The Photorefractive and Photovoltaic Properties of a Composite Based on Ferroelectric Polymer Doped with Carbon Nanotube
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ACE AND THIN FILMS
The Photorefractive and Photovoltaic Properties of a Composite Based on Ferroelectric Polymer Doped with Carbon Nanotubes K. A. Verkhovskayaa,*, S. P. Chumakovaa, V. V. Savelevb, and T. V. Krivenkob a
Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia b Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119071 Russia *e-mail: [email protected] Received April 2, 2018
Abstract—The photorefractive effect is obtained in a composite of a ferroelectric polymer (polyvinylidene fluoride with trifluoroethylene (P(VDF/TrFE)) and single-walled carbon nanotubes (SWCNTs), which serve simultaneously as non-linear optical chromophores and spectral sensitizers to a 1064-nm laser beam. It is established that the presence of internal field in the ferroelectric makes it possible to measure the photorefractive effect in the absence of applied external field. Undoped films of VDF/TrFE copolymer are not photosensitive and do not exhibit the bulk photovoltaic effect (BPE). BPE is observed (along with the photosensitivity) in polarized samples, when carbon nanotubes are introduced into the copolymer. DOI: 10.1134/S1063774518050309
INTRODUCTION Ferroelectric copolymer films of vinylidene fluoride with trifluoroethylene (VDF-TrFE) were used in this study. The structure, phase transition, and ferroelectric properties of VDF-TrFE copolymer films were described in detail in [1, 2]. The PVDF structural unit–(СH2–СF2)– has a dipole moment μ = 7 × 10–30 C m (2 D), related to the presence of electronegative fluorine and electropositive hydrogen. Copolymer samples of the 70 : 30 composition were studied. The PVDF-TrFE films (obtained by centrifugation) exhibit spontaneous polarization Ps ~ 0.065 C/m2 in the polar orthorhombic phase 2mm. The copolymer consists of zigzag carbon chains with a distance of ~2.6 Å between СH2–СF2 groups. The dipole moment is oriented perpendicularly to the polymer chain. In the ferroelectric phase, polymer molecules are characterized by trans–trans conformations. The internal electric field Ein induced by spontaneous polarization in ferroelectric copolymer VDF-TrFE (70 : 30) was estimated in [2] to be (1) Еin = (Рind + Ps )/3ε0 = 2.5 × 10 V/m. The Рind value for the 70 : 30 copolymer is less than Ps = 0.065 C/m2. We consider below the photorefractive (PR) effect in a –(СH2–СF2)n–(СHF–СF2)m– composite, containing single-walled carbon nanotubes (SWCNTs), which serve simultaneously as non-linear optical chromophores and spectral sensitizers to an IR laser 9
beam. Measurements were performed both with application of external electric field and in its absence. EXPERIMENTAL PVDF-TrFE containing 0.52 wt % SWCNTs was used in our study. Nanotubes were initially sonicated in tetrachloroethane (TCE) for 30 min. Then PVDFTrFE was added to the dispersed solution, and the mixture was additionally treated by ultrasound for 5 min. The obtained viscous mixture w
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