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Highly Thermally Conductive Yet Electrically Insulating Polymer/Boron Nitride Nanosheets Nanocomposite Films for Improved Thermal Management Capability ACS Nano 2019.13:337-345. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/23/19. For personal use only.

Jin Chen, Xingyi Huang,* Bin Sun, and Pingkai Jiang Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Thermally conductive yet electrically insulating polymer composites are urgently required for thermal management applications of modern electrical systems and electronic devices because of their multifunctionality and ease of processing. However, the thermal conductivity enhancement of polymer composites is usually at the price of the loss of lightweight, the deterioration of flexibility, and electrical insulation. Here we report advanced polymer nanocomposites containing orientated boron nitride nanosheets (BNNSs), which simultaneously exhibit high thermal conductivity enhancement, excellent electrical insulation, and outstanding flexibility. These nanocomposite films can be easily constructed by electrospinning polymer/BNNSs nanocomposite fibers, vertically folding the electrospun nanocomposite fibers and the subsequent pressing. The nanocomposite films exhibit thickness-dependent in-plane thermal conductivity, which can reach 16.3 W/(m·K) in the 18 μm thick nanocomposite film with 33 wt % BNNSs. In addition, the nanocomposite films have superior electrically insulating properties compared with the pristine polymer, such as reduced dielectric loss, increased electrical resistivity, and enhanced breakdown strength. The strong thermal management capability of the nanocomposite film was demonstrated in switching power supply, which showed the importance of high in-plane thermal conductivity in thermal management of high-power density electronic devices. KEYWORDS: electrospinning, boron nitride nanosheets, in-plane thermal conductivity, nanocomposites, PVDF

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design freedom. However, the low intrinsic thermal conductivity significantly limits their applicability in thermal management. Thus, the composite materials combining the merits of polymer and the high thermal conductivity of fillers are considered as the ideal solution.14−18 Accordingly, the thermal conductivity enhancement induced by the fillers should be kept as much as possible to keep the flexibility, processing, lightweight and electrical insulation performance of the composites.19,20 So far, many efforts have been devoted to increasing the thermal conductivity enhancement efficiency of fillers in composites. Typical methods include using high-aspect-ratio nanofiller, self-assembly, orientation, formation of 3D interconnected structure and preferential localization.21−27 How-

hermally conductive but electrically insulating polymer materials have been extensively applied in light-emitting diodes (LEDs), integrated electronic devices, energy storage and conversion systems, military weapons, and aerospace industry to realize proper thermal management.1−8 With the rapid performance evolutionary of electrical systems and electronic devices, the traditional polymer composites cannot meet the high requirement for thermal management.9−13 For example, nowadays much more heat can be generated in the metal-oxide-semiconductor field effect transistors (MOSFETs) due to their miniaturization and the higher power density of power supplies or power adapter modules. In this case, the thermal interface material between the MOSFETs and the heat sinks should possess sufficient heat transfer capability and electrical insulation level, otherwise the excessive temperature may shorten the lifetime of the modules. Polymeric materials own excellent electrical insulation performance, flexibility, and © 2018 American Chemical Society

Received: August 18, 2018 Accepted: December 19, 2018 Published: December 19, 2018 337

DOI: 10.1021/acsnano.8b06290 ACS Nano 2019, 13, 337−345

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standard procedure described in our previous work, and the majority of the BNNSs are 1−2 μm in lateral size and 2−3 nm in thickness.45 The precursor solution for electrospinning was prepared by first dissolving PVDF in the mixture solvent of DMF and acetone, then a specific amount of BNNSs were added before stirring for 2 h. During electrospinning, a drum covered with aluminum foil having a high revolving speed was used as the collector of the PVDF/BNNS nanocomposite fibers. After electrospinning, the oriented PVDF/BNNS nanofiber membranes were generated because of the high rotation speed and the match between electrical and force centrifugal during electrospinning. The oriented PVDF/BNNS nanocomposite fibers membranes were vertically folded into each other into a stack array. Then the PVDF/BNNS nanocomposites films were fabricated by pressing the stack array at different temperatures. In the following, the nanocomposite fiber stack array was first pressed at a temperature below the melting point of PVDF in order to remain the vertical cross structure with narrow gap among fibers, and subsequently the pressing was performed at a temperature above the melting point of PVDF to form an integrated film. Finally, the flexible PVDF/BNNS nanocomposite films were obtained after annealing. Figure 2 presents the microstructure or morphology of the fibers and nanocomposites films. As evidenced in the SEM images (Figure 2a−c), the pure polymer fibers show a smooth surface with a narrow diameter distribution of 300−600 nm. In the nanocomposite fibers, BNNSs are uniformly dispersed along the oriented PVDF fibers and the concentration of BNNSs increases as the BNNS loading enhances from 20 to 33 wt %. As imaged by TEM (Figure 2 and Figure S1), the BNNSs are interconnected and linearly ordered along the PVDF fibers at the loading of 33 wt %, looking like the churrasco. It has been demonstrated that BNNSs could be uniformly dispersed in DMF because of their matched surface energy.46 During electrospinning, the BNNSs could stack in sequence along the oriented direction of the fibers, resulting in large contact area and reduced thermal contact resistance between BNNSs. After vertically folding of the oriented PVDF/BNNS fibers, the grid structure can be formed and shown in Figure 2f,g. Figure 2h and Figure S2 show the representative SEM images of the PVDF/ BNNS film. One can see that the oriented and interconnected BNNS structure is remained well after pressing. In addition, no obvious interfacial void or pores can be observed. It is expected that these features would be conducive to boost the in-plane thermal conductivity of the PVDF/BNNS nanocomposite films. Thermal and Dielectric Properties of the Nanocomposite Films. Figure 3 shows the in-plane (=) and through-plane (⊥) thermal conductivity of directly hot-pressed BNNS/PVDF nanocomposites, nanocomposites with interconnection oriented BNNSs, and nanocomposites with randomly dispersed BNNSs. One can see that the nanocomposites with interconnection oriented BNNSs show sharply increased inplane thermal conductivity compared with randomly dispersed BNNSs and directly hot-pressed composites, while each composite has a much lower through-plane thermal conductivity. It is found that the oriented PVDF/BNNS (33 wt %) composites show ultrahigh in-plane thermal conductivity of 10.4 W/(m·K), which is more than 4 times of randomly distributed BNNS composites and 2 times of directly hotpressed composites with the same BNNS loading at room temperature. It is believed the interconnected and grid-shaped BNNS chains can construct abundant thermal conductive pathways with small BNNS/BNNS interfacial thermal resist-

ever, the realization of high thermal conductivity enhancement while remaining the excellent electrical insulation and flexibility is still a big challenge.28−32 It is believed that the key to resolve this challenge is constructing interconnected thermally conductive paths using highly electrically insulating and highaspect-ratio nanofillers. Electrospinning under high electric field is a simple and effective way to manufacture polymer fibers from solutions, which makes it possible to construct high-aspect-ratio and aligned polymer or nanocomposite fibers.33−39 Hexagonal boron nitride nanosheets (BNNSs) are promising thermally conductive fillers due to the ultrahigh thermal conductivity, wide band gap (about 5.9 eV), and high-aspectratio two-dimensional (2D) morphology, making them the most ideal nanofillers for constructing polymer composites with high thermal conductivity enhancement and excellent electrical insulation property.40−44 Here, using electrospinning, simple folding, and hot-pressing highly thermally conductive but electrically insulating thermoplastic polymer-based BNNS nanocomposite films were prepared, which have the advantage of simplicity and adaptability for a commercial scaleup. Polyvinylidene fluoride (PVDF) was used as the matrix and BNNSs were oriented and interconnected along the in-plane direction of polymer film, making the PVDF/BNNS nanocomposite films exhibit ultrahigh in-plane thermal conductivity of 16.3 W/(m·K) at 33 wt % BNNS loading. Furthermore, the nanocomposite films have a better electrically insulating performance than the pristine PVDF. The potential application of such nanocomposite films in thermal management of power supplies was demonstrated by experiment and simulation in this work.

RESULTS AND DISCUSSION Preparation and Characterization of the Nanocomposite Films. The entire synthesis pathway of the PVDF/BNNS nanocomposite films was presented in Figure 1. BNNSs were prepared by liquid phase exfoliation of h-BN according to the

Figure 1. Scheme of the preparation process of the PVDF/BNNS nanocomposite films. 338

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Figure 2. SEM images of PVDF fibers (a); SEM images of nanocomposite fibers with 20 wt % (b) and 33 wt % (c) BNNS; TEM images of nanocomposite fibers with 33 wt % BNNS (d,e). Inset in (d) is simulated morphology of interconnected and linearly ordered BNNSs. SEM images of vertically folded nanocomposite fibers with 33 wt % (f) and 20 wt % BNNS; SEM images (h) and photograph (i) of the nanocomposite film with 33 wt % BNNS;.

ance, where phonons can transmit along the BNNS chains with small scattering, as revealed in Figure 2b. However, in the nanocomposites with randomly dispersed BNNSs, the thermal conductivity enhancement is quite low because most BNNSs are isolated by the low thermal conductivity polymer matrix. In this case, phonons transfer between fillers and matrix become difficult because of the large matrix−filler interfacial thermal resistance.47 In the case of the nanocomposite with 33 wt % random BNNSs, its in-plane thermal conductivity (K=) is about 2.6 times higher than that of the through-plane one (K⊥), indicating that the molding process resulted in a certain extent of orientation of the BNNSs. However, the nanocomposites with oriented BNNS demonstrated an exceptionally stronger thermal conductivity anisotropy, where the K=/K⊥ can reach 21. It is worth recognizing that the K= sharply increases with the enhancement of filler loading in the nanocomposites with oriented BNNSs, suggesting significant impact of orientation and internal connection on the in-plane thermal conductivity of the PVDF/BNNS film. Figure 3d shows the thickness dependent in-plane thermal conductivity of oriented BNNS nanocomposites. One can see that the in-plane thermal conductivity sharply increases as the film thickness decreases from 50 μm, reaching up to 16.3 W/(m·K) at about 18 μm. However, the in-plane thermal conductivity only exhibits a slight decrease as the film thickness increases from 50 to 170 μm. This result can be understood by the impact of film thickness on the orientation and interconnection of BNNS along the in-plane direction of the films. The hot-pressing was performed at 185 °C, which is above

the melting point of PVDF. In this case, the BNNSs have larger freedom degree along the through-thickness direction in thicker samples, which results in lower in-plane orientation and interconnection of BNNSs and thus the in-plane thermal conductivity becomes lower as the increase of film thickness. When the film thickness is higher than 50 μm, the BNNSs start to have enough freedom degree, and the thermal conductivity of the films exhibits weak dependence on thickness. The previously reported thermal conductivities of BN or BNNS based polymer composites13,48−55 are summarized in Figure 3e, which illustrate the superiority of our oriented and grid stacking BNNS structure in boosting the thermal conductivity of the polymer nanocomposites. Our PVDF/ BNNS nanocomposite film exhibits outstandingly high thermal conductivity above the reported composites with similar BN concentrations. This result illustrates that the oriented and grid stacking BNNS structure has the prominent advantage to boost heat transfer capability of the polymeric nanocomposites. To understand the thermal conduction process of the PVDF composites with different BNNS structure, using the Foygel’s theory and effective medium theory (EMT)56,57 the interfacial thermal resistance in nanocomposites with oriented and random dispersed BNNSs were respectively calculated. The calculation process could be found from the Supporting Information. The computed interfacial thermal resistance in the nanocomposite with 33 wt % oriented BNNSs is about 1.26 × 10−6 m2·K/W, which is 1 order of magnitude smaller than that (1.81 × 10−5 m2· K/W) of the nanocomposite with randomly dispersed BNNSs. These calculated values are well matched with the measured 339

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Figure 3. (a) In-plane (K=) and through-plane (K⊥) thermal conductivity of the nanocomposites at 25 °C. (b) The diagram of in-plane transfer of heat flow in the composite films. (c) Thermal conductivity anisotropic factor of the composite films. (d) Thickness dependent in-plane thermal conductivity of the PVDF/BNNS composites. (e) Thermal conductivity of our work and other representative published work about BNNS or BN-based polymer composites.

characteristic Eb has been also rationalized by the low dielectric loss (Figure 4c) and high electrical resistivity of the nanocomposites. The suppressed dielectric loss, especially at lowfrequency region in the dielectric spectra, and the improvement in volume electrical resistivity illustrate that the BNNSs can act as an effective insulating shield against the space charge conduction and the leakage current. Nevertheless, increasing BNNSs to 33 wt % results in decreased characteristic Eb in the case of the nanocomposites with oriented BNNSs. This is reasonable because at high loading levels the increase of BNNS loading results in interfacial defects like holes. Meanwhile, at a concentration of 33 wt % BNNSs, the volume electrical resistivity and breakdown strength of the randomly dispersed BNNS nanocomposites are slightly higher in comparison with those with oriented BNNSs. One possible explanation is that impurities were introduced during the electrospinning. Despite this, the nanocomposites with oriented BNNSs are still high insulation since the volume resistivity is 3.87 × 1014 Ω·cm at a BNNS loading of 33 wt %.

thermal conductivity, accounting for the importance of low interfacial thermal resistance in achieving ultrahigh thermal conduction of the nanocomposites with oriented BNNS. For electrical and electronic applications, a reliable electrical insulation performance of the cooling materials is usually of crucial importance. Figure 4a presents the volume electrical resistivity of the nanocomposites. The introduction of BNNSs into PVDF can gives rise to nearly 1 order of magnitude enhancement of volume electrical resistivity, resulting in enhanced electrical insulation property of the nanocomposites. This phenomenon could be ascribed to the 2D morphology of BNNSs and the ultrahigh electrical resistivity. The Weibull plots of breakdown strength (Eb) of different PVDF/BNNS composites are shown in Figure 4b. Apparently, the characteristic Eb is increased with the incorporation of BNNSs from 357 kV/mm for the electrospun PVDF film to 432 kV/mm for the nanocomposite film with 20 wt % oriented BNNSs. This is because the BNNSs can generate a robust scaffold hampering the onset of electromechanical failure.58 The enhancement of 340

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Figure 4. (a) Volume electrical resistivity of the nanocomposites. (b) Weibull plots for breakdown strength of different nanocomposites. (c) Frequency-dependent dielectric loss tangent of different nanocomposites. (d) DSC heating curves of nanocomposites with 33 wt % BNNS.

Figure 5. (a) Optical photograph of the MOSFETs integrated with thermal interface materials. (b) Infrared thermal images of MOSFETs integrated with different thermal interface materials. (c) Schematic diagram of a MOSFET integrated with a thermal interface material. (d) The surface temperature variations of MOSFETs versus time.

Melting temperature is a significant parameter to illustrate the thermal behavior of the thermoplastic nanocomposites, which refers to the ultimately service temperature of PVDF/BNNS composite films. Figure 4d shows that the melting points of the

nanocomposites follow the order of PVDF < randomly dispersed BNNS nanocomposite < oriented BNNS nanocomposite, further demonstrating the excellent thermal properties of the nanocomposites with orientated BNNTs. 341

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ACS Nano Application of the Nanocomposite Films in Cooling MOSFETs. The switching power supply is one of most crucial systems in determining the performance and guaranteeing stable operation of electronics. With the rapidly increasing power density of modern electronics, the higher power of electrical devices brings forward a more serious heat dissipation problem. MOSFETs are the main heat-generating elements in a switching power supply, which should work at limited temperatures for the remaining high enough working efficiency (Figure 5a). The thermal interface materials (TIMs) between the MOSFETs and the heat sinks are the key medium to transfer the heat generated by the MOSFETs to the heat sinks and finally the heat was dissipated out by the fan above the heat sinks (Figure 5c). In order to evaluate the practical performance in thermal management, instead of original silicone pad,59 the nanocomposite films were used as thermal interface materials between the MOSFETs and the heat sinks integrated in a switching power supply of personal computers. Here, PVDF, PVDF/random BNNS, and PVDF/oriented BNNS composites were used as thermal interface materials between MOSFETs and heat sinks. In addition, their performance was compared with those using commercial silicone pad as thermal interface materials. All the thermal interface materials have the same size and all MOSFETs start to work at room temperature. The surface temperatures of the MOSFETs were captured by an infrared thermograph. The infrared thermal images of the overall process were recorded and are shown in Figure 5b. Throughout the operation process, it can be seen that the nanocomposite film with oriented BNNSs displays obviously slower temperature rise and lower stable temperature compared with the commercial silicone pad, that is, the nanocomposite film with randomly dispersed BNNS. In the case of the nanocomposite film with oriented BNNSs, the equilibrium temperature is only around 65 °C, which has dropped by about 5 and 11 °C in comparison with the commercial silicone pad (or nanocomposite film with randomly dispersed BNNS) and the pure PVDF film, respectively. This result illustrates that the nanocomposite films with oriented BNNSs have prominent thermal management capability, which is consistent with the result of their high thermal conductivity. A finite element model was used to demonstrate the excellent heat transfer effects of the oriented BNNSs nanocomposite films on the cooling applications of the MOSFETs. In the simulation model, the oriented BNNSs nanocomposite films or the silicone pad were contacted with the top surface of a heat source, which were considered as a thermal barrier from the MOSFET to the heat sink. More information about the simulation model can be seen from the Supporting Information. During the heat transfer process from the MOSFET, the in-plane and through-plane thermal conductivities play a vital role in maintaining the reliability of the MOSFET. Herein, according to the aforementioned test results, the in-plane and through-plane thermal conductivities of the nanocomposite films with oriented BNNSs are set to be 10 and 0.5 W/(m·K), and the isotropic thermal conductivity of the silicone pad is 3 W/(m·K). According to the computation results (Figure 6), compared with the commercial silicone pad the nanocomposite film with oriented BNNSs exhibits much higher edge temperature and similar center temperature, despite its much lower throughplane thermal conductivity. This should be attributed to the ultrahigh in-plane thermal conductivity of the nanocomposite film with oriented BNNSs. For cooling of MOSFETs, the thermal interface materials were used with the shape of the thin

Figure 6. Modeling and calculation of the temperature of (a) the nanocomposite film with oriented BNNSs and (b) silicone pad above the MOSFETs (the temperature of MOSFET is set to be 90 °C).

plate and their size is much larger than the transistor. This means there are little through-plane thermal resistance disparities between different TIMs. In this case, the through-plane thermal conductivity is not the decisive factor; the increase of in-plane thermal conductivity has a larger positive impact on the edge heat dissipation of a thermal interface material. Overall, the nanocomposite films with oriented BNNSs display much better thermal management capability for the cooling of MOSFETs.

CONCLUSIONS In summary, highly thermally conductive yet electrically insulating polymer nanocomposite films filled with BNNSs were fabricated by three steps: electrospinning, vertically folding, and subsequent press molding. The nanocomposites not only exhibit ultrahigh in-plane thermal conductivity at relatively low BNNS loading but also have excellent electrically insulating properties, such as higher breakdown strength, higher volume electrical resistivity, and reduced dielectric loss in comparison with the pure polymer. The strong cooling capability of the nanocomposite films with oriented BNNSs for MOSFET was demonstrated by experiment and simulation, indicating a promising application in the thermal management of emerging electrical systems and electronic devices. EXPERIMENTAL SECTION Materials. The BN powders were obtained from 3M Technical Ceramics (U.S.A.) with an average diameter of 3 μm. The isopropanol (99%) was supplied from Adamas Reagent Ltd., China. The PVDF (Solef 6010) used in this work was purchased from SOLVAY (Belgium). The N,N-dimethylformamide (99%) and acetone (99%) were supplied from Sigma-Aldrich (U.S.A.). Preparation of Aligned PVDF/BNNS Nanofibers. The liquid phase exfoliation of BN particles was conducted in accordance with the previous work.45,60 The precursor solution for electrospinning was prepared by dissolving PVDF in mixture solvent of N,N-Dimethylformamide and acetone (2:3), and then mixing them at 40 °C. After stirring thoroughly for 1 h, a specific amount of BNNSs was added with stirring for 2 h. During the electrospinning process, the work distance between the spinneret and collector was 15 cm and the voltage was set as 15 kV. The solution flow was controlled at 0.5 mL per hour by a syringe pump. The revolving speed of cylindrical collector is 1600 rpm. The electrospinning proceeded at 25 ± 2 °C, and the ambient humidity was from 45 ± 5% relative humidity (RH). After electrospinning, the PVDF/BNNS nanofiber membrane was peeled out from aluminum foil 342

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ACS Nano and placed into 70 °C vacuum oven for 12 h drying. The PVDF nanofibers were prepared using the similar process. Preparation of PVDF/BNNS Films. The aligned PVDF/BNNS nanofiber membranes were cut into 30 mm narrow strips along the fiber orientation. As shown in Figure 1, the PVDF/BNNS fiber strips were vertically folded into a stack array. The PVDF/BNNS films were fabricated by mold pressing the stack array under 20 MPa for 0.5 h at 25 °C and 0.5 h at 185 °C subsequently. The directly hot-pressed BNNS/ PVDF composites were fabricated by grinding the BNNS/PVDF powder and hot-pressing for 0.5 h at 185 °C. For comparison, randomly dispersed BNNSs based PVDF composite films were also prepared by solution blending. Characterization. The morphology of the PVDF fibers, PVDF/ BNNS fibers, and PVDF/BNNS composite films was performed by a scanning electron microscope (SEM) (Nova NanoSEM 450, FEI Corporation, U.S.A.). A transmission electron microscope (TEM) (JEM-2010, JEOL Corporation, Japan) was used to observe the morphology of the PVDF/BNNS fibers at 200 kV acceleration voltage. The fiber samples were obtained by electrospinning PVDF/BNNS fibers on the carbon-coated cooper grids and then air-dried. The optical photos of PVDF/BNNS nanocomposites and power supply device were taken by a digital camera (A6000, Sony Corporation, Japan). The thermal diffusivity (D, mm2/s) and specific heat (Cp, J/(g·K)) were measured by the laser flash method (LFA-467, NETZSCH, Germany), and the density (ρ, g/cm) was assessed by the water displacement method. Thermal conductivity λ (W/(m·K)) was calculated as λ = ρ × Cp × D. Differential scanning calorimetry (DSC) (NETZSCH 200 F3, Germany) was used to measure the melting point of nanocomposites at a heating rate of 20 °C/min and range of 50−250 °C under nitrogen atmosphere. The dc breakdown strength was measured by a dielectric strength tester (DH, Shanghai Lanpotronics Co., China) with a 10 mm ball-to-plate stainless electrode system. Volume resistivity of nanocomposites was characterized using electrometer (Keithley 9008, Tektronix, U.S.A.). The samples, cut into the same size with commercial silicone pad (13 × 18 × 0.8 mm), were put between MOSFET and heat sinks, then MOS devices ran at room temperature. The surface temperature of MOSFET was captured by an infrared thermograph (FOTRIC-226, China). The frequency-dependent electrical conductivity was tested by a Novocontrol Alpha-N highresolution dielectric analyzer (GmbH Concept 40) Hz at room temperature with the frequency range of 10−1−106. A layer of gold was evaporated on both sides of the samples to serve as electrodes.

Multiphysics software was performed at the Institute of Electrical Engineering, Chinese Academy of Sciences.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06290. The TEM images of PVDF/BNNS fibers (20 wt %); SEM images of PVDF/BNNS (20 wt %) film; the calculation of interfacial thermal resistances; finite element model (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jin Chen: 0000-0002-5063-1180 Xingyi Huang: 0000-0002-8919-6884 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Special Fund of the National Priority Basic Research of China (No. 2014CB239503) and National Natural Science Foundation of China (No. 51522703, No. 51477096) for financial support. The simulation by 343

DOI: 10.1021/acsnano.8b06290 ACS Nano 2019, 13, 337−345

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