A Semiconducting Organic–Inorganic Hybrid Metal Halide with

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A Semiconducting Organic-Inorganic Hybrid Metal Halide with Switchable Dielectric and High Phase Transition Temperature Yan Sui, Wen-Tong Chen, Shu-Xia OuYang, Wen-Qian Wang, Gui-Xin Zhang, and Dong-Sheng Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00228 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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A Semiconducting Organic-inorganic Hybrid Metal Halide with Switchable Dielectric and High Phase Transition Temperature Yan Sui*, Wen-Tong Chen, Shu-Xia Ouyang, Wen-Qian Wang, Gui-Xin Zhang, Dong-Sheng Liu* School of Chemistry and Chemical Engineering, The Key Laboratory of Coordination Chemistry of Jiangxi Province, Jinggangshan University, Ji'an, Jiangxi, P. R. China *Corresponding author. E-mail address: [email protected] (Y. Sui), [email protected] (D.S. Liu)

ABSTRACT: Organic-inorganic hybrid metal halides with temperature-triggered responsive switchable dielectric properties have evoked great attention for their potential application in the field of optoelectronic information. Here, we present a new organicinorganic hybrid metal halide switchable dielectric material [(C3H5)2N(CH3)2]2SnCl6 (1), which is obtained by the reaction of diallyldimethylaminium chloride and tin(IV) chloride pentahydrate. Compound 1 exhibits excellent high-temperature switchable dielectric performance and antifatigue. Notablely, 1 also exhibits multifunctionality with semiconducting and photoluminescent property. Such a Sn(IV)-based organicinorganic hybrid with outstanding switchable dielectric, semiconducting and photoluminescent characteristics will pave a new approach in the development of Sn(IV)-based metal halides with practical applications.

1. INTRODUCTION Organic-inorganic hybrid metal halides have recently received extraordinary research community attention in the field of chemistry and materials sciences due to their potential applications in photovoltaic (PV)

1-6

and related optoelectronic devices.7-10

Among them, temperature-triggered responsive switchable materials, undergoing dielectric transitions between high and low levels near a phase transition temperature (Tc) as a response to a temperature stimulus, have been paid increasing attention for their potential application in the field of optoelectronic information.11-22 1

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Using organic ammonium and metal halide to prepare organic-inorganic hybrid metal halide is often an effective strategy obtain temperature-triggered responsive switchable materials, in which the flexible organic cations and/or metal halide anions can also bring about motional changes with increasing temperature.23-25 For the organic component, most of them belong to organic ammoniums with saturated alkyl substituents in reported literatures, such as bicyclo[2.2.2]octane and its derivatives,26,27 1,5-pentanediammonium,28 bromoethylammonium29 and so on, but organic ammonium salts with unsaturated substituents are rarely considered. In fact, some organic ammonium salts with unsaturated substituents such as allyl groups still possess good thermal stability. Meanwhile, the optical properties will be interesting due to the existence of unsaturated substituents. As for the inorganic component, Sn(IV)-based metal halides are usually stable in the air, and could be good candidates to prepare switchable dielectric semiconductors. Meanwhile, we have reported a method that incorporating single-protonated 1,4diazabicyclo[2.2.2]octane fluoborate (DabcoHBF4) into β-phase PVDF to prepare flexible switchable dielectric film, which exhibits enhanced switchable dielectric property because of the dielectric confinement effect and induced orientation function of β-phase PVDF.30 As our continuous interest, it is necessary that utilizing different molecular switchable dielectric compounds to prepare flexible switchable dielectric films. In this paper, tin(IV) chloride pentahydrate was reacted with diallyldimethylaminium chloride in HCl aqueous solution to obtain a new organic-inorganic hybrid metal halide switchable dielectric material [(C3H5)2N(CH3)2]2SnCl6 (1). The compound 1 exhibits not only the excellent reproducibility of switchable dielectric property with high phase transition temperature, but also the semiconducting and photoluminescent property. Flexible composite film 1@PVDF was also prepared by incorporating 1 into PVDF matrix, which could exhibit enhanced switchable dielectric property.

2. EXPERIMENTS AND METHOD 2.1 Materials and Methods. All chemicals are used as received without further purification and are analytical grade. 2.2 Preparation of single crystal of 1. Diallyldimethylaminium chloride (1.61 g, 0.01 mol), SnCl4·6H2O (1.75 g, 0.005 mol) and excess concentrated hydrochloric acid 2

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(4 g, 0.04 mol) were mixed in deionized water (50 mL) to get a clear solution. By a slow evaporation of the solution at room temperature, colourless single crystals of 1 were obtained after several days. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC reference number 1858259. Details of crystallographic data are summarized in Table S1. Selected bond length, angle and torsion angle are given in Table S2. 2.3 Preparation of composite films 1@PVDF. The following procedure was used to prepare PVDF composite films with 1 as fillers. Certain amount of 1 was added into a 10 w.t.% of PVDF solution in DMF, magnetic agitated at 60 °C for 30 min and ultrasonicated for 30 min, then degassed under vacuum overnight. The mixed solution was cast on quartz glass substrates, evaporated at 70 °C for 2 h to remove solvents.

3. RESULTS AND DISCUSSION 3.1 Thermal analysis

Figure 1. DSC cycle curves of 1 with rate of 10 K min-1 (A) and TG curve of 1 in the temperature range of 300-900 K (B). The DSC cycle measurement of heating and cooling is a useful way to judge a temperature-triggered phase transition. As shown in Figure 1A, the DSC curve shows one couple of peaks at 371 K (Tc) and 355 K (Tc′) with a thermal hysteresis of ~16 K. Here, high-temperature phase (HTP) is used to represent the structural phase above Tc and room-temperature phase (RTP) is used to label the phase below Tc. The sharp peaklike anomalies and large thermal hysteresis are indicative of a first-order phase transition. In addition, the entropy changes (ΔS) for the phase transition is calculated as 3

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ΔS=ΔH/T, in which the value of enthalpy change ΔH is obtained from the integral of the peak area. The ΔH is equal to 5.038 J g-1 at 371 K for the heating phase transition, therefore, ΔS is calculated to be 7.927 J mol-1K-1. According to the Boltzmann equation, ΔS = RlnN, N is calculated as 2.60, suggesting that 1 undergoes a typical order-disorder phase transition. Thermal stability of 1 was also investigated and illustrated in Figure 1B. Only one obvious weight loss process is found over 555 K, which should be due to the decomposition of 1 and transformation to SnO2 or other complicated substance. The high thermal stability also indicates that 1 is stable enough to endure the repeatedly phase transition process. 3.2 Structural characterization. At room temperature, single crystal X-ray diffraction analyses reveal that 1 crystallizes in centrosymmetric P21/c space group which belongs to monoclinic crystal system and the point group C2h, with a = 8.4404(4) Å, b = 9.3829(5) Å, c = 17.6920(10) Å, β = 115.385(4)° and V = 1265.84(12) Å3. Its unit cell consists of one diallyldimethylaminium cation and half of [SnCl6]2- anion. Sn(IV) atom is coordinated with six Cl atoms adopting regular octahedron coordination mode to form [SnCl6]2- anion cluster (Figure 2A). Each cluster is connected with three diallyldimethylaminium cations through C-H…Cl hydrogen bonds interactions to form a one-dimensional chain (Figure 2B). Neighbour chains have no H-bond interactions, which are collected together through van der Waals force between aliphatic organic compounds. The packing structure shows that inorganic anion chain is surrounded by insulating organic shells with their aliphatic "arms" (Figure 2C). When inorganic anions are well wrapped with organic cations in the structure of organic-inorganic hybrid, the dielectric mismatch between a much lower dielectric constant in the organic cation layer and a higher dielectric constant in the inorganic anion will produce dielectric confinement effect. 31-33 This kind of structure usually lead to sharp and intense exciton resonances in the optical absorption spectra.34

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Figure 2. Molecular structure of 1 at RTP (A), H-Bonds interaction (B) and view of packing structure (C). In order to figure out the microscopic mechanism of the phase transition, it is essential to obtain the crystal structure at HTP. Unfortunately, we have tried several times and failed to obtain satisfied crystal data. Usually, it is difficult to obtain the crystal structure when the temperature is higher than 373 K due to the serious disorder. As provided in the supporting information, the basic unit cell parameters of 1 at 383 K (HTP) may be as follows, a = 9.868(3) Å, b = 13.999(5) Å, c = 19.786(5) Å, α = β = γ = 90.0 °, V = 2733(2) Å3, showing remarkable change compared with those at RTP. To further confirm the phase transition of 1, variable-temperature PXRD measurements were performed and illustrated in Figure 3. When the temperature is increased up to 363 K, the obvious decrease of the diffraction peak numbers at 11.0°, 11.9° and 18.1° and new peak at 22.7 ° indicates the occurring of structural transition from low to high symmetry. When the temperature is back to 303 K, the PXRD patterns go back to original status again.

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Figure 3. Powder XRD patterns of 1 as-synthesized samples at different temperatures by grinding. 3.3 Optical properties

Figure 4. Solid-state UV-vis diffuse reflectance spectra of 1. The inset shows the α/S versus energy plot for 1. To explore the semiconducting properties, 1 was subjected to optical solid-state UVvis diffuse reflectance measurement. As shown in Figure 4, in the ultraviolet spectral region, 1 obviously shows a sharp peak, indicating a direct band gap and dielectric confinement effect due to exciton self-trapping.35,

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The solid-state UV-

vis diffuse reflectance data was treated with the Kubelka-Munk function which is known as α/S = (1-R)2/2R. Where α refers to the absorption coefficient, S is the 6

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scattering coefficient, and the R is related to the reflectance. The band gap Eg can be obtained from a diagram (inset of Figure 4) of α/S versus energy. The estimated Eg of 1 is 4.02 eV, which is comparable to analogues (benzylammonium)2PbCl4 (3.65 eV),37 but larger than those of (MA)1-x(en)x(Sn)1-0.7x(I)3-0.4x (1.25-1.51 eV), xI3 nanowires

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CsPbxSn1-

from1.3 to 1.78 eV.39

Furthermore, 1 exhibits an interesting photoluminescence property. As illustrated in Figure 5, upon excitation at λex =248 nm, 1 exhibits dual broadband emission at 288 and 390 nm, respectively, which is also indicative of dielectric confinement effect. Further investigation, it can be found that the two emission peaks should be resulted from the same photoluminescent center according to the excitation spectra (inset of Figure 5). The emission of 390 nm has stronger 1uminescent intensity and longer lifetimes than those of 288 nm. The lifetime of the former is calculated to be 109,044 ns, but the latter is only 1045 ns (Figure 6).

Figure 5. Solid-state photoluminescence emission spectrum of 1 (λex=248 nm) at room temperature. The inset shows the excitation spectra at λem= 288 and 390 nm, respectively.

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Figure 6. The photoluminescence decays of 1 measured at the emission wavelength of 390 nm (A) and 288 nm (B), respectively. 3.4 Switchable dielectric properties

Figure 7. (A) Variable temperature dielectric constant (ε′) of 1 at 1 MHz (inset: measured at four frequencies during the cooling process); (B) Variable temperature dielectric loss (ε″) of 1 measured at four frequencies during the cooling process. Variable temperature dielectric measurement is considered as a useful tool to determine phase transition. The complex dielectric permittivity measured at selected four frequencies from 310 K to 380 K are shown in Figure 7A. Obvious sharp step-like striking dielectric anomalies upon heating and cooling process are obtained at the frequency of 1 MHz. The Tc temperature (around 360 K) matches well with DSC result, which clearly indicates the reversible thermally-triggered structural phase transitions. Moreover, the dielectric constant (ε′) values remain steadily until 360 K upon heating process, and afterward it quickly shift to 30.6 from 14.5 around Tc. Such a sharp dielectric anomaly around Tc is a typical characteristic for switchable dielectric 8

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materials. From 5 kHz to 1 MHz, the dielectric real parts at high dielectric state decrease with increasing frequencies, indicating a little frequency dependent properties. The temperature dependent dielectric loss of 1 was given in Figure 7B, similar step-like change was also found.

Figure 8. Dielectric switchable reversibility and antifatigue of 1 at 1 MHz. More importantly, 1 exhibits excellent dielectric switching reversibility and remarkable switching antifatigue. As illustrated in Figure 8, ε′-switching of 1 is retained after at least five cycles without obvious deterioration. In the cycle measurements, the low and high dielectric state represents “switch off” and “switch on”, respectively. When the temperature rises up to Tc, ε′ increases quickly from the low dielectric state to the high dielectric state, and when the temperature is decreased back to Tc′, ε′ decreases quickly from the high dielectric state to the low dielectric state. With the temperature repeatedly changing above or below the phase transition point, the dielectric constant switchover between the high and low dielectric states. The repetition of “ON” / “OFF” cycles indicates the sustainable utilization of 1. 3.5 Flexible PVDF composite films

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Figure 9. Temperature dependent dielectric constants of 1@PVDF composite films with different contents of compound 1 at the frequency of 1 MHz. (the inset shows the macroscopic image of PVDF composite film to demonstrate its superior flexibility) Most dielectric materials should be made into thin film form to meet the demand of integration and miniaturization.40-43 As our continuing interest in converting molecular switchable dielectric materials into PVDF-based composite films,

30, 44

flexible

1@PVDF composite films are prepared according to our previous reported methods. The prepared films were referred as 0.10Sn, 0.20Sn, 0.30Sn, 0.40Sn and 0.50Sn according to their mass contents of 1 in films. These films are characterized by temperature dependent dielectric constants, and the results are illustrated in Figure 9. Obvious step-like dielectric anomalies upon heating and cooling process can also be observed at the frequency of 1 MHz around the Tc temperature of 1, which indicates that the dielectric anomalies should be resulted from the reversible phase transition of 1, but not from PVDF. As we known, PVDF cannot exhibit switchable dielectric property in itself, 45 but it has a slight influence upon the property of composite films. The dielectric constants of 1@PVDF composite films slightly increase with the increases of temperature before and after phase transition temperature, which does not like those of pure sample of 1. The dielectric change between high and low dielectric state increases with increasing mass content of 1. For 0.50Sn (50% mass content of 1), the dielectric constant contrast is about two times at the frequency of 1 MHz, which is comparable with pure sample of 1. That is to say, using only 50% of 1 could obtain 10

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almost the same level of switchable dielectric property of pure sample, so the switchable dielectric property is enhanced by forming PVDF composite films.

Figure 10. SEM images of PVDF composite film 0.50Sn with a magnification of 500 (left) and 1000 (right) times, respectively. SEM technique is used to investigate the dispersion state of 1 in PVDF matrix. As given in Figure 10, it was found that 1 was homogeneously dispersed within polymer matrix without obvious agglomeration. Large amount of small globules were found with the diameter varied from 5 to 8 μm, which indicates the characteristic of β-phase dominated PVDF composite films, because α- and γ-phase dominated PVDF should have diameter greater than 10 μm.46, 47

4. CONCLUSIONS In

summary,

a

new

tin(IV)-based

organic-inorganic

hybrid

compound

[(C3H5)2N(CH3)2]2SnCl6 (1), which possess excellent high-temperature switchable dielectric properties and antifatigue. Systematic characterizations including DSC, TG, Solid-state UV-vis diffuse reflectance, photoluminescence, dielectric measurements and variable-temperature PXRD were performed to determine the phase transition and multifunctionality of 1. Using 1 as the fillers, flexible composite films 1@PVDF are obtained and exhibit enhanced switchable dielectric property, which will be valuable in practical application.

SUPPORTING INFORMATION

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Characterization method, Table S1, Table S2, and X-ray crystallographic information files, CCDC reference number 1858259.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (21661016, 21361012 and 21461013), Science and Technology Research Project of Jiangxi Provincial Department of Education (GJJ160734), Young Scientist Program of Jiangxi Province (20144BCB23038), Key Program of Scientific Research of Jinggangshan University (JFD1801) and Undergraduate Training Programs for Innovation and Entrepreneurship (201810419003).

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(25) Liu, Y.; Zhou, H.-T.; Chen, S.-P.; Tan, Y.-H.; Wang, C.-F.; Yang, C.-S.; Wen, H.-R.; Tang, Y.-Z. Reversible Phase Transition and Switchable Dielectric Behaviors Triggered by Rotation and Order-disorder Motions of Crowns. Dalton Trans. 2018, 47, 3851-3856. (26) Zeb, A.; Sun, Z.; Khan, A.; Zhang, S.; Khan, T.; Asghar, M. A.; Luo, J. [C6H14N]PbI3: a Onedimensional Perovskite-like Order-disorder Phase Transition Material with Semiconducting and Switchable Dielectric Attributes. Inorg. Chem. Front. 2018, 5, 897-902. (27) Wang, Z.-X.; Li, P.-F.; Liao, W.-Q.; Tang, Y.-Y.; Ye, H.-Y.; Zhang, Y. Structure-Triggered High Quantum Yield Luminescence and Switchable Dielectric Properties in Manganese(II) Based Hybrid Compounds. Chem.-Asian J. 2016, 11, 981-985. (28) Mei, G.-Q.; Zhang, H.-Y.; Liao, W.-Q. Symmetry Breaking Phase Transition-Triggered Hightemperature Solid-state Quadratic Nonlinear Optical Switch Coupled with Switchable Dielectric Constant in An Organic–inorganic Hybrid Compound. Chem. Commun. 2016, 52, 11135-11138. (29) Chen, H.-P.; Wang, Z.-X.; Chen, C.; Lu, Y.; Yin, Z.; Sun, X.-F.; Fu, D.-W. High-Temperature Structural Phase Transition Coupled with Dielectric Switching in An Organic-inorganic Hybrid Crystal:[NH3(CH2)2Br]3CdBr5. Dalton Trans. 2017, 46, 4711-4716. (30) Sui, Y.; Liu, D.-S.; Chen, W.-T.; Zhao, G.; Xing, M.-M. Enhanced Switchable Dielectric Performance of β-Phase-Dominated PVDF Composite Films Modified with Single-Protonated 1,4-Diazabicyclo[2.2.2]octane Fluoborate. J. Phys. Chem. C 2017, 121, 13586-13592. (31) Tanaka, K.; Takahashi, T.; Kondo, T.; Umebayashi, T.; Asai, K.; Ema, K. Image Charge Effect on Two-dimensional Excitons in An Inorganic-organic Quantum-Well Crystal. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 045312(1-6). (32) Wu, X. X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B.; Zhu, X.-Y. Trap States in Lead Iodide Perovskites. J. Am. Chem. Soc. 2015, 137, 20892096. (33) Tomimoto, S.; Saito, S.; Suemoto, T.; Takeda, J.; Kurita, S. Ultrafast Dynamics of Lattice Relaxation of Excitons in Quasi-one-dimensional Halogen-bridged Platinum Complexes. Phys. Rev. B 2002, 66, 155112(1-10). (34) Hong, X. T.; Ishihara, A. V.; Nurmikko, D. Dielectric Confinement Effect on Excitons in Lead Tetraiodide-based Layered Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 6961-6964. (35) Dohner, E. R.; Hoke, E. T.; Karunadasa, H. I. Self-assembly of Broadband White-light Emitters. J. Am. Chem. Soc. 2014, 136, 1718-1721. (36) Hu, T.; Smith, M. D.; Dohner, E. R.; Sher, M.-J.; Wu, X.; Trinh, M. T.; Fisher, A.; Corbett, J.; Zhu, X.-Y.; Karunadasa, H. I.; et al. Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites. J. Phys. Chem. Lett. 2016, 7, 2258-2263.

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(37) Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.-F.; Huang, S. D.; Xiong, R.G. A Lead-halide Perovskite Molecular Ferroelectric Semiconductor. Nat. Commun. 2015, 6, 7338. (38) Spanopoulos, I.; Ke, W.; Stoumpos, C. C.; Schueller, E. C.; Kontsevoi, O. Y.; Seshadri, R.; Kanatzidis, M. G. Unraveling the Chemical Nature of the 3D "Hollow" Hybrid Halide Perovskites. J. Am. Chem. Soc. 2018, 140, 5728-5742. (39) Lei, T.; Lai, M. L.; Kong, Q.; Lu, D.; Lee, W.; Dou, L.; Wu, V.; Yu, Y.; Yang, P. D. Electrical and Optical Tunability in All-inorganic Halide Perovskite Alloy Nanowires. Nano Lett. 2018, 18, 3538-3542. (40) Wang, J.; Neaton, J.; Zheng, H.; Nagarajan, V.; Ogale, S.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D.; Waghmare, U. Epitaxial BiFeO3 Multiferroic Thin-film Heterostructures. Science, 2003, 299, 1719-1722. (41) Zhang, Y.; Liu, Y.; Ye, H. Y.; Fu, D. W.; Gao, W.; Ma, H.; Liu, Z.; Liu, Y.; Zhang, W.; Li, J.; et al. A Molecular Ferroelectric Thin Film of Imidazolium Perchlorate That Shows Superior Electromechanical Coupling. Angew. Chem., Int. Ed. 2014, 53, 5064-5068. (42) Noda, Y.; Yamada, T.; Kobayashi, K.; Kumai, R.; Horiuchi, S.; Kagawa, F.; Hasegawa, T. Few-Volt Operation of Printed Organic Ferroelectric Capacitor. Adv. Mater. 2015, 27, 64756481. (43) Lu, H.; Li, T.; Poddar, S.; Goit, O.; Lipatov, A.; Sinitskii, A.; Ducharme, S.; Gruverman, A. Statics and Dynamics of Ferroelectric Domains in Diisopropylammonium Bromide. Adv. Mater. 2015, 27, 7832-7838. (44) Sui, Y.; Chen, W.-T.; Ma, J.-J.; Hu, R.-H.; Liu, D.-S. Enhanced Dielectric and Ferroelectric Properties in PVDF Composite Flexible Films through Doping with Diisopropylammonium Bromide. RSC Adv. 2016, 6, 7364-7369. (45) Liu, S.; Xue, S.; Zhang, W.; Zhai, J.; Chen, G. Significantly Enhanced Dielectric Property in PVDF Nanocomposites Flexible Films through A Small Loading of Surface-hydroxylated Ba0.6Sr0.4TiO3 Nanotubes. J. Mater. Chem. A 2015, 2, 18040-18046. (46) Mandal, D.; Henkel, K.; Schmeisser, D. The Electroactive β-phase Formation in Poly(vinylidene fluoride) by Gold Nanoparticles Doping. Mater. Lett. 2012, 73, 123-126. (47) Ince-Gunduz, B. S.; Alpern, R.; Amare, D.; Crawford, J.; Dolan, B.; Jones, S.; Kobylarz, R.; Reveley, M.; Cebe, P. Impact of Nanosilicates on Poly(vinylidene fluoride) Crystal Polymorphism: Part 1. Melt-crystallization at High Supercooling. Polymer 2010, 51, 14851493.

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Figure 1. DSC cycle curves of 1 with rate of 10 K min-1 (A) and TG curve of 1 in the temperature range of 300-900 K (B). 333x150mm (300 x 300 DPI)

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Figure 2. Molecular structure of 1 at RTP (A), H-Bonds interaction (B) and view of packing structure (C). 266x150mm (300 x 300 DPI)

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Figure 3. Powder XRD patterns of 1 as-synthesized samples at different temperatures by grinding. 195x150mm (300 x 300 DPI)

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Figure 4. Solid-state UV-vis diffuse reflectance spectra of 1. The inset shows the α/S versus energy plot for 1. 195x150mm (300 x 300 DPI)

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Figure 5. Solid-state photoluminescence emission spectrum of 1 (λex=248 nm) at room temperature. The inset shows the excitation spectra at λem= 288 and 390 nm, respectively. 202x150mm (300 x 300 DPI)

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Figure 6. The photoluminescence decays of 1 measured at the emission wavelength of 390 nm (A) and 288 nm (B), respectively. 338x190mm (300 x 300 DPI)

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Figure 7. (A) Temperature dependent of the real part (ε′) of the dielectric permittivity of 1: The powder samples measured at 1 MHz upon heating and cooling. (inset: measured at 5 KHz, 10 KHz, 100 KHz and 1 MHz upon cooling); (B) Temperature dependent dielectric loss (ε″) of 1 measured at 5 KHz, 10 KHz, 100 KHz and 1 MHz upon cooling. 362x150mm (300 x 300 DPI)

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Figure 8. Dielectric switching reversibility and antifatigue of compound 1 at the frequency of 1 MHz. 216x150mm (300 x 300 DPI)

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Figure 9. Temperature dependent dielectric constants of PVDF composite films with different contents of compound 1 at the frequency of 1 MHz. (the inset shows the macroscopic image of PVDF composite film to demonstrate its superior flexibility) 240x177mm (300 x 300 DPI)

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Figure 10. SEM images of PVDF composite film 0.50Sn with a magnification of 500 (left) and 1000 (right) times, respectively. 225x99mm (300 x 300 DPI)

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