Nano Energy 58 (2019) 227–233

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Full paper Metal –polymer matrix hybrid layer for triboelectric nanogenerator T

Hyungseok Kanga,1, Hyoung Taek Kimb,1, Hwi Je Wooa, Han Kimb, Do Hwan Kimc, Sungjoo Leea, ⁎⁎ ⁎ SeongMin Kimb, Young Jae Songa, Sang-Woo Kima,b, , Jeong Ho Chod, a SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea b School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea c Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea d Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea

ARTICLE INFO ABSTRACT

Keywords: In this work, we studied the surface potential of a metal–polymer hybrid layer and its effect on the performance Triboelectric nanogenerator of a triboelectric nanogenerator (TENG). Ag (AgNWs) separately embedded in two different poly- Surface potential mers–one with a positive tribopotential and the other with a negative tribopotential–were prepared as model Kelvin probe force microscopy hybrid systems. The surface potentials of the hybrid system were systematically investigated by Kelvin probe Silver nanowire force microscopy. The results demonstrated that each component of the hybrid layer affected the other com- Hybrid ponent because of the difference in their work functions. The following two important findings were obtained. First, the surface potential of each polymer shifted drastically toward that of Ag and the surface potential of Ag shifted toward that of each polymer. Second, higher density of AgNWs led to higher Ag-induced charge density in the polymer, which consequently resulted in larger shift in the surface potential of the polymer. TENG per- formance measurements revealed that the tribopotential difference between the contact surfaces of the AgNW–polymer hybrid layer and the perfluoroalkoxy alkane (or Nylon) used as the top triboelectric layer governed the TENG performance. Our systematic investigation of the surface potential of a hybrid surface consisting of two materials with different surface potentials provides insight into the design of triboelectric layers for high-performance TENGs.

1. Introduction enhance the density of induced charges. The first factor is the effective contact area between the two triboelectric layers [25–30]. Considerable Environmental is a promising approach for ad- efforts have been devoted to developing one-dimensional (1D) and two- dressing global energy issues and for realizing self-powered operation dimensional (2D) micro/nanopatterned surface reliefs via various pat- of various electronic devices such as flexible displays, elastic circuits, terning techniques such as photolithography, soft lithography, e-beam and e-skin sensors [1–8]. Technologies for the conversion of environ- lithography, nanoimprinting, and nanoparticle deposition. The second mental energy into through mechanical sources such as factor is the triboelectric potential difference between the two tribo- wind, water flow, vibration, and human body motions have been de- electric layers [31–39]. The TENG performance is governed by the veloped [9–18]. Recently, triboelectric nanogenerators (TENGs) have choice of materials in the triboelectric contact pair, where the selection undergone rapid development as a technology for harvesting electricity of two different materials far apart in the triboelectric series is neces- through contact triboelectrification and sary for achieving a high performance. To date, various fluorinated [19–24]. The use of TENGs in practical applications necessitates that dielectric materials such as polyvinylidene difluoride (PVDF), poly- their output performance be as high as possible; their output perfor- tetrafluoroethylene (PTFE), and perfluoroalkoxy alkane (PFA) have mance is critically dependent on the density of charge induced on the been utilized as negative triboelectric layers, whereas metals such as surface of the triboelectric layer. Two factors can primarily be tuned to aluminum and copper have been widely employed as positive

⁎ Corresponding author at: Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea. ⁎⁎ Corresponding author at: SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea. E-mail addresses: [email protected] (S.-W. Kim), [email protected] (J.H. Cho). 1 H. Kang and H. T. Kim contributed equally this work. https://doi.org/10.1016/j.nanoen.2019.01.046 Received 28 September 2018; Received in revised form 1 January 2019; Accepted 15 January 2019 Available online 15 January 2019 2211-2855/ © 2019 Published by Elsevier Ltd. H. Kang et al. Nano Energy 58 (2019) 227–233 triboelectric layers [40–43]. Kelvin probe force microscopy (KPFM), completely embedded in the polymer matrix (Figs. 1b and 1c). The which measures the contact potential difference between the probe tip height profile obtained from the AFM images indicated that the surface and the sample surface, has been demonstrated to be a powerful tech- of the AgNW-embedded polymer film was significantly flattened after nique for fundamental analysis of the electrostatic potential properties embedding: the root-mean-square roughness (Rrms) decreased from during contact electrification [44–51]. The relationship between the 25.4 nm to 3.1 nm. The prepared model system of a hybrid film com- electrostatic surface potential and triboelectric charges can be under- prising two materials with different tribopotentials was used for sys- stood by comparing the surface potential values obtained by KPFM tematically investigating the surface potentials and their contribution measurements. to the TENG performance. Fig. 1d shows the schematic device structure Ag nanowires (AgNWs) have attracted much attention as conductive of contact-separation-mode TENGs. AgNWs embedded in either of two electrodes owing to their remarkable flexibility and stretchability that different polymers (PVC or PMMA) were utilized as the bottom tribo- result from the formation of a percolation network. In addition, AgNWs electric layer in the TENGs. Nylon (with a strong positive triboelectric can be simply deposited by solution-coating methods such as spin potential) attached to an Al electrode or PFA (with a strong negative coating, dip coating, spray coating, and Meyer-rod coating [52–54]. triboelectric potential) attached to an Al electrode was utilized as the However, the weak adhesion between AgNWs and the substrate results top triboelectric layer in the TENGs. in delamination of the AgNWs from the substrate during TENG opera- In order to understand the electronic influence of the AgNWs in the tion. Very recently, our group proposed an embedded structure of polymer matrix, 2D mapping of the surface potentials of the AgNWs in a polymer matrix for the fabrication of mechanically stable AgNW–polymer hybrid surface was performed via noncontact-mode TENGs [55]. In the present work, we systematically studied a KPFM. For all the KPFM measurements, the same Pt tip was utilized to AgNW–polymer hybrid structure by KPFM and evaluated the relation- ensure that the measured values were referenced to a common energy ship between the electrostatic surface potential of the hybrid surface level. Six films were prepared in total: a bare PVC film, AgNW–PVC and the TENG performance. As model hybrid systems, AgNWs with two films with two different AgNW densities, AgNW–PMMA films with two different areal factors were embedded in polymer matrixes of poly- different AgNW densities, and a bare PMMA film. Fig. 2a shows vinylchloride (PVC), having a negative surface potential, and poly 2 μm×2μm maps of the surface potential difference of the AgNW–- (methyl methacrylate) (PMMA), having a positive surface potential. polymer hybrid films, which was referenced to Pt. The bare PVC film KPFM results showed that each of the two components of the hybrid showed a negative surface potential, whereas the bare PMMA film system affected the other component because of the difference in their showed a positive surface potential. These results indicated that the work functions. For example, the surface potentials of the polymer surface potential of the Pt tip was located between those of PVC and matrixes shifted toward that of Ag (that is, the shift directions of the PMMA. The hybrid films showed a distinct color contrast between the surface potentials of PVC and PMMA relative to the surface potential of AgNW region and the polymer region. The brighter region in the images Ag were positive and negative, respectively). Additionally, in each corresponds to a location with a more positive surface potential in the hybrid system, the surface potential of Ag shifted toward that of the film, whereas the darker region corresponds to that with a more ne- polymer matrix. In the hybrid system with a larger number of AgNWs, gative potential. In particular, in the AgNW–PVC film, the AgNW region more charges were transferred from the AgNWs to the polymer matrix, was brighter than the PVC region, which indicated that the surface which resulted in a larger shift in the surface potential of the polymer potential of Ag was higher than that of PVC. In contrast, in the matrix toward that of Ag. The observed directions of surface potentials AgNW–PMMA film, the AgNW region was darker than the PMMA re- were closely related to the device performance of TENGs. This sys- gion, which meant that the surface potential of Ag was lower than that tematic investigation of the surface potential of a hybrid surface con- of PMMA. This difference in the relative surface potentials was closely sisting of two materials with different surface potentials provides in- related to the electron-withdrawing characteristics of PVC and the sight into the design of triboelectric layers for high-performance electron-donating characteristics of PMMA. TENGs. More detailed information about the surface potentials of the hybrid films was obtained from a quantitative analysis of the colors in the 2. Results and discussion KPFM images. Fig. 2b shows the histogram of the surface potential obtained from the total area of all the KPFM images in Fig. 2a. In this A schematic of the fabrication procedure of the AgNW–polymer histogram, the surface potential difference referenced to Pt was plotted hybrid layer for application as an electrode in a TENG is shown in on the x-axis and the counts were plotted on the y-axis. The peaks of the Fig. 1a. A large-area, uniform AgNW network film was deposited onto a hybrid films were deconvoluted by using a sum of Lorentzian–Gaussian hydrophobic octadecyltrichlorosilane (ODTS)-treated glass substrate by functions. The red peak corresponded to the signal originating from the the Meyer-rod coating method. Two Meyer rods (#7 and #14) were AgNWs, whereas the green and blue peaks corresponded to the signals utilized to control the deposition density of the AgNWs (Figs. S1 and originating from PVC and PMMA, respectively. The surface potential of S2). PMMA, having positive tribopotential relative to Ag, and PVC, bare PVC was measured to be around − 2.51 V, but it shifted sig- having negative tribopotential relative to Ag, were selected as model nificantly to the positive direction when PVC came into contact with polymers for embedding the AgNWs. When the prepared viscous Ag, whose surface potential was higher than that of PVC. At the same polymer solution was spin-coated onto the AgNW/ODTS-treated glass time, the surface potential of Ag (+0.01 V, see Fig. S3) shifted slightly substrate, the solution filled the gaps among the AgNWs. The poly to the negative direction. Unlike in the case of the AgNW–PVC hybrid (ethylene terephthalate) (PET) film was then laminated using two films, the surface potential of PMMA (+3.45 V) shifted negatively after compression rollers before solvent drying. During evaporation of the PMMA came into contact with Ag, whose surface potential was lower solvent, the AgNWs were embedded and mechanically interlocked in than that of PMMA. From the KPFM results of the hybrid systems with the penetrated polymer solution. Both the polymer concentration and two different AgNW densities, it was found that the surface potential of the lamination pressure were optimized to adjust the thickness of the PVC increased from − 0.08 V to − 0.06 V as the areal factor the AgNWs embedding polymer layer to around 5 μm. Finally, the PET fi lm con- increased from 0.23 to 0.55 and the surface potential of PMMA in- taining the AgNW-embedded polymer matrix was peeled off from the creased from + 0.18 V to + 0.19 V as the areal factor of the AgNWs mother ODTS-treated glass substrate. Both the mechanical interlocking decreased from 0.54 to 0.42. Fig. 2c summarizes the change in the of AgNWs in the polymer matrix and the weak adhesion between ODTS surface potential induced by the electrostatic interaction between the and the AgNWs resulted in clean and complete delamination of the PET AgNWs and the polymer matrix on the hybrid surface. film from the ODTS substrate. Scanning electron microscopy (SEM) and From the histograms of the surface potential distributions, a sche- (AFM) images confirmed that the AgNWs were matic energy-band diagram of the hybrid films was proposed, as shown

228 H. Kang et al. Nano Energy 58 (2019) 227–233

Fig. 1. (a) Schematic of fabrication procedure of AgNW–polymer hybrid triboelectric layer. (b) SEM and (c) AFM images of as-coated AgNW films (left) and AgNW films embedded in polymer matrix (right). (d) Schematic device structure of TENGs based on PFA (or nylon) top triboelectric layer and AgNW–PMMA (or AgNW–PVC) bottom triboelectric layer. The lower panel shows the chemical structures of PMMA and PVC used in this study. in Fig. 3a. The surface potential variation could be understood from the shows schematic structures that explain the effect of the areal factor of dipole formation caused by the electron transfer process at the AgNWs on the charge transfer between them and the polymer matrix. AgNW–polymer interface. Because of the higher of Ag When more AgNWs were embedded in the polymer matrix, the density than of PVC in the AgNW–PVC hybrid film, the electrons in Ag were of charges transferred from the AgNWs to the polymer matrix was transferred to PVC, which caused the other side of Ag to become more higher. This resulted in a larger shift in the surface potential of the positive; this consequently increased the surface potential of PVC. In polymer matrix. the AgNW–PMMA hybrid film, however, the electrons in PMMA were Next, the relationship between the surface potentials of the transferred to Ag because of the higher work function of PMMA than of AgNW–polymer hybrid films and the TENG performance was in- Ag; this consequently decreased the surface potential of PMMA. Fig. 3b vestigated. PFA, having a strong negative triboelectric potential, or

Fig. 2. (a) KPFM images and (b) histograms of measured surface potentials of bare PVC film, AgNW–PVC films with two different areal factors of AgNWs, AgNW–PMMA films with two different areal factors of AgNWs, and bare PMMA film. (c) Variation of surface potentials of PVC, PMMA, and Ag as a function of areal factor of AgNWs.

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Fig. 3. Schematic illustration and band structures ex- plaining charge transfer between AgNWs and polymer matrix on hybrid surface. The blue region represents the polymer with a negative tribopotential whereas the red region represents the polymer with a positive tribopoten- tial, relative to Ag ((a) effect of polymer matrix on charge transfer and (b) effect of AgNW density on charge transfer).

Fig. 4. (a) Schematic diagram illustrating operation of contact-separation-mode TENGs with AgNW–polymer bottom triboelectric layer and PFA top triboelectric layer. (b) Output voltage and current of TENGs with AgNW–polymer/PFA configuration. (c) Maximum output voltage and current extracted from (b) as functions of surface potential. (d) Schematic diagram illustrating operation of contact-separation-mode TENGs with AgNW–polymer bottom triboelectric layer and nylon top triboelectric layer. (e) Output voltage and current of TENGs with AgNW–polymer/nylon configuration. (f) Maximum output voltage and current extracted from (e) as functions of surface potential. nylon, having a strong positive triboelectric potential, was utilized as performance of this TENG device. The device showed a negative vol- the top triboelectric layer. AgNWs with two different areal factors that tage/current output during contact, but it showed a positive voltage/ were embedded in either PVC or PMMA served as the bottom tribo- current output during separation because PFA was positioned on the electric layer. Fig. 4a shows a schematic of a TENG structure with the negative side relative to the AgNW–polymer hybrid film in the tribo- AgNW–polymer hybrid film as the bottom triboelectric layer and PFA as electric series. Fig. 4c summarizes the maximum voltage/current output the top triboelectric layer. Fig. 4b shows the voltage/current output as a function of the surface potential of the AgNW–polymer hybrid film.

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The TENG output performance increased as the surface potential in- respectively. Specifically, Figs. 5a and 5b show results of three-di- creased from − 0.07 V to + 0.19 V. For example, an output voltage of mensional (3D) numerical simulation of the potential distribution of 64 V and an output current of 4.9 μA were obtained at the surface po- TENGs with the AgNW–polymer/PFA and AgNW–polymer/nylon con- tential of − 0.07 V, and these values improved to 145 V and 9.8 μA, figurations, respectively. The AgNW network embedded in the polymer respectively, at the surface potential of + 0.19 V. A higher surface matrix was simply described as a mesh structure. The relationship be- potential of the hybrid film resulted in a larger triboelectric potential tween the simulated Voc and the surface potential of the AgNW–po- difference when PFA was used as the top triboelectric layer. This larger lymer hybrid surface was consistent with the measured TENG perfor- triboelectric potential difference resulted in a better TENG perfor- mance (see Fig. 2). Fig. 5c shows the simulated Voc as a function of the mance. Next, nylon, having a strong negative triboelectric potential, surface potential of the hybrid films. The Voc values of the TENGs were was used as the top triboelectric layer in the TENG, as shown in Fig. 4d. strongly related to the differences in the triboelectric potentials of Fig. 4e plots the voltage/current output of the TENG with nylon as the contact surfaces, where these differences were, in turn, dependent on top triboelectric layer. The direction of the voltage/current output of how far apart the contacting materials were located in the triboelectric the nylon TENGs was opposite to that of the PFA TENGs. A positive series. The simulated Voc of the TENG with the PFA top triboelectric output was observed in the contact mode and a negative output was layer increased with an increase in surface potential of the hybrid observed in the separation mode because nylon was positioned on the surface; this trend was attributed to the fact that the strong negative positive side relative to the hybrid film in the triboelectric series. The tribopotential of PFA resulted in a larger tribopotential difference with voltage/current output of the TENGs decreased as the surface potential the hybrid film having a higher surface potential. In contrast, the si- of the hybrid film increased (Fig. 4f). A lower surface potential of the mulated Voc of the TENG with the nylon top triboelectric layer showed hybrid film resulted in a larger triboelectric potential difference when the opposite tendency: Voc decreased with an increase in surface po- the top triboelectric layer was nylon, and this consequently led to a tential of the hybrid film. A lower surface potential of the hybrid film better TENG performance. As a result, the TENG performance could be resulted in a larger tribopotential difference between this film and the deterministically controlled by varying the type of polymer matrix and nylon top triboelectric layer with a strong positive tribopotential. The the areal factors of AgNWs embedded in the polymer matrix. FEM simulation results further supported the conclusion that a larger To further investigate the effect of the surface potential of the tribopotential difference between the contact surfaces resulted in a AgNW–polymer hybrid film on the TENG output, we performed a finite higher triboelectric potential. Consequently, it can be deduced that element method (FEM) simulation using the COMSOL Multiphysics rational design of a triboelectric layer is crucial to enhancing the software. Under the open-circuit condition, the output voltage (Voc)of electrical performance of TENGs. the contact-separation-mode TENG can be expressed as Voc = σ·d/ε0, where σ is the surface charge density, d is the gap between two tribo- 3. Conclusion electric layers, and ε0 is the vacuum permittivity [56–60]. Voc and σ of the TENG was successfully calculated as shown in Fig. 5 and Fig. S4, In conclusion, we systematically studied the surface potential of

Fig. 5. (a), (b) Results of 3D numerical simulation of potential distribution of TENGs with (a) AgNW–polymer/PFA and (b) AgNW–polymer/nylon configurations. (c) Comparison of simulated and measured TENG potentials as functions of surface potential for AgNW–polymer/PFA (left) and AgNW–polymer/nylon (right) con- figurations.

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AgNW–polymer hybrid films by both KPFM and numerical simulations. Appendix A. Supporting information AgNWs with two different areal densities were embedded in either of two different polymers: PVC or PMMA. Because of the work function Supplementary data associated with this article can be found in the difference between the polymer and the AgNWs, the surface potential of online version at doi:10.1016/j.nanoen.2019.01.046. PVC shifted positively whereas that of PMMA shifted negatively upon the embedding of the AgNWs. A higher density of AgNWs embedded in References the polymer matrix resulted in the transfer of more charges from the AgNWs to the polymer matrix. The surface potential of the [1] D.H. Ho, R. Song, Q. Sun, W.-H. Park, S.Y. Kim, C. Pang, D.H. Kim, S.-Y. Kim, J. Lee, AgNW–polymer hybrid film could be modulated deterministically by J.H. Cho, ACS Appl. Mater. Interfaces 9 (2017) 44678–44686. [2] H. Kang, I. Kang, J. Han, J.B. Kim, D.Y. Lee, S.M. Cho, J.H. Cho, J. Phys. Chem. 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[44] N. Knorr, S. Vinzelberg, Microsc. Microanal. 26 (2012) 7–12. Hyoung Taek Kim received his BS in the Department of [45] K.-E. Byun, Y. Cho, M. Seol, S. Kim, S.-W. Kim, H.-J. Shin, S. Park, S. Hwang, ACS Advanced Materials Science and Engineering from SKKU in ’ Appl. Mater. Interfaces 8 (2016) 18519–18525. 2016. Now he is Ph.D. candidate in Sang-Woo Kim s group [46] J. Liu, A. Goswami, K. Jiang, F. Khan, S. Kim, R. McGee, Z. Li, Z. Hu, J. Lee, at Advanced Material Science and Engineering from SKKU. T. Thundat, Nat. Nanotechnol. 13 (2018) 112. His research interest includes triboelectric nanogenerators [47] N. Balke, P. Maksymovych, S. Jesse, I.I. Kravchenko, Q. Li, S.V. Kalinin, ACS Nano 8 and 2D nanomaterials. (2014) 10229–10236. [48] S.-H. Shin, Y.E. Bae, H.K. Moon, J. Kim, S.-H. Choi, Y. Kim, H.J. Yoon, M.H. Lee, J. Nah, ACS Nano 11 (2017) 6131–6138. [49] Y.S. Zhou, S. Wang, Y. Yang, G. Zhu, S. Niu, Z.-H. Lin, Y. Liu, Z.L. Wang, Nano Lett. 14 (2014) 1567–1572. [50] L. Vieira, F. Lucas, S. Fisssmer, L. dos Santos, M. Massi, P. Leite, C. Costa, E. Lanzoni, R. Pessoa, H. Maciel, Surf. Coat. Technol. 260 (2014) 205–213. [51] J. Liu, M. Miao, K. Jiang, F. Khan, A. Goswami, R. McGee, Z. Li, L. Nguyen, Z. Hu, J. Lee, Nano Energy 48 (2018) 320–326. Prof. Sang-Woo Kim is a professor in the Department of [52] V. Scardaci, R. Coull, P.E. Lyons, D. Rickard, J.N. Coleman, Small 7 (2011) Advanced Materials Science and Engineering at SKKU. His 2621–2628. recent research interest focuses on piezoelectric/tribo- [53] H. Kang, S.-J. Song, Y.E. Sul, B.-S. An, Z. Yin, Y. Choi, L. Pu, C.-W. Yang, Y.S. Kim, electric nanogenerators, sensors, and photovoltaics using S.M. Cho, ACS Nano 12 (2018) 4894–4902. nanomaterials. He has published over 200 peer-reviewed [54] H. Kang, Y. Kim, S. Cheon, G.-R. Yi, J.H. Cho, ACS Appl. Mater. Interfaces 9 (2017) papers and holds over 80 domestic/international patents. 30779–30785. Now, he is a director of SAMSUNG-SKKU /2D [55] H. Kang, H. Kim, S. Kim, H.J. Shin, S. Cheon, J.H. Huh, D.Y. Lee, S. Lee, S.W. Kim, Research Center and leading National Research Laboratory J.H. Cho, Adv. Funct. Mater. 26 (2016) 7717–7724. for Next Generation Hybrid Energy Harvester. He is cur- [56] B. Yang, W. Zeng, Z.-H. Peng, S.-R. Liu, K. Chen, X.-M. Tao, Adv. Energy Mater. 6 rently serving as an Associate Editor of Nano Energy and an (2016) 1600505. Executive Board Member of Advanced Electronic Materials. [57] S. Niu, Y. Liu, S. Wang, L. Lin, Y.S. Zhou, Y. Hu, Z.L. Wang, Adv. Funct. Mater. 24 (2014) 3332–3340. [58] S. Niu, S. Wang, L. Lin, Y. Liu, Y.S. Zhou, Y. Hu, Z.L. Wang, Energy Environ. Sci. 6 (2013) 3576–3583. – [59] S. Niu, Z.L. Wang, Nano Energy 14 (2015) 161 192. Prof. Jeong Ho Cho obtained his BS in chemical en- [60] L. Polak, R.J. Wijngaarden, Phys. Rev. B 93 (2016) 195320. gineering from Sogang University in 2001 and his MS and Ph.D. in Chemical Engineering from POSTECH in 2006. He was a postdoctoral researcher in Department of Chemical Hyungseok Kang received his BS in the Department of Engineering and Materials Science at University of Polymer Engineering from Sungkyunkwan University Minnesota (2006–2008) and then joined as a faculty at (SKKU) in 2014. Now, he is M.Sc.-Ph.D. combined student Soongsil University (2008–2012) and Sunkyunkwan in Jeong Ho Cho’s group at SKKU Advanced Institute of University (2012–2018). He now is a professor at Yonsei Nanotechnology (SAINT) from SKKU. His research interest University with an appointment in Department of Chemical includes silver nanowires and its application. and Biomolecular Engineering. His research interests in- clude organic electronic devices (transistor, memory, and sensor) and 2-dimentional nanomaterials.

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