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Control of Triboelectrification by Engineering Surface Dipole and Surface Electronic State Kyung-Eun Byun, Yeonchoo Cho, Minsu Seol, Seongsu Kim, SangWoo Kim, Hyeon-Jin Shin, Seongjun Park, and Sung Woo Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02802 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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ACS Applied Materials & Interfaces

Control of Triboelectrification by Engineering Surface Dipole and Surface Electronic State Kyung-Eun Byun†§, Yeonchoo Cho†§, Minsu Seol†, Seongsu Kim‡, Sang-Woo Kim‡*, HyeonJin Shin†*, Seongjun Park†*, and Sungwoo Hwang† †Samsung Advanced Institute of Technology, Suwon 443-803, Republic of Korea, ‡School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440–746, Republic of Korea §These authors contributed equally to this work. *Correspondence and requests for materials should be addressed to S.-W.K ([email protected]), H.-J.S.([email protected]) and S.P.([email protected])

KEYWORDS Triboelectrification; self-assembled monolayer; electron-donating group; electronwithdrawing group; Kelvin force microscopy

ACS Paragon Plus Environment

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Abstract

Although triboelectrification is a well-known phenomenon, fundamental understanding of its principle on a material surface has not been studied systematically. Here, we demonstrated that the surface potential, especially the surface dipoles and surface electronic states, governed the triboelectrification by controlling the surface with various electron-donating and -withdrawing functional groups. The functional groups critically affected the surface dipoles and surface electronic states followed by controlling the amount of and even the polarity of triboelectric charges. As a result, only one monolayer with a thickness of less than 1 nm significantly changed the conventional triboelectric series. First-principles simulations confirmed the atomistic origins of triboelectric charges and helped elucidate the triboelectrification mechanism. The simulation also revealed for the first time where charges are retained after triboelectrification. This study provides new insights to understand triboelectrification.


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Introduction Triboelectrification, which is defined as the separation of charges by the contact of two materials, has been considered as an interesting phenomenon since the time of ancient Greece.1 Triboelectric charges generated by triboelectrification are used in diverse applications, including for increasing the selective adhesion of the toner particles in printers,2 separating minerals and coals in electrostatic separators based on the charge polarity,3 and coating whole objects with charged paint droplets.4 In particular, a triboelectric nanogenerator (TENG), a device that can convert mechanical energy into electrical energy by exploiting the coupling between triboelectrification and electrostatic induction, has recently been drawing attentions as a key solution for energy harvesting from the waste energy in the environment. Thanks to recent efforts, the output power of the TENG has increased dramatically. Further, it has been demonstrated that the TENG is capable of powering touch pad sensors,5 mobile electronics,6 and self-charged vehicle entry systems.7 These efforts include the physical modification of the relevant surface to increase the frictional movement as well as the actual contact area by forming pyramid-like patterns,6 fabricating vertically aligned nanowires on surface,8 or nanoparticlescoated surface.9 Fine-tuning the device structure by forming an arch for effective charge separation,6 multi-layers to increase the contact area10 or a grating to improve the energy conversion efficiency11 has also resulted in improvements in the output power. However, the fundamental understanding of triboelectrification in a material’s point of view has not been pursued systematically. One of the main mechanisms to explain triboelectrification is that the amount of charges exchanged is proportional to the surface potential (effective work function) difference of the two materials being contacted together.12,13 The surface potential of a material is the overall


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consequence of the following: bulk electronic structures, surface dipoles, and surface electronic states.14,15 While bulk electronic structures are determined by the given material components and its atomic structures, the surface dipoles and surface electronic structures can be affected by various uncontrollable surface conditions, such as surface reconstructions, adsorbates in air, and surface defects.15 Thus, many contradictory and ambiguous results are observed during the triboelectrification measurements.16,17 In recent, several researches have reported the surface potential altered by the surface functional group affects triboelectrification. These results have focused on the enhancement of triboelectric charges, and just explained a phenomenological level.18-20 The fundamental understanding regarding a relationship between triboelectrification and surface potential of materials was insufficient. In this work, we report effects of the surface potential on triboelectrification by systematically controlling surface dipoles and surface electronic states through the surface modification with various electron-donating and -withdrawing functional groups. We characterized the surface potential of self-assembled monolayer (SAM) modified SiO2 surfaces21 with Kelvin probe force microscopy (KPFM)22 to confirm the relationship between the surface potential and triboelectric charges. The SiO2 with an electron-donating layer such as an NH2 group, tends to donate electrons to the counter metal surface, while the SiO2 with an electron-withdrawing layer such as a CF3 group, tends to accept electrons from the counter surface. Consequently, SAM altered the conventional triboelectric series of original substrates according to the functional group of SAM. We also calculated the surface dipole and surface electronic states of the SAM-modified materials using first-principles simulations to understand the atomistic origins of triboelectric charges and to elucidate the triboelectrification mechanism. This study provides a better


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understanding of the triboelectrification: the surface dipole and surface electronic states are the critical factors that control the amount of and even convert the polarity of triboelectric charges. Results and Discussion Figure 1 shows a schematic diagram depicting the triboelectrification of various surfacemodified materials. The uniform and well-ordered SAM in nanoscale effectively controlled surface defects and reduced the adsorption of additional adsorbates in air.23 In this way, the effect of surface potential on triboelectrification can be monitored without any artifacts. The functional groups in the SAM, which included groups such as -NH2, -SH, -CH3, and -CF3, determined whether the surface were electron-donating or electron-withdrawing.24 The lone pair of the NH2 and the SH functional groups tends to donate electrons (Figure 1a i), while the CF3 group accepts electrons from an adjacent system, in one case, metal, owing to the presence of the highly electronegative atom F (Figure 1a iv).25 A weakly electron-donating or a neutral functional group such as SH or CH3 donates fewer electrons to the metal (Figure 1a ii and iii). Surface modification by SAMs on the materials could increase or decrease triboelectric charges generated on surfaces. To verify our hypothesis on the triboelectricity affected by the surface modification, we employed the KPFM mode in atomic force microscopy (AFM) measurements.12,26 The KPFM mode allowed the surface potential of the substrate to be visualized, on the basis of the contact potential difference (CPD) between the probe and the substrate.22 The surface potential of the substrate, ߶௦௨௕௦௧௥௔௧௘ , is defined as ߶௦௨௕௦௧௥௔௧௘ = ߶௣௥௢௕௘ − ܸ݁஼௉஽

(1)

where ߶௣௥௢௕௘ is the work function of the probe, ܸ஼௉஽ is the measured CPD, and e is the electronic charge. After the substrate has been contacted, the surface potential should be changed


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owing to the triboelectric charges generated. The triboelectric potential can be defined by the difference in the surface potential before and after contact. Figure 1b shows the CPD images of various SAM-modified substrates after they had been contacted by a Rh-coated AFM probe. To define the relationship between the functional groups of the SAMs and the triboelectric charge, the carbon chain length of SAM molecules was fixed to that of propyl (C3), and the experiments performed in dry N2. The square in the middle of the images represents the spatial distribution of the triboelectric charge. The contact area for the as-received SiO2, NH2-, SH-, and CH3-SiO2 (Figure 1b i – iv) is much brighter than the surrounding area; this indicates that positive triboelectric charges were generated in these cases, while negative triboelectric charges were generated in the contact area on CF3-SiO2 (Figure 1b v). As shown in Figure 1b, the CPD distributions of the SAM-modified SiO2 were notably different from that of the as-received SiO2. This meant that the surface potentials and triboelectric potentials of the SAM-modified substrates were different from those of the as-received SiO2. The changes in the CPD before (empty symbols) and after contact (filled symbols) for the SAM-modified SiO2 substrates are shown in Figure 1c. Even though the SAMs consisted of only one monolayer with a thickness of less than 1 nm, the surface potentials of the SAM-modified surfaces were significantly different. Furthermore, the triboelectric potential was also significantly different and depended on the functional group of the SAM materials. The layer that donated the most electrons, namely, NH2-SiO2, exhibited a triboelectric potential (1.42 ± 0.13 V) almost 4 times higher than that of a neutral layer, CH3-SiO2 (0.36 ± 0.03 V). In case of the CF3-SiO2, which was the strongest electron-withdrawing layer, the triboelectric potential was -0.18 ± 0.04 V and had the opposite polarity of that of the neutral layer. It clearly shows that the triboelectric potential increased as the surface potential before contact increased (Figure 1d).


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These results support the conclusion that the surface potential strongly affects both the amount and polarity of the triboelectric charge. Furthermore, the surface modification affected the diffusion of the charge. Since the main contribution to the diffusion process is from the surface, the diffusion coefficient sensitively depended on the surface functional groups of SAM (Figure S1, Supporting Information). We also confirmed the triboelectrification behavior in air (Figure S2, Supporting Information). The amount of triboelectric charges generated is affected by the presence of atmospheric moisture, given that water has a high dielectric constant and/or the mobile ions in it form a water bridge.27–29 However, the changes in the triboelectric potential caused by the SAM were similar to those observed in dry N2. In addition, even though the SAM had identical functional group, the carbon chain length of the SAM was also a factor affecting triboelectrification owing to the formation of radicals along the carbon chains (Figure S3, Supporting Information).30 To elucidate the triboelectrification process on the SAM-modified SiO2 surfaces, the surface dipole and surface states induced by the SAMs were estimated using first-principles method. Atomic structures were constructed by attaching the SAMs to the hydrogenated quartz (111) surface (Figure S4, Supporting Information). Then, the density of states (DOS) was calculated and plotted with respect to the vacuum level (Figure 2a). The black curves represent the total DOS, while the red ones represent the SAM-related portion of the total DOS. There is around 1 eV difference between the total DOS of the NH2-SiO2 and CF3-SiO2 owing to the surface dipole difference. The negative surface dipoles of NH2, SH, and CH3 groups reduced the surface potential, while the positive surface dipole of CF3-SiO2 was along the direction in which the surface potential increased (Table 1). Interestingly, among the SAM-modified surfaces with electron donating groups, the NH2-SiO2 and SH-SiO2 have the extra surface states (red peaks) at


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approximately 5.5 eV in the gap, which further modify its surface potential. These peaks were related to the lone electron pair of the functional groups and we believe these surface states acted as the donor states. Although the dipole of the SH-SiO2 is smaller than that of the CH3-SiO2, the triboelectric potential of the SH-SiO2 is larger than that of the CH3-SiO2 owing to these donor states. We confirmed the surface dipole and the electronic states were varied by the functional group of SAM-modified surfaces, and the calculated results were in good agreements with the KPFM results. The key parameters from the experimental and computational analyses are listed in Table 1. Despite many studies on triboelectrification, it still remains an unsolved issue where triboelectric charges are retained after triboelectrification. We investigated the spatial distribution of an excess and a deficit electron after triboelectrification (Figure 2b). It is extremely difficult to identify it even with the current state-of-art experimental techniques, but the first-principles analysis allows us to look over the charge distribution in the SAM-modified surfaces after triboelectrification. This charge distribution was estimated from the difference in the electron densities of the negatively/positively charged state and the neutral state. According to the KPFM results, the NH2-SiO2 lost electrons and became positively charged, while the CF3SiO2 gained electrons and became negatively charged when the Rh probe contacted. When the NH2-SiO2 becomes positively charged, the lone-pair state of the nitrogen gives away electrons, as evidenced by the peak corresponding to the position of the nitrogen (Figure 2b i). On the contrary, when the CF3-SiO2 accepts electrons, electrons are accepted by the SiO2 surface, as the largest peak is at the surface of SiO2 and not at -CF3 or within the SiO2 substrate (Figure 2b iv). On the other hand, electrons are accepted from and donated to the surface of SiO2 when the NH2SiO2 and the CF3-SiO2 becomes negatively and positively charged, respectively (Figure 2b iii


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and ii). Thus, the surface strongly affects not only the transfer but also the retention of triboelectric charges after triboelectrification. Based on our results, a plausible model of the triboelectrification of a SAM-modified surface is proposed in Figure 2c. The surface dipole introduced by the electron-donating and -withdrawing groups (∆φ in Figure 2c) can decrease or increase the surface potential (Figure 2c i and iii). The additional surface electronic state (the red and violet bars in Figure 2c i) and iii) also participates in the donation of electrons. These affect the triboelectric charges generated. We note that inherent defect-induced states (black bars in Figure 2c) of the material can influence the amount of triboelectric charges, but their contribution to the triboelectric potential does not vary depending on the SAM functional group. The direction and magnitude of the electron flow may depend on the relative position of the surface and the counter materials in energy level. However, the electron-donating or withdrawing tendency caused by SAM functional group is not changed, thus the model can be generally applied to triboelectrification. We demonstrated that the triboelectrification was affected by the surface modification, which varied the surface electronic state and surface dipoles. In addition, the bulk electronic structure is also an important factor determining the triboelectrification. To investigate how surface modification depends on bulk electronic structures, Indium tin oxide (ITO), SiO2 and Al2O3 were employed as the bulk materials, and NH2- and CF3- were chosen as the surface functional groups. The SAM-modified substrates contacted with counter materials such as perfluoroalkoxy alkane (PFA), polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyimide (PI), polyethylene terephthalate (PET), Al, polycarbonate (PC), nylon and mica to identify whether the materials exhibited a negative or positive triboelectric charging tendency. The triboelectric charging tendencies were determined from the direction of the first peak of the open-circuit


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voltage and the short-circuit current of a triboelectric device composed of the SAM-modified substrate and the counter material.8 Figure 3a and 3b show the representative open-circuit voltage and short-circuit current of the device consisting of the SAM-modified ITO and PI. The first peak of the open-circuit voltage and the short-circuit current of the bare ITO (black) has a negative direction, indicating that PI was negatively charged, and the bare ITO substrate was positively charged. When the NH2-ITO was made to come in contact with PI, it induced extra charges of the same polarity (red) as that in the case of the bare ITO substrate. On the other hand, the CF3-ITO (blue) was negatively charged, and it induced extra charges of the opposite polarity to the NH2-and bare ITO. These results were in good agreement with the KPFM data on the SAM-modified SiO2. Figure 3c shows the modified triboelectric series including the various SAM-modified substrates. All of the output voltage between SAM-modified substrates and the counter materials are displayed in Figure S5 and S6. Interestingly, irrespective of the substrate species used, the electron-donating NH2-modifed substrates exhibited a relatively positive charge compared to the corresponding as-received substrate, while the electron-withdrawing CF3modified substrates exhibited a relatively negative charge. However, even though the substrates were modified by NH2 functional group, the each position of the NH2-modified substrate in the triboelectric series was different. These results confirmed that the bulk materials are also important factor to control the triboelectrification. Conclusion In conclusion, we demonstrated that the surface potential, especially the surface dipoles and surface electronic states, governed the triboelectrification. We systematically modulated the surfaces of materials using the SAM with electron-donating groups, -NH2, -SH, a neutral group CH3, and an electron-withdrawing group, -CF3. Even though the thickness of the SAM was less


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than 1 nm, it critically affected the surface dipoles and surface electronic states followed by the polarity and amount of triboelectric charges. Accordingly, the triboelectric series was changed depending on the properties of the SAM rather than the bulk materials. We also monitored the DOS, surface dipole, and charge distribution after the triboelectrification of the SAM-modified SiO2 using first-principles calculations. The surface dipole and DOS confirmed that the surface dipole and surface electronic states altered the transfer of triboelectric charges and the charge distribution after triboelectrification. It also revealed that the surface accommodated triboelectric charges. These results confirmed triboelectrification related with the surface potential of materials. This study provides new insights to understand triboelectrification and new strategies to achieve high performance of TENG. Methods Surface functionalization: All the chemicals used were purchased from Sigma-Aldrich. The substrate used, which was a wafer with a 100-nm-thick SiO2 layer, was treated by with ultraviolet/ozone

plasma

for

10

min

before

forming

SAM.

In

case

of

(3-

aminopropyl)triethoxysilane (APTES, NH2-SAM), the substrate was immersed in a 1% (v/v) APTES/ethanol

solution

for

1

h.

The

other

SAMs,

namely,

layers

of

(3-

mercaptopropyl)trimethoxysilane (MPTMS, SH-SAM), n-propyltriethoxysilane (PTES, CH3SAM), and (3,3,3-trifluoropropyl)trimethoxysilane (FAS3, CF3-SAM), were formed by the chemical vapor deposition method as follows.31,32 The cleaned substrate and 200 µL of the corresponding SAM chemical were placed in the middle and bottom, respectively, of a Teflon container. The Teflon container was incubated at 150 °C for 3 h. Next, the chemically functionalized substrates were rinsed with isopropyl alcohol and dried in a N2 flow.


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KPFM measurements: All the KPFM measurements were performed using an E-sweep AFM system (Seiko Instruments Inc.) with Rh-coated probes (SI-DF3-R, Seiko Instruments Inc.) To generate the triboelectric charges, the Rh-coated probe was scanned over the substrates with a contact force of 19 nN in the contact mode. The triboelectric charges were measured in the KPFM mode using oscillation amplitude of 5 V and resonance frequencies of 24-26 kHz. All the measurements were performed in dry N2 at room temperature (22-26 °C). Fabrication of TENG: The triboelectric device was constructed using the functionalized substrates (ITO, SiO2, and Al2O3) and the counter materials (PFA, PTFE, PDMS, PI, PET, PC, Al, nylon, and mica). For the functionalized Al2O3, a 100-nm-thick layer of Al was deposited on the 100-nm-thick SiO2 via e-beam evaporation. Next, a 50-nm-thick Al2O3 layer was grown on the Al layer by atomic layer deposition. The ITO and Al2O3 were functionalized with FAS3 and APTES in the same way as was the SiO2. Cyclic contact and separation operations of the triboelectric device were performed using a pushing tester (JIPT-100, JUNIL Tech). The contact area was 0.785 cm2, and the contact force was 0.7 kgf. The open-circuit voltage and short-circuit current signals were measured using an oscilloscope (Wave runner 640Zi, LeCroy) and a current preamplifier (DDPCA-300, Femto). Simulation: The Vienna ab-initio simulation package was employed for the first-principles calculations, which were based on the density functional theory and the projector augmented wave method

33,34

. We used the Perdew-Burke-Ernzerhof functional35 to describe the exchange

and correlation effects of electrons. The plane-wave cutoff was 400 eV. Only the gamma point was sampled for ionic relaxation, whereas a 5 × 5 × 1 Monkhorst-Pack k-point grid

36

was

sampled for the DOS. The SiO2 slab was adopted from the literature37. The unit cell included a vacuum of at least 15 Å to minimize spurious interactions between the cells. The surface dipoles


12

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were computed from the charge distribution after correcting the potential due to spurious interaction between cells.


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Figure 1. Triboelectrification on the SAM-modified substrates with electron-donating and electron-withdrawing groups. (a) Schematic diagram of the triboelectrification on i) a strongly electron-donating layer (-NH2), ii) a moderate electron-donating layer (-SH), iii) a neutral layer (-CH3), and iv) an electron-withdrawing layer (-CF3). The chemical structures of the respective molecules are shown in the dotted circles. (b) CPD images of i) as-received SiO2 and the ii) NH2-


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, iii) SH -, iv) CH3-, and v) CF3-modified SiO2. (c) CPD of each substrate. The empty and filled symbols represent the CPD before and after contact, respectively. (d) Triboelectric potentials of the various surfaces as a function of the CPD before contact.


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Figure 2. Atomistic origin of the generation of the triboelectric potential. (a) DOS of NH2-SiO2, SH-SiO2, CH3-SiO2, and CF3-SiO2 relative to the vacuum level. (b) An excess electron of i) NH2-SiO2 and ii) CF3-SiO2, and a deficit electron of iii) NH2-SiO2 and iv) CF3-SiO2. (c) Energy band diagrams of substrates modified with a i) NH2-SiO2, an electron-donating layer; ii) SiO2, a neutral layer; iii) CF3-SiO2, an electron-withdrawing layer and that of the metal. EVAC and ∆φ denote the vacuum level and the change of the surface dipole, respectively. The small black bars in the gap are the defect-induced surface states. The red bar in (i) and the violet bar in (iii) denote


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the SAM-induced surface states. The SAM-induced surface dipoles are represented by the green circles. The directions of electron transfer are shown by arrows.


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Table 1. Parameters related to triboelectrification of the various SAM-modified SiO2 substrates SAM functional group

Donating

CPD before contact[a]

CPD after contact[a]

Triboelectric potential[a]

Diffusion coefficient[a]

/Withdrawing [V]

[V]

[V]

[10

-17

Dipole[b] [e-Å]

m/sec]

State position[b] [eV]

NH2

Donating

0.40 ± 0.10

1.71 ± 0.09

1.42 ± 0.13

4.69 ± 0.05

-0.67

-5.45

SH

Moderately Donating

-0.06 ± 0.05

0.39 ± 0.06

0.44 ± 0.08

0.73 ± 0.03

-0.37

-5.51, -5.67

CH3

Neutral

-0.73 ± 0.09

-0.38 ± 0.07

0.36 ± 0.03

0.49 ± 0.01

-0.50

N/A

CF3

Withdrawing

-1.33 ± 0.18

-1.52 ± 0.18

-0.18 ± 0.04

2.90 ± 0.03

0.56

N/A

[a] KPFM measurements using a Rh tip; [b] Simulation results


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Figure 3. Triboelectric properties of SAM-modified materials (a) Open circuit voltage and (b) short circuit current of the triboelectric device composed of PI and SAM-modified ITO. (c) Modified triboelectric series consisting of SAM-modified ITO, SiO2 and Al2O3.


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ASSOCIATED CONTENT Supporting Information. Diffusion of triboelectric charges on various surfaces, triboelectrification in air, triboelectrification of CH3-SiO2 with different carbon chain length, unit cell used in density of functional theory calculations, output voltage between SAM-modified substrates and the counter materials. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Correspondence and requests for materials should be addressed to S.-W.K. ([email protected]), H.-J.S. ([email protected]) and S.P. ([email protected]) Author Contributions K.-E.B. and Y.C. contributed equally to this work. H.-J.S. and S.P. designed the study and were responsible for project planning. K.-E.B. performed most of the surface treatment and the fabrication of the TENG. Y.C. performed the computational simulations. M.S. and S.K. helped with the fabrication of the TENG and data analysis. S.-W.K. and S.H. participated in the discussion. S.-W.K., H.-J.S. and S.P. co-wrote the manuscript. ACKNOWLEDGMENT This

work

was

financially

supported

by

Basic

Science

Research

Program

(2015R1A2A1A05001851) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning.


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