Piezoelectric Energy Harvesting from Two-Dimensional BoronNitride NanoflakesGyoung-Ja Lee,† Min-Ku Lee,† Jin-Ju Park,† Dong Yeol Hyeon,‡ Chang Kyu Jeong,§,∥

and Kwi-Il Park*,‡

†Sensor System Research Team, Korea Atomic Energy Research Institute, 111 Daedeok-daero, 989 Beon-gil, Yuseong-gu, Daejeon34057, Republic of Korea‡School of Materials Science and Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic ofKorea§Division of Advanced Materials Engineering and ∥Hydrogen and Fuel Cell Research Center, Chonbuk National University, 567Baekje-daero, Deokjin-gu, Jeonju 54896, Jeonbuk, Republic of Korea

*S Supporting Information

ABSTRACT: Two-dimensional (2D) piezoelectric hexagonalboron nitride nanoflakes (h-BN NFs) were synthesized by amechanochemical exfoliation process and transferred onto anelectrode line-patterned plastic substrate to characterize theenergy harvesting ability of individual NFs by external stress.A single BN NF produced alternate piezoelectric outputsources of ∼50 mV and ∼30 pA when deformed bymechanical bendings. The piezoelectric voltage coefficient(g11) of a single BN NF was experimentally determined to be2.35 × 10−3 V·m·N−1. The piezoelectric composite composedof BN NFs and an elastomer was spin-coated onto a bulk Sisubstrate and then transferred onto the electrode-coatedplastic substrates to fabricate a BN NFs-based flexiblepiezoelectric energy harvester (f-PEH) which converted a piezoelectric voltage of ∼9 V, a current of ∼200 nA, and aneffective output power of ∼0.3 μW. This result provides a new strategy for precisely characterizing the energy generation abilityof piezoelectric nanostructures and for demonstrating f-PEH based on 2D piezomaterials.

KEYWORDS: flexible energy harvester, piezoelectric, two-dimensional boron nitride, self-powered nanogenerator,boron nitride nanoflake, flexible electronics

1. INTRODUCTION

Energy harvesting based on piezoelectric nanostructures fromambient and tiny mechanical stimulations has provided abreakthrough of flexible self-powered systems.1−9 Piezoelectricnanogenerators constructed by transferring one-dimensional(1D) nanowires with a noncentrosymmetric structure (e.g.,ZnO,2,4 ZnSnO3,

10 CdTe,11 BaTiO3,12 and PMN-PT13

nanowires) have been intensively studied for flexible andwearable energy harvesting/sensing applications owing to theirexcellent piezoelectricity and mechanical flexibility. In addition,new approaches that demonstrate vertical or lateral config-urations have enabled highly efficient flexible nanogener-ators.14−18 Recently, two-dimensional (2D) piezoelectricnanostructures such as hexagonal boron nitride (h-BN),19−21

molybdenum sulfide (MoS2),20,22,23 and tungsten diselenide

(WSe2)20,24 have been examined for energy conversion

because they have excellent flexibility compared with thetraditional 1D piezoelectric nanostructures.25

A number of 2D layered bulky materials, including h-BN,MoS2, WSe2, and many transition-metal dichalcogenides

(TMDCs), have centrosymmetric crystal structures, whereas,when thinned into a single layer, they show differentsymmetries. Unlike graphene, in which all atoms are identicaland have a center of symmetry, the inversion symmetry isbroken in monolayer h-BN, MoS2, WSe2, and TMDCs; thisbehavior causes a strong piezoelectricity.20,26,27 Recently,experimental observation of the intrinsic piezoelectricityindicated that the piezoelectric coefficients of the 2Dnanostructures were 2.9 × 10−10 C·m−1 (e11 for MoS2)

22 and3.26 ± 0.3 pm·V−1 (d11 for WSe2)

24 by using piezoresponseforce microscopy, which enables an expanded study of 2Dmaterials in a variety of piezoelectric applications.h-BN is typically a nonconductor (a wide-band semi-

conductor), the bandgap of which is reportedly 5.3−5.9 eV.The remarkable advantages of h-BN are excellent mechanicalproperties, high thermal conductivity, chemical inertness,

Received: July 11, 2019Accepted: September 24, 2019Published: September 24, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 37920−37926

© 2019 American Chemical Society 37920 DOI: 10.1021/acsami.9b12187ACS Appl. Mater. Interfaces 2019, 11, 37920−37926

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nontoxicity, environmental safety, and so forth. Mele et al.reported theoretically the existence of piezoelectricity in planarBN nanotubes owing to the polarization of the B−N chemicalbond in terms of geometric phase.28 Although reports haverevealed that h-BN shows a piezoelectric response whensubjected to external deformation, there are still limitations onprevious characterizations of the energy harvesting ability of2D h-BN. Moreover, the product yields of monolayer h-BN arescarce because of the limited fabrication methods adopted,which restricts actual applications.This study is of great significance for its first attempt to

measure the piezoelectric voltage coefficient of a single BNnanoflake (NF) and to evaluate the energy harvesting ability of2D h-BN NFs-based flexible piezoelectric energy devices. BNNFs with a large yield and high efficiency were prepared byadopting a simple ball-milling process with the aid of sodiumhydroxide. By simple processes involving transfer and focusedion beam (FIB)-Pt deposition, a piezoelectric single BN NFwas successfully connected with Au electrodes on a flexiblesubstrate to precisely investigate its efficiency of energyconversion during mechanical agitation. During repeatedmechanical deformation of the flexible substrate, the recordedvoltage signals and current pulse from the single BN NF were∼50 mV and ∼30 pA, respectively. These values correspondedto the piezoelectric voltage coefficient (g11) of 2.35 × 10−3 V·m·N−1. We also fabricated a flexible piezoelectric energyharvester (f-PEH) by a simple spin-casting of the piezoelectric

composite based on BN NFs and a polymeric matrix. BN NFs-based f-PEH with an electrode area of 3 × 3 cm2 converted anopen-circuit voltage (Voc) of ∼9 V, a short-circuit current of∼200 nA, and an instantaneous output power of ∼0.3 μW atan external resistance of 200 MΩ.

2. EXPERIMENTAL SECTION2.1. Preparation of BN NFs. For mechanochemical exfoliation of

BN, 2 g of micron-sized h-BN powder (Kojundo Korea) and a 2 Maqueous NaOH solution (Junsei Chemical) were put together with 8mm steel balls in a steel jar and subjected to milling in a planetary mill(P100, KM Tech, Korea) at 600 rpm for 24 h with a ball-to-powderratio of 50:1. The milled product was rinsed several times until the pHwas near neutral with HCl solution (Sigma-Aldrich) and deionized(DI) water to remove by-products such as Fe3+. The resultant wasdried in an oven and then dispersed in isopropyl alcohol (IPA,Merck) at a concentration of 0.5 mg/mL, followed by sonication for 1h. Finally, the BN NFs were obtained by centrifugation (Supra 22K,Hanil Science Industrial, Korea) for 30 min at a speed of 2000 rpm.

2.2. Material Characterization. The surface functional groupswere investigated using a Fourier transform infrared (FT-IR)spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA) in anattenuated total reflectance mode. The zeta potential of BN NFsdispersed in IPA was detected using an ELSZ-1000 (OtsukaElectronics, Japan). The crystallographic structure of the powderswas analyzed by X-ray diffraction (XRD; MiniFlex600, Rigaku, Japan)with Cu Kα radiation (λ = 1.54 Å). The specific surface area of theraw BN powder and BN NFs was measured by the Brunauer−Emmett−Teller method (BET; ASAP2020, Micromeritics Inc., USA).

Figure 1. (a) Schematic illustration of the sequential process for fabricating 2D h-BN NFs (BN NFs). (b) Schemes showing the piezoelectricresponse of 2D h-BN when subjected to mechanical deformation. The dipole moments induced by external bending can produce the piezoelectricpotential difference around the sides of hexagonal structure. (c,d) XRD patterns (c) and FT-IR spectra (d) observed from the h-BN NFs and raw h-BN powders. (e) TEM image of h-BN NFs with a low-magnification and corresponding SAED patterns (i), high-resolution TEM images obtainedfrom the surface (ii) and the edge region (iii) of single BN NF.

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The morphology of BN NFs was observed using transmission electronmicroscopy (TEM; JEM-2100F, JEOL, Japan). The thickness andlateral size of BN NFs were investigated by atomic force microscopy(AFM; NX20, Park Systems, Korea), with the sample prepared bytransferring the NFs on a mica substrate.2.3. Fabrication Process for a Single BN NF on a Plastic

Substrate. An electrode layer (Au, 100 nm) was deposited onto aflexible plastic substrate (polyimide, 125 μm) and patterned using astandard microfabrication procedure to form electrode lines and pads.A thin chromium layer (<10 nm thickness) was also deposited ontothe electrode line-coated plastic substrate to avoid the problem ofcharging under the FIB-Pt deposition process. Subsequently, a drop ofa solution of well-distributed BN NFs in ethanol and DI water wasspin-coated onto the plastic substrate. To connect both ends of theselected single BN NF with the Au electrode lines, in situ Ptdeposition was conducted by employing the FIB process. Cu wireswere affixed to the electrode pads using a Ag-based conductive epoxyto detect the electric signals produced from the single BN NF. Thesingle BN NF on the plastic substrate suffered a high electric field of∼5 MV cm−1 for 3 h at 120 °C; as a result, the surface of the singleBN NF was electrically poled in plane.2.4. Fabrication Steps for BN NFs-Based f-PEH. To

demonstrate a flexible energy device made of 2D piezoelectric BNNFs, we used the well-known nanocomposite-based generator (NCG)fabrication technique.29−32 First, a pure polydimethylsiloxane(PDMS) elastomer, to serve as a dielectric layer, was preparedusing a base (Sylgard 184, Dow Corning) and a hardener in the ratioof 10:1; subsequently, PDMS as a dielectric layer was spin-casted ontoa bulk substrate at 1500 rpm for 30 s. A 50 μm thick PDMS was curedat 85 °C for 10 min in an oven. Next, the piezoelectric nanocomposite(p-NC) composed of inorganic piezoelectric BN NFs and organicelastomers at a ratio of 5 wt % was coated onto the PDMS-coatedbulk substrate with the same coating/curing conditions. A purePDMS was also placed onto p-NC as the top dielectric layer;subsequently, the fully hardened three layers were sliced into areas of3 × 3 cm2. For the fabrication of f-PEHs, the BN NFs-basedpiezoelectric rubber was peeled off from the bulk substrate andinserted into the two transparent electrode (indium tin oxide)-coated

plastic substrates (polyethylene terephthalate). The top and bottomelectrodes were connected with a high-voltage source meter and poledwith a high voltage from 0.5 to 2 kV for ∼12 h to enhance thepiezoelectric effect.

2.5. Measurement of the Harvested Output Signals. Acustomized bending machine was used to repeatedly deform thesingle BN NF on the plastic substrate and f-PEHs with variousdisplacements and strain rates. The electrical output signals generatedfrom the single BN NF and f-PEHs during periodic mechanicaldeformations were measured and simultaneously recorded by meansof a precise electrometer unit. To avoid disturbance from any externalsource, a Faraday cage was used during the process of measuring theoutput signals produced from the energy harvesters.

3. RESULTS AND DISCUSSION

Figure 1a shows the fabrication procedure for the preparationof BN NFs by the ball-milling method with the assistance of anhydroxide.33 This process involving chemical exfoliation andmechanical shear forces allows to improve the output of NFsand to prevent the agglomeration between the particles. The h-BN monolayer has a honeycomb lattice structure constructedof boron (B) and nitrogen (N) atoms in an ordered mode. Asshown in the schemes of Figure 1b, by the mechanical forcealong the horizontal direction, the inversion symmetry of theh-BN layer is broken which causes a nonzero dipole momentin the strain direction; this behavior leads to the generation ofa piezoelectric potential.34 The raw h-BN powder hadcharacteristic hexagonal diffraction peaks at 26.7, 41.6 43.9,50.2, and 55.1: these peaks are indexed to the (002), (100),(101), (102), and (004) planes, respectively, with the latticeconstants of a = 0.2504 nm and c = 0.6661 nm (ICDD card01-073-2095). No impurity peaks observed in Figure 1cindicate that the produced BN NFs are chemically pure. Therelatively high intensities of the (002) and (004) peakscompared to the raw BN powder show that the exfoliation

Figure 2. (a) Schematic diagram (i) of the preparation process for demonstrating the 2D h-BN NF on an electrode patterned-flexible plasticsubstrate. The right-top panels (ii,iii) show the SEM images of a h-BN single NF connected with two Au lines (electrode gap of 800 nm) using theFIB-Pt deposition process. A right-bottom panel (iv) shows a photograph of flexible substrate with a piezoelectric single NF deformed by humanfingers. (b,c) The measured open-circuit voltage (b) and short-circuit current (c) from a single h-BN NF under an applied external stress in forwardand reverse connections, respectively.

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process of BN NFs occurs along the (002) plane withoutdestroying its crystal structure, so that the (002) crystal planesare much more exposed. The experimentally measured BETsurface areas of raw BN and BN NFs were 2.56 and 48.81 m2·g−1, respectively, which also confirm that the exfoliation bymilling reduces the thickness and exposes more basal (ab)-plane surfaces, thereby increasing the specific surface area.The FT-IR spectra (Figure 1d) of both raw BN and BN NFs

show the E1u mode (B−N stretching: in-plane ring vibration)around 1348 cm−1 and the A2u mode (B−N bending: out-of-plane vibration) around 790 cm−1. In addition, the BN NFshave an additional peak at 1190 cm−1 different from the rawBN powder, which is due to the B−O deformation caused bythe hydroxylation of the BN NFs.33,35 The zeta potentials ofraw BN and BN NFs dispersed in IPA solution were 3.23 and−61.4 mV, respectively. A newly appeared B−O deformationpeak in the FT-IR spectra and the highly negative zetapotential are due to the hydroxyl groups attached to the ab-planes and the edge sites of the BN NFs.The morphology of BN NFs was investigated by using TEM

and AFM. As seen in the TEM image [Figure 1e(i)] and theheight profiles (Figure S1) of BN NF using AFM, the BN NFshave an average lateral size of ∼0.82 μm and a thickness of∼25 nm; these values were taken from the statistical data of 60NFs. The selected-area electron diffraction (SAED) pattern[the inset of Figure 1e(i)] of BN NFs indicated the typicalsixfold symmetry with a [0001] viewing zone axis which meansthat BN NFs maintain the hexagonal lattices during the harshexfoliation processes. Figure 1e(ii,iii) shows the high-resolution TEM images obtained from the surface and theedge region of a single BN NF, respectively. From the profilingresults (the insets) of the TEM contrast intensity recordedalong the red dotted lines, fringe separation and interlayerdistance were found to be ∼0.25 and ∼0.33 nm, respectively.By the geometrical relationship, the spacing between B and Natoms was estimated to be ∼1.43 Å; this value agrees well tothe length of the B−N bond of 1.44 Å.33,36

To evaluate the piezoelectric output performance of anindividual BN NF under mechanical deformation conditions,we demonstrated a single BN NF connected to the electrodepads on a plastic substrate and explored the power generationfrom the single NF. Figure 2a(i) shows the fabrication designused to characterize the intrinsic piezoelectricity of a single BNNF: these procedures were detailed in the ExperimentalSection. The right panels of Figure 2a show the scanningelectron microscopy (SEM) images and the photograph of asingle BN NF on a plastic substrate (5 × 5 cm2), respectively,in which both edges of the single NF (lateral size of about 1μm) were connected with the electrode lines by FIB-Ptdeposition. When the single BN NF was periodically bentalong the direction indicated in Figure 2a(ii), the outputsignals were measured with open-circuit voltage (Voc) andshort-circuit current (Isc) signals, as shown in Figure 2b,c.During the mechanical bendings with a strain of 0.283% whichcorresponds to the induced force of 17 μN with an elasticmodulus of 865 GPa37 at a strain rate of 2.32%·s−1, a single BNNF at an electrode gap of 800 nm generated piezoelectricoutputs of ∼50 mV and ∼30 pA; these values were obtainedfrom the top surface of the single BN NF and can be convertedto a piezoelectric voltage coefficient (g11) of 2.35 × 10−3 V·m·N−1. The piezoelectric properties of BN NFs are expected tobe improved by optimizing the exfoliation processes becausethe piezoelectric outputs of the 2D piezoelectric nanomaterialsdepend on the number of atomic layers. According to thereported results, the odd-layer sheets show intrinsic piezo-electricity owing to the inversion symmetry; on the contrary,the piezoelectricity disappeared for even-layer samples and thebulk three-dimensional form with a centrosymmetric struc-ture.20,38,39 We also performed a verification process, the so-called switching polarity test, to verify that the measured tinyelectricity originated from the piezoelectricity of the single BNNF. The alternate electric signals measured in response to thebending and releasing in a forward connection (left signals ofFigure 2b,c) were inverted in reverse connection (rightsignals). As a result, we checked that the detected electric

Figure 3. (a) Schematic of the fabrication step for f-PEH made of piezoelectric h-BN NFs/polymeric matrix and photograph (right panel) of thefinal fabricated f-PEH. (b,c) The alternate output voltage (b) and current (c) generated from the h-BN NFs-f-PEH in forward (left signals) andreverse (right signals) connection with an electrometer.

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pulses were introduced from the piezoelectric effect of thesingle BN NF.To fabricate f-PEH devices by adopting 2D piezoelectric BN

NFs, we used the widely used NCG fabrication technique witha spin-coating process. Figure 3a presents the schemesinvolving the fabrication procedures of BN NFs-based f-PEHs, which were detailed in the Experimental Section. Thethick piezoelectric rubber layer formed from the BN NFs thatwere embedded in the polymeric matrix was inserted betweentwo transparent electrode-coated plastics (see the actualfabricated f-PEH in the right panel of Figure 3a) andmechanically bent by a bending machine at a precisedisplacement and strain rate. As shown in Figure 3b,c, thefabricated flexible energy harvester with a real area of 3 × 3cm2 converted Voc of ∼9 V and Isc of ∼200 nA from therepeated mechanical deformations created by bending with 5mm displacement at a strain rate of 5 cm·s−1. From theswitching polarity test results, we verified that the recordedpeaks truly originated from piezoelectric energy harvesting bythe f-PEH.We also characterized the dependence of the output

performance on the poling process with an induced highvoltage from 0.5 to 2.0 kV. The as-fabricated f-PEH withoutthe poling process showed a negligible output (Voc < 1 V andIsc < 10 nA), whereas the piezoelectric output values of thepoled f-PEHs were improved, as shown in Figure 4a. Todetermine the instantaneous power of BN NFs-based f-PEHs,the load voltage (VL) and current (IL) were measured withload resistors ranging from 100 kΩ to 1 GΩ (Figure 4b). Theload voltage of a resistor slowly increased to 10 MΩ and thenrapidly rose with increasing resistance; on the contrary, theload current through the resistor gradually decreased.Consequently, by multiplying VL and IL under differentresistances, we found an effective power with different loadsand a maximum value of ∼0.3 μW at an external load ofaround 200 MΩ (see Figure 4c). Figure 4d shows themeasured output voltage of BN NF-based f-PEHs whensubjected to bending deformation, with the displacementvarying from 1 to 5 mm at a fixed strain rate of 5 cm·s−1. Fromthese results, the values of the electric potential difference can

be improved by the degree of deformation. To investigate thestrain rate dependence of the f-PEHs on output performance,the produced output voltage peaks were recorded duringperiodic bending, with the strain rates ranging from 1 to 5 cm·s−1 at a consistent displacement of 5 mm (see Figure 4e).Because the rapid bending deformation enhances the chargesthat contribute to the output signals because of fast electronflows, the f-PEHs deformed by fast bending can providesuperior performance compared to the output generated byslow bending. We performed the mechanical durability test ofan f-PEH during 5000 repeat bending cycles with adisplacement of 5 mm at a strain rate of 5 cm·s−1. As shownin Figure 4f, the BN NF-based f-PEH is mechanically stableand maintains the amplitude of the electrical output peaks.

4. CONCLUSIONS

In summary, we adopted a mechanochemical exfoliationprocess to achieve 2D h-BN NF with an average lateral sizeof ∼0.82 μm and a thickness of ∼25 nm. To characterize theenergy harvesting ability of the synthesized single BN NF, weused the simple transfer and FIB-Pt deposition processes anddemonstrated the single BN NF on a flexible substrate, whichproduced alternate piezoelectric output signals of ∼50 mV and∼30 pA under repeated mechanical bendings. A piezoelectricrubber, by dispersing the BN NFs into an elastomer matrix,was inserted between two transparent electrode-coated flexiblesubstrates to fabricate f-PEHs: the harvested Voc and Iscreached up to ∼9 V and ∼200 nA, respectively. An effectiveoutput power was calculated at ∼0.3 μW by using load voltageand current values with various external resistors. Ourtechnology allows the precise characterization of energygeneration from piezoelectric nanostructures and provides aninnovative progress toward the realization of 2D piezoelectricnanomaterial-based f-PEHs.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b12187.

Figure 4. (a) Output voltage and current values measured from f-PEH as a function of an applied poling voltage. (b) Measured load voltage andcurrent under various external resistors in the range from 100 kΩ to 1 GΩ. (c) Effective output power calculated by multiplying load voltage andcurrent. (d,e) Open-circuit voltage of the f-PEH stressed by a bending machine with varied displacements (d) and strain rates (e). (f) Mechanicalstability test results of the fabricated f-PEH under repeat bending up to 5000 cycles.

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AFM topography image of the BN NFs and statisticaldata obtained from the selected 60 BN NFs (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: kipark@knu.ac.kr. Phone: +82-53-950-5564. Fax:+82-53-950-6559.ORCIDChang Kyu Jeong: 0000-0001-5843-7609Kwi-Il Park: 0000-0002-9140-6641NotesThe 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 Education (NRF-2019R1I1A2A01057073) and the Ministry of Science andICT (NRF-2018R1A4A1022260). This study was alsosupported by the Korean Nuclear R&D program organizedby the National Research Foundation of Korea (NRF) grantfunded by the Korea government (MSIT) (NRF-2017M2A8A4017220) and in part by the Korea AtomicEnergy Research Institute (KAERI) R&D program.

■ REFERENCES(1) Wang, Z. L.; Song, J. H. Piezoelectric Nanogenerators based onZinc Oxide Nanowire Arrays. Science 2006, 312, 242−246.(2) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Power Generation withLaterally Packaged Piezoelectric Fine Wires. Nat. Nanotechnol. 2008,4, 34−39.(3) Choi, M.-Y.; Choi, D.; Jin, M.-J.; Kim, I.; Kim, S.-H.; Choi, J.-Y.;Lee, S. Y.; Kim, J. M.; Kim, S.-W. Mechanically Powered TransparentFlexible Charge-Generating Nanodevices with Piezoelectric ZnONanorods. Adv. Mater. 2009, 21, 2185−2189.(4) Yang, R.; Qin, Y.; Li, C.; Zhu, G.; Wang, Z. L. ConvertingBiomechanical Energy into Electricity by a Muscle-Movement-DrivenNanogenerator. Nano Lett. 2009, 9, 1201−1205.(5) Wang, Z. L.; Wu, W. Nanotechnology-Enabled EnergyHarvesting for Self-Powered Micro-/Nanosystems. Angew. Chem.,Int. Ed. 2012, 51, 11700−11721.(6) Park, K.-I.; Jeong, C. K.; Kim, N. K.; Lee, K. J. StretchablePiezoelectric Nanocomposite Generator. Nano Convergence. 2016, 3,Article number 12. DOI: 10.1186/s40580-016-0072-z(7) Lim, J.; Jung, H.; Baek, C.; Hwang, G.-T.; Ryu, J.; Yoon, D.; Yoo,J.; Park, K.-I.; Kim, J. H. All-Inkjet-Printed Flexible PiezoelectricGenerator made of Solvent Evaporation Assisted BaTiO3 HybridMaterial. Nano Energy 2017, 41, 337−343.(8) Jeong, C. K.; Baek, C.; Kingon, A. I.; Park, K.-I.; Kim, S.-H.Lead-Free Perovskite Nanowire-Employed Piezopolymer for HighlyEfficient Flexible Nanocomposite Energy Harvester. Small 2018, 14,1704022.(9) Park, K.-I. Recent Progress in Flexible Energy HarvestingDevices based on Piezoelectric Nanomaterials. J. Korean PowderMetall. Inst. 2018, 25, 263−272.(10) Wu, J. M.; Xu, C.; Zhang, Y.; Wang, Z. L. Lead-FreeNanogenerator Made from Single ZnSnO3 Microbelt. ACS Nano2012, 6, 4335−4340.(11) Hou, T.-C.; Yang, Y.; Lin, Z.-H.; Ding, Y.; Park, C.; Pradel, K.C.; Chen, L.-J.; Lin Wang, Z. Nanogenerator based on Zinc BlendeCdTe Micro/Nanowires. Nano Energy 2013, 2, 387−393.(12) Wang, Z.; Hu, J.; Suryavanshi, A. P.; Yum, K.; Yu, M.-F. VoltageGeneration from Individual BaTiO3 Nanowires under PeriodicTensile Mechanical Load. Nano Lett. 2007, 7, 2966−2969.

(13) Moorthy, B.; Baek, C.; Wang, J. E.; Jeong, C. K.; Moon, S.;Park, K.-I.; Kim, D. K. Piezoelectric energy harvesting from a PMN-PT single nanowire. RSC Adv. 2017, 7, 260−265.(14) Huang, C.-T.; Song, J.; Lee, W.-F.; Ding, Y.; Gao, Z.; Hao, Y.;Chen, L.-J.; Wang, Z. L. GaN Nanowire Arrays for High-OutputNanogenerators. J. Am. Chem. Soc. 2010, 132, 4766−4771.(15) Xu, S.; Hansen, B. J.; Wang, Z. L. Piezoelectric-Nanowire-Enabled Power Source for Driving Wireless Microelectronics. Nat.Commun. 2010, 1, 93.(16) Xu, S.; Qin, Y.; Xu, C.; Wei, Y.; Yang, R.; Wang, Z. L. Self-Powered Nanowire Devices. Nat. Nanotechnol. 2010, 5, 366−373.(17) Koka, A.; Zhou, Z.; Sodano, H. A. Vertically Aligned BaTiO3

Nanowire Arrays for Energy Harvesting. Energy Environ. Sci. 2014, 7,288−296.(18) Hyeon, D. Y.; Park, K.-I. Vertically Aligned PiezoelectricPerovskite Nanowire Array on Flexible Conducting Substrate forEnergy Harvesting Applications. Adv. Mater. Technol. 2019, 4,1900228.(19) Michel, K. H.; Verberck, B. Theory of Elastic and PiezoelectricEffects in Two-Dimensional Hexagonal Boron Nitride. Phys. Rev. B:Condens. Matter Mater. Phys. 2009, 80, 224301.(20) Duerloo, K.-A. N.; Ong, M. T.; Reed, E. J. IntrinsicPiezoelectricity in Two-Dimensional Materials. J. Phys. Chem. Lett.2012, 3, 2871−2876.(21) Duerloo, K.-A. N.; Reed, E. J. Flexural ElectromechanicalCoupling: A Nanoscale Emergent Property of Boron Nitride Bilayers.Nano Lett. 2013, 13, 1681−1686.(22) Zhu, H.; Wang, Y.; Xiao, J.; Liu, M.; Xiong, S.; Wong, Z. J.; Ye,Z.; Ye, Y.; Yin, X.; Zhang, X. Observation of Piezoelectricity in Free-Standing Monolayer MoS2. Nat. Nanotechnol. 2014, 10, 151−155.(23) Song, X.; Hui, F.; Knobloch, T.; Wang, B.; Fan, Z.; Grasser, T.;Jing, X.; Shi, Y.; Lanza, M. Piezoelectricity in Two Dimensions:Graphene vs. Molybdenum Disulfide. Appl. Phys. Lett. 2017, 111,083107.(24) Lee, J.-H.; Park, J. Y.; Cho, E. B.; Kim, T. Y.; Han, S. A.; Kim,T.-H.; Liu, Y.; Kim, S. K.; Roh, C. J.; Yoon, H.-J.; Ryu, H.; Seung, W.;Lee, J. S.; Lee, J.; Kim, S.-W. Reliable Piezoelectricity in Bilayer WSe2for Piezoelectric Nanogenerators. Adv. Mater. 2017, 29, 1606667.(25) Guo, J.; Wen, R.; Liu, Y.; Zhang, K.; Kou, J.; Zhai, J.; Wang, Z.L. Piezotronic Effect Enhanced Flexible Humidity Sensing ofMonolayer MoS2. ACS Appl. Mater. Interfaces 2018, 10, 8110−8116.(26) Blonsky, M. N.; Zhuang, H. L.; Singh, A. K.; Hennig, R. G. AbInitioPrediction of Piezoelectricity in Two-Dimensional Materials.ACS Nano 2015, 9, 9885−9891.(27) Song, H.; Karakurt, I.; Wei, M.; Liu, N.; Chu, Y.; Zhong, J.; Lin,L. Lead Iodide Nanosheets for Piezoelectric Energy Conversion andStrain Sensing. Nano Energy 2018, 49, 7−13.(28) Mele, E. J.; Kral, P. Electric Polarization of HeteropolarNanotubes as a Geometric Phase. Phys. Rev. Lett. 2002, 88, 056803.(29) Jung, J. H.; Lee, M.; Hong, J.-I.; Ding, Y.; Chen, C.-Y.; Chou,L.-J.; Wang, Z. L. Lead-Free NaNbO3 Nanowires for a High OutputPiezoelectric Nanogenerator. ACS Nano 2011, 5, 10041−10046.(30) Park, K.-I.; Lee, M.; Liu, Y.; Moon, S.; Hwang, G.-T.; Zhu, G.;Kim, J. E.; Kim, S. O.; Kim, D. K.; Wang, Z. L.; Lee, K. J. FlexibleNanocomposite Generator Made of BaTiO3 Nanoparticles andGraphitic Carbons. Adv. Mater. 2012, 24, 2999−3004.(31) Park, K.-I.; Jeong, C. K.; Hwang, G.-T.; Lee, K. J. Flexible andLarge-Area Nanocomposite Generators Based on Lead ZirconateTitanate Particles and Carbon Nanotubes. Adv. Energy Mater. 2013, 3,1539−1544.(32) Baek, C.; Yun, J. H.; Wang, J. E.; Jeong, C. K.; Lee, K. J.; Park,K.-I.; Kim, D. K. A Flexible Energy Harvester based on a Lead-Freeand Piezoelectric BCTZ Nanoparticle-Polymer Composite. Nanoscale2016, 8, 17632−17638.(33) Lee, D.; Lee, B.; Park, K. H.; Ryu, H. J.; Jeon, S.; Hong, S. H.Scalable Exfoliation Process for Highly Soluble Boron NitrideNanoplatelets by Hydroxide-Assisted Ball Milling. Nano Lett. 2015,15, 1238−1244.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b12187ACS Appl. Mater. Interfaces 2019, 11, 37920−37926

37925

(34) Droth, M.; Burkard, G.; Pereira, V. M. Piezoelectricity in PlanarBoron Nitride via a Geometric Phase. Phys. Rev. B 2016, 94, 075404.(35) Moon, O. M.; Kang, B.-C.; Lee, S.-B.; Boo, J.-H. TemperatureEffect on Structural Properties of Boron Oxide Thin Films Depositedby MOCVD Method. Thin Solid Films 2004, 464-465, 164−169.(36) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilizationin Polymeric Composites with Improved Thermal and MechanicalProperties. Adv. Mater. 2009, 21, 2889−2893.(37) Falin, A.; Cai, Q.; Santos, E. J. G.; Scullion, D.; Qian, D.;Zhang, R.; Yang, Z.; Huang, S.; Watanabe, K.; Taniguchi, T.; Barnett,M. R.; Chen, Y.; Ruoff, R. S.; Li, L. H. Mechanical Properties ofAtomically Thin Boron Nitride and the Role of InterlayerInteractions. Nat. Commun. 2017, 8, 15815.(38) Michel, K. H.; Verberck, B. Phonon Dispersions andPiezoelectricity in Bulk and Multilayers of Hexagonal Boron Nitride.Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 115328.(39) Wu, W.; Wang, L.; Li, Y.; Zhang, F.; Lin, L.; Niu, S.; Chenet,D.; Zhang, X.; Hao, Y.; Heinz, T. F.; Hone, J.; Wang, Z. L.Piezoelectricity of Single-Atomic-Layer MoS2 for Energy Conversionand Piezotronics. Nature 2014, 514, 470−474.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b12187ACS Appl. Mater. Interfaces 2019, 11, 37920−37926

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