S. G. Yudin

Two-dimensional ferroelectric films

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films,, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 Å in diameter, in perovskite films 100 Å thick and in crystalline ferroelectric polymers as thin as 25 Å (refs 7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 Å (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

Piezoelectric and pyroelectric properties of ferroelectric Langmuir-Blodgett polymer films

The piezoelectric and pyroelectric responses of ferroelectric Langmuir–Blodgett polymer films are less than the largest values measured with bulk films of the same composition. The films of the crystalline copolymer poly(vinylidene fluoride trifluoroethylene) fabricated by the Langmuir–Blodgett technique are 30 ML thick (15 nm) and are highly crystalline and oriented with polarization perpendicular to the film. Both piezoelectric and pyroelectric measurements show reversible ferroelectric switching. The films are suitable for use in pyroelectric infrared imaging and in piezoelectric acoustic transducers.

Determination of the optical dispersion in ferroelectric vinylidene fluoride (70%)/trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report measurements of the optical dispersion in ferroelectric Langmuir–Blodgett films of polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, using variable-angle spectroscopic ellipsometry over a wide spectral range from infrared to ultraviolet. Film thickness averaged 1.78±0.07 nm per deposition layer for films ranging from 5 to 125 deposition layers as determined from multi-sample analysis. This deposition rate was consistent with capacitance measurements, yielding a dielectric constant of 9.9±0.4 normal to the film, by quartz microbalance measurements, and by atomic force microscopy.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films

An investigation was carried out on thin permanent phase polarization gratings recorded in Langmuir–Blodgett films using two orthogonal circularly polarized Ar-ion laser beams. The films are composed of amphiphilic azo-dye molecules and manifest an extremely large value of photoinduced optical anisotropy, Δn≈0.36. The experimental results are in excellent accord with the theoretical model based on the Jones matrix representation of the polarization pattern. The gratings are stable at least, for half a year.

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 properties of Langmuir-Blodgett copolymer films at the nanoscale

Langmuir-Blodgett (LB) films are well known structures prepared as a result of successive transfer of monolayers (ML) from the gas-liquid interface onto solid substrates. One impressive possibility of LB method is the opportunity to vary the thickness of the film to an accuracy of one transfer, which could coincide with one ML. The ferroelectric properties of a copolymer of vinylidene fluoride and trifluorethylene P[VDF-TrFE] prepared by LB deposition are investigated in the region of a few monolayers.

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.

Thickness dependence of switching for ferroelectric Langmuir films

Abstract Switchable ferroelectric Langmuir-Blodgett films prepared from the ferroelectric copolymer of vinylidene fluoride with trifluoroethylene P(VDF-TrFE) manifest the true bistability for films thicker than 15 monolayers; thinner films are still switchable but monostable. The crossover of the switching behaviour presents the first estimation of the ferroelectric correlation length, ξ ≈ 75 A .

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.