RESUMEN
It is well known that there are two possible switching mechanisms in ferroelectric crystals and films (see, e.g., Tagantsev et al 2010 Domains in Ferroic Crystals and Thin Films (Berlin: Springer)). The first mechanism, which follows from the mean-field theory of Landau-Ginzburg, is a homogeneous one and does not connect domains. This mechanism was never observed before 1998. The second mechanism, connected with nucleation and domain movement, is common for the ferroelectrics and is well known from the time of domain discovery (1956). In the present paper the existence of a homogeneous mechanism of switching in ultrathin copolymer films is confirmed by piezoresponse force microscopy. The results of the present paper permit us to suppose that homogeneous switching exists in other ultrathin ferroelectric films.
RESUMEN
High-resolution studies of domain configurations in Langmuir-Blodgett films of ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), have been carried out by means of piezoresponse force microscopy (PFM). Changes in film thickness and morphology cause significant variations in polarization patterns. In continuous films and nanomesas with relatively low thickness/grain aspect ratio (<1/10), the relationship between the average domain size and thickness follows the Kittel law. Nanomesas with high aspect ratio (>1/5) exhibit significant deviations from this law, suggesting additional surface-energy-related mechanisms affecting the domain patterns. Polarization reversal within a single crystallite has been demonstrated and local switching parameters (coercive voltage and remnant piezoresponse) have been measured by monitoring the local hysteresis loops. Reliable control of polarization at the sub-grain level demonstrates a possibility of studying the mechanism of the intrinsic switching behavior down to the molecular scale.
RESUMEN
Thin films of amphiphilic vinylidene fluoride oligomers prepared by Langmuir-Blodgett deposition on silicone substrates were investigated by comparing experimental and theoretical mid-infrared (IR) spectra. The experimental spectra were obtained using infrared spectroscopic ellipsometry. Theoretical spectra were calculated using density functional theory. Excellent correspondence of major IR bands in both data sets shows that the molecular backbone is oriented with the long axis normal to the substrate plane. This is in contrast to poly(vinylidene fluoride) LB films, in which the polymer chains are parallel to the substrate.
RESUMEN
Optical second-harmonic generation (SHG) is used as a noninvasive probe of two-dimensional (2D) ferroelectricity in Langmuir-Blodgett (LB) films of the copolymer vinylidene fluoride with trifluoroethylene. The surface 2D ferroelectric-paraelectric phase transition in the topmost layer of the LB films and a thickness-independent (almost 2D) transition in the bulk of these films are observed in temperature studies of SHG.
RESUMEN
Anthracene was the first photoconductor on which Carlson obtained electrophotographic images in 1938. Kallmann and his co-workers demonstrated that, using electrolytes as contacts for aromatic hydrocarbons, carrier injection into the crystal takes place. In the present paper, conditions for injection of electrons and holes into organic semiconductors from electrolytic contacts in which photochemical reactions are going on are analyzed. Specifically, anthracene and iodous hydrogen acid are examined. It has been shown that, using iodous hydrogen acid as a positive electrode, the photochemical reaction of decomposing molecular iodine into atoms leads to the injection of holes into anthracene. This phenomenon has been used to establish a new method of obtaining the latent electrophotographic image in anthracene. In these experiments, a solution of partly decomposed iodous hydrogen acid with various iodine contents was used as a substratum of the electrophotographic layer (anthracene). The surface of the layer was charged with the negative oxygen ions from the corona discharge in the air. The layer was exposed in the visible region (lambda=6500 A), where the photodissociation takes place. As this dissociation leads to the increasing density of the injection current from the electrolyte into the crystal, the illumination of the system in the visible region results in the fast discharge of the layer although the conductivity of the anthracene proper is insignificant in the visible region. Thus, the phenomenon of the photoactive electrolyte injecting holes into a crystal enables the formation of latent electrophotographic images in the visible part of the spectrum. In this paper, the kinetics of the discharge of the anthracene in the conditions of injection and the fulfillment of the reciprocity law have been investigated.
RESUMEN
A study was made of the shift of the absorption edge in BaTiO(3) caused by an applied electric field near the phase transition from the tetragonal phase to the cubic one. The absorption edge shifted to shorter wavelengths is independent of the direction of the field. This shift was measured as a function of temperature and field strength and shows a sharp maximum at the Curie temperature theta, a linear field dependence below theta, and a quadratic field dependence above theta.
