Photo-Cross-Linkable Organic–Inorganic Hybrid Gate Dielectric for

Article pubs.acs.org/JPCC

Photo-Cross-Linkable Organic−Inorganic Hybrid Gate Dielectric for High Performance Organic Thin Film Transistors Kyunghun Kim,† Hyun Woo Song,‡ Kwonwoo Shin,*,§ Se Hyun Kim,*,‡,∥ and Chan Eon Park*,† †

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784 South Korea ‡ Department of Advanced Organic Materials Engineering and ∥School of Chemical Engineering, Yeungnam University, Gyeongsan, 712-749 South Korea § Energy Nano Materials Research Center, Korea Electronics Technology Institute (KETI), Seongnam 463-816 South Korea ABSTRACT: In this study, we have demonstrated a novel organic− inorganic hybrid gate dielectric material, zirconium tetraacrylate (ZrTA). ZrTA gate dielectric, where inorganic Zr elements are embedded in organic acrylate matrix, takes advantage of the complementary properties of single organic or inorganic gate dielectrics. A simple spin-coating and UV-assisted cross-linking reaction of acrylate moieties allowed ZrTA film to be photopatterned. The cross-linked ZrTA film by UV and heat treatments (UV, 365 nm for 3 min; heat, 120 °C for 30 min) showed high dielectric strength (10−7 A/cm2 at 2 MV/cm), and dielectric constant (5.48). In addition, surface properties of the ZrTA film (surface energy, surface roughness) were favorable for the growth of overlying pentacene organic semiconductor. Consequently, the organic thin-film transistor composed of a pentacene semiconductor and a cross-linked ZrTA gate dielectric displayed a moderately high field-effect mobility of 0.50 cm2/(V·s) with a negligible hysteresis transfer characteristic.

1. INTRODUCTION Recently, organic thin film transistors (OTFTs) have attracted great attention from academic and industrial fields due to their various applications in displays, radio frequency identification tags (RFIDs), and memory.1,2 In particular, the emerging technologies such as flexible and wearable electronics provide a remarkable opportunity to commercialize organic electronic materials and devices, as well as to expect a quantum leap in the organic electronic market. To this end, there has been considerable progress in the improvement of materials synthesis, device physics, and processing methodologies.3−7 For example, high field-effect mobility (μFET) up to 10−20 cm2/(V·s) has been achieved in solution-processed polymer semiconductor materials (e.g., thiadiazole-based copolymer).7 New electrode materials such as graphene, metal nanoparticles, and nanowires showed good physical properties with high conductivity, mechanical strength, transparency, and solution processability.8,9 Furthermore, the development of roll-to-roll and diverse printing processes (e.g., inkjet, aerosol jet printing) made it possible to fabricate organic devices on large plastic substrates easily and cheaply.10,11 Regarding gate dielectric materials in OTFTs, there are several requirements from emerging flexible and wearable applications: high dielectric constant (κ), high dielectric strength, high mechanical strength, and good surface properties (e.g., proper surface tension and low surface roughness). Highκ gate dielectric materials enable low voltage operation of © XXXX American Chemical Society

OTFTs by accumulating charge carriers in the channel by a factor of κ/κ0 (κ0 is space permittivity). The contact resistance problem occurred in short channel transistors could be also overcome by adapting high-κ gate dielectric materials.12 High gate leakage current causes extra noise in power amplifiers and additional loss at the OFF state in power supplies, which are the reason for the high dielectric strength. Therefore, several inorganic gate dielectrics such as vapor and solution phase metal oxides (Al2O3, ZrO2, Ta2O5, HfO2, and TiO2) have been widely investigated, and the corresponding OTFTs have operated at low operating voltage below 5 V.13−15 However, these metal oxide gate dielectrics exhibit high Young’s modulus to impede their flexibility, and high surface tension that not only induces charge traps but also degrades the growth of organic semiconductors.16 On the other hand, organic gate dielectrics based on polymers can provide several advantages over inorganic ones, considering the aspect of their flexibility and compatibility with organic semiconductors, as well as solution and printing processes. Nevertheless, relatively low κ values and weak dielectric strength of organic gate dielectrics are still a bottleneck to replacing their inorganic counterparts. In fact, several polymers such as poly(vinyl alcohol),17,18 cyanoethylated pullulan,19 and cyanoethylated poly(vinyl Received: January 8, 2016 Revised: February 28, 2016

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Figure 1. (a) UV−vis transmittance spectrum of as-spun ZrTA film. A chemical structure of ZrTA is presented in the inset. (b) FT-IR absorption spectra of ZrTA film according to UV-irradiation time.

strength (10−7 A/cm2 at 2 MV/cm), good surface properties, and photopatternability under UV and low temperature (120 °C) treatment. Base on the cross-linked ZrTA gate dielectrics, the OTFTs employing pentacene as a semiconductor exhibited low voltage operation (below −3 V) and good device performances: μFET (0.50 cm2/(V·s)), threshold voltage (−0.92 V), and subthreshold swing (110 mV/decade).

alcohol) (CR-V)20,21 possess κ values higher than 10; however, high-κ polymers as gate dielectrics in OTFTs often show abnormal device operation behavior (e.g., artificial increase of drain current and large hysteresis during a dual sweep of the transfer curve), which may be due to the presence of chemical species that slowly polarized.21 Specific treatment such as thermal cross-linking should be required to improve the abnormal device operation of the OTFTs employing high-κ polymers. Recently, research attempts to improve the κ and dielectric strength of organic gate dielectrics have been frequently carried out with an organic−inorganic hybrid concept: nanocomposite, bilayer, and core−shell structures composed of organic and inorganic constituents.22−26 Although several research groups have reported the organic−inorganic hybrid gate dielectrics that enable the reliable operation of OTFTs within a few volts, there are still problems from the viewpoint of processing and device performance. For example, a nanocomposite gate dielectric that incorporates inorganic nanoparticles into polymer matrix could enhance its κ value, but it was found that surface roughness and leakage current increased with increasing the content of nanoparticles in the polymer matrix.23,24 Ultrathin bilayer gate dielectrics comprised of an organic-top- and inorganicbottom-layer structure made it possible to obtain the surface properties compatible with organic semiconductor film and a high capacitance.26 However, the process to fabricate inorganic gate dielectrics generally needs high temperature thermal energy or vacuum condition that restricts cheap and large area processing for flexible electronics. Hence, it is necessary to develop high quality gate dielectrics (high κ, high dielectric strength, and good surface properties) through low temperature and solution processing. In the present study, we introduce zirconium tetraacrylate (ZrTA) as a new organic−inorganic hybrid gate dielectric for high performance, low-voltage operated OTFTs. ZrTA is comprised of Zr(4+) cation and four acrylate(1−) anions, which are held together by ionic bonds. This fact means that high thermal energy is not needed to cross-link organic− inorganic hybrid gate dielectrics. Instead, the cross-linking of ZrTA can be obtained from the chemical reaction between acrylate moieties under UV illumination and relatively low temperature thermal treatments (below 120 °C), enabling its application into flexible electronics. In addition, the present of inorganic Zr element in the organic matrix increased the κ or dielectric strength of the film. Consequently, the cross-linked ZrTA gate dielectric film yielded high κ value (5.48), dielectric

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. ZrTA, poly(4vinylphenol) (PVP), poly(melamine-co-formaldehyde), methylated (PMFA), n-butanol, N,N′-dimethylformamide (DMF), and pentacene were purchased from Aldrich, respectively. Heavily doped silicon and quartz glass substrates were cleaned using acetone, isopropyl alcohol, and UV−ozone exposure. A 4 wt % ZrTA solution that contains 0.1 wt % Irgacure 184 (BASF) photoinitiator was made in n-butanol, and then the solution was spin-cast on the substrate to fabricate a crosslinked ZrTA gate dielectric. Subsequently, UV exposure (peak λ = 365 nm and energy density of ca. 25 mJ/cm2) was performed for 3 min, and heating the substrates at 120 °C for 30 min hardened the films. The thickness of the resulting film was 50− 60 nm. The total procedures for the fabrication of ZrTA gate dielectric were carried out in ambient condition (ca. 25 °C and relative humidity of 45%). Cross-linked PVP gate dielectric (ca. 60 nm) was fabricated by blending the cross-linker PMFA into the PVP with a weight ratio of 5:1 in DMF, followed by a spincasting and thermal annealing process at 180 °C in a vacuum oven. Pentacene active layers were deposited onto the gate dielectrics using the organic molecular beam deposition method (with a deposition rate of 0.1−0.2 Å/s; vacuum pressure = 10−6 Torr; substrate temperature of 25 °C). Thermal evaporation of Au source/drain electrodes onto the pentacene layers with shadow masks yielded bottom-gate, top-contact OTFTs (channel length and width were 50 and 1000 μm, respectively). 2.2. Characterization. Atomic force microscopy (AFM) (Multimode SPM, Digital Instruments) was performed to investigate various surface morphologies. Film thicknesses were determined by an ellipsometer (J. A. Woollam Co. Inc.). The surface energies of the gate dielectric films were evaluated by measuring the contact angles using water and diiodomethane test liquids. From the dispersion (γsd) and polar (γsp) components of the surface energy, the total surface energy (γs) was calculated based on the equation B

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Figure 2. (a) UV-assisted cross-linking reaction of ZrTA with a photoinitiator. (b) Schematic illustration of the cross-linked ZrTA film structure. The ZrTA contains four coordination numbers originated from Zr(4+) cation.

Figure 3. (a) Optical microscopy images of 100 (top) and 5 μm (bottom) photopatterned ZrTA film. (b) Current density−electric field characteristics of as-spun and cross-linked ZrTA gate dielectrics. The architecture in the inset represents the device structure. Atomic force microscopy images of (c) as-spun and (d) cross-linked ZrTA films.

2(γsd)1/2 (γlvd)1/2

2(γsp)1/2 (γlvp)1/2 (1)

Accelerator Laboratory in Korea. The electrical characteristics of pentacene OTFTs were measured using a Keithley 4200 SCS.

where γlv is the surface energy of the test liquids, and and γplv indicate the dispersive and polar components, respectively. The Fourier transform infrared spectroscopy (FT-IR) spectrum was obtained using a Nicoret 6700 (Thermo Electron) to trace the acrylate groups in ZrTA gate dielectric films. UV−vis transmittance characteristics of ZrTA films were measured using a PerkinElmer LAMBDA-900. Synchrotron-based onedimensional X-ray diffraction (XRD) experiment was performed on pentacene films at the 5A beamline of the Pohang

3. RESULTS AND DISCUSSION 3.1. Gate Dielectric Properties. Figure 1a shows the UV− vis transmittance spectrum of as-spun ZrTA film on the quartz glass substrate, and the inset represents its own chemical structure. Pure ZrTA only absorbed UV light with wavelength (λ) below 300 nm (deep UV (DUV)) in the applied λ range from 200 to 1000 nm, implying that the cross-linking reaction between acrylates occurred by the irradiation of DUV (λ < 300

1 + cos θ =

γlv

+

γlv

γdlv

C

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the trap formation at the semiconductor/gate dielectric interface. Parts c and d of Figures 3 show surface topographic images for as-spun and cross-linked ZrTA films, respectively, characterized by AFM. Both gate dielectric films indicated amorphous surface morphologies and very low root-meansquare surface roughness (Rq): 0.561 nm (as-spun) and 0.449 nm (cross-linked ZrTA), respectively. In general, a rough surface hinders the diffusion of growing semiconductors, thereby inducing grain boundaries and lowering the crystallinity. It has been reported that Rq below 1 nm is sufficient to induce efficient semiconductor growth.29−31 Furthermore, the γs values for as-spun and the cross-linked ZrTA films were 45.66 and 39.13 mJ/m2, respectively (Table 1). Our cross-

nm). However, this high energy DUV irradiation may accompany the decomposition of organic molecules, and therefore the irradiation process should be carried out in vacuum or inert condition. Instead, the use of conventional near-UV (λ of 300−400 nm) can provide several benefits to the aspect of processing and materials. Hence, Irgacure 184 was incorporated as a photoinitiator which can make radicals under the irradiation of near-UV to initiate chemical cross-linking between acrylates of ZrTA. Figure 1b shows FT-IR spectra for ZrTA film containing 0.1 wt % photoinitiator, depending on UV irradiation time and thermal annealing. The UV light (peak λ = 365 nm and energy density of ca. 25 mJ/cm2) was irradiated to the sample in ambient air. As-spun ZrTA film clearly indicates FT-IR peaks corresponding to the acrylate moiety at 1640 cm−1 (CC stretching). After exposure of the ZrTA film to UV light for 1 min, the CC stretching peaks (1640 cm−1) reduced significantly, and UV irradiation longer than 3 min made the peak dissipate in the FT-IR spectrum. The complete UV cross-linking of ZrTA allowed achieving a densely packed network structure because the Zr core contains coordination numbers of 4 as proposed in Figure 2. In particular, UV-irradiated ZrTA film was selectively patterned through a conventional glass photomask and the following developing process. Figure 3a exhibits optical microscope images of the patterned ZrTA film obtained by 3 min UV irradiation and developing with n-butanol. The line pattern widths from 5 to 100 μm were successfully obtained without discernible defects. To characterize gate dielectric properties of the ZrTA film, metal−insulator−metal (MIM) structures were fabricated by sandwiching gate dielectric layers between Au and Si electrodes. The cross-linked ZrTA film was used to define leakage current and κ after 3 min UV irradiation and thermal annealing (120 °C for 30 min) to remove residual solvent and to harden the film. Figure 3b shows the leakage current density (Ileakage)−electric field (E) characteristics of Au/gate dielectrics/Si electrode. Asspun ZrTA indicated an electrical breakdown at only E of 0.5 MV/cm2; however, the cross-linked ZrTA film maintained Ileakage of 10−7 A level up to E of 2.0 MV/cm2. As mentioned above, low Ileakage is a vital element for reliable operation of OTFTs, and it could be resulted from the dense gate dielectric film via UV and thermal cross-linking process. The κ value of the cross-linked ZrTA film was measured to 5.48 at 20 Hz. A poly(methyl methacrylate) which possesses a similar chemical structure shows the κ value of about 3.5, and the general polymer materials used as a gate dielectric layer of OTFTs (e.g., poly(4-vinylphenol),24 polystyrene,27 CYTOP,28 and benzocyclobenzene16) showed the κ value ranged from 2.5 to 4.0, given that special additives (e.g., ceramic and carbon nanoparticles or nanosheets) are not contained in the film. In comparison, the higher κ value of ZrTA could be attributed to the incorporation of metallic Zr core into the acrylate matrix. Although ZrTA still exhibited lower κ compared with other ceramic materials such as Al2O3 (κ = 7), ZrO2, (κ = 25), and HfO2 (κ = 10), its own processing advantages (e.g., low temperature, solution processability, photopatternability) were enough to redeem the lower κ value. The organic matrix part of ZrTA film can provide additional advantages in the aspect of organic semiconductor growth or mechanical flexibility of the film. In addition to the bulk property of a gate dielectric, the surface ones (such as surface energy and roughness) play a significant role in determining the device performance of OTFTs, considering the crystal growth of semiconductors and

Table 1. Contact Angles and Surface Energies (γs’s) for AsSpun and UV-Treated (3 min) ZrTA Films contact angle (deg) film

water

diiodomethane

γps (mJ m−2)

γds (mJ m−2)

γs (mJ m−2)

as-spun UV-treated

66 85

35 41

10.51 2.31

35.15 36.82

45.66 39.13

linking condition (UV, 365 nm for 3 min; heat, 120 °C for 30 min) made the ZrTA film surface more hydrophobic: the increase in water contact angle (from 66 to 85°) and the decrease in the γps (from 10.51 to 2.31 mJ/m2). This result may stem from the formation of alkyl networks surrounding Zr cores in the film, and the presence of alkyls at the surface enhanced the hydrophobicity. Such an improved hydrophobicity of the cross-linked ZrTA film can exert a favorable influence on the charge carrier transport. General oxide gate dielectrics have been well-known to cause charge traps due to the polar surface functionalities such as hydroxyl groups, degrading the electrical performances of OTFTs. In contrast, the organic−inorganic hybrid ZrTA gate dielectric in this study is expected to provide not only favorable morphologies of overlying semiconductors but also a high electrical performance of corresponding OTFTs due to its moderately high hydrophobicity (will be discussed). 3.2. Growth Behavior of an Overlying Semiconductor. The growth behavior of a vacuum-sublimated pentacene on the cross-linked ZrTA gate dielectric was investigated by AFM. To evaluate the quality of the crystalline morphology, the pentacene film grown on the cross-linked PVP was suggested as control samples because the cross-linked PVP is one of the most commonly used polymer gate dielectrics.32 Figure 4 shows surface morphologies for pentacene films on the crosslinked ZrTA and PVP gate dielectrics with the nominal film thicknesses of 5 and 50 nm. From the 5 nm thick pentacene films, both the ZrTA and PVP samples exhibited twodimensional (2D) layered grains during the first stage of film growth, and then island-type grains were formed on the top. Finally, terrace-like grains with average size from 0.5 to 1.2 μm are completed via the layer−island (Stranski−Krastanov) mode.33 The layer−island mode of pentacene growth has been frequently observed in the case where the surface tension of a dielectric surface is comparable to, or higher than, that of pentacene (48.1 mJ/m2).34 In the early stages of pentacene growth, strong π−π interaction between pentacene molecules and the dielectric surface induces percolated 2D layered grains. Then pentacene molecules have formed islands on the 2D layered grains because the interaction between adsorbed D

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drain current−drain voltage (ID−VD) output characteristics of bottom-gate, top-contact OTFTs with vacuum-deposited pentacene on the cross-linked ZrTA and PVP gate dielectrics. The device structure is illustrated in Figure 6a. Device measurements were carried out in a N2-purged glovebox, and the capacitances of the two gate dielectrics were 93 (ZrTA) and 65 nF/cm2 (PVP), respectively. The μFET and threshold voltage (Vth) were calculated by fitting the experimental data to the following equation: ID = (WCi/2L)μFET(VG − Vth)2. The on− off ratio (Ion/Ioff) and subthreshold slope (SS) were extracted from the ID−VG transfer curves on a logarithmic scale. The extracted electrical parameters of these OTFTs are summarized in Table 2. Pentacene OTFTs on the cross-linked ZrTA and PVP gate dielectrics were well operated within +0.5 to −3 V, in transfer characteristics (Figure 6b). In addition, These OTFTs exhibited typical p-type characteristics with a clear transition from linear to saturation behavior in output characteristics (Figure 6c,d). The OTFT with a cross-linked ZrTA gate dielectric was found to exhibit an average μFET of 0.50 cm2/(V· s), which is approximately 2 times higher than that of the OTFT with a cross-linked gate dielectric (0.24 cm2/(V·s)). In addition, other device parameters of the ZrTA-based OTFT (e.g., Vth, SS, and Ion/Ioff) were far better than those of the PVPbased OTFT (see Table 2). The significant difference between the two OTFTs may be due to the chemical compositions of the semiconductor/gate dielectric interface that can determine the trap formation at the interface. Generally, polar moieties such as hydroxyl groups have been well-known to create trap sites of free carriers (both holes and electrons), thereby leading to the degradation of device performances.16 For this reason, there have been many attempts to synthesize hydroxyl-free, hydrophobic gate dielectric materials, and to passivate, eliminate, the hydroxyl groups of gate dielectrics on PVP or inorganic oxide gate dielectrics. By contrast, the cross-linked ZrTA dielectric does not include hydroxyl groups in its chemical structure and maintains a hydrophobic surface, as previously mentioned in the description of the surface energy. Such a hydrophobic surface characteristic of the cross-linked ZrTA gate dielectric made it possible to provide higher device performances compared to the cross-linked PVP one. It should be noted that ZrTA-based device showed a negligible hysteresis. The hysteresis present during device operation leads to a shift in Vth when the device is switched from off to on, and back to off. This phenomenon prevents the OTFT

Figure 4. Atomic force microscopy images of 5 (left) and 50 nm (right) thick pentacene films deposited on cross-linked ZrTA (top) and PVP (bottom) gate dielectrics.

molecules is comparable to that between molecules and the underlying surface. The crystalline structures of 50 nm thick pentacene film grown on the cross-linked ZrTA and PVP gate dielectrics were examined by an X-ray diffraction experiment (Figure 5). Both the pentacene films on the two gate dielectrics mainly formed a thin film crystalline phase, as indicated by the intense (00l) peaks and a layer spacing of 15.4 Å from the outof-plane XRD profile (Figure 5a). However, the peak intensity for the pentacene on the cross-linked ZrTA was twice as high as the intensity for the pentacene film on the cross-linked PVP. Moreover, grazing incident angle XRD (GIXD) studies, with a fixed grazing incident angle of 0.13°, characterized the in-plane structure (parallel to the lateral π-conjugation of the assembled pentacene molecules) of these pentacene films (Figure 5b). The GIXD profiles showed that pentacene films grown on both gate dielectrics were packed in a herringbone geometry along the in-plane direction with a thin film crystalline phase.35 3.3. Electrical Characteristics. Figure 6 shows the drain current−gate voltage (ID−VG) transfer characteristics and the

Figure 5. One-dimensional (a) out-of-plane and (b) in-plane X-ray diffraction patterns of 30 nm thick pentacene films prepared on cross-linked ZrTA and PVP gate dielectrics. E

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Figure 6. (a) Schematic illustration of bottom gate, top-contact OTFTs. (b) ID−VG transfer characteristics of pentacene OTFTs prepared on crosslinked ZrTA and PVP gate dielectrics. ID−VD output characteristics of corresponding OTFTs on cross-linked (c) ZrTA and (d) PVP gate dielectrics.

Table 2. Electrical Characteristics of Pentacene OTFTs Prepared on Cross-Linked ZrTA and PVP Gate Dielectrics semiconductor

gate dielectric

μFET (cm2/(V·s))

Vth (V)

SS (mV/decade)

Ion/off

pentacene

ZrTA PVP

0.50 ± 0.10 0.24 ± 0.04

−0.92 −0.95

110 125

1.92 × 105 1.31 × 104

of Korea (NRF), funded by MSIP (2014R1A2A1A05004993 and 2014R1A1A1005896), as well as Industrial Strategic Technology Development Program (10048255) funded by the Ministry of Trade, Industry and Energy (MI) of Korea.

being used as a driving unit device in display backplanes or logic circuitry in RFID tags, and thus Vth must be stable.

4. CONCLUSION In conclusion, we have demonstrated the cross-linked ZrTA gate dielectric for high performance OTFTs. ZrTA, which consists of metallic Zr(4+) core and four acrylates, was photochemically cross-linked by adding Irgacure 184 photoinitiator under 365 nm UV light within 3 min. Such a photocross-linking enabled selective patterning of ZrTA through a glass mask, resulting in the well-defined line pattern widths from 5 to 100 μm. UV and thermally cross-linked ZrTA film exhibited a high dielectric strength that can maintain low leakage current density of 10−7 A/cm2 at 2 MV/cm, as well as a relatively high κ of 5.48 compared with conventional polymers. In addition, the surface properties (surface energy and roughness) of the cross-linked ZrTA film were suitable to the growth of vacuum deposition. Therefore, the OTFTs employing the cross-linked ZrTA gate dielectrics showed good electrical performance at low operating voltage (+0.5 to −3 V). Pentacene OTFT yielded an average μFET of 0.50 cm2/(V· s), a threshold voltage of −0.92 V, a subthreshold swing of 110 mV/decade, an on−off current ratio of >105, and negligible hysteresis.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-31-789-7463. Fax: +82-31-789-7469. E-mail: [email protected] (K.S.). *Tel.: +82-53-810-2788. Fax: +82-53-810-4686. E-mail: [email protected] (S.H.K.). *Tel.: +82-54-279-2269. Fax: +82-54-279-8298. E-mail: cep@ postech.ac.kr (C.E.P.). Author Contributions

K.K. and H.W.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Basic Science Research Program through the National Research Foundation F

DOI: 10.1021/acs.jpcc.6b00213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b00213 J. Phys. Chem. C XXXX, XXX, XXX−XXX