V. M. Fridkin

Bulk photovoltaic effect in noncentrosymmetric crystals

The theory and the experimental characteristics of the bulk photovoltaic effect in ferroelectric and piezoelectric crystals first observed at the Institute of Crystallography have been reviewed. I dedicate this review to the memory of B.K. Vainshtein whose help in performing these works was really inestimable.

Lattice and electronic band structure changes across the surface ferroelectric transition

Abstract The origin of the surface ferroelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) is explored. We report a uniaxial doubling of the surface Brillouin zone in the conduction band dispersion across the surface ferroelectric phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientation.

Nanoscale polarization patterning of ferroelectric Langmuir–Blodgett P(VDF-TrFE) films

This paper reports nanoscale piezoelectric measurements on ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films prepared by the Langmuir–Blodgett (LB) technique. Polarization patterning, piezoelectric hysteresis and relaxation after poling are studied in this work by piezoresponse force microscopy. High quality P(VDF-TrFE) films with a copolymer content of 30% were fabricated using a Schaefer monolayer transfer setup permitting precise control of the film microstructure. The thickness of the films deposited with 100 transfers was ~64 nm. Local switching resulted in written polarization lines with the lateral size in the range 70–300 nm depending on polarization time. Local hysteresis loop (at a fixed tip position) demonstrated clear ferroelectric switching with the coercive voltage ≈8–10 V that corresponds to a macroscopic switching field (~1.5 MV cm−1) at a ~10 nm depth below the tip. Relatively slow ageing after poling was observed with the characteristic relaxation time of about 1500–2000 s depending on the polarization direction. The obtained results demonstrate that the stable polarization patterns can be created in LB P(VDF-TrFE) films and attest them as suitable candidates for memory and nanotemplate applications.

Ferroelectric polymer langmuir-blodgett films

Publisher Summary This chapter reviews ferroelectricity and ferroelectric polymers. The chapter describes the fabrication techniques based on the Langmuir–Blodgett (LB) deposition used to fabricate the ultrathin ferroelectric polymer films and summarizes film structure and morphology. The fabrication of ultrathin ferroelectric polymer films by the Langmuir–Blodgett deposition has provided an exciting system for detailed study, revealing valuable insights into the nature of the vinylidene-fluoride polymers specifically and ferroelectricity in general. The LB films prove superior to the polymorphous films for many purposes—both scientific and technological—because of the ability to control thickness with monolayer precision, the natural orientational alignment of straight polymer chains without amorphous material or lamellae, and the convenient crystallographic orientation with the polarization axis perpendicular to the film. The ability to make such thin ferroelectric films has led to two key discoveries. There is no minimum critical thickness for the ferroelectric state—contrary to many predictions based on three-dimensional mean-field theory—demonstrating the essentially two-dimensional nature of the ferroelectric state, and closing the dimensionality gap between ferromagnetism and ferroelectricity. The finite-size effect is evident in the coercive field and other switching properties for films thicker than 15 nm, but not in thinner films, thereby demonstrating that intrinsic switching is not significantly influenced by nucleation and domain dynamics.

Polarization switching at the nanoscale in ferroelectric copolymer thin films

The polarization switching kinetics were measured at the nanoscale in continuous thin films of a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene. The dependence of the switching rate on voltage for a 54-nm thick film exhibits extrinsic nucleation and domain-growth type kinetics with no true threshold coercive field, and is qualitatively different from the behavior of an 18-nm thick film, which exhibits intrinsic switching kinetics, and a true threshold field. The results are consistent with studies of thin film capacitors of much larger area and with a recent refinement of the theory of the critical size for intrinsic switching.

Polarization switching kinetics of ferroelectric nanomesas of vinylidene fluoride-trifluoroethylene copolymer

The polarization switching kinetics of ferroelectric polymer nanomesas was investigated using piezoresponse force microscopy. The nanomesas were made by self-organization from Langmuir–Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage that is consistent with nucleation-type switching dynamics.

Photoemission band symmetries and dipole active modes of crystalline films of vinylidene fluoride (70%) with trifluoroethylene (30%) across the ferroelectric transition(s)

The symmetry of the occupied electronic bands of crystalline films of vinylidene fluoride (70%) with trifluoroethylene (30%) and the vibrational dipole modes are identified using angle resolved photoemission and high resolution electron energy loss respectively. The symmetry of the occupied valence bands is preserved across the surface `ferroelectric phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientation and a surface Brillouin zone doubling across the surface transition, thus these results strongly support a first order transition with a uniaxial polarization direction in these crystalline ferroelectric polymer films. There is compelling evidence that this surface phase transition is an improper ferroelectric transition and discontinuous.

Photorefractive grating formation in piezoelectric La3Ga5SiO14:Pr3+ crystals

Photorefractive grating formation and erasure in piezoelectric crystals of La3Ga5SiO14:Pr3+ are presented. The specific photoconductivity and the photorefractive sensitivity are determined. The polarization dependence of the grating formation due to the bulk photovoltaic effect is shown and compared favorably with the theoretical expression. This photorefractive material provides a possibility for separate investigations of the charge migration processes responsible for the photorefractive effect.

Polarization of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) thin films revealed by emission spectroscopy with computational simulation during phase transition

The electronic structure and self-polarization of P(VDF-TrFE) Langmuir-Blodgett nanofilms were analyzed under temperature-driven phase transitions, according to their thickness, composition, and structural conformation. Both thermo-stimulated exoelectron emission (TSEE) spectroscopy and computational simulation, including quantum-chemical calculations from first principles, were carried out. PVDF and composite P(VDF-TrFE) (70:30) molecular chains as Trans and Gauche conformers, as well as crystal cells, were modeled for these TSEE analyses. The quantum-chemical calculations and the computational simulation were based on the density functional theory (DFT) as well as semi-empirical (PM3) methods. It was demonstrated that the energy of electron states, as well as the total energies of the studied P(VDF-TrFE) molecular clusters during phase transformation, is influenced by electron work function and electron affinity. Analysis was performed by combining TSEE experimental data with the computational data of t...

Physics of two-dimensional ferroelectric polymers

Unique insight into the nature of ferroelectricity is emerging from the study of Langmuir-Blodgett films of vinylidene fluoride copolymers. These films are the first truly two-dimensional ferroelectrics, with thickness-independent bulk ferroelectric properties and a separate surface phase transition.