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Nature of the Enhancement in Ferroelectric Properties by Gold Nanoparticles in Vinylidene Fluoride and Trifluoroethylene Copolymer Naoto Tsutsumi,*,† Ryusei Kosugi,‡ Kenji Kinashi,† and Wataru Sakai† †

Faculty of Materials Science and Engineering and ‡Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: Ferroelectric polymers are a candidate for versatile and cheap data storage memory devices, with easy processing for a large-scale device. Easy switching and large remanent polarization of preferentially formed βcrystal dipoles in a copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) are promising properties for versatile memory devices. At higher frequency switching, however, the remanent polarization is reduced and a high coercive field, an electric field for polarization switching is required. The addition of a small amount of nanoparticles (NPs) significantly improves these ferroelectric properties in fluoropolymers. Here, we show that the addition of NPs of gold (Au), silver (Ag), and silicon oxide (SiO2) enhanced the ferroelectric properties in P(VDF-TrFE). AuNPs significantly affected a 40% increase of the remanent polarization, 14% reduction of the coercive field, and 100% increase of the switching speed of ferroelectric polarization. The nature of these enhancements due to the addition of NPs is verified. A higher shift of the binding energy of Au (4f7/2 and 4f5/2) and an increase of the fluorine ion (F−) was observed in AuNP-doped P(VDF-TrFE). Strong interactions between the AuNPs and the ferroelectric backbone gave rise to the formation of the interfacial polarization, which induced the local electric field to enhance the ferroelectric properties of the increment of the remanent polarization, the reduction of the coercive field, and faster switching speed. KEYWORDS: ferroelectric polymers, Au nanoparticles, copolymer of vinylidene fluoride and trifluoroethylene, binding energy, interfacial polarization, local electric field PVDF doped with ZnO nanoparticles was also reported.10 Preferential formation of β-phase crystals was reported in palladium-doped PVDF thin films11 and gold nanoparticledoped PVDF thin films.12 Gold nanoparticle-induced polymorphism was investigated in PVDF.13 The enhancement of ferroelectric switching in P(VDF-TrFE) by the addition of gold nanoparticles was reported,14 but the origin has not been clarified yet. From the point of view of power energy storage, high permeability polymer composites with nanoparticles have also been investigated.15,16 We show that gold (Au), silver (Ag), and silicon oxide (SiO2 ) nanoparticles (NPs) enhanced the ferroelectric performance of P(VDF-TrFE). In particular, the addition of AuNPs significantly induced 40% increase of remanent polarization (Pr), 13−14% reduction of coercive field (Ec), and 60−100% increase of switching speed of the ferroelectric polarization. To clarify the nature of the enhancement of ferroelectricity by the addition of NPs, we performed X-ray

1. INTRODUCTION Following early studies of polar and nonpolar crystal structures, phase transitions, and ferroelectricity in poly(vinylidene fluoride) (PVDF) in the 1970s, the discovery of the preferential formation of ferroelectric β-phase crystals in a copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE))1 has significantly aided the understanding of ferroelectric polymers. Furthermore, the easy formation of ferroelectric β-phase crystals in P(VDF-TrFE) by the simple processing of meltpress quenching, spin-coating from solution, and solvent casting, and its reversible spontaneous polar polarization have also spread the research interest to extended fields. In the 1990s and 2000s, ferroelectric memory devices using P(VDF-TrFE) were investigated.2,3 Recently, field-effect transistors with P(VDF-TrFE) gate structures4 and organic photovoltaic cells5,6 were reported. Preferential formation of the δ-phase in PVDF was also recently reported.7 The recent development and easy handling of nanoparticles have triggered the enhancement of electrical and optical properties of materials. The effect of silver nanoparticle on the electrical and ferroelectric properties of P(VDF-TrFE) has been reported.8,9 The enhancement of pyroelectric response in © 2016 American Chemical Society

Received: May 18, 2016 Accepted: June 16, 2016 Published: June 16, 2016 16816

DOI: 10.1021/acsami.6b05897 ACS Appl. Mater. Interfaces 2016, 8, 16816−16822

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ACS Applied Materials & Interfaces

100% increase of the switching speed is achieved by the addition of AuNPs As described in the Experimental section, the dielectric constant of each sample was evaluated from the capacitance term of switching current and shown in Table 1d. The dielectric constant increased by the addition of AuNPs, AgNPs, and SiO2NPs. It is well-known that introducing conductive particles in a polymer host induces an increase of the dielectric constant of the polymer due to interfacial polarization phenomena (low frequency). This effect leads to a higher local electric field on the crystalline phase. Thus, the increase in remnant polarization and the increase in polarization switching rate can be ascribed to an increase in interfacial effects between inorganic and organic phases. The crystalline states of a ferroelectric domain are wellknown to influence the ferroelectric behaviors and properties in PVDF, P(VDF-TrFE) and odd-Nylon ferroelectrics; larger crystallinity prefers a larger remanent polarization. Crystallinity evaluated from the X-ray diffraction pattern is shown for pristine and NP-doped P(VDF-TrFE) thin films in Table 2. Table S1 summarizes the information from the crystal structures measured using X-ray diffraction in the present thin films. The crystallinity of the ferroelectric domain in P(VDF-TrFE) is slightly reduced by the addition of NPs, but crystallite size and lattice interval are not. Surface image of each sample observed using an atomic force microscope (AFM) is shown in Figure 2. No significant difference was measured among samples. Figure 3a shows the FT-IR ATR spectra in the range between 700 and 1500 cm−1 for each thin film. The FT-IR ATR method is a useful tool to detect the vibrational mode parallel to the substrate.17 Characteristic peaks due to the crystal’s aaxis are the absorption peak at 1177 cm−1, assigned to the asymmetric stretching vibration of CF2 (νaCF2) coupled with the rocking vibration of CF2 (rCF2), and the peak at 890 cm−1, assigned to the rocking vibration of CH2 (rCH2) coupled with rCF2 and νaCF2. The characteristic peaks due to the crystal’s baxis are the absorption peak at 1290 cm−1, assigned to the symmetric stretching vibration of CF2 (νsCF2) coupled with the symmetric vibration of C−C (νsCC) and the bending vibration of C−C−C (δCCC), and the peak at 852 cm−1, assigned to the νsCF2 coupled with νsCC. The characteristic peaks due to the caxis (chain axis) are the absorption peak at 1400 cm−1, assigned to the wagging vibration of CH2 (wCH2) coupled with the asymmetric vibration of C−C (νaCC), and the peak at 1080 cm−1, assigned to νaCC coupled with wCH2 and the wagging vibration of CF2 (wCF2). The absorbance ratio between the 1400 cm−1 (c-axis) and 1177 cm−1 (a-axis) peaks are summarized in Table 2. The absorbance ratio is 0.92 for AuNP-doped P(VDF-TrFE), 0.79 for AgNP-doped P(VDFTrFE), 0.63 for SiO2NP-doped P(VDF-TrFE), and 0.62 for pristine P(VDF-TrFE). The larger absorbance ratio indicates that the molecular chain of polymers near the interface between the sample bulk and substrate tends to align parallel to the substrate. Thus, AuNPs significantly assist the parallel alignment of molecular chains of P(VDF-TrFE). Figure 3b shows the predicted schematic pictures of molecular chain alignment in pristine P(VDF-TrFE) and AuNP-doped P(VDF-TrFE). Because of the preferential alignment of the molecular chains parallel to the substrate, the dipole moments of ferroelectric domain is subjected to an effective large electric field. 2.2. Binding Energy Measurement. XPS is a useful tool to directly measure the binding energy between atoms and molecules. C 1s binding energies appeared in the range of 282

photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), and wide-angle X-ray diffraction (WAXD) measurements on thin films of pristine and NPdoped P(VDF-TrFE). XPS directly measures the state of the binding energy of atoms, which relates to the ferroelectric quantities and explains the enhancement of the remanent polarization by the addition of AuNPs. To the best of our knowledge, the direct measurement of the interaction between AuNPs and the ferroelectric domain in P(VDF-TrFE) has not been reported.

2. RESULTS AND DISCUSSION 2.1. NPs Enhanced Ferroelectric Response. The determination of the appropriate size and concentration of NPs are summarized in the Supporting Information. An appropriate NPs size of 10 nm and a NP concentration of 1 × 10−3 wt % were determined. Typical ferroelectric switching current and hysteresis loop are shown for pristine P(VDFTrFE) and NP-doped P(VDF-TrFE) thin films in Figure 1a, b,

Figure 1. (a) Ferroelectric switching current and (b) hysteresis loop of pristine and NP-doped P(VDF-TrFE) thin films. A 1 kHz cycling frequency with an amplitude of 170 MV m−1 was employed. Remanent polarization values, Pr, the displaced charge density at zero electric field, of AuNP and AgNP-doped P(VDF-TrFE) are 98.5 and 97.3 mC m−2, respectively, which are 40% higher than that of pristine P(VDFTrFE). Respective coercive fields are 85.0 and 84.7 MV m−1, which are 13−14% lower than that of pristine P(VDF-TrFE).

respectively. The switching cycle is 1 kHz, and the maximum amplitude of cycle is 170 MV m−1. Compared with the hysteresis loop of a pristine P(VDF-TrFE) thin film, slim hysteresis loops with lower Ec and larger Pr are observed in AuNP and AgNP-doped P(VDF-TrFE) thin films. Table 1 summarizes Pr, Ec, the speed of polarization switching, and dielectric constant for each sample measured at switching frequencies between 1 to 100 kHz with an amplitude of 170 MV m−1. Pr is largely enhanced and Ec is reduced by the addition of AuNPs and AgNPs. The switching speed of polarization reversal is significantly enhanced by the addition of AuNPs. A 40% increase of Pr, 13−14% reduction of Ec and 60− 16817


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Table 1. Summary of Ferroelectric Properties (Pr, Ec, and Switching Speed) and Dielectric Properties (Dielectric Constant)a (a) Remanent Polarization Pr (mC m−2) sample

10 Hz

pristine P(VDF-TrFE) AuNPs/P(VDF-TrFE) AgNPs/P(VDF-TrFE) SiO2NPs/P(VDF-TrFE)

76.3 108.1 108.1 95.1

100 Hz

1 kHz

10 kHz

100 kHz

69.7 98.5 97.3 85.8

64.1 94.0 94.2 71.1

62.3 81.4 80.6 21.6

1 kHz

10 kHz

100 kHz

98.1 85.0 84.7 111.4

122.3 108.4 107.7 135.4

145.2 142.2 142.2 150.6

72.2 103.0 104.3 90.3 (b) Coercive Field

Ec (MV m−1) sample

10 Hz


60.4 61.7 60.4 82.6

100 Hz 72.2 70.7 69.8 93.3 (c) Switching Speed

switching speed (s−1)

a

sample

10 Hz


× × × ×

3.26 5.40 3.96 2.94

100 Hz

1 kHz

2.30 × 10 4.19 × 103 2.83 × 103 2.35 × 103 (d) Dielectric Constant

2

3

10 102 102 102

1.60 3.09 2.13 1.88

× × × ×

10 kHz 4

10 104 104 104

1.26 2.10 1.50 1.62

× × × ×

100 kHz 5

10 105 105 105

1.47 1.79 1.32 2.30

× × × ×

106 106 106 106

sample

10 Hz

100 Hz

1 kHz

10 kHz

100 kHz


10.55 10.79 15.25 9.61

11.61 12.57 14.08 10.04

9.85 12.39 14.82 10.53

10.81 10.93 13.40 11.61

10.6 10.84 13.75 10.88

Ferroelectric response data were taken with various cycling frequencies with an amplitude of 170 MV m−1 for every thin film.

Table 2. Crystallinity, Absorption Ratio, Percentage of Fluoride Ion F−, and Summary of Binding Energies in Pristine and NPDoped P(VDF-TrFE) (a) Crystallinity, Absorption Ratio, and Percentage of Fluoride Ion F− sample

abs (1400 cm−1)/abs(1177 cm−1)

crystallinity (%)


F− (%)

70.0 0.62 63.0 0.92 66.6 0.79 58.8 0.63 (b) Summary of C 1s, O 1s, and F 1s Binding Energies

sample

C

CH2

CF

CF2

O

COO−


285.0 285.0 285.0 285.0

286.4 286.4 286.8 286.8

288.4 288.6 289.3 288.8

290.8 291.0 291.4 291.3

532.5 532.8 532.5

531.0

5.94 11.06 7.37 7.39 F−

CH2CF2

CHFCF2

685.7 686.7 686.5 686.8

687.1 687.9 688.0 688.4

688.1 689.0 688.9 689.3

a

Crystallinity was evaluated from the X-ray diffraction pattern. Diffraction peaks are separated into crystalline and noncrystalline parts, and crystallinity was evaluated from the percentage of the crystalline part. The ratio between absorbance due to the c-axis and that due to the a-axis was measured from the FT-IR ATR method. The percentage of F− was evaluated from the XPS spectra due to the binding energy of F 1s.

significant change or shift in C 1s binding energies, as shown in Table 2. XPS spectra of Au (4f7/2 and 4f5/2) are shown in neat AuNPs and AuNPs in P(VDF-TrFE) matrix in Figure 4a. The same type of spectra for AgNPs and SiO2NPs are shown in Figure 4b, c, respectively. Au (4f7/2 and 4f5/2) binding energies appeared at 84.4 and 88 eV in the P(VDF-TrFE) thin film, respectively, whereas energies of appeared at 83.4 and 86.8 eV in neat AuNPs, respectively. The Au (4f7/2 and 4f5/2) binding energies clearly shifted to 1−1.2 eV higher energies in the P(VDFTrFE) thin film. The Ag (3d5/2 and 3d3/2) binding energies

to 292 eV in each sample. XPS spectra of C 1s are shown in Figure S3. Overlapped XPS signals were separated using Gaussian peaks. For example, the obtained XPS spectra can be divided into four Gaussian peaks with different binding energies centered at 285.0, 286.4, 288.4, and 290.8 eV in pristine P(VDF-TrFE). C 1s binding energies appearing at 286.4, 288.4, and 290.8 eV are assigned to those due to CH2, CF, and CF2 in P(VDF-TrFE), respectively.18,19 Table 2 summarizes the binding energies for pristine P(VDF-TrFE) and NP-doped P(VDF-TrFE). The addition of NPs does not result in any 16818


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Figure 2. AFM images of sample surface. (a) Pristine P(VDF-TrFE), (b) AuNPs/P(VDF-TrFE), (c) AgNPs/P(VDF-TrFE), and (d) SiO2NPs/P(VDF-TrFE). The horizontal and vertical scale is shown in figure.

Figure 4. XPS spectra of Au, Ag, Si, and O 1s. (a) AuNPs, (b) AgNPs, and (c) SiO2NPs. (a) Au (4f7/2 and 4f5/2) binding energies appearing at 83.4 and 86.8 eV, respectively, in neat AuNPs are shifted 1 eV higher in the P(VDF-TrFE) thin film. (b) Ag binding energies are not shifted in P(VDF-TrFE). (c) SiO2 binding energy. O 1s binding energy in AuNPs/P(VDF-TrFE) (d), in AgNPs/P(VDF-TrFE) (e), and in SiO2NPS/P(VDF-TrFE) (f). Each sample shows O 1s binding energy in the range of 532.5 to 532.8 eV. In addition, AuNPs/P(VDFTrFE) gives O 1s binding energy due to COO− at 531 eV. Figure 3. FT-IR absorption spectra and schematic picture of molecular chains alignment on the substrate. (a) FT-IR absorption spectra in the fingerprint region of pristine and NP-doped P(VDF-TrFE) thin films. a, b, and c-axes are axes of the crystal lattices: c-axis is the axis along the molecular chain, a and b-axes are axes between the intermolecular chains. Blue, a-axis; green, b-axis; and red, c-axis. The characteristic absorption peak due to the c-axis (chain axis) is that at 1400 cm−1, assigned to the wagging vibration of CH2 (wCH2) coupled with the asymmetric vibration of C−C (νaCC). The characteristic absorption peak due to the a-axis is that at 1177 cm−1, assigned to the asymmetric stretching vibration of CF2 (νaCF2) coupled with the rocking vibration of CF2 (rCF2). Thus, the absorbance ratio between the 1400 cm−1 (caxis) and 1177 cm−1 (a-axis) peaks is a measure of the molecular orientation of the chain to the substrate. (b) Schematic picture of molecular chains alignment on the substrate. Dashed arrow: applied electric field. Solid arrow: effective field subjected to the dipoles.

(2p) binding energy appeared at 99.0 eV in P(VDF-TrFE), which is 0.3 eV higher in neat SiO2NPs. A distinct, large shift is measured in the Au (4f7/2 and 4f5/2) binding energies. The shift of the binding energies of Au (4f7/2 and 4f5/2) is ascribed to electron withdrawing by the fluorine atom in P(VDF-TrFE) through the carboxyl anion electrostatically adsorbed to the AuNPs. O 1s binding energy is appeared in the range of 528 to 536 eV for each thin film in Figure 4d−f. The O 1s binding energy curves in AgNPs/P(VDF-TrFE) and SiO2NPs/P(VDF-TrFE) samples are fitted well by single Gaussian peak centered at 532.8 and 532.5 eV (the dashed red curve in Figure4 e, f), respectively, whereas in the O 1s spectrum of the AuNPs/ P(VDF-TrFE) sample, a clear shoulder appears at 531 eV in addition to the single Gaussian peak centered at 532.5 eV. The clear shoulder at 531 eV is ascribed to the O 1s binding energy due to COO− is derived from the citric acid buffer solution in

appeared at 367.2 and 373.3 eV in P(VDF-TrFE), respectively. These values are almost the same as in neat AgNPs. The Si 16819


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ACS Applied Materials & Interfaces AuNPs/P(VDF-TrFE). O 1s binding energy due to COO− was reported to appear at 531 eV.20 The F 1s binding energy appears in the range of 684 to 690 eV for P(VDF-TrFE). The F 1s binding energy is also affected by the binding condition of the fluorine atom in the matrix.16 The XPS spectra due to F 1s binding energy are the summation of F 1s binding energies due to the fluoride ion F−, CH2CF2, and CHFCF2 which appear at 685.7, 687.1, and 688.1 eV, respectively, in pristine P(VDF-TrFE). The separated XPS spectra is shown in Figure 5. The addition of NPs (AuNPs,

coevaporated composites.21 As described above, the electron was withdrawn from the Au NPs to the fluorine atoms. Furthermore, the existence of COO− in AuNPs/P(VDF-TrFE) is estimated from the O 1s XPS spectrum. From these results of the binding energies in AuNPs/P(VDF-TrFE), a significant interaction between Au NPs and fluorine atoms can be proposed. The charge transferred electronic states were assumed. The schematic model of Au NPs and P(VDFTrFE) through citric acid buffer is shown in Figure 5e. The electron is donated from Au NPs to the fluorine atom in P(VDF-TrFE) through the citric acid buffer. This result is consistent with the above discussion in which the addition of nanoparticles leads to the interfacial polarization on the crystalline phase. Namely the interfacial polarization formed between positively charged AuNPs and negatively charged fluorine atoms in the polymer backbone induced the local electric field, which enhanced the remanent polarization and fast switching speed. It is noted that Pr ≈ 133 mC m−2 is observed in the 10 nm Au NP-doped P(VDF-TrFE) sample, as shown in Figure S1. Using the measured crystallinity of 63.0%, the maximum remanent polarization of 211 mC m−2 is calculated for the present AuNPs/P(VDF-TrFE). This value is comparable to the remanent polarization of the β-crystal form of PVDF calculated between 130 mC m−2 without Lorentz local electric field and 220 mC m−2 with Lorentz local electric field.22

3. CONCLUSIONS AND OUTLOOK We have demonstrated the enhancement of the ferroelectric properties of P(VDF-TrFE) by the addition of Au, Ag, and SiO2 nanoparticles. AuNPs gave a significant enhancement of ferroelectric properties: a 40% increase of the remanent polarization, 13−14% reduction of the coercive field, and 60−100% increase of the switching speed. The size of AuNPs, 10 nm, is the same order of the crystallite size of β-crystals in P(VDF-TrFE), and thus the significant interaction between the Au NPs and the ferroelectric crystals of P(VDF-TrFE) promotes these ferroelectric enhancements. The addition of NPs are the new trends for the further improvements and the enhancements of the properties of well-established materials. The fundamental analysis is also important to understand their mechanisms, which extends the future developments of the materials.

Figure 5. XPS spectra of F 1s and proposed interaction between Au NPs and P(VDF-TrFE). (a) Pristine P(VDF-TrFE), (b) AuNPs/ P(VDF-TrFE), (c) AgNPs/P(VDF-TrFE), (d) SiO2NPs/P(VDFTrFE). Red curve is the summation of the three separated curves (green, F−; blue, CH2F2; light blue, CHFCF2). (e) Schematic picture of the proposed model. The fraction of F− was calculated from the integration of the F− peak area vs total integration of the peak area. In AuNPs/P(VDF-TrFE), the fraction of F− is higher.

4. EXPERIMENTAL SECTION A copolymer of vinylidene fluoride at 75 mol % and trifluoroethylene at 25 mol % (P(VDF-TrFE) (75/25)) (Kureha, Japan) was used as received. Three types of nanoparticles, gold, silver and silicon oxide nanoparticles (AuNPs, AgNPs, and SiO2NPs, respectively) were used. The particle size of the AuNPs was in the range of 10−60 nm, that of the AgNPs was 10 nm, and that of the SiO2NPs was 60 nm. P(VDF-TrFE) was dissolved with the nanoparticles in methyl ethyl ketone (MEK, Nakalai Tesque, Japan) to prepare a 4 wt % of P(VDFTrFE) MEK solution with nanoparticles. The concentrations of the nanoparticles are 1.00 × 10−6, 1.00 × 10−5, 1.00 × 10−4, 1.00 × 10−3, and 1.00 × 10−2 wt %. The P(VDF-TrFE) MEK solution with nanoparticles was spin-coated at 1000 rpm for 30 s onto a 5 mm φ gold electrode evaporated onto a silicon substrate; the resulting sample was subsequently dried at ambient conditions for 24 h, followed by thermal annealing at 135 or 140 °C for 2 h under a vacuum. Another tiny top gold electrode was evaporated onto an annealed P(VDFTrFE) and nanoparticles doped P(VDF-TrFE) thin films using a 117 μm × 117 μm mesh mask. For the measurements of ferroelectric switching and hysteresis, AFM, X-ray diffraction pattern, and infrared

AgNPs, and SiO2NPs) led to a 1.0−1.4 eV higher energy shift of the F 1s binding energy of CH2CF2 and CHFCF2, and a 0.5−1.1 eV higher energy shift of the F− ion. The fraction of F− is calculated from the integration of the F− peak area vs total integration of the peak area. In this calculation, the fraction ratio between CH2CF2 and CHFCF2 was set to 3/1. The fraction of F− is shown for pristine and NP-doped P(VDFTrFE) in Table 2. The fraction of F− in AuNPs/P(VDF-TrFE) is two times higher than that in pristine P(VDF-TrFE), and one and half higher than those in the other NP-doped P(VDFTrFE) samples. It means that the electron was donated to fluorine atom from gold nanoparticles via carboxyl group adsorbed around gold nanoparticles. Fluoride ion F− content increased with increasing AuNPs content in Teflon−gold 16820


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ACS Applied Materials & Interfaces spectroscopy were performed for the sample films annealed at 140 °C for 2 h under a vacuum. For XPS measurement was performed for the sample films annealed at 135 °C for 2 h under a vacuum. Ferroelectric switching was measured using a FCE-1/1A ferroelectric measurement system (Toyo Corporation, Tokyo, Japan) in combination with a Nano-R AFM (Pacific Nanotechnology, Santa Clara, CA, USA).23 Details for these measurements were described in a previous report.23,24 AFM images were observed using a Nano-R AFM. Film thickness was ca. 280 nm, which was determined using an AFM. Surface image of each sample was measured using an AFM. X-ray diffraction pattern, WAXD, of the sample film was recorded using a Bruker MX-lab X-ray diffractometer with nickel filtered Cu Kα radiation. Infrared spectra of the sample film were recorded with an attenuated total reflection (ATR) method using a PerkinElmer model Spectrum GX FTIR equipped with an optical microscope. 4.1. Determination of ferroelectric and dielectric properties. Switching current J(t) follows the equation of J(t ) =

(4) Kobayashi, T.; Hori, N.; Nakajima, T.; Kawae, T. Electrical Characteristics of MoS2 Field-Effect Transistor with Ferroelectric Vinylidene Fluoride-Trifluoroethylene Copolymer Gate Structure. Appl. Phys. Lett. 2016, 108, 132903. (5) Yuan, Y.; Reece, T. J.; Sharma, P.; Poddar, S.; Ducharme, S.; Gruverman, A.; Yang, Y.; Huang, J. Efficiency Enhancement in Organic Solar Cells with Ferroelectric Polymers. Nat. Mater. 2011, 10, 296− 302. (6) Shin, K. − S.; Kim, T. Y.; Yoon, G. C.; Gupta, M. K.; Kim, S. K.; Seung, W.; Kim, H.; Kim, S.; Kim, S.; Kim, S. − W. Ferroelectric Coupling Effect on the Energy-Band Structure of Hybrid Heterojunctions with Self-Organized P(VDF-TrFE) Nanomatrices. Adv. Mater. 2014, 26, 5619−5625. (7) Li, M.; Wondergem, H. J.; Spijkman, M. − J.; Asadi, K.; Katsouras, I.; Blom, P. W. M.; de Leeuw, D. M. Revisiting the δ-phase of Poly(vinylidene fluoride) for Solution-Processed Ferroelectric Thin Films. Nat. Mater. 2013, 12, 433−438. (8) Huang, X.; Jiang, P.; Xie, L. Ferroelectric Polymer/Silver Nanocomposites with High Dielectric Constant and High Thermal Conductivity. Appl. Phys. Lett. 2009, 95, 242901. (9) Paik, H.; Choi, Y.-Y.; Hong, S.; No, K. Effect of Ag Nanoparticle Concentration on the Electrical and Ferroelectric Properties of Ag/ P(VDF-TrFE) Composite Films. Sci. Rep. 2015, 5, 13209. (10) Tan, K. S.; Gan, W. C.; Velayutham, T. S.; Abd Majid, W. H. Pyroelectricity Enhancement of PVDF Nanocomposite Thin Films Doped with ZnO Nanoparticles. Smart Mater. Struct. 2014, 23, 125006. (11) Mandal, D.; Kim, K. J.; Lee, J. S. Simple Synthesis of Palladium Nanoparticles, β-Phase Formation, and the Control of Chain and Dipole Orientations in Palladium-Doped Poly(vinylidene fluoride) Thin Films. Langmuir 2012, 28, 10310−10317. (12) Mandal, D.; Henkel, K.; Schmeißer, D. The Electroactive βPhase Formation in Poly(vinylidene fluoride) by Gold Nanoparticles Doping. Mater. Lett. 2012, 73, 123−125. (13) Wang, W.; Zhang, S.; Srisombat, L.; Lee, T. R.; Advincula, R. C. Gold-Nanoparticle- and Gold-Nanoshell-Induced Polymorphism in Poly(vinylidene fluoride). Macromol. Mater. Eng. 2011, 296, 178−184. (14) Kusuma, D. Y.; Nguyen, C. A.; Lee, P. S. Enhanced Ferroelectric Switching Characteristics of P(VDF-TrFE) for Organic Memory Devices. J. Phys. Chem. B 2010, 114, 13289−13293. (15) Dang, Z. − M.; Yuan, J. − K.; Yao, S. − H.; Liao, R. − J. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25 (44), 6334−6365. (16) Dang, Z. − M.; Yuan, J. − K.; Zha, J. − W.; Zhou, T.; Li, S. − T.; Hu, G.-H. Fundamentals, Processes and Applications of HighPermittivity Polymer-Matrix Composites. Prog. Mater. Sci. 2012, 57, 660−723. (17) Tsutsumi, N.; Yoda, S.; Sakai, W. Infrared Spectra and Ferroelectricity of Ultra-Thin Films of Vinylidene Fluoride and Trifluoroethylene Copolymer. Polym. Int. 2007, 56, 1254−1260. (18) Otsuka, T.; Endo, K.; Suhara, M.; Chong, D. P. Theoretical Xray Photoelectron Spectra of Polymers by Demon DFT Calculations Using the Model Dimers. J. Mol. Struct. 2000, 522, 47−60. (19) Walker, M.; Baumgartner, K. M.; Ruckh, M.; Kaiser, M.; Schock, H. W.; Rauchle, E. XPS and IR Analysis of Thin Barrier Films Polymerized from C2H4 /CHF3 ECR-Plasmas. J. Appl. Polym. Sci. 1997, 64, 717−722. (20) Cossaro, A.; Cvetko, D.; Floreano, L. Amino−Carboxylic Recognition on Surfaces: from 2D to 2D + 1Nnano-Architecture. Phys. Chem. Chem. Phys. 2012, 14, 13154−13162. (21) Cioffi, N.; Losito, I.; Torsi, L.; Farella, I.; Valentini, A.; Sabbatini, L.; Zambonin, P. G.; Bleve-Zacheo, T. Analysis of the Surface Chemical Composition and Morphological Structure of VaporSensing Gold-Fluoropolymer Nanocomposites. Chem. Mater. 2002, 14, 804−811. (22) Ogura, H.; Chiba, A. Calculation of the Equilibrium Polarization of Vinylidene Fluoride-Trifluoroethylene Copolymers Using the Iteration Method. Ferroelectrics 1987, 74, 347−355.

dP dE E + ϵϵ0 + dt dt ρ

when the sample film was subjected to cyclic applied electric field. Here P is polarization, ε is dielectric constant, ε0 is permeability in vacuum, ρ is resistivity, and E is applied electric field. Dielectric constant of the sample film was evaluated from the capacitance term parallel to the dE/dt. The integration of the current subtracted the capacitance and the resistivity terms gave the polarization as a function of applied electric field (hysteresis loop) from which Pr and Ec were evaluated. Speed of polarization switching, inverse of switching time, was determined by the same procedure reported in previous studies.23,24 4.2. XPS Measurements. XPS (JEOL JPS-9010MC/SP equipped with a Mg Kα radiation source) was used to investigate the interaction between P(VDF-TrFE) and the nanoparticles at room temperature. The obtained spectra were calibrated for a carbon C 1s excitation at binding energy of 285.0 eV. The obtained XPS spectra were separated into several Gaussian peaks using a Peak Fit Ver 4.0.

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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/acsami.6b05897. Additional information, Table S1, and Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

N.T. designed the study. R.K. made samples and performed experiments and measurements. K.K. suggested the XPS measurements and assisted with the data analysis of the XPS spectra. N.T., R.K., K.K., and W.S. discussed the study. N.T. helped in the preparation of the manuscript. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Yagi, T.; Tatemoto, M.; Sako, J. Transition Behavior and Dielectric Properties in Trifluoroethylene and Vinylidene Fluoride Copolymers. Polym. J. 1980, 12, 209−223. (2) Furukawa, T.; Takahashi, Y.; Nakajima, T. Recent Advances in Ferroelectric Polymer Thin Films for Memory Applications. Current Appl. Phys. 2010, 10, e62−e67. (3) Asadi, K.; Li, M.; Blom, P. W. M.; Kemerink, M.; de Leeuw, D. M. Organic Ferroelectric Opto-Electronic Memories. Mater. Today 2011, 14, 592−599. 16821


Research Article

ACS Applied Materials & Interfaces (23) Tsutsumi, N.; Bai, X.; Sakai, W. Towards Nonvolatile Memory Devices Based on Ferroelectric Polymers. AIP Adv. 2012, 2, 012154. (24) Tsutsumi, N.; Kitano, T.; Kinashi, K.; Sakai, W. Ferroelectric Switching of Vinylidene and Trifluoroethylene Copolymer Thin Films on Au Electrodes Modified with Self-Assembled Monolayers. Materials 2014, 7, 6367−6376.

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Nature of the Enhancement in Ferroelectric ... - ACS Publications

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