Self-Positioned Nanosized Mask for Transparent ... - ACS Publications

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Self-Positioned Nanosized Mask for Transparent and Flexible Ferroelectric Polymer Nanodiodes Array Seung Hyun,† Owoong Kwon,‡ Chungryong Choi,† Kanniyambatti L. Vincent Joseph,† Yunseok Kim,*,‡ and Jin Kon Kim*,† †

National Creative Research Initiative Center for Smart Block Copolymer Self-Assembly, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk 790-784, Republic of Korea ‡ School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 440-746, Republic of Korea S Supporting Information *

ABSTRACT: High density arrays of ferroelectric polymer nanodiodes have gained strong attention for next-generation transparent and flexible nonvolatile resistive memory. Here, we introduce a facile and innovative method to fabricate ferroelectric polymer nanodiode array on an ITO-coated poly(ethylene terephthalate) (PET) substrate by using block copolymer selfassembly and oxygen plasma etching. First, polystyrene-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) micelles were spin-coated on poly(vinylidene fluoride-ran-trifluoroethylene) copolymer (P(VDF-TrFE)) film/ ITO-coated PET substrate. After the sample was immersed in a gold precursor (HAuCl4) containing solution, which strongly coordinates with nitrogen group in P2VP, oxygen plasma etching was performed. During the plasma etching, coordinated gold precursors became gold nanoparticles (GNPs), which successfully acted as self-positioned etching mask to fabricate a high density array of P(VDF-TrFE)) nanoislands with GNP at the top. Each nanoisland shows clearly individual diode property, as confirmed by current−voltage (I−V) curve. Furthermore, due to the transparent and flexible nature of P(VDF-TrFE)) nanoisland as well as the substrate, the P(VDF-TrFE) nanodiode array was highly tranparent, and the diode property was maintained even after a large number of bendings (for instance, 1000 times). The array could be used as the next-generation tranparent and flexible nonvolatile memory device. KEYWORDS: self-positioned nanosized mask, ferroelectric polymer nanodiodes array, block copolymer micelle, electrical properties (I−V curve), ferroelectric hysteresis loops, transparent and flexible memory device

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lithography,22,23 and anodic aluminum oxide (AAO) maskassisted pulsed laser deposition (PLD).5,6 However, both FIB and EB lithography give lower throughput, which is unsuitable for developing nanostructure in a large area in addition to potential damage of the crystal structure of a ferroelectric material because of using high energy irradiation during etching.15−22 On the other hand, for AAO mask-assisted PLD method, a very thin AAO mask should be used for depositing metal on the top of ferroelectric nanoislands. However, a very thin AAO mask is brittle, which prevents floating of a large size of AAO mask on a conducting substrate; thus, the covering area of the nanoislands array is limited (at most ∼mm2).5,6 For easily fabricating ferroelectric nanostructures on a large area, self-organizing diblock copolymers micelles are a unique method just by selective solvents of one block. Because inorganic precursor positioned at the one block could be

INTRODUCTION

Switchable ferroelectric diodes, by utilizing the change of polarization direction depending on applied electric field, have attracted strong attention for their potential applications to next-generation nonvolatile electroresistive memory.1−3 Especially, individually switchable ferroelectric nanoislands have tremendous advantages due to realization of ultrahigh-density memory devices, minimization of cross-talk effect,4−6 and low voltage data bit operation.5−13 The ferroelectric nanoislands could be individually addressed by a precisely controlled tool. For instance, atomic force microscopy (AFM) based storage combined with the ferroelectric nanoislands could be one of good examples. To fabricate the nanoisland diode array, the geometry of each nanodiode should be metal/ferroelectric nanoisland/metal (MFM) heterostructure, because it allows a uniform electric field to individual ferroelectric material in a single cell5,6 and blocking the depolarization.14,15 Several methods for preparing the array of MFM type nanoislands have been introduced in the literature, for example, focused ion beam (FIB) lithography,16−21 electron beam (EB) © 2016 American Chemical Society

Received: July 11, 2016 Accepted: September 16, 2016 Published: September 16, 2016 27074

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic for fabricating a high density array of Au/P(VDF-TrFE) ferroelectric heteronanoislands on a conductive substrate. (a) UVozone treated P(VDF-TrFE) film, (b) PS-b-P2VP micelles were spin coated onto UV-ozone treated P(VDF-TrFE) film, (c) Au precursors were coordinated with P2VP chains by dipping the film into Au precursor solution, (d) Au precursors within each micelle core become several tiny GNPs, whereas PS and P2VP chains are completely etched out at earlier stage of oxygen plasma treatment, and (e) high density array of Au/P(VDF-TrFE) heteronanoislands on a conducting substrate by further oxygen plasma etching.

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changed to metal oxide ferroelectric nanostructures at above 400 °C high temperature, only ferroelectric nanostructures, not metal/ferroelectric heterostructured nanoislands, could be manufactured on hard conductive electrode.24,25 Because wearable devices have become more and more important, MFM diodes for this purpose should be optically transparent and mechanically flexible.26−34 One of the promising ferroelectric materials is poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene [P(VDFTrFE)], because of low-cost fabrication process (e.g., solution coating) and mechanical flexibility. For a continuous film and a macroscopic-sized cell device, which are easily strained under mechanical forces such as bending, piezoresponse and electrical conductance would greatly change upon bending, which results in memory corruption and loss.32−34 On the other hand, for very small size and discrete P(VDF-TrFE) nanoislands, ferroelectric properties could be maintained even under a large macroscopic bending. Therefore, a facile method is utmost needed to fabricate a high density array of individually switchable MFM type diodes on a large area flexible substrate (for example, a wafer size) and the maintenance of diode property under a large bending. In this study, we fabricated a high density array of Au/P(VDF-TrFE) heterostructured nanoislands on a flexible conductive substrate in a wafer size. We first prepared a high density array of polystyrene-blockpoly(2-vinylpyridine) (PS-b-P2VP) micelles on a continuous P(VDF-TrFE) film on ITO-coated poly(ethyleneterephthalate) (PET) substrate. Then, the sample was dipped into gold precursor (HAuCl4) containing solution. Gold precursor is strongly coordinated with nitrogen in the P2VP chains.35,36 Oxygen plasma treatment makes gold precursors to gold nanoparticles (GNPs), which simultaneously act as a selfpositioned etching mask to obtain P(VDF-TrFE) nanoislands. Fabricated Au/P(VDF-TrFE) heteronanoisland structures were investigated by energy-dispersive X-ray spectroscopy (EDS) and element mapping analysis of scanning transmission electron microscopy (STEM).37 We used piezoresponse force microscopy (PFM) and conductive AFM (CAFM) for investigating polarization switching and diode effect of a single ferroelectric nanoisland, respectively. The diode property was even maintained even at 1000 times macroscopic bending.

EXPERIMENTAL SECTION

Preparation of Hydrophilic P(VDF-TrFE) Film. The P(VDFTrFE) copolymer (72/28 mol % of VDF/TrFE, the number-average molecular weight (Mn) = 92 000, polydispersity index (PDI) = 1.87) was purchased from Solef and used as-received.12 P(VDF-TrFE) in methyl ethyl ketone (MEK) at two different concentrations (1 and 2 wt %) was spin-coated on an ITO-coated PET substrate at a rotating speed 3000 rpm for 60 s. The final thickness of P(VDF-TrFE) thickness obtained from the above two different concentrations was 19 and 62 nm, respectively. Because the roughness of the film increases with increasing annealing temperature, we annealed the film at 65 °C for 2 h at the air and then cooled down to room temperature to minimize the film roughness. Even though the sample was annealed at a low temperature of 65 °C, it clearly shows crystallinity and ferroelectricity as confirmed by PFM images and 2D grazing incident wide-angle X-ray scattering (GIWAXS) patterns. (Figure S1, S2a, and Table S1 of Supporting Information).38 To increase wettability against block copolymer micelles, we changed hydrophobic P(VDF-TrFE) film to hydrophilic surface by exposing the film under to UV-ozone for 40 min (see Figure S3 of Supporting Information). Preparation of PS-b-P2VP Micelles Array on P(VDF-TrFE) Film. PS-b-P2VP copolymers (S2VP1 with Mn of PS block = 440 kg mol−1, Mn of P2VP block = 353 kg mol−1, PDI = 1.19, and S2VP2 with Mn of PS block = 102 kg mol−1, Mn of P2VP block = 97 kg mol−1, PDI = 1.12) were purchased from Polymer Source and used without further purification. 0.4 wt % S2VP1 in in o-xylene (Sigma-Aldrich) was spin-coated at 3000 rpm for 1 min on P(VDF-TrFE) film with a thickness of 62 nm. 0.5 wt % of S2VP2 in o-xylene was spin-coated at 6000 rpm for 1 min on P(VDF-TrFE) film with thickness of 19 nm. Fabrication of Gold Metal Coated P(VDF-TrFE) Nanoislands. PS-b-P2VP micelles on P(VDF-TrFE) film were dipped into an Au precursor solution (0.3 wt % HAuCl4 in ethanol) for 1 h and then washed by DI-water. During earlier times of oxygen plasma, gold precursors were reduced to tiny GNPs, whereas PS and P2VP were completely etched out. Further oxygen plasma treatment resulted in the aggregation of tiny GNPs into a single GNP confined within each micelle (see Figure S4 of Supporting Information). This selfpositioned GNPs successfully act as the etching mask for P(VDFTrFE) to make Au/P(VDF-TrFE) hetero nanoislands. Oxygen plasma was performed during 20 s for thinner P(VDF-TrFE) film (19 nm) and S2VP2), and 50s for a thicker film (62 nm) at a flow of 20 sccm, and source power of 100 W. Characterizations. Surface topography of the Au/P(VDF-TrFE) heterostructured nanoislands was observed by field-emission scanning electron microscopy (FE-SEM:S4800, Hitachi) and AFM (Dimension 27075

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces

Figure 2. AFM topography images of (a1 and b1) the array of S2VP micelles on UV-Ozone treated P(VDF-TrFE) film, (a2 and b2) the array of Au precursor incorporated inside P2VP micelle cores, and (a3 and b3) the array of Au/P(VDF-TrFE) heteronanoislands after oxygen plasma etching. (a4 and b4) AFM line profile of each AFM image. (a5 and b5) SEM images of high density array of Au/P(VDF-TrFE) heteronanoislands. The left panels (a1−a5) correspond to AFM and SEM images prepared by using a large molecular weight S2VP1, and the right panels (b1−b5) correspond to AFM and SEM images prepared by using a lower molecular weight S2VP2.

The film was immersed in a gold precursor solution to coordinate strongly with P2VP chains. When gold precursors incorporated P2VP cores were slightly exposed to oxygen plasma, they immediately became tiny GNPs in each P2VP core, whereas PS and P2VP chains were completely etched out. Further oxygen plasma treatment resulted in these tiny GNPs to a single GNP on the top of P(VDF-TrFE) thin film. (see Figure S4 in Supporting Information). The self-positioned GNPs act successfully as an etching mask during plasma etching to produce Au/P(VDF-TrFE) heteronanoislands on a conducting substrate. Namely, the regions of P(VDF-TrFE) thin film without having GNPs layer are completely etched out. Figure 2 shows AFM topography images and line profiles corresponding to each fabrication process. We used two different molecular weights of PS-b-P2VPs (S2VP1 and S2VP2) to control the size of block copolymer micelles; and thus the size of Au/P(VDF-TrFE) heteronanoislands. A high density array of well-ordered block copolymer micelles was formed on the P(VDF-TrFE) film (Figures 2a1 and 2b1). The diameter (D) and height (H) of micelles for S2VP1 (S2VP2) were 200 ± 50 (40 ± 5) nm and 20 ± 10 (7 ± 5) nm, respectively. During immersing PS-b-P2VP micellar film in gold precursor solution, there was no reconstruction of the micelles such as nanoporous structures which are often observed when PS-b-P2VP micelles are simply immersed in ethanol solution without gold precursors. This is because of fast coordination of gold precursor with P2VP chains, which prevents the reconstruction of block copolymer chains (Figures 2a2 and 2b2).35,36 The size of the micelles containing gold precursor was slightly increased compared with that of micelles without

3100 with Nanoscope V, Veeco) with tapping mode. High resolution TEM (JEOL, JEM-2100F) image was obtained to observe the location of GNP in P(VDF-TrFE) nanoisland. PFM was used to observe piezoresponse properties of individual Au/P(VDF-TrFE) nanoislands by an AFM (NX10, Park Systems) combined with a lock-in amplifier (SR830, Stanford Research Systems). In the measurement of piezoresponse hysteresis loop, an AC voltage of 1 Vrms at a frequency of 17 kHz with DC voltage of +10 to −10 V was applied to a conductive probe (Multi75E-G, BudgetSensors). We note that the hysteresis loop was obtained for the off-field state. Electrical properties (current versus voltage (I−V) curve) of a single Au/P(VDF-TrFE) heterostructured nanoisland were measured by using CAFM (MultiMode 8, Bruker). The maximum and minimum currents for the CAFM measurement limit were +12 and −10 nA, respectively. GIWAXS measurements were performed at room temperature on beamline 9A (beam energy of 11.12 keV at the Pohang Accelerator Laboratory (PAL) (Korea) to investigate the chain orientations of the P(VDF-TrFE) thin films. The incident angle was 0.15° and exposure time was 10 s.

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RESULTS AND DISCUSSION Figure 1 gives a scheme to fabricate a high density array of MFM type P(VDF-TrFE) nanodiodes in a large area. First, P(VDF-TrFE) thin film was prepared by spin coating on a flexible conductive substrate (ITO-coated PET). To increase the wettability between P2VP chains and P(VDF-TrFE), the P(VDF-TrFE) thin film was treated by ozone (see Figure S3 in Supporting Information). Then, PS-b-P2VP micelles in toluene were spin coated on the preformed P(VDF-TrFE) film. Because of increased hydrophilicity of P(VDF-TrFE) surface after ozone treatment, the P2VP chains were well wetted on the film surface and dimple-type micellar structures were obtained. 27076

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces gold precursors: D = 220 ± 50 nm and H = 50 ± 10 nm for S2VP1, and D = 45 ± 5 nm and H = 15 ± 5 nm for S2VP2. Figures 2a3 and 2b3 clearly show a high density array of Au/ P(VDF-TrFE) heteronanoislands after oxygen plasma treatment. This indicates that self-positioned GNP on the top of P(VDF-TrFE) successfully acts as an etching mask. The size of Au/P(VDF-TrFE) heteronanoislands after oxygen plasma etching was: D = 230 ± 50 nm and H = 90 ± 10 nm prepared from S2VP1, and D = 50 ± 5 nm and H = 25 ± 5 nm prepared from S2VP2. SEM image in Figures 2a5 and 2b5 show that a high density array of Au/P(VDF-TrFE) heteronanoislands was successfully obtained in a large area (see also Figure S5 of Supporting Information). Figure 3a gives 60° tilted SEM image of a single Au/P(VDFTrFE) nanoisland. Because we were not able to know directly

oxygen plasma treatment. The distance of the lattice planes is ∼2.33 Å corresponding to Au(111) plane.37 Figure 4a gives amplitude, phase, and piezoresponse hysteresis loops measured by PFM of a single Au/P(VDFTrFE) heteronanoisland with GNP at the top. A typical butterfly shape of amplitude loops and 180° variation of phase loops are clearly observed, indicating all of individual nanoisland showed ferroelectric switching at coercive voltages of −5.5 and +4.5 V. We also measured I−V characteristics of a single nanoisland by using CAFM to investigate the effect of polarization direction on charge conductions. I−V measurements were carried out by sweeping the bias voltage from 0 to +8 V, back to −8 V, and returned 0 V. A large hysteresis in I−V curve is clearly observed (Figure 4b). The initial electric current through the nanoisland with upward polarization shows diode-like behavior. By continuously sweeping to positive voltage (from 0 to +8 V), the current rises rapidly at around +5.8 V. This indicates that low conducting state (OFF state) was changed to high conducting state (ON state). Inversely, by sweeping the negative voltage (from 0 to −8 V), the electric current changed rapidly at around −6.1 V, indicating change in the conducting states from OFF to ON states. Comparing between ferroelectric hysteresis loops (Figure 4a) and I−V curve (Figure 4b), transition voltages in I−V diode-like curve are similar to coercive voltages obtained from PFM hysteresis loops. This means that the conductivity of the P(VDF-TrFE) nanoisland might be strongly dependent on the direction of polarization. As previously reported,10 the Schottky barriers or depletion regions at the electrode/semiconducting ferroelectric interfaces can be modulated by the polarization charges. Thus, the observed current behavior might dominantly result from the polarization dependent charge conduction. As shown in Figure 5a, Au/P(VDF-TrFE) nanodiodes array on ITO-coated PET substrate is highly flexible. To investigate the mechanical stability of the array of Au/P(VDF-TrFE) nanodiode under bending, SEM and AFM images and electrical conductivity before and after 500 and 1000 times bending were obtained at the center region of the substrate shown in the red line box of Figure 5a (the enlarged SEM image is given in Figure 5b), where the largest bending force is expected. At this location, the tensile strain of a single nanodiode was roughly estimated as 5.2 × 10−3 (see Figure S6 of Supporting Information). As shown in Figure 5c and d, there was no change in size and shape of Au/P(VDF-TrFE) heteronanoislands before and after 1000 times bending, indicating excellent maintenance against a large number of bending. As shown in Figure 5e, the I−V curve of single Au/P(VDF-TrFE) heteronanoisland remains maintained even after 1000 bendings. Namely, the transition voltages of diode current switching show nearly the same (Figure 5e). This is because nanosized P(VDFTrFE) nanoislands are not significantly affected by macroscopic bending and cross-talk effect. Furthermore, the array shows highly transparent (the average transmittance in visible wavelengths (380−750 nm) is as high as 96.3% (Figure 5f), which is even higher than that (94.8%) of continuous P(VDFTrFE) film (Figure S7 of Supporting Information). Thus, a high density array of Au/P(VDF-TrFE) heteronanoislands fabricated on a flexible substrate (ITO-coated PET) is applicable for transparent wearable memory device.

Figure 3. (a) 60° tilted SEM image of Au/P(VDF-TrFE) heteronanoislands array. Inset is a magnified SEM image. (b1) STEM image of a single Au/P(VDF-TrFE) heteronanoisland. STEMEDS element mapping images of (b2) Au, (b3) fluorine, and (b4) Au/ fluorine elements. (c) HRTEM image of a single GNP showing gold crystalline. (d) Lattice parameter of (111) plane of Au crystal. (e) SAED pattern of Au GNP.

the position of GNP in the nanoisland in SEM image, we took HRTEM and STEM images and performed EDS element mapping analysis. Figure 3a shows a single Au/P(VDF-TrFE) heteronanoisland prepared by S2VP2. As shown in Figures 3a2−a4, the GNP was located at the top of the P(VDF-TrFE) nanoisland. HRTEM and selected area electron diffraction (SAED), as shown in Figure 3c and d, respectively, show that crystalline Au metals are obtained from Au precursor after 27077

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Amplitude, phase, and piezoresponse hysteresis loops measured by PFM of a single Au/P(VDF-TrFE) heteronanoisland. (b) I−V curves of a single Au/P(VDF-TrFE) heteronanoisland measured at two different polarization directions: positive-forward (downward) direction (red arrow) and negative-forward (upward) direction (blue arrow). We note that hysteresis loops and I−V curves were measured at different single Au/ P(VDF-TrFE) heteronanoislands.

Figure 5. (a) Photograph and (b) SEM images of bent Au/P(VDF-TrFE) heteronanoislands on flexible ITO-coated PET substrate. (c) SEM and (d) AFM images (c1 and d1) before and after (c2 and d2) 500 times and (c3 and d3) 1000 times bending. Scale bar is 100 nm. (e) I−V curves before and after bending. (f) Transmittance spectra of Au/P(VDF-TrFE) heteronanoislands at the visible light wavelengths (380−750 nm).

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CONCLUSION

1000 times), and this array show excellent transparency. It could be used as the next-generation wearable memory device.

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We fabricated the self-positioned GNP confined within nanosized block copolymer micelle cores, which successfully act as an etching mask to prepare a high density array of Au/ P(VDF-TrFE) heteronanoislands on a flexible ITO-coated PET substrate in a large area. The single Au/P(VDF-TrFE) heteronanoisland shows a clear ferroelectric piezoresponse hysteresis loop and diode behavior. Because of the nanosized diode and flexible substrate, I−V characteristics were maintained even at a large number of bending (for instance,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08459. AFM topography, PFM amplitude and PFM phase images, and 2D GIWAXS patterns of the P(VDF-TrFE) thin films depending on annealing temperature, contact 27078

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces

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(11) Hyun, S.; Seo, H.; Yang, I.-K.; Kim, Y.; Jeon, G.; Lee, B.; Jeong, Y. H.; Kim, Y.; Kim, J. K. High Density Array of Multiferroic Nanoislands in a Large Area. J. Mater. Chem. C 2015, 3, 2237−2243. (12) Hyun, S.; Kwon, O.; Lee, B.; Seol, D.; Park, B.; Lee, J. Y.; Lee, J. H.; Kim, Y.; Kim, J. K. Multi-Floor Cascading Ferroelectric Nanostructures: Multiple Data Writing-Based Multi-Level NonVolatile Memory Devices. Nanoscale 2016, 8, 1691−1697. (13) Kim, Y.; Han, H.; Lee, W.; Rodriguez, B. J.; Vrejoiu, I.; Lee, W.; Baik, S.; Hesse, D.; Alexe, M. Individual Switching of Film-based Nanoscale Epitaxial Ferroelectric Capacitors. J. Appl. Phys. 2010, 108, 042005. (14) Cai, R.; Jonas, A. M. Local Maps of the Polarization and Depolarization in Organic Ferroelectric Field-Effect Transistors. Sci. Rep. 2016, 6, 22116. (15) Dubourdieu, C.; Bruley, J.; Arruda, T. M.; Posadas, A.; JordanSweet, J.; Frank, M. M.; Cartier, E.; Frank, D. J.; Kalinin, S. V.; Demkov, A. A.; Narayanan, V. Switching of Ferroelectric Polarization in Epitaxial BaTiO3 Films on Silicon without a Conducting Bottom Electrode. Nat. Nanotechnol. 2013, 8, 748−754. (16) Ganpule, C. S.; Stanishevsky, A.; Aggarwal, S.; Melngailis, J.; Williams, E.; Ramesh, R.; Joshi, V.; Paz de Araujo, C. Scaling of Ferroelectric and Piezoelectric Properties in Pt/SrBi2Ta2O9 /Pt Thin Films. Appl. Phys. Lett. 1999, 75, 3874−3876. (17) Alexe, M.; Harnagea, C.; Hesse, D. Non-Conventional Microand Nanopatterning Techniques for Electroceramics. J. Electroceram. 2004, 12, 69−88. (18) Ganpule, C. S.; Stanishevsky, A.; Su, Q.; Aggarwal, S.; Melngailis, J.; Williams, E.; Ramesh, R. Scaling of Ferroelectric Properties in Thin Films. Appl. Phys. Lett. 1999, 75, 409−411. (19) Nagarajan, V.; Stanishevsky, A.; Ramesh, R. Ferroelectric Nanostructures via a Modified Focused Ion Beam Technique. Nanotechnology 2006, 17, 338−343. (20) Nagarajan, V.; Roytburd, A.; Stanishevsky, A.; Prasertchoung, S.; Zhao, T.; Chen, L.; Melngailis, J.; Auciello, O.; Ramesh, R. Dynamics of Ferroelastic Domains in Ferroelectric Thin Films. Nat. Mater. 2003, 2, 43−47. (21) Schilling, A.; Byrne, D.; Catalan, G.; Webber, K. G.; Genenko, Y. A.; Wu, G. S.; Scott, J. F.; Gregg, J. M. Domains in Ferroelectric Nanodots. Nano Lett. 2009, 9, 3359−3364. (22) Bühlmann, S.; Dwir, B.; Baborowski, J.; Muralt, P. Size Effect in Mesoscopic Epitaxial Ferroelectric Structures: Increase of Piezoelectric Response with Decreasing Feature Size. Appl. Phys. Lett. 2002, 80, 3195−3197. (23) Lee, K.; Baik, S. Ferroelastic Domain Structure and Switching in Epitaxial Ferroelectric Thin Films. Annu. Rev. Mater. Res. 2006, 36, 81−116. (24) Pang, X.; Zhao, L.; Han, W.; Xin, X.; Lin, Z. A General and Robust Strategy for the Synthesis of Nearly Monodisperse Colloidal Nanocrystals. Nat. Nanotechnol. 2013, 8, 426−431. (25) Kim, Y.; Han, H.; Kim, Y.; Lee, W.; Alexe, M.; Baik, S.; Kim, J. K. Ultrahigh Density Array of Epitaxial Ferroelectric Nanoislands on Conducting Substrates. Nano Lett. 2010, 10, 2141−2146. (26) Cauda, V.; Stassi, S.; Bejtka, K.; Canavese, G. Nanoconfinement: an Effective Way to Enhance PVDF Piezoelectric Properties. ACS Appl. Mater. Interfaces 2013, 5, 6430−6437. (27) Bhavanasi, V.; Kumar, V.; Parida, K.; Wang, J.; Lee, P. S. Enhanced Piezoelectric Energy Harvesting Performance of Flexible PVDF-TrFE Bilayer Films with Graphene Oxide. ACS Appl. Mater. Interfaces 2016, 8, 521−529. (28) Alluri, N. R.; Saravanakumar, B.; Kim, S.-J. Flexible, Hybrid Piezoelectric Film (BaTi(1-x)Zr(x)O3)/PVDF Nanogenerator as a Self-Powered Fluid Velocity Sensor. ACS Appl. Mater. Interfaces 2015, 7, 9831−9840. (29) Lee, J.; van Breemen, A. J. J. M.; Khikhlovskyi, V.; Kemerink, M.; Janssen, R. A. J.; Gelinck, G. H. Pulse-Modulated Multilevel Data Storage in an Organic Ferroelectric Resistive Memory Diode. Sci. Rep. 2016, 6, 24407.

angle (CA) and AFM topography images of P(VDFTrFE) films before and after UV-ozone treatment, AFM topography images of formation of GNPS during oxygen plasma etching, SEM images covering a large area of the array of Au/P(VDF-TrFE) heteronanoislands, tensile strain of a single P(VDF-TrFE) nanoisland during the bending, and transmittance spectra (visible range: 380− 750 nm) of P(VDF-TrFE) film and nanoislands array (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. K. Kim). *E-mail: [email protected] (Y. Kim). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF) grant (no. 2013R1A3A2042196) funded by the Korean government. It was also supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the “IT Consilience Creative Program” (NIPA-2014H0201-14-1001) supervised by the NIPA (National IT Industry Promotion Agency).

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REFERENCES

(1) Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.; Cheong, S.-W. Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3. Science 2009, 324, 63−66. (2) Guo, R.; You, L.; Zhou, Y.; Lim, Z. S.; Zou, X.; Chen, L.; Ramesh, R.; Wang, J. Non-Volatile Memory Based on the Ferroelectric Photovoltaic Effect. Nat. Commun. 2013, 4, 1990. (3) Asadi, K.; de Leeuw, D. M.; de Boer, B.; Blom, P. W. M. Organic Non-Volatile Memories from Ferroelectric Phase-Separated Blends. Nat. Mater. 2008, 7, 547−550. (4) Kim, Y.; Han, H.; Vrejoiu, I.; Lee, W.; Hesse, D.; Alexe, M. Cross Talk by Extensive Domain Wall Motion in Arrays of Ferroelectric Nanocapacitors. Appl. Phys. Lett. 2011, 99, 202901. (5) Han, H.; Kim, Y.; Alexe, M.; Hesse, D.; Lee, W. Nanostructured Ferroelectrics: Fabrication and Structure−Property Relations. Adv. Mater. 2011, 23, 4599−4613. (6) Lee, W.; Han, H.; Lotnyk, A.; Schubert, M. A.; Senz, S.; Alexe, M.; Hesse, D.; Baik, S.; Gösele, U. Individually Addressable Epitaxial Ferroelectric Nanocapacitor Arrays with Near Tb inch−2 Density. Nat. Nanotechnol. 2008, 3, 402−407. (7) Kassa, H. G.; Nougaret, L.; Cai, R.; Marrani, A.; Nysten, B.; Hu, Z.; Jonas, A. M. The Ferro- to Paraelectric Curie Transition of a Strongly Confined Ferroelectric Polymer. Macromolecules 2014, 47, 4711−4717. (8) Kassa, H. G.; Cai, R.; Marrani, A.; Nysten, B.; Hu, Z.; Jonas, A. M. Structure and Ferroelectric Properties of Nanoimprinted Poly(vinylidene fluoride-ran-trifluoroethylene). Macromolecules 2013, 46, 8569−8579. (9) Kim, Y.; Kim, Y.; Han, H.; Jesse, S.; Hyun, S.; Lee, W.; Kalinin, S. V.; Kim, J. K. Towards the Limit of Ferroelectric Nanostructures: Switchable Sub-10 nm Nanoisland Arrays. J. Mater. Chem. C 2013, 1, 5299−5302. (10) Hong, S.; Choi, T.; Jeon, J. H.; Kim, Y.; Lee, H.; Joo, H. Y.; Hwang, I.; Kim, J. S.; Kang, S. O.; Kalinin, S. V.; Park, B. H. Large Resistive Switching in Ferroelectric BiFeO3 Nano-Island Based Switchable Diodes. Adv. Mater. 2013, 25, 2339−2343. 27079

DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080

Research Article

ACS Applied Materials & Interfaces (30) Stingelin, M.; Li, N.; Michels, J. J.; Spijkman, M.-J.; Asadi, K.; Feldman, K.; Blom, P. W. M.; de Leeuw, D. M. Ferroelectric Phase Diagram of PVDF:PMMA. Macromolecules 2012, 45, 7477−7485. (31) Kim, R. H.; Kim, H. J.; Bae, I.; Hwang, S. K.; Velusamy, D. B.; Cho, S. M.; Takaishi, K.; Muto, T.; Hashizume, D.; Uchiyama, M.; Andre, P.; Mathevet, F.; Heinrich, B.; Aoyama, T.; Kim, D.; Lee, H.; Ribierre, J.; Park, C. Non-volatile Organic Memory with SubMillimetre Bending Radius. Nat. Commun. 2014, 5, 3583. (32) Lee, J.-H.; Lee, K. Y.; Kumar, B.; Tien, N. T.; Lee, N.-E.; Kim, S.-W. Highly Sensitive Stretchable Transparent Piezoelectric Nanogenerators. Energy Environ. Sci. 2013, 6, 169−175. (33) Lee, J.-H.; Ryu, H.; Kim, T.-Y.; Kwak, S.-S.; Yoon, H.-J.; Kim, T.-H.; Seung, W.; Kim, S.-W. Thermally Induced Strain-Coupled Highly Stretchable and Sensitive Pyroelectric Nanogenerators. Adv. Energy Mater. 2015, 5, 1500704. (34) Jung, W.-S.; Kang, M.-G.; Moon, H. G.; Baek, S.-H.; Yoon, S.-J.; Wang, Z.-L.; Kim, S.-W.; Kang, C.-Y. High Output Piezo/Triboelectric Hybrid Generator. Sci. Rep. 2015, 5, 9309. (35) Cho, H.; Choi, S.; Kim, J. Y.; Park, S. Fabrication of Gold Dot, Ring, and Corpuscle Arrays from Block Copolymer Templates via a Simple Modification of Surface Energy. Nanoscale 2011, 3, 5007− 5012. (36) Wang, L.; Montagne, F.; Hoffmann, P.; Pugin, R. Gold Nanoring Arrays from Responsive Block Copolymer Templates. Chem. Commun. 2009, 3798−3800. (37) Shen, Y.; Liu, Y.; Wang, W.; Xu, F.; Yan, C.; Zhang, J.; Wang, J.; Yuan, A. Au Nanocluster Arrays on Self-Assembled Block Copolymer Thin Films as Highly Active SERS Substrates with Excellent Reproducibility. RSC Adv. 2016, 6, 38716−38723. (38) Wahid, M. H. M.; Dahan, R. M.; Sa’ad, S. Z.; Arshad, A. N.; Sarip, M. N.; Mahmood, M. R.; Chen, G. W.; Haliza, A. M. W. Effect of Annealing Temperature on the Crystallinity, Morphology and Ferroelectric of Polyvinylidenefluoride-rifluoroethylene (Pvdf-Trfe) Thin Film. Adv. Mater. Res. 2013, 812, 60−65.

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DOI: 10.1021/acsami.6b08459 ACS Appl. Mater. Interfaces 2016, 8, 27074−27080