Nano Energy 22 (2016) 483–489

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Directional dependent piezoelectric effect in CVD grown monolayerMoS2 for flexible piezoelectric nanogenerators

Sung Kyun Kim a,1, Ravi Bhatia a,1, Tae-Ho Kim a, Daehee Seol a, Jung Ho Kimb, Hyun Kim b,Wanchul Seung a, Yunseok Kim a,n, Young Hee Lee b,n, Sang-Woo Kim a,c,n

a School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Koreab Department of Energy Science, Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University (SKKU),Suwon 440-746, Republic of Koreac SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

a r t i c l e i n f o

Article history:Received 2 February 2016Accepted 21 February 2016Available online 27 February 2016

Keywords:Molybdenum disulfideDirectional dependent piezoelectricityPiezoresponse force microscopyElectromechanical couplingNanogenerator

x.doi.org/10.1016/j.nanoen.2016.02.04655/& 2016 Elsevier Ltd. All rights reserved.

esponding authors.ail addresses: Yunseokkim@skku.edu (Y. Kim)g@skku.edu (Y.H. Lee), kimsw1@skku.edu (S.-ese authors contributed equally to this work

a b s t r a c t

Due to the interesting semiconducting and optical properties of transition metal dichalcogenides, theyhave received particular attention for novel electronics and optoelectronics. In addition it is expected thatpiezoelectric properties of two-dimensional (2D) layered materials are very useful to realize next gen-eration mechanically powered transparent flexible charge-generating devices. Here we report directionaldependent piezoelectric effects in chemical vapor deposition grown monolayer MoS2 for flexible pie-zoelectric nanogenerators (NGs). It was found that the output power obtained from the NG with thearmchair direction of MoS2 is about two times higher than that from the NG with the zigzag direction ofMoS2 under the same strain of 0.48% and the strain velocity of 70 mm/s. This study provides a new wayto effectively harvest mechanical energy using novel flexible piezoelectric NGs based on 2D semi-conducting piezoelectric MoS2 for powering low power-consuming electronics and realizing self-pow-ered sensors.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Considerable scientific efforts are being expanded towardsrealizing electronic components for transparent flexible self-powered electronic switches, skins, sensors, etc. Experimentalstudies on the physical properties of two-dimensional (2D) ma-terials have grown exponentially since 2D materials offer uniqueadvantages for use in such next-generation devices [1–5]. Varioussemiconducting 2D materials have been studied, including tran-sition metal dichalcogenides (TMDs) such as molybdenum dis-ulfide (MoS2), molybdenum diselenide (MoSe2), tungsten dis-elenide (WSe2), which are likely to bring breakthroughs in futureelectronic and optoelectronic devices [6–11]. The physical prop-erties of 2D MoS2 nanosheets have been actively explored parti-cularly as a result of their possible integration in both nano/micro-electromechanical devices and energy harvesting devices [4,7].Monolayer MoS2 has a direct band gap and high mobility [6,11]and has been used to successfully fabricate field-effect transistors[8,11–14] so it has emerged as an interesting complement to

,W. Kim)..

graphene in various semiconducting applications.In previous theoretical studies, most of 2D monolayer materials

may exhibit piezoelectric properties, unlike its bulk parent crystal[15–17]. Remarkably, the calculation of the piezoelectric coefficientfor monolayer MoS2 according to density-functional theory re-vealed that the monolayer structure exhibits a stronger piezo-electric coupling than the bulk wurtzite structured materials [15].Nevertheless, experimental evidence of the piezoelectricity of 2Dmonolayer MoS2 has not yet been sufficiently provided althoughvery recently few studies on the experimental observation of in-trinsic piezoelectric properties of MoS2 reported that the piezo-electricity from MoS2 only exists when there are an odd number oflayers in the 2D crystal [5,8]. Here we report directional dependentpiezoelectric effects in chemical vapor deposition (CVD) grownmonolayer MoS2 using lateral piezoresponse force microscopy(PFM) [18,19] measurements. In addition, it was found that thepiezoelectric power output from piezoelectric nanogenerators(NGs) fabricated with monolayer MoS2 is strongly dependent onthe MoS2 atomic orientation along either armchair or zigzag di-rection, which further confirms that the magnitude of the piezo-electric polarization in monolayer MoS2 significantly depends onthe atomic orientation axis of MoS2.

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2. Methods

2.1. Synthesis of monolayer MoS2 by CVD

Triangular-shaped monolayer MoS2 flakes has been synthe-sized in the conventional atmospheric pressure CVD method[20,21]. In detail, 5 mg of MoO3 (Sigma-Aldrich, 203815) and200 mg of solid S (Sigma-Aldrich, 344621) were loaded into thetube reactor which are at 210 °C (zone 1) and 780 °C (zone 2). As asynthesized template, SiO2 (300 nm)/Si wafer was faced-downabove MoO3 crucible and total growth time is 15 min includingramping up period for both zone 1 and 2. After cooling step, thetriangular monolayer MoS2 flakes were directly synthesized onSiO2/Si substrate.

2.2. PFM measurement

The atomic force microscopy (AFM)-based investigations werecarried out using an AFM (Park Systems, XE-100). The piezoelectricproperty of the monolayer MoS2 flake samples was confirmed byPFM equipped with non-conductive silicon tips (with about 3 N/mof spring constant) (Multi 75-G, Budget Sensors), operating incontact mode for imaging of topography and relative polarizationby PFM. A lock-in amplifier (Stanford Research, SR830) was used todetect the piezoresponse signal.

2.3. Electrical characterization measurement

The output voltage signals of MoS2 piezoelectric NGs weremeasured using a programmable electrometer (Keithley, 6514B)with 200 TΩ input impedance. A picoammeter (Keithley, 5-1/2digit Model 6485) was used for low-noise output current mea-surements, with an input impedance of 1 kΩ. A bending tester wasused to induce strain between the electrodes depending onbending radius.

Fig. 1. Atomic structure and image of a single-crystalline monolayer MoS2 flake. (a) Atomonolayer MoS2 flake on a SiO2/Si substrate with schematically overlapping lattice orienSiO2/Si substrate through a CVD process. (c) Piezoelectricity of the monolayer MoS2 usingon the monolayer MoS2. (d,e) Topography and friction force microscopy images of the m

3. Results and discussion

2D layered MoS2 is expected to exhibit piezoelectric effects dueto the non-centro-symmetric arrangement of the Mo and S atoms(Fig. 1a). Piezoelectricity in 2D MoS2 structures arises from thedevelopment of asymmetrical electrical dipoles that are inducedwhen the material is subjected to an external stress [22]. An op-tical microscope image in Fig. 1b shows that the present work isbased on CVD-grown triangular-shaped single-crystalline mono-layer MoS2 flakes [23–25]. Geometric features of the MoS2 causethe magnitude of the piezoelectric property to significantly de-pend on the crystallographic orientation of MoS2. In particular, ifwe measure the piezoelectric property in parallel to the armchairdirection of MoS2, the value of the piezoelectricity can be differentfrom that which is perpendicular to the zigzag direction of MoS2.However, the directional dependent piezoelectric properties of the2D layered material makes cannot be experimentally studied in astraightforward manner due to its atomic-scale thickness.

The 2D monolayer MoS2 was transferred onto a SiO2 (300 nmthickness)/Si substrate, and contact pads were deposited onto theindividual monolayer MoS2 by using standard electron beam li-thographic techniques. Although the tip of the AFM can usually actas the top electrode during PFM measurements, lateral measure-ment of the piezoresponse is difficult to measure reliably with aconsideration of the field distribution if an AFM tip is used as thetop electrode [26]. Furthermore, since the target material is thinon an atomic scale, the field concentration under the AFM tip cancause unexpected issues, such as water-mediated electrochemicalreactions or electrochemical reactions of the SiO2 layer under theMoS2 layer [27]. Thus we deposited two lateral electrodes onto theMoS2 in order to apply an electric field laterally through themonolayer MoS2.

We used a non-conductive AFM tip as a mechanical sensor todetect the lateral piezoresponse (Fig. 1c). As mentioned above, twolateral electrodes were used for applying electric field in order to

mic structure of the monolayer MoS2. (b) Optical microscope image of a triangulartation of Mo and S atoms. The monolayer MoS2 flake was directly synthesized on alateral PFM. Schematic image of the measurement configuration for the lateral PFMonolayer MoS2 on the SiO2/Si substrate, respectively.

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measure the lateral piezoresponse of the monolayer MoS2. Thus,the AFM tip should not be conductive because the AFM tip wasutilized to only measure the deformed strain value of MoS2 byapplying extrinsic bias. The external electric field was applied toone end of the monolayer MoS2 to conduct the PFM measurement,the data of the piezoresponse was recorded by scanning thesample with the AFM tip. Since the AFM tip is non-conductive, theelectric field distribution along the monolayer MoS2 is not dis-turbed by the AFM tip.

The contact pads width of approximately 5 mm ensures thehigh-quality pads when applying the electric field while providingan ample sample area for the PFM studies. Considering van derWaals interaction with a weak physical contact between MoS2 andSiO2 and the formation of wrinkles in the monolayer MoS2 flaketransferred on the other substrate [6,8], it is reasonable that thereis a room for a single monolayer MoS2 flake to be effectivelystretched or compressed by applying electrical field through thetwo lateral electrodes. Fig. 1d presents a topographical image ofthe MoS2 flakes after the lateral electrodes were deposited. Al-though the MoS2 monolayer is difficult to observe in the topo-graphical image, it can be clearly observed in the friction image(Fig. 1e). We also prepared another sample so that the electric fieldcould be applied in the zigzag direction of MoS2.

The monolayer MoS2 is hard to be distinguished from SiO2/Sisubstrate in the AFM topography image. Thus, friction force mi-croscopy was used to distinguish the monolayer MoS2 because ofdistinct friction difference between the MoS2 and the SiO2. In or-der to further observe the piezoelectric property of the monolayerMoS2, we carried out the lateral and vertical PFM measurements ofthe sample. As shown in Fig. 2b, the lateral PFM phase showed aclearly different contrast of MoS2 and SiO2 areas. In contrast, thevertical PFM phase did not show any distinguishable contrast aspresented in Fig. 2c because of absence in the vertical piezo-response for both materials. Mo atom and S atom shift much largerfor the A1g mode than for the E2g in monolayer MoS2 by extrinsicbias, indicating that the lateral PFM is an effective way to in-vestigate the piezoelectric property of the MoS2 monolayer.

MoS2 has a hexagonal crystal structure, and either Mo or Satoms can be accommodated along each side of the triangularmonolayer MoS2 flake [14]. Therefore, as previously noted, thepiezoelectric coefficient is expected to be dissimilar when it ismeasured along the “armchair” (Mo and S parallel) and “zigzag”(Mo and S in the same line) directions of the triangular monolayerMoS2 flake (Fig. 3a and b). Fig. 3c and d respectively shows thevariations in the piezoresponse as a function of the electric fieldapplied across the MoS2 samples with these two different devicegeometries. Fig. 3c shows the piezoresponse as a function of themagnitude of the voltage that is applied in the armchair directionfor the monolayer MoS2 and the α-quartz, revealing a distinctpiezoresponse from the monolayer MoS2 when compared to that

Fig. 2. Friction force and PFM phase images of the edge of the monolayer MoS2 on the(c) vertical PFM phase images of the MoS2/SiO2 boundary area which is blue-dotted rec

of the α-quartz, which increases as the applied voltage increases,indicating piezoelectric properties from the MoS2 monolayer. Thepiezoelectric coefficient can therefore be obtained from the slopeof the solid lines that represents the fitted linear equation becausethe piezoelectricity has a linear relation to the electric and me-chanical status. Based on the previous reports [28,29], it can besuggested that this force–distance curve can provide a method toestimate the lateral piezoelectric coefficient, d11. The slope ob-tained for MoS2 by fitting the data is 17.93�10�6 arbitrary unit/V,and that for α-quartz is 10.41�10�6. Since the d11 of the quartz isknown to be approximately 2.3 pm/V [28], it can be used to cali-brate the obtained piezoresponse considering piezoelectric tensor.Accordingly, the d11 of MoS2 in the armchair direction is calibratedinto 3.78 pm/V by using the ratio between the materials. In con-trast, the piezoresponse in the zigzag direction exhibited a lowerresponse, with a piezoelectric coefficient of 1.38 pm/V (Fig. 3d).The d11 measured for the 2D monolayer MoS2 flake is compara-tively larger than that for quartz in the case of the armchair di-rection while it is smaller in the zigzag direction. In fact, theseanisotropic piezoelectric properties can be understood by con-sidering the atomic arrangement of the triangular 2D MoS2 flakes.The d11 calibrated along the armchair direction for the monolayerMoS2 is 1.5 times greater than that of quartz [30]. The experi-mental findings obtained in this work are consistent with the re-sults of simulations [15] that were previously reported.

Therefore, the PFM results confirm that 1) MoS2 is piezoelectric,and the 2) piezoelectricity of MoS2 depends on the atomic orienta-tion axis. The same experimental process was used to confirm that adistinct piezoresponse was not observed in the bare SiO2 substratewith no piezoelectric property (Fig. S2), which further confirms thatthe experimental results reported in this work are reliable. The directpiezoresponse observations from the PFM experiments were furthervalidated using Raman measurements as well (Fig. S3). The piezo-electric effect of the monolayer MoS2 depending on the orientation isexplained by three-fold rotational symmetry of piezoresponse. Theexistence of the piezoelectricity in TMDs studied by the optical sec-ond-harmonic generation (SHG) has been reported [31]. In particular,the SHG intensity in the monolayer MoS2 shows the polarizationdepending on angle θ.

To further confirm the piezoelectric properties of the mono-layer MoS2 depending on the atomic orientation axis, we fabri-cated a monolayer MoS2-based flexible piezoelectric NG (Fig. 4a).Triangular shape monolayer MoS2 flakes were transferred on apolyethylene terephthalate as a flexible polymer substrate [32].The monolayer MoS2 flakes were connected with lateral electrodesvery carefully in order to clearly compare the power output fromthe piezoelectric NGs depending on the MoS2 atomic orientationalong either armchair or zigzag direction. Fig. 4c and d showsoptical images showing energy harvesting active regions with thearmchair and zigzag atomic orientations of the piezoelectric NGs

SiO2/Si substrate. (a) Friction force image of the monolayer MoS2. (b) Lateral andtangular region in (a).

Fig. 3. Piezoelectric coefficient (d11) according to the electric field applied in the direction of the monolayer MoS2 flake. (a and b) Lateral electrode configurations to measurethe piezoelectric coefficient (d11) according to the electric field applied in the direction of (a) armchair and (b) zigzag directions. (c and d) Corresponding piezoresponse forthe monolayer MoS2 in the (c) armchair and (d) zigzag directions. The solid lines represent the fitted linear equation.

Fig. 4. (a) Photographic image of the CVD grown monolayer MoS2 based flexible piezoelectric NG. (b) Top (upper) and cross-section (lower) atomic structures for themonolayer MoS2 strained by an external stress. (c and d) Optical images showing energy harvesting active regions with the armchair and zigzag atomic orientations of thepiezoelectric NGs respectively. (e–h) Power output from the piezoelectric NGs depending on the MoS2 atomic orientation along either armchair or zigzag direction. Thepiezoelectric power output performances were investigated by applying mechanical strain. (e and f) Voltage output signals from the NG by applying strain along armchairand zigzag directions, respectively. (g and h) Output currents of the NG by applying same strain along armchair and zigzag directions, respectively.

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respectively. The piezoelectric power output performances wereinvestigated by applying mechanical strain (Fig. 4e–h). The voltageand current output shown in Fig. 4e–h were obtained withbending strain of 0.48% at a frequency of 0.5 Hz. The measuredoutput voltage approached up to 20 mV (Fig. 4e) and the output

current was over 30 pA (Fig. 4g) from the NG with the armchairdirection of MoS2. On the other hand, the output voltage andcurrent are less than 10 mV and 20 pA, respectively, from the NGwith the zigzag direction of MoS2. This result suggests that ma-nipulation of the MoS2 atomic orientation along an armchair

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direction in a large scale is very critical to dramatically enhancepiezoelectric power output performance from single-crystallineMoS2 monolayer-based piezoelectric NGs.

Fig. 5. Output voltage obtained from the monolayer MoS2 NGs at a fixed velocity of70 mm/s as a function of applied strain. (a and b) Change of voltage output with thestrain applied along the armchair direction and the zigzag direction, respectively.

Fig. 6. Mechanical durability test of the output voltage (a) and output current (b) obtaineNG.

Moreover, we also investigated the power output change fromthe MoS2 NG as a function of the applied strain (Fig. 5) and theload resistance (Fig. S6). It was found that the output performanceis increased with increasing the applied strain, revealing typicalpiezoelectric power output behavior and good mechanical dur-ability of the flexible piezoelectric MoS2 NGs in this work. Asshown in Fig. S6, output voltage from the monolayer MoS2 NG as afunction of the external load resistance under the 0.48% strainalong the armchair direction is almost constant for a load re-sistance up to �1 MΩ. However, the output voltage is dramati-cally increased over 10 MΩ in the load resistance because an in-crease of the load resistance leads to a decrease of the outputcurrent due to the ohmic loss.

We further explored the output voltages and output currentswith continuous application of cycled compressive force to theMoS2 NG. Notably, there were no significant differences in theoutput voltages measured from the MoS2 NG over 7500 cycleswith bending strain of 0.42% at a frequency of 0.5, confirming thegood mechanical durability of our MoS2 NG in this work (Fig. 6).Again, ‘switching polarity’ tests were also conducted to confirmthat the measured output signals were generated from themonolayer MoS2 NG rather than from the measuring system (Fig.S7). The output signals were reversed when we reversed the po-larity of the voltage and current meters.

4. Conclusions

In conclusion, the unique directional dependent piezoelectric ef-fect of the CVD-grown triangular-shaped single-crystalline monolayerMoS2 flake was qualitatively studied by using lateral PFM. We suc-cessfully perform an experiment where the piezoelectric coefficient,d11, was measured, showing the anisotropic piezoresponse in thesingle-crystalline monolayer MoS2. It was found that the d11 of MoS2in the armchair direction is 3.78 pm/V, while the d11 of MoS2 in the

d from the MoS2 NG with continuous application of cycled compressive force to the

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zigzag direction is 1.38 pm/V, clearly revealing its distinct anisotropicpiezoelectric properties. In addition, flexible piezoelectric NGs weresuccessfully fabricated using the CVD-grown single-crystallinemonolayer MoS2 flakes. In accordance with the PFM result, the pie-zoelectric power output from the monolayer MoS2 NGs was stronglydependent on the MoS2 atomic orientation. It was found that theoutput power obtained from the NG with the armchair direction ofMoS2 is about two times higher than that from the NGwith the zigzagdirection of MoS2 under the same strain of 0.48% and the strain ve-locity of 70 mm/s. This study provides a new way to effectively har-vest mechanical energy using novel flexible piezoelectric NGs basedon 2D semiconducting piezoelectric MoS2 for powering low power-consuming devices and realizing self-powered electronics.

Acknowledgments

S. K. Kim and R. Bhatia contributed equally to this work. Thiswork was financially supported by the Center for Advanced Soft-Electronics as the Global Frontier Project (2013M3A6A5073177)and Institute for Basic Science Program (IBS-R011-D1) through theNational Research Foundation (NRF) of Korea Grant funded by theMinistry of Science, ICT & Future Planning.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.nanoen.2016.02.046.

References

[1] J.B. Oostinga, H.B. Heersche, X.L. Liu, A.F. Morpurgo, L.M.K. Vandersypen, Nat.Mater. 7 (2008) 151–157.

[2] S. Park, J. An, J.W. Suk, R.S. Ruoff, Small 6 (2010) 210–212.[3] Y.B. Zhang, T.T. Tang, C. Girit, Z. Hao, M.C. Martin, A. Zettl, M.F. Crommie, Y.

R. Shen, F. Wang, Nature 459 (2009) 820–823.[4] W.Z. Wu, L. Wang, Y.L. Li, F. Zhang, L. Lin, S.M. Niu, D. Chenet, X. Zhang, Y.

F. Hao, T.F. Heinz, J. Hone, Z.L. Wang, Nature 514 (2014) 470–474.[5] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi,

B.H. Hong, Nature 457 (2009) 706–710.[6] T. Eknapakul, P.D.C. King, M. Asakawa, P. Buaphet, R.H. He, S.K. Mo, H. Takagi,

K.M. Shen, F. Baumberger, T. Sasagawa, S. Jungthawan, W. Meevasana, NanoLett. 14 (2014) 1312–1316.

[7] J.G. Song, J. Park, W. Lee, T. Choi, H. Jung, C.W. Lee, S.H. Hwang, J.M. Myoung, J.H. Jung, S.H. Kim, C. Lansalot-Matras, H. Kim, ACS Nano 7 (2013) 11333–11340.

[8] A. Castellanos-Gomez, R. Roldan, E. Cappelluti, M. Buscema, F. Guinea, H.S.J. van der Zant, G.A. Steele, Nano Lett. 13 (2013) 5361–5366.

[9] B. Radisavljevic, A. Kis, Nat. Mater. 12 (2013) 815–820.[10] J.W. Jiang, Nanoscale 6 (2014) 8326–8333.[11] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat. Nanotechnol.

6 (2011) 147–150.[12] H. Qiu, T. Xu, Z.L. Wang, W. Ren, H.Y. Nan, Z.H. Ni, Q. Chen, S.J. Yuan, F. Miao, F.

Q. Song, G. Long, Y. Shi, L.T. Sun, J.L. Wang, X.R. Wang, Nat. Commun. 4 (2013)2642.

[13] W.J. Zhao, R.M. Ribeiro, M.L. Toh, A. Carvalho, C. Kloc, A.H.C. Neto, G. Eda, NanoLett. 13 (2013) 5627–5634.

[14] S. Najmaei, Z. Liu, W. Zhou, X.L. Zou, G. Shi, S.D. Lei, B.I. Yakobson, J.C. Idrobo, P.M. Ajayan, J. Lou, Nat. Mater. 12 (2013) 754–759.

[15] K.A.N. Duerloo, M.T. Ong, E.J. Reed, J. Phys. Chem. Lett. 3 (2012) 2871–2876.[16] S. Chandratre, P. Sharma, Appl. Phys. Lett. 100 (2012) 023114.[17] M.T. Ong, E.J. Reed, Acs Nano 6 (2012) 1387–1394.[18] A. Gruverman, O. Auciello, H. Tokumoto, Annu. Rev. Mater. Sci. 28 (1998)

101–123.[19] S.V. Kalinin, B.J. Rodriguez, S. Jesse, E. Karapetian, B. Mirman, E.A. Eliseev, A.

N. Morozovska, Annu. Rev. Mater. Res. 37 (2007) 189–238.[20] X. Ling, Y.H. Lee, Y.X. Lin, W.J. Fang, L.L. Yu, M.S. Dresselhaus, J. Kong, Nano Lett.

14 (2014) 464–472.[21] G.H. Han, N.J. Kybert, C.H. Naylor, B.S. Lee, J. Ping, J.H. Park, K. Kang, S.Y. Lee, Y.

H. Lee, R. Agarwal, A.T.C. Johnson, Nat. Commun. 6 (2015) 6128.[22] D.Y. Fu, J. Zhou, S. Tongay, K. Liu, W. Fan, T.J.K. Liu, J.Q. Wu, Appl. Phys. Lett. 103

(2013) 183105.[23] Y.H. Lee, L.L. Yu, H. Wang, W.J. Fang, X. Ling, Y.M. Shi, C.T. Lin, J.K. Huang, M.

T. Chang, C.S. Chang, M. Dresselhaus, T. Palacios, L.J. Li, J. Kong, Nano Lett. 13

(2013) 1852–1857.[24] Y.N. Liu, R. Ghosh, D. Wu, A. Ismach, R. Ruoff, K.J. Lai, Nano Lett. 14 (2014)

4682–4686.[25] A.M. van der Zande, P.Y. Huang, D.A. Chenet, T.C. Berkelbach, Y.M. You, G.

H. Lee, T.F. Heinz, D.R. Reichman, D.A. Muller, J.C. Hone, Nat. Mater. 12 (2013)554–561.

[26] F. Peter, A. Rudiger, R. Waser, Rev. Sci. Instrum. 77 (2006) 036103.[27] S. Jesse, B.J. Rodriguez, S. Choudhury, A.P. Baddorf, I. Vrejoiu, D. Hesse,

M. Alexe, E.A. Eliseev, A.N. Morozovska, J. Zhang, L.Q. Chen, S.V. Kalinin, Nat.Mater. 7 (2008) 209–215.

[28] Y. Kim, M. Alexe, E.K.H. Salje, Appl. Phys. Lett. 96 (2010) 032904.[29] Y. Kim, H. Han, Y. Kim, W. Lee, M. Alexe, S. Baik, J.K. Kim, Nano Lett. 10 (2010)

2141–2146.[30] J.H. Si, T. Mitsuyu, P.X. Ye, Y.Q. Shen, K. Hirao, Appl. Phys. Lett. 72 (1998)

762–764.[31] L.M. Malard, T.V. Alencar, A.P.M. Barboza, K.F. Mak, A.M. de Paula, Phys. Rev. B

87 (2013) 201401.[32] C.Y. Su, D.L. Fu, A.Y. Lu, K.K. Liu, Y.P. Xu, Z.Y. Juang, L.J. Li, Nanotechnology 22

(2011) 185309.

Sung Kyun Kim is a Ph.D. student under the super-vision of Prof. Sang-Woo Kim at School of AdvancedMaterials Science & Engineering, SungkyunkwanUniversity (SKKU). His research interests includeatomic force microscopy studies of piezoelectric, tri-boelectric and ferroelectric materials and character-ization of 2D materials.

Dr. Ravi Bhatia worked as a Postdoctoral Fellow withProf. Sang-Woo Kim at Sungkyunkwan University(SKKU), South Korea. He earned his doctorate degreefrom Department of Physics, Indian institute of Science(IISc), Bangalore in 2012. During Ph.D. tenure, heworked on the low temperature charge transport andmagnetic properties of iron-filled multiwall carbonnanotube (MWCNT), and MWCNT based compositesystems. His current research interests are focused ongrowth and applications of two dimensional materials.

Tae-Ho Kim is a Ph.D. student under the supervision ofProf. Sang-Woo Kim at School of Advanced MaterialsScience & Engineering, Sungkyunkwan University(SKKU). His research interests include synthesis 2Dmaterials such as graphene, h-BN and 2D materialsbased piezoelectric nanogenerators.

Daehee Seol is a Ph.D. student under the supervision ofProf. Yunseok Kim at School of Advanced MaterialsScience & Engineering, Sungkyunkwan university(SKKU). His research interests include scanning probemicroscopy based studies of electromechanical, ferro-electric, ionic phenomena at the nanoscale.

S.K. Kim et al. / Nano Energy 22 (2016) 483–489 489

Jung Ho Kim is a Ph.D. student under the supervisionof Prof. Young Hee Lee at Center for Integrated Na-nostructure Physics (CINAP) of Institute of Basic Science(IBS) and Department of Energy Science, Sungkyunk-wan University (SKKU). His research interests includefabrication and characterization of electrical devicesusing two-dimensional materials such as graphene,h-BN, and transition metal dichalcogenides.

Hyun Kim is a Ph.D. student under the supervision of Prof.Young Hee Lee at Center for Integrated NanostructurePhysics (CINAP) of Institute of Basic Science (IBS) and De-partment of Energy Science, Sungkyunkwan University(SKKU). His research interests include synthesis of twodimensional materials such as transition metal dichalco-genides and black phosphorus.

Wanchul Seung is a Ph.D. student under the super-vision of Prof. Sang-Woo Kim at School of AdvancedMaterials Science and Engineering, SungkyunkwanUniversity (SKKU). His research interests are fabrica-tions and characterizations of piezoelectric and tribo-electric nanogenerator energy harvesting and theirapplications in self-powered devices.

Dr. Yunseok Kim was born in Korea, 1979. He receivedthe M.S. and the Ph.D. degrees in Materials Science andEngineering from Korea Advanced Institute of Scienceand Technology (KAIST), Korea, in, respectively, 2004and 2007. From 2008 to 2010, he was awarded theHumboldt research fellowship from the Alexander vonHumboldt foundation which allowed him to work as apostdoctoral researcher at Max Planck Institute of Mi-crostructure Physics, Germany. Then, from 2011 to2012, he worked as a postdoctoral researcher at OakRidge National Laboratory, USA. Since 2012, YunseokKim is Assistant Professor at the School of Materials

Science and Engineering, Sungkyunkwan University

(SKKU), Korea. His research interests include scanning probe microscopy studies ofelectromechanical, ferroelectric, transport and ionic phenomena at the nanoscale.

Dr. Young Hee Lee is a director of the Center for In-tegrated Nanostructure Physics, Institute for Basic Sci-ence. He is also a Professor in the Department of EnergyScience and Department of Physics at SungkyunkwanUniversity, Korea. He received his B.Sc. degree in phy-sics from Chonbuk National University, Korea, and hisPh.D. degree in physics from Kent State University, USA.His research interests include exploration of un-precedented physical and chemical properties of 2Dlayered materials and carbon-based materials and theirapplications to electronic devices and energy storage.

Dr. Sang-Woo Kim is a Professor in School of AdvancedMaterials Science and Engineering at SungkyunkwanUniversity (SKKU). He received his Ph.D. from KyotoUniversity in Department of Electronic Science andEngineering in 2004. After working as a postdoctoralresearcher at Kyoto University and University of Cam-bridge, he spent 4 years as an assistant professor atKumoh National Institute of Technology. He joined theSchool of Advanced Materials Science and Engineering,SKKU Advanced Institute of Nanotechnology (SAINT) atSKKU in 2009. His recent research interests includepiezoelectric/triboelectric/pyroelectric nanogenerators,

hybrid energy harvesting/storage devices, flexible sen-

sors, etc. Now he is an Associate Editor of Nano Energy and an Executive BoardMember of Advanced Electronic Materials.