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RAPID COMMUNICATION

Freestanding ZnO nanorod/graphene/ZnOnanorod epitaxial double heterostructurefor improved piezoelectric nanogenerators

Dong-Myeong Shina, Ermias Libnedengel Tsegea, Seok Hee Kangb,Wanchul Seungc, Sang-Woo Kimc, Hyung Kook Kima,Suck Won Hongb,n, Yoon-Hwae Hwanga,n

aDepartment of Nanomaterials Engineering & BK21 Plus Nanoconvergence Technology Division, PusanNational University (PNU), Miryang 627–706, Republic of KoreabDepartment of Cogno-Mechatronics Engineering, Pusan National University (PNU), Busan 609–735,Republic of KoreacSKKU Advanced Institute of Nanotechnology (SAINT) & Center for Human Interface Nanotechnology(HINT), SKKU-Samsung Graphene Center, Sungkyunkwan University (SKKU) Suwon 440–746,Republic of Korea

Received 5 November 2014; received in revised form 27 December 2014; accepted 27 December 2014Available online 6 January 2015

KEYWORDSDouble-sided hetero-structures;ZnO nanorods;Preheating hydrother-mal;Graphene;Nanogenerator

0.1016/j.nanoen.2lsevier Ltd. All rig

thors.: swhong@pusan.ar (Y.-H. Hwang).

AbstractIt has been intensively studied that vertically aligned ZnO nanorods over the last decade due totheir remarkable piezoelectric properties and a potential for various applications. Here, wepresent a simple strategy for fabricating the freestanding ZnO nanorods/graphene/ZnOnanorods double sided heterostructures. The characterization of the double sided hetero-structures by using SEM, and Raman scattering spectroscopy reveals the key process andworking mechanism of a formation of the heterostructure. The mechanism is discussed in detailin term of the decomposed seed layer and the vacancy defect of graphene. The approachconsists of a facile one-step fabrication process and could achieve ZnO coverage with a highernumber density than that of the epitaxial single heterostructure. The resulting improvement inthe number density of nanorods has a direct beneficial effect on the double side hetero-structured nanogenerator performance. The total output voltage and current density areimproved up to�2 times compared to those of a single heterostructure due to the coupling ofthe piezoelectric effects from both upward and downward grown nanorods. The facile one-stepfabrication process suggests that double sided heterostructures would improve the performance

014.12.040hts reserved.

c.kr (S.W. Hong),

269Freestanding ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure for improved

of electrical and optoelectrical device, such as touch pad, pressure sensor, biosensor and dye-sensitized solar cells.& 2015 Elsevier Ltd. All rights reserved.

Introduction

One-dimensional nanostructures based on carbon nanotubes[1,2], zinc oxide [3,4] and silicon [5,6] have recently beenreceiving considerable attention because of their remarkablephysical properties and potential applications. Vertically alignedZnO nanorods (NRs) are employed as building blocks forelectronics and optoelectronics, including field-effect transis-tors [7], piezoelectric nanogenerators [8], UV light-emittingdiodes [9], solar cells [10] and biosensors [11]. Single-layergraphene (SLG) is a promising component for heterostructureswith ZnO NRs due to its attractive properties, e.g., its highelectron mobility (250,000 cm2V�1 s�1) [12], high elastic mod-ulus (�1 TPa) [13] and optical transparency [14]. Thesesuperior properties make SLG highly attractive for numerousapplications in electronics [15–18], photonics [19–21], energystorage [22–24], and bioelectronics [25–27]. Graphene electro-des are beneficial for compressible, flexible and rollableelectronics applications due to the outstanding mechanicalendurance and low sheet resistance of graphene. As anelectrode material, graphene far exceeds any other availablecandidate with respect to endurance; for example, the fracturestrength of graphene is 10 times greater than that of indium tinoxide [28]. This property ensures that the implementation ofgraphene in flexible electronic devices will most likely impartflexibility and durability. Heterostructures containing graphenealong with 1D nanostructures of wurtzite materials, includingZnO [29,30], GaAs,[31], and InAs [32], have attracted consider-able attention in recent years because heterostructuring hasbeen able to improve the required properties of nanostructuresin several nanodevices [33–36].

Although a wide range of techniques have been developedto synthesize 1D ZnO nanostructures [37–42], the hydrother-mal method is the most attractive technique for obtainingwell-aligned ZnO nanorods due to its low cost, low hazard, lowsynthesis temperature, compatibility with organic substrates,and potential for easy scaling up. However, for syntheses thatrequire prolonged reaction times, the conventional hydrother-mal method involves the repeated addition of fresh precursorsolution to compensate for the depletion of the reagents andto tune the pH in the range from �10.5 to 7.2 [43,44]. Addingfresh solution to the previous solution decreases the reprodu-cibility of the resulting ZnO nanostructures. We previouslyreported the fabrication of ZnO nanostructures with a longreaction time by using the preheating hydrothermal approach[10,45], which is a consecutive hydrothermal technique with-out and with target substrates. Compared with the conven-tional hydrothermal route, the preheating hydrothermaltechnique for long-term growth results in not only higherreproducibility of the resulting nanostructures but also morenovel nanostructures, such as nanoleaves [46], nanoflowers[47] and nanotubes [48]. The preheating hydrothermal methodis likely to have important implications for the synthesis of

unique ZnO nanostructures and can be extended to synthesizethese materials for practical applications.

In this communication, we demonstrate a facile route forsuccessfully fabricating large-scale freestanding epitaxialdouble heterostructures in situ, which consist of two arraysof ZnO nanorods and single-layer graphene. Our approachfor controlling the desired ZnO NR construct, including theepitaxial single heterostructure (ZnO NRs/SLG) or theepitaxial double heterostructure (ZnO NRs/SLG/ZnO NRs),relies on regulating the growth time. The mechanismunderlying the nanostructure obtained during the hydro-thermal growth process is discussed in detail in terms of thedissolution of the seed layer and the intrinsic defects ofgraphene. This synthetic mechanism is supported by pre-cisely analyzed Raman spectra and validated by a shift ofthe 2D mode, which indicates that zinc complex ionspermeate through the vacancy defects of the single-layered graphene surface and facilitate the downwardgrowth of ZnO NRs. A piezoelectric nanogenerator inte-grated with these epitaxial double heterostructures pro-vided output voltage and current density performance thatwere enhanced compared to that those of a nanogeneratorwith embedded epitaxial single heterostructures.

Experimental section

Synthesis of freestanding ZnO NR/graphene/ZnO NRepitaxial double heterostructure

Graphene was synthesized using thermal chemical vapordeposition (CVD) on a Cu foil substrate (purchased from AlfaAesar). After the cleaned Cu foil substrate was positioned ina quartz tube, the quartz tube was heated from roomtemperature to 995 1C with H2 and Ar gases at flow rates of100 and 50 sccm, respectively. Graphene was synthesizedunder a 20 sccm flow of CH4 gas for 10 min. After the growthwas completed, the quartz tube chamber was rapidly cooledto room temperature. The as-grown graphene sheet wascoated with polymethyl methacrylate (PMMA) and sepa-rated from the underlying Cu foil using a metal etchant(Trascene, Type I). The PMMA/graphene sheet floating onthe water was transferred onto polyethylene terephthalate(PET) using the fishing method. Next, the ZnO NRs weregrown as follows. The seed particle solution was preparedusing 5 mM zinc acetate [Zn(C2H3O2)2] in absolute ethanol.The solution of ZnO seed particles was spin coated ontographene. The ZnO seed particles were thermally annealedat 200 1C for 30 min. The spin casting and subsequentannealing process were repeated twice. The ZnO NRs weregrown using the typical preheating hydrothermal approach.The growth solution was prepared by dissolving 40 mM zincnitrate hexahydrate (98%, Aldrich), 40 mM hexamethylene-tetramine (HMT) (99%, Aldrich) and 9 mM polyethylenimine

D.-M. Shin et al.270

(PEI) (Mw�25,000, Aldrich) in deionized water. Then, thesolution was heated at 95 1C for 1 h, i.e., the preheatingprocess. The ZnO-seeded graphene on PET was then rapidlyimmersed in the hot preheated solution by placing thesubstrate face down at 95 1C. The morphologies of the ZnONRs were observed using a field-emission scanning electronmicroscope (FESEM, Hitachi S-4700). The crystalline struc-ture of the sample was characterized using an X-raydiffraction system (PANalytical X'Pert PRO) with Cu Kα of1.5406 Å. Raman measurements were performed in thebackscattering geometry using 2.33 eV laser lines below3 mW to avoid damage or heating. In all cases, a 100�objective was used, which provided a laser spot size ofapproximately 1,000 nm. Raman spectra were measuredusing an Andor SR303i spectrometer equipped with acharge-coupled device (Andor CCD DV401A). The spectro-meter used a 1,800 lines/mm grating, providing a spectralmeasurement range from 1500 to 2800 cm�1 with a spectralresolution of 3 cm�1. The G and 2D peaks were fitted withLorentzian functions.

Figure. 1 (a) Schematic of the procedures for growing an epitaxiheterostructure of ZnO NRs/SLG/ZnO NRs. i) The process begins witas a seed layer is spin coated in the graphene region. iii) Hydrothervertically aligned on the graphene surface, i.e., epitaxial single heta long period of time creates additional aligned ZnO NRs on the bheterostructure of ZnO NRs/SLG/ZnO NRs. (b) The transparencyepitaxial single and double heterostructures, in which the peaks cowith red asterisks.

Characterization of the piezoelectric outputperformance

A linear motor (LS Mechapion APM-SB02ADK) was used toapply a periodic change in strain to the device. An oscillo-scope (Agilent DSO-X-2014 A) and low-noise current pream-plifier (Stanford research system SR570) were used for low-noise voltage and current measurements. The piezoelectricoutput voltage was measured under compression andrelease conditions with a constant strain (20%) and strainrate (100 mm/s).

Results and discussions

Figure. 1a shows a schematic of the procedures for growingepitaxial double heterostructures of ZnO NRs/SLG/ZnO NRs.Briefly, a single-layer graphene sheet was synthesized usingthermal chemical vapor deposition (CVD) and transferredonto a polyethylene terephthalate (PET) film using the

al single heterostructure of ZnO NRs/SLG and epitaxial doh transferring graphene onto a PET substrate. ii) A ZnO thinmal growth for a short amount of time yields ZnO NRs thaterostructure of ZnO NRs/graphene. iv) Hydrothermal growthottom side of graphene, thereby forming the epitaxial doof the as-transferred graphene on PET. (c) XRD results ofrresponding to standard ZnO from the JCPDS card are indic

fis-

ublefilmarefor

ublethe

ated

271Freestanding ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure for improved

hing technique [49] (see the experimental section). Next, aZnO seed layer was uniformly coated only on a regioncovered by graphene. Then, ZnO nanorods were grown fromthe catalytic seed layer by placing the substrate (i.e.,graphene/PET) face down in a prepared precursor solutionat 95 1C (see experimental, Supporting Information). TheRaman spectra (Supporting Information, Figureure S1) ofgraphene on glass clearly show the G and 2D modes ofgraphene. The intensity ratio of the 2D and G modesremains at �2.1, indicating that the graphene used in thisexperiment is of high quality. The photograph presented inFigure. 1b shows that the as-transferred graphene on PETwas sufficiently transparent to observe the university logounderneath the graphene/PET substrate. The crystallinestructures of the as-grown ZnO nanorods on a sheet ofsingle-layered graphene for epitaxial single and doubleheterostructures were investigated using X-ray diffraction(XRD), as shown in Figure. 1c. All of the diffraction peakscorrespond to the standard pattern for ZnO powder, asindicated with asterisks, and can be indexed to the wurtzitestructure of ZnO with lattice constants of a=3.25 andc=5.17 Å, which is in accordance with the reported values(a=3.25 and c=5.207 Å) in the JCPDS files (card no. 36-1451). The patterns presented a strong peak at approxi-mately 34.71 that corresponds to the (0002) spacing,indicating good alignment in the c-axis direction.

The scanning electron microscopy (SEM) images pre-sented in Figure. 2 show the typical morphologies of ZnONRs grown on a sheet of graphene following the procedurein Figure. 1a. The SEM images indicate that the preheatinghydrothermal method resulted in the successful growth ofZnO NRs not only on a single face of graphene (i.e.,epitaxial single heterostructure, ZnO NRs/SLG) but also ontwo faces of graphene (i.e., epitaxial double heterostruc-ture, ZnO NRs/SLG/ZnO NRs). Figure. 2a presents images ofthe epitaxial single heterostructure (i.e., Figure. 1a-iii),and Figures. 2b and 2c show the epitaxial double hetero-structure (i.e., Figure. 1a-iv). The inset image in Figure. 2bconfirms that the aligned array of nanorods and SLGconstruct were self-elevated from the PET substrate dueto the growth of the nanorod array on the bottom surface ofSLG, thereby forming freestanding epitaxial double hetero-structures. Analogously, Figure. 2c reveals that the epitaxialdouble heterostructure was achieved over a large scale, andthe arrays of upward- and downward-grown ZnO NRs wereseparated by single-layer graphene (yellow color). As shownin Figureure S2, a high density of ZnO NRs, with diametersranging from �60 to 100 nm, were uniformly grown anddistributed at a large scale over the graphene surface.Additional SEM measurements were performed to calculatethe number densities of ZnO NRs on the top and bottomsurfaces of the graphene (presented in Figureure S3). In allcases, the number density of downward-grown NRs(�38.8� 108 cm�2) was comparable to that of upward-grown NRs (�42.8� 108 cm�2) as a result of controlling thegrowth time. The epitaxial double heterostructure had ahigh density of ZnO NR arrays on both surfaces of thegraphene, and the number density was almost two-foldgreater than that in the epitaxial single heterostructure.

Figureure S4a-c present SEM images of ZnO NRs/SLG/ZnONRs epitaxial double heterostructures synthesized usingdifferent growth times. Nearly all of the ZnO NRs on the

top of the graphene (i.e., upward-grown ZnO NRs) wereperpendicular to the substrate, with average lengths of2.58, 2.61 and 2.30 μm, respectively. Furthermore, thelengths of the other ZnO NRs (i.e., downward-grown ZnONRs) linearly increased from 0.61 to 2.23 μm at a growthrate of 79 nm hr�1. Figure. 2d presents the average lengthsof the upward- and downward-grown ZnO NRs as the growthtime was increased from 1 to 36 h. The lengths of theupward-grown ZnO NRs were linearly dependent on thegrowth time in a short time range from 1 to 8 h, with agrowth rate of 254 nm hr�1. This result was expected basedon our previous works [10], in which the length of the ZnONRs was dependent on the growth time in the hydrothermalroute. When increasing the growth time from 8 to 36 h, thelength of the ZnO nanorods reached a maximum of approxi-mately 2.50 μm. Thereafter, the NR arrays began to grow atthe interface between the graphene and PET substratebecause the surface of the graphene was exposed to theprecursor solution as a result of surface delamination fromthe PET substrate.

The primary principle underlying the growth of theseepitaxial double heterostructures can be rationalizedthrough two main scenarios: i) the dissolution of the seedlayer during the hydrothermal route and ii) the existence ofintrinsic vacancy defects in graphene [50], as illustrated inFigure. 3a. During the hydrothermal process, zinc ions andhydroxide were provided by the hydration of zinc nitratehexahydrate and hexamethylenetetramine, thereby formingmore soluble Zn(OH)2 complexes. Then, the growth of ZnOwas initiated by the decomposition of Zn(OH)2 according tothe growth environment, in which zinc ions preferentiallymove toward the polar (001) facet. Therefore, the chemicalreactions involved in the hydrothermal synthesis can beexpressed as

ðCH2Þ6N4þH2O⟹Δ4NH3ðgÞ þ6HCHOðgÞ ð1Þ

NH3þH2O-NH4þ þOH� ð2Þ

Zn2þ þ2OH�2ZnðOHÞ2ðsÞ ð3Þ

ZnO sð Þ þ2OH�2ZnO22� þH2O ð4Þ

The ZnO nanorods grew rapidly in a supersaturated precursorsolution that contained an abundance of Zn(OH)2, whichindicates that the growth of nanorods is dominant during theinitial stage (i.e., Figure. 3a-i). As the reaction proceeded, thezinc ions were gradually consumed due to the formation of ZnOprecipitates, resulting in an increase in the pH (see Figure. 3b).When the concentration of OH─ ions exceeded the criticalconcentration, the growth of the nanorods began to competewith the dissolution of ZnO. The growth rate of the ZnO NRsexhibited a crossover at a growth time close to 12 h (Figure.2d), which is consistent with the crossover point in the pHvalues. Thus, the ZnO in the reaction solution could bedecomposed by the negative species (i.e., Figure. 3a-ii), asindicated in Eq. 4. The SEM images in the inset of Figure. 3bclearly demonstrate that dissolution occurred in the hydro-thermal solution. This dissolution of ZnO resulted in theepitaxial double heterostructures produced with growth timesof 12, 24 and 36 h having pencil-shaped tips, indicating that theperimeter of the NR's polar surface has a more intense

Figure. 2 SEM images of epitaxial single and double heterostructures on graphene. a) ZnO NRs/SLG epitaxial singleheterostructure. b) ZnO NRs/SLG/ZnO NRs epitaxial double heterostructure. Scale bar=1 μm. c) Image similar to that shown in(b) but with epitaxial double heterostructures that are uniformly grown and distributed over the entire graphene area (yellow colorscorrespond to single-layer graphene in the inset image). d) The lengths of the upward (blue) and downward (red) ZnO NRs areplotted as a function of the growth time. The calculated growth rates are 254 and 79 nm h�1, respectively.

D.-M. Shin et al.272

Figure. 3 a) Schematic of the growth mechanism for the ZnO NRs/SLG/ZnO NRs epitaxial double heterostructure. b) Measured pHvalues as a function of the growth time. c) Evolution of the 2D peak positions (open symbol) and I(2D)/I(G) ratio (solid symbol) withincreasing growth time.

273Freestanding ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure for improved

electrical field than the central region surround by theperimeter. Notably, the diameters of the upward-grown ZnONRs (12, 24 and 36 h) were approximately 1.14–1.43 timessmaller than those at 8 h. This dissolution occurred on thesurface of the early grown nanorods and on the seed layer onthe graphene; therefore, the graphene covered by the seedlayer was exposed to the hydrothermal precursor solution. Theinflux of zinc ions enables the growth of ZnO NRs on the bottomside of graphene facing the PETsubstrate through two plausibleroutes: i) through the vacancy defects in the graphene or ii)through the interfacial gap, which originated from the delami-nation between the graphene and PET. Recently, Vijayakumaret al. provided fundamental insight into the molecular behaviorat the interface between the electrolyte and graphene whenthe graphene possesses defects that could interact withcations, i.e., positive ions, in the electrolyte [51]. Theseauthors found that zinc complex ions could attach to thedefects on graphene and initiate nucleation on the surface ofgraphene. The zinc oxide ions could be placed above the centerof the carbon ring or the bridge between two carbon atoms inthe graphene with four different atomic configurations, ascomputed by Munshi et al [31]. Therefore, the hexagonal

carbon ring in graphene induces the formation of ZnO NRs witha hexagonal crystal structure. In other words, the ZnO NRscould heteroepitaxially grow on the graphene sheet due to thelow lattice mismatch between the ZnO crystal and graphenebond center [31]. Consequently, downward-grown ZnO NRscould self-elevate the epitaxial single heterostructure, leadingto the formation of the epitaxial double heterostructure (i.e.,Figure. 3-iii).

Raman spectroscopy has been used to investigate dis-order in carbon-based materials because of its sensitivity todefects [52–55]. In graphene, the G and 2D modes alwayssatisfy the selection rule, whereas the forbidden D and D'modes appear when defects are present to satisfy momen-tum conservation [56]. Figureure S5 shows the Ramanspectra of epitaxial single and double heterostructures withdifferent growth times. Three peaks appear at approxi-mately 1619, 1729 and 2624 cm�1, which correspond to PET.The 2D mode of as-transferred graphene on PET is clearlyobserved as a sharp (�35 cm�1) and symmetric peak atapproximately 2691 cm�1. Although the G, D and D' modesof graphene overlap with a strong peak from PET, the Gmode appears as a weak shoulder at approximately

D.-M. Shin et al.274

1585 cm�1. Focusing on the 2D peak, the peak positionclearly shifts to lower frequencies and saturates at approxi-mately 2682 cm�1 as the growth time increases (Figure.3c). The down-shift of the 2D peak is related to theconcentration of defects in graphene [52], which results inthe transverse optical (TO) phonons dispersing in the vicinityof the K point. Moreover, the variation in the intensity ratioof I(2D)/I(G) also indicated that the defect concentration ingraphene increases with increasing growth time. Theseincreases in the defect concentration are associated witha consecutive reaction as follows: zinc complex ions acci-dentally percolate through naturally formed vacancydefects in graphene and then begin to nucleate and formZnO NRs on both surfaces of the graphene. The vacancydefects are completely filled with ZnO NRs, which causesthe TO phonons to decelerate and disperse. Using thissynthesis scheme, we successfully fabricated an epitaxial

Figure. 4 a) Schematic of the device in which epitaxial doublec) Piezoelectric output voltage of the device under compress-releasoutput voltages (d) and current densities (e) from the upward-grow(2)–(1) connection), and the total output voltage and current densityboth NRs (i.e., (2)–(1), (3) connection)

double heterostructure of ZnO NRs/SLG/ZnO NRs with a highdensity of ZnO NRs on both graphene surfaces. Furthermore,these epitaxial double heterostructures could enhance notonly the number density of nanorods but also the specificsurface area of the resulting heterostructure in a givenregion.

To demonstrate the utility of this freestanding epitaxialdouble heterostructure for devices, we constructed nano-generators to harvest electrical energy. Figure. 4a shows aschematic of the flexible nanogenerator. The device isprimarily composed of three layers: the epitaxial doubleheterostructure as a piezoelectric material, which includesthe graphene sheet as the working electrode in the middle,and two ITO-coated PET layers as the top and bottomelectrodes. The matrix of ZnO nanorods was immersed ina polydimethylsiloxane (PDMS) elastomeric layer, whichserved not only as a potential barrier but also as a buffer

heterostructures are installed. b) Photograph of the device.e conditions in the opposite connection polarities. Piezoelectricn NRs (i.e., (2)–(3) connection) and downward-grown NRs (i.e.,resulting from the coupling of the piezoelectric effect between

275Freestanding ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure for improved

layer to improve the robustness and durability of thenanogenerator (Figureure S6 and S7) [57,58]. An opticalimage of a fabricated nanogenerator is presented in Figure.4b that shows the high flexibility of our energy-harvestingdevice. This high flexibility provides potential for applica-tions such as wearable and implantable electronic devices,biomedical sensors and flexible displays.

Figure. 4c shows the piezoelectric output voltage from ournanogenerator under compression and release. The outputvoltage reached 0.10 V under an applied load of F=49 N, andthe voltage signal was perfectly reversed when the devicepolarity was switched, indicating that the piezoelectric signalswere from the device and not from environmental noise.Figures. 4d and 4e show the enhancement in the output voltageand current density from the nanogenerator when the piezo-electric effect for the upward- and downward-grown ZnO NRs iscoupled. The output voltage from the upward-grown ZnO NRswas observed up to 0.10 V (i.e., (2)-(3) connection indicated inFigure. 4a), and the output voltage from the downward-grownZnO NRs was observed up to 0.13 V (i.e., (2)-(1) connectionindicated in Figure. 4a). The output voltages from both ZnO NRswere successfully integrated to provide a total voltage of up to0.17 V (i.e., (2)-(1), (3) connection). Furthermore, the outputcurrent densities from both ZnO NRs were also integrated, anda peak output power density of 4.68 nW cm�2 was obtained.Each nanorod in the compressed area acts as a charging pump,and subsequently, the output voltage could be constructivelyintegrated if the polarization and depolarization processes ofmany nanorods could be synchronized under periodic mechan-ical deformation [59]. The number of ZnO NRs has a crucial rolefor the generated potential from a piezoelectric perspective;therefore, the electrical energy-harvesting performance couldbe improved through the epitaxial double heterostructure.Thus, we could improve the piezoelectric energy-harvestingproperties by using the epitaxial double heterostructure inwhich the number density of ZnO NRs increased almost two-foldunder compression and release. As the applied force increased,the output voltage clearly increased linearly (Figure S8). Thisresult indicates that the most of the applied stress is consumedaround the epitaxial double heterostructure ZnO NRs in ournanogenerator.

Conclusion

In summary, this communication presents a simple strategyfor fabricating an epitaxial double heterostructure of ZnONRs/SLG/ZnO NRs. The rigorous characterization of theepitaxial double heterostructure through precise surfacemorphology observations and Raman scattering spectro-scopy measurements reveals the key process and workingmechanism for forming the epitaxial double heterostruc-ture. The mechanism is discussed in detail in terms of thedecomposition of the seed layer and the vacancy defects ingraphene. The resulting improvement in the number densityof NRs has a direct beneficial effect on the performance of ananogenerator constructed using the epitaxial double het-erostructure. The total output voltage and current densityare improved by up to �2-fold compared to those asso-ciated with the epitaxial single heterostructure due to thecoupling of the piezoelectric effects from both upward- anddownward-grown NRs. Such an approach with a facile one-

step fabrication process suggests that the epitaxial doubleheterostructure of ZnO NRs/SLG/ZnO NRs would contributeto advancements in the performance of electrical andoptoelectrical devices, such as touchpad electronics, smartgloves, implantable devices, biosensors and solar cells.

Acknowledgments

We thank Dr. Seunghun Lee and Prof. Se-Young Jeong for theirhelpful discussions about the instrument set-up and for provid-ing the instruments for the piezoelectric output voltage andcurrent measurements. This work was supported by theNational Research Foundation of Korea(NRF) grant funded bythe Korea government(MSIP) (No. 2014R1A2A1A11051146). Thisresearch was also supported by the Civil & Military TechnologyCooperation Program through the National Research Foundationof Korea (NRF) funded by the Ministry of Science, ICT & FuturePlanning (No. 2013M3C1A9055407).

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.12.040.

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Dong-Myeong Shin is a Ph.D. candidate inthe Department of Nanomaterials Engineer-ing at Pusan National University, Busan,Republic of Korea. He received his BSdegree (2009) and MS degree (2011) fromPusan National University. His main researchinterests include the synthesis of piezoelec-tric and triboelectric materials for energyharvesting.

Ermias Libnedengel Tsege was born inAddis Ababa, Ethiopia, in 1985. He receivedhis BS degree (2005) from Jimma Universityand MS degree (2007) from Addis AbabaUniversity (Ethiopia). Since then he studiedas a PhD student under the supervision ofProf. Yoon-Hwae Hwang in Pusan NationalUniversity, Busan, Republic of Korea. Hiscurrent research interests focus on nano-materials synthesis, optical spectroscopy

and energy harvesting nanomaterials.

Seok Hee Kang is a master candidate in theDepartment of Cogno-Mechatronics Engi-neering at Pusan National University, Busan,Republic of Korea. He received his BSdegree (2013) from Pusan National Univer-sity. His main research interest is focused onthe two-dimensional nanomaterials includ-ing graphene and molybdenum disulfidenanosheets and their application in electro-nic devices.

Wanchul Seung is a Ph.D. student under thesupervision of Prof. Sang-Woo Kim at School ofAdvanced Materials Science and Engineering,Sungkyunkwan University (SKKU). His researchinterests are fabrications and characteriza-tions of piezoelectric and triboelectric nano-generator energy harvesting and their appli-cations in self-powered devices.

Prof. Sang-Woo Kim is an Associate Professorin School of Advanced Materials Science andEngineering at Sungkyunkwan University(SKKU). He received his Ph.D. from KyotoUniversity in Department of Electronic Scienceand Engineering in 2004. After working as apostdoctoral researcher at Kyoto Universityand University of Cambridge, he spent fouryears as an assistant professor at KumohNational Institute of Technology. He joined

the School of Advanced Materials Science and Engineering, SKKUAdvanced Institute of Nanotechnology (SAINT) at SKKU in 2009. His

277Freestanding ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure for improved

recent research interest is focused on piezoelectric/triboelectricnanogenerators, photovoltaics, and two-dimensional nanomaterialsincluding graphene and hexagonal boron nitride nanosheets. Now heis an Associate Editor of Nano Energy.

Prof. Hyung Kook Kim is a Professor inDepartment of Nanomaterials Engineering atPusan National University (PNU). He receivedhis Ph.D. from Pennsylvania State University inDepartment of Physics in 1985 under super-vision of Prof. Moses H. W. Chen, and joinedthe Pusan National University in 1986. From2012 to 2014, he was a president in Institutefor Research and Industry Cooperation at PNU.He now is the Chairman in Department of

Advanced Integrated Circuit at PNU. His recent research interest isfocused on piezoelectric and triboelectric nanogenerators, photovol-taics, and phosphor nanomaterials.

Prof. Suck Won Hong is an Associate Profes-sor in Department of Cogno-MechatronicsEngineering at Pusan National University(PNU). He received his Ph.D. from Iowa StateUniversity in Department of Materials Scienceand Engineering in 2008 under supervision ofProf. Zhiqun Lin. He spent two years as apostdoctoral researcher in Prof. John A. Rogersgroup at University of Illinois at Urbana-Champaign. He joined the Department of

Cogno-Mechatronics at PNU in 2011. His recent research interest isfocused on piezoelectric and triboelectric nanogenerators, transistor,biosensor, and two dimensional nanomaterials including graphene andmolybdenum disulfide nanosheets.

Prof. Yoon-Hwae Hwang is a Professor inDepartment of Nanomaterials Engineeringat Pusan National University (PNU). Hereceived his Ph.D. from the University ofPittsburgh in Department of Physics andAstronomy in 1995 under the supervisionof Prof. Xiaomin Wu. After working as thepostdoctoral researcher at the City Collegeof New York, City University of New Yorkunder the supervision of Prof. Herman Z.

Cummins he joined the Pusan National University in 1997. Now he isDirector of BK21PLUS Nanoconvergence Technology Division at PNU.His recent research interest is focused on piezoelectric and tribo-electric nanogenerators, photovoltaics, phosphor nanomaterials.