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Nano Energy

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Metal nanowire–polymer matrix hybrid layer for triboelectricnanogenerator

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 Koreab School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Koreac Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of KoreadDepartment of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea

A R T I C L E I N F O

Keywords:Triboelectric nanogeneratorSurface potentialKelvin probe force microscopySilver nanowireHybrid

A B S T R A C T

In this work, we studied the surface potential of a metal–polymer hybrid layer and its effect on the performanceof a triboelectric nanogenerator (TENG). Ag nanowires (AgNWs) separately embedded in two different poly-mers–one with a positive tribopotential and the other with a negative tribopotential–were prepared as modelhybrid systems. The surface potentials of the hybrid system were systematically investigated by Kelvin probeforce microscopy. The results demonstrated that each component of the hybrid layer affected the other com-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 Agshifted toward that of each polymer. Second, higher density of AgNWs led to higher Ag-induced charge densityin 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 theAgNW–polymer hybrid layer and the perfluoroalkoxy alkane (or Nylon) used as the top triboelectric layergoverned the TENG performance. Our systematic investigation of the surface potential of a hybrid surfaceconsisting of two materials with different surface potentials provides insight into the design of triboelectriclayers for high-performance TENGs.

1. Introduction

Environmental energy harvesting is a promising approach for ad-dressing global energy issues and for realizing self-powered operationof various electronic devices such as flexible displays, elastic circuits,and e-skin sensors [1–8]. Technologies for the conversion of environ-mental energy into electricity through mechanical sources such aswind, water flow, vibration, and human body motions have been de-veloped [9–18]. Recently, triboelectric nanogenerators (TENGs) haveundergone rapid development as a technology for harvesting electricitythrough contact triboelectrification and electrostatic induction[19–24]. The use of TENGs in practical applications necessitates thattheir output performance be as high as possible; their output perfor-mance is critically dependent on the density of charge induced on thesurface of the triboelectric layer. Two factors can primarily be tuned to

enhance the density of induced charges. The first factor is the effectivecontact area between the two triboelectric layers [25–30]. Considerableefforts have been devoted to developing one-dimensional (1D) and two-dimensional (2D) micro/nanopatterned surface reliefs via various pat-terning techniques such as photolithography, soft lithography, e-beamlithography, nanoimprinting, and nanoparticle deposition. The secondfactor is the triboelectric potential difference between the two tribo-electric layers [31–39]. The TENG performance is governed by thechoice of materials in the triboelectric contact pair, where the selectionof two different materials far apart in the triboelectric series is neces-sary for achieving a high performance. To date, various fluorinateddielectric materials such as polyvinylidene difluoride (PVDF), poly-tetrafluoroethylene (PTFE), and perfluoroalkoxy alkane (PFA) havebeen utilized as negative triboelectric layers, whereas metals such asaluminum and copper have been widely employed as positive

https://doi.org/10.1016/j.nanoen.2019.01.046Received 28 September 2018; Received in revised form 1 January 2019; Accepted 15 January 2019

⁎ 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: kimsw1@skku.edu (S.-W. Kim), jhcho94@yonsei.ac.kr (J.H. Cho).

1 H. Kang and H. T. Kim contributed equally this work.

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Available online 15 January 20192211-2855/ © 2019 Published by Elsevier Ltd.

T

triboelectric layers [40–43]. Kelvin probe force microscopy (KPFM),which measures the contact potential difference between the probe tipand the sample surface, has been demonstrated to be a powerful tech-nique for fundamental analysis of the electrostatic potential propertiesduring contact electrification [44–51]. The relationship between theelectrostatic surface potential and triboelectric charges can be under-stood by comparing the surface potential values obtained by KPFMmeasurements.

Ag nanowires (AgNWs) have attracted much attention as conductiveelectrodes owing to their remarkable flexibility and stretchability thatresult from the formation of a percolation network. In addition, AgNWscan be simply deposited by solution-coating methods such as spincoating, dip coating, spray coating, and Meyer-rod coating [52–54].However, the weak adhesion between AgNWs and the substrate resultsin delamination of the AgNWs from the substrate during TENG opera-tion. Very recently, our group proposed an embedded structure ofAgNWs in a polymer matrix for the fabrication of mechanically stableTENGs [55]. In the present work, we systematically studied aAgNW–polymer hybrid structure by KPFM and evaluated the relation-ship between the electrostatic surface potential of the hybrid surfaceand the TENG performance. As model hybrid systems, AgNWs with twodifferent areal factors were embedded in polymer matrixes of poly-vinylchloride (PVC), having a negative surface potential, and poly(methyl methacrylate) (PMMA), having a positive surface potential.KPFM results showed that each of the two components of the hybridsystem affected the other component because of the difference in theirwork functions. For example, the surface potentials of the polymermatrixes shifted toward that of Ag (that is, the shift directions of thesurface potentials of PVC and PMMA relative to the surface potential ofAg were positive and negative, respectively). Additionally, in eachhybrid system, the surface potential of Ag shifted toward that of thepolymer matrix. In the hybrid system with a larger number of AgNWs,more charges were transferred from the AgNWs to the polymer matrix,which resulted in a larger shift in the surface potential of the polymermatrix toward that of Ag. The observed directions of surface potentialswere closely related to the device performance of TENGs. This sys-tematic investigation of the surface potential of a hybrid surface con-sisting of two materials with different surface potentials provides in-sight into the design of triboelectric layers for high-performanceTENGs.

2. Results and discussion

A schematic of the fabrication procedure of the AgNW–polymerhybrid layer for application as an electrode in a TENG is shown inFig. 1a. A large-area, uniform AgNW network film was deposited onto ahydrophobic octadecyltrichlorosilane (ODTS)-treated glass substrate bythe Meyer-rod coating method. Two Meyer rods (#7 and #14) wereutilized to control the deposition density of the AgNWs (Figs. S1 andS2). PMMA, having positive tribopotential relative to Ag, and PVC,having negative tribopotential relative to Ag, were selected as modelpolymers for embedding the AgNWs. When the prepared viscouspolymer solution was spin-coated onto the AgNW/ODTS-treated glasssubstrate, the solution filled the gaps among the AgNWs. The poly(ethylene terephthalate) (PET) film was then laminated using twocompression rollers before solvent drying. During evaporation of thesolvent, the AgNWs were embedded and mechanically interlocked inthe penetrated polymer solution. Both the polymer concentration andthe lamination pressure were optimized to adjust the thickness of theembedding polymer layer to around 5 μm. Finally, the PET film con-taining the AgNW-embedded polymer matrix was peeled off from themother ODTS-treated glass substrate. Both the mechanical interlockingof AgNWs in the polymer matrix and the weak adhesion between ODTSand the AgNWs resulted in clean and complete delamination of the PETfilm from the ODTS substrate. Scanning electron microscopy (SEM) andatomic force microscopy (AFM) images confirmed that the AgNWs were

completely embedded in the polymer matrix (Figs. 1b and 1c). Theheight profile obtained from the AFM images indicated that the surfaceof the AgNW-embedded polymer film was significantly flattened afterembedding: the root-mean-square roughness (Rrms) decreased from25.4 nm to 3.1 nm. The prepared model system of a hybrid film com-prising two materials with different tribopotentials was used for sys-tematically investigating the surface potentials and their contributionto the TENG performance. Fig. 1d shows the schematic device structureof contact-separation-mode TENGs. AgNWs embedded in either of twodifferent polymers (PVC or PMMA) were utilized as the bottom tribo-electric layer in the TENGs. Nylon (with a strong positive triboelectricpotential) attached to an Al electrode or PFA (with a strong negativetriboelectric potential) attached to an Al electrode was utilized as thetop triboelectric layer in the TENGs.

In order to understand the electronic influence of the AgNWs in thepolymer matrix, 2D mapping of the surface potentials of theAgNW–polymer hybrid surface was performed via noncontact-modeKPFM. For all the KPFM measurements, the same Pt tip was utilized toensure that the measured values were referenced to a common energylevel. Six films were prepared in total: a bare PVC film, AgNW–PVCfilms with two different AgNW densities, AgNW–PMMA films with twodifferent AgNW densities, and a bare PMMA film. Fig. 2a shows2 μm×2 μm maps of the surface potential difference of the AgNW–-polymer hybrid films, which was referenced to Pt. The bare PVC filmshowed a negative surface potential, whereas the bare PMMA filmshowed a positive surface potential. These results indicated that thesurface potential of the Pt tip was located between those of PVC andPMMA. The hybrid films showed a distinct color contrast between theAgNW region and the polymer region. The brighter region in the imagescorresponds to a location with a more positive surface potential in thefilm, whereas the darker region corresponds to that with a more ne-gative potential. In particular, in the AgNW–PVC film, the AgNW regionwas brighter than the PVC region, which indicated that the surfacepotential of Ag was higher than that of PVC. In contrast, in theAgNW–PMMA film, the AgNW region was darker than the PMMA re-gion, which meant that the surface potential of Ag was lower than thatof PMMA. This difference in the relative surface potentials was closelyrelated to the electron-withdrawing characteristics of PVC and theelectron-donating characteristics of PMMA.

More detailed information about the surface potentials of the hybridfilms was obtained from a quantitative analysis of the colors in theKPFM images. Fig. 2b shows the histogram of the surface potentialobtained from the total area of all the KPFM images in Fig. 2a. In thishistogram, the surface potential difference referenced to Pt was plottedon the x-axis and the counts were plotted on the y-axis. The peaks of thehybrid films were deconvoluted by using a sum of Lorentzian–Gaussianfunctions. The red peak corresponded to the signal originating from theAgNWs, whereas the green and blue peaks corresponded to the signalsoriginating from PVC and PMMA, respectively. The surface potential ofbare PVC was measured to be around − 2.51 V, but it shifted sig-nificantly to the positive direction when PVC came into contact withAg, whose surface potential was higher than that of PVC. At the sametime, the surface potential of Ag (+0.01 V, see Fig. S3) shifted slightlyto the negative direction. Unlike in the case of the AgNW–PVC hybridfilms, the surface potential of PMMA (+3.45 V) shifted negatively afterPMMA came into contact with Ag, whose surface potential was lowerthan that of PMMA. From the KPFM results of the hybrid systems withtwo different AgNW densities, it was found that the surface potential ofPVC increased from− 0.08 V to− 0.06 V as the areal factor the AgNWsincreased from 0.23 to 0.55 and the surface potential of PMMA in-creased from +0.18 V to + 0.19 V as the areal factor of the AgNWsdecreased from 0.54 to 0.42. Fig. 2c summarizes the change in thesurface potential induced by the electrostatic interaction between theAgNWs and the polymer matrix on the hybrid surface.

From the histograms of the surface potential distributions, a sche-matic energy-band diagram of the hybrid films was proposed, as shown

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in Fig. 3a. The surface potential variation could be understood from thedipole formation caused by the electron transfer process at theAgNW–polymer interface. Because of the higher work function of Agthan of PVC in the AgNW–PVC hybrid film, the electrons in Ag weretransferred to PVC, which caused the other side of Ag to become morepositive; this consequently increased the surface potential of PVC. Inthe AgNW–PMMA hybrid film, however, the electrons in PMMA weretransferred to Ag because of the higher work function of PMMA than ofAg; this consequently decreased the surface potential of PMMA. Fig. 3b

shows schematic structures that explain the effect of the areal factor ofAgNWs on the charge transfer between them and the polymer matrix.When more AgNWs were embedded in the polymer matrix, the densityof charges transferred from the AgNWs to the polymer matrix washigher. This resulted in a larger shift in the surface potential of thepolymer matrix.

Next, the relationship between the surface potentials of theAgNW–polymer hybrid films and the TENG performance was in-vestigated. PFA, having a strong negative triboelectric potential, or

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 AgNWfilms embedded in polymer matrix (right). (d) Schematic device structure of TENGs based on PFA (or nylon) top triboelectric layer and AgNW–PMMA (orAgNW–PVC) bottom triboelectric layer. The lower panel shows the chemical structures of PMMA and PVC used in this study.

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 arealfactor of AgNWs.

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nylon, having a strong positive triboelectric potential, was utilized asthe top triboelectric layer. AgNWs with two different areal factors thatwere embedded in either PVC or PMMA served as the bottom tribo-electric layer. Fig. 4a shows a schematic of a TENG structure with theAgNW–polymer hybrid film as the bottom triboelectric layer and PFA asthe top triboelectric layer. Fig. 4b shows the voltage/current output

performance of this TENG device. The device showed a negative vol-tage/current output during contact, but it showed a positive voltage/current output during separation because PFA was positioned on thenegative side relative to the AgNW–polymer hybrid film in the tribo-electric series. Fig. 4c summarizes the maximum voltage/current outputas a function of the surface potential of the AgNW–polymer hybrid film.

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

Fig. 4. (a) Schematic diagram illustrating operation of contact-separation-mode TENGs with AgNW–polymer bottom triboelectric layer and PFA top triboelectriclayer. (b) Output voltage and current of TENGs with AgNW–polymer/PFA configuration. (c) Maximum output voltage and current extracted from (b) as functions ofsurface potential. (d) Schematic diagram illustrating operation of contact-separation-mode TENGs with AgNW–polymer bottom triboelectric layer and nylon toptriboelectric layer. (e) Output voltage and current of TENGs with AgNW–polymer/nylon configuration. (f) Maximum output voltage and current extracted from (e) asfunctions of surface potential.

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The TENG output performance increased as the surface potential in-creased from − 0.07 V to + 0.19 V. For example, an output voltage of64 V and an output current of 4.9 μA were obtained at the surface po-tential of − 0.07 V, and these values improved to 145 V and 9.8 μA,respectively, at the surface potential of + 0.19 V. A higher surfacepotential of the hybrid film resulted in a larger triboelectric potentialdifference when PFA was used as the top triboelectric layer. This largertriboelectric potential difference resulted in a better TENG perfor-mance. Next, nylon, having a strong negative triboelectric potential,was used as the top triboelectric layer in the TENG, as shown in Fig. 4d.Fig. 4e plots the voltage/current output of the TENG with nylon as thetop triboelectric layer. The direction of the voltage/current output ofthe nylon TENGs was opposite to that of the PFA TENGs. A positiveoutput was observed in the contact mode and a negative output wasobserved in the separation mode because nylon was positioned on thepositive side relative to the hybrid film in the triboelectric series. Thevoltage/current output of the TENGs decreased as the surface potentialof the hybrid film increased (Fig. 4f). A lower surface potential of thehybrid film resulted in a larger triboelectric potential difference whenthe top triboelectric layer was nylon, and this consequently led to abetter TENG performance. As a result, the TENG performance could bedeterministically controlled by varying the type of polymer matrix andthe areal factors of AgNWs embedded in the polymer matrix.

To further investigate the effect of the surface potential of theAgNW–polymer hybrid film on the TENG output, we performed a finiteelement method (FEM) simulation using the COMSOL Multiphysicssoftware. Under the open-circuit condition, the output voltage (Voc) ofthe 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-electric layers, and ε0 is the vacuum permittivity [56–60]. Voc and σ ofthe TENG was successfully calculated as shown in Fig. 5 and Fig. S4,

respectively. Specifically, Figs. 5a and 5b show results of three-di-mensional (3D) numerical simulation of the potential distribution ofTENGs with the AgNW–polymer/PFA and AgNW–polymer/nylon con-figurations, respectively. The AgNW network embedded in the polymermatrix was simply described as a mesh structure. The relationship be-tween the simulated Voc and the surface potential of the AgNW–po-lymer hybrid surface was consistent with the measured TENG perfor-mance (see Fig. 2). Fig. 5c shows the simulated Voc as a function of thesurface potential of the hybrid films. The Voc values of the TENGs werestrongly related to the differences in the triboelectric potentials ofcontact surfaces, where these differences were, in turn, dependent onhow far apart the contacting materials were located in the triboelectricseries. The simulated Voc of the TENG with the PFA top triboelectriclayer increased with an increase in surface potential of the hybridsurface; this trend was attributed to the fact that the strong negativetribopotential of PFA resulted in a larger tribopotential difference withthe hybrid film having a higher surface potential. In contrast, the si-mulated Voc of the TENG with the nylon top triboelectric layer showedthe opposite tendency: Voc decreased with an increase in surface po-tential of the hybrid film. A lower surface potential of the hybrid filmresulted in a larger tribopotential difference between this film and thenylon top triboelectric layer with a strong positive tribopotential. TheFEM simulation results further supported the conclusion that a largertribopotential difference between the contact surfaces resulted in ahigher triboelectric potential. Consequently, it can be deduced thatrational design of a triboelectric layer is crucial to enhancing theelectrical performance of TENGs.

3. Conclusion

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.AgNWs with two different areal densities were embedded in either oftwo different polymers: PVC or PMMA. Because of the work functiondifference between the polymer and the AgNWs, the surface potential ofPVC shifted positively whereas that of PMMA shifted negatively uponthe embedding of the AgNWs. A higher density of AgNWs embedded inthe polymer matrix resulted in the transfer of more charges from theAgNWs to the polymer matrix. The surface potential of theAgNW–polymer hybrid film could be modulated deterministically byvarying either the polymer matrix or the AgNW density; the surfacepotential, in turn, influenced the electrical properties of TENGs with aPFA (or nylon) top triboelectric layer. The proposed model system of ahybrid surface consisting of two materials with different surface po-tentials provided insight into the optimization of the triboelectric layerin TENGs. The simple, cheap, and large-area fabrication of mechani-cally robust AgNWs electrodes provides a platform for the engineeringof the flexible and stretchable TENGs.

4. Methods

Fabrication of AgNW–polymer hybrid system Ag nanowires (AgNWs)dispersed in isopropyl alcohol were purchased from Nanopyxis Co.(diameter: 27–37 nm, length: 20–30 µm). The AgNW suspension wascoated onto a glass substrate treated with octadecyltrichlorosilane(ODTS) through the Meyer-rod coating method (RD Specialist Inc., #7and #14 rods). The AgNWs films prepared with #7 and #14 rods ex-hibited the electrical conductivities of 76 and 10Ω/sq, respectively.Separately, two polymer solutions–poly(methyl methacrylate) (PMMA,Aldrich, Mw = 120,000) and polyvinylchloride (PVC, Aldrich, Mw

=62,000) –were prepared by dissolving polymers in solvent (30mg/mlPMMA in chlorobenzene and 50mg/ml PVC in a mixed solvent of cy-clohexane and tetrahydrofuran with a volume ratio of 2:1). The pre-pared viscous polymer solution was spin-coated onto the AgNW-coatedglass substrate, and then, the poly(ethylene terephthalate) (PET) filmwas laminated using two compression rollers before solvent drying.Both the polymer concentration and the lamination pressure were op-timized to adjust the thickness of the embedding polymer layer to 5 μm.Finally, the PET films were peeled off smoothly from the ODTS-treatedglass substrate. The size of the TENG device was 2 cm×2 cm.

Measurements The surface morphologies of the hybrid films weremeasured by both contact-mode atomic force microscopy (AFM, XE-100, Park Systems) with a Pt/Cr-coated silicon tip (radius< 25 nm) andfield-emission scanning electron microscopy (FESEM, JSM-7600F,JEOL, Ltd.). Kelvin probe force microscopy (KPFM) images were ac-quired using the XE-100 at. force microscope (Park Systems) with thePt/Cr-coated silicon tip. The scan area and scan rate were 5 µm×5 µmand 0.3 Hz, respectively. The applied voltage, phase, and frequencywere 2 V ac, − 90°, 17 kHz, respectively. The sheet resistances of theAgNW films were measured using Keithley 2182 A and 6221 instru-ments. The TENG performance was measured using both a TektronixDPO 3052 digital phosphor oscilloscope and a low-noise current pre-amplifier (SR570, Stanford Research Systems, Inc.) under ambientconditions. Before measurement, all the sample surfaces were cleanedwith ethanol and dried for 1min to ensure electroneutrality. The forceof 5.5 kgf was applied for the TENG output measurements.

Acknowledgements

This work was supported by a grant from the Basic ResearchProgram (2017R1A4A1015400) of the National Research Foundation ofKorea (NRF) funded by the Ministry of Science, ICT& Future Planning,the Korea Institute of Industrial Technology (Kitech JA-18-0002), andthe Gyeongi-Do Technology Development Program (Kitech IZ-18-0001), Korea.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2019.01.046.

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Hyungseok Kang received his BS in the Department ofPolymer Engineering from Sungkyunkwan University(SKKU) in 2014. Now, he is M.Sc.-Ph.D. combined studentin Jeong Ho Cho’s group at SKKU Advanced Institute ofNanotechnology (SAINT) from SKKU. His research interestincludes silver nanowires and its application.

Hyoung Taek Kim received his BS in the Department ofAdvanced Materials Science and Engineering from SKKU in2016. Now he is Ph.D. candidate in Sang-Woo Kim’s groupat Advanced Material Science and Engineering from SKKU.His research interest includes triboelectric nanogeneratorsand 2D nanomaterials.

Prof. Sang-Woo Kim is a professor in the Department ofAdvanced Materials Science and Engineering at SKKU. Hisrecent research interest focuses on piezoelectric/tribo-electric nanogenerators, sensors, and photovoltaics usingnanomaterials. He has published over 200 peer-reviewedpapers and holds over 80 domestic/international patents.Now, he is a director of SAMSUNG-SKKU Graphene/2DResearch Center and leading National Research Laboratoryfor Next Generation Hybrid Energy Harvester. He is cur-rently serving as an Associate Editor of Nano Energy and anExecutive Board Member of Advanced Electronic Materials.

Prof. Jeong Ho Cho obtained his BS in chemical en-gineering from Sogang University in 2001 and his MS andPh.D. in Chemical Engineering from POSTECH in 2006. Hewas a postdoctoral researcher in Department of ChemicalEngineering and Materials Science at University ofMinnesota (2006–2008) and then joined as a faculty atSoongsil University (2008–2012) and SunkyunkwanUniversity (2012–2018). He now is a professor at YonseiUniversity with an appointment in Department of Chemicaland Biomolecular Engineering. His research interests in-clude organic electronic devices (transistor, memory, andsensor) and 2-dimentional nanomaterials.

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