu2O:Sb/ZnO/Aupiezoelectric nanogenerators (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +82 31 299 4733. Fax: +82 31 290 7410. E-mail:chohk@skku.edu (H.K.C.).

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and FuturePlanning (Grant No. 2015R1A2A2A01007409).

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Figure 7. NG performance of the ZnO NG with a relatively thin layerof Cu2O:Sb (300 nm): (a) piezopotential and (b) piezocurrent.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b03649ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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