M. Molotskii

Submicron ferroelectric domain structures tailored by high-voltage scanning probe microscopy

We have developed a high voltage atomic force microscope that allowed us to tailor submicrometer ferroelectric domains in bulk ferroelectrics. One- and two-dimensional domain configurations have been fabricated in LiNbO3, RbTiOPO4, and RbTiOAsO4 ferroelectric crystals. It is found that the application of superhigh electric fields (reaching 5×107 V/cm) by the atomic force microscope tip leads to a unique polarization reversal mechanism, and open the way to a technology for photonic and acoustic devices.

Blue Luminescence Based on Quantum Confinement at Peptide Nanotubes

We report on observation of photoluminescence (PL) in blue and UV regions of exciton origin in bioinspired material-peptide nanotubes (PNTs). Steplike optical absorption and PL measurements have allowed finding quantum confined (QC) phenomenon in PNTs. The estimations show that QC in these nanotubes occurs due to a crystalline structure of subnanometer scale dimension formed under the self-assembly process. Our new findings pave the way for the integration of PNT in a new generation of optical devices. A blue PL array of a PNT-patterned device is demonstrated.

Generation of ferroelectric domains in atomic force microscope

A theory of an equilibrium shape of domains formed in an electric field of atomic force microscope (AFM) is proposed. The domain shape depends on parameters of the ferroelectric and on the applied voltage. Under low voltages the length and the diameter of the domain are of the same order of magnitude. With voltage increase the ratio between the length and the diameter increases. A correlation between the lateral sizes and the spontaneous polarization value is considered. It is shown that under the same voltage the thinnest domains are formed in ferroelectrics with high spontaneous polarization. The concept of the domain shape invariant as a combination of the domain length and lateral size, which is constant when changing the AFM parameters, is introduced. Results of the calculation of the invariant value in barium titanate as well as of the domain dimensions and the shape in GASH are in good agreement with the experiment.

Dynamics of ferroelectric domain growth in the field of atomic force microscope

Application of very high voltage to atomic force microscope tip leads to the growth of narrow, stringlike domains in some ferroelectrics, a phenomenon that was named “ferroelectric domain breakdown.” In this work the dynamics of domain breakdown have been studied experimentally and theoretically in stoichiometric lithium niobate (LN). The theory has been found to be in a good agreement with the measured domain radius temporal dependence. Dynamics of domain growth has also been studied in ultrathin LN crystals, where the domain breakdown phenomenon does not take place. It is also shown that domain formation processes occurring in bulk and ultrathin crystals are very different, and this is ascribed to the observed difference in depolarization energy dependence on the domain length.

Self-assembled bioinspired quantum dots: Optical properties

Until now, the wide research field of quantum dots (QDs) focused on inorganic structures. In the present study, we report on quantum confinement phenomena found in peptide nanocrystalline regions formed within self-assembly peptide nanospheres. These bioinspired nanostructures exhibit the optical absorption characteristics of QDs with pronounced luminescence of excitons whose origin is at the UV region. Theoretical estimations based on experimental data show that the radius of the self assembled peptide QDs is 1.3 nm.

Ferroelectric domain inversion: The role of humidity

The authors report on the effect of ambient humidity on domain inversion in ferroelectrics using atomic force microscopy. It is shown that the size of single domains inverted under low humidity in stoichiometric lithium tantalate single crystals is much smaller relative to ambient conditions. These differences are due to the much smaller tip-sample capacitance under low humidity. This phenomenon paves the way for the use of atomic force microscopy to tailor various nanodomain configurations for nonlinear optical applications.

Theoretical basis for electro- and magnetoplasticity

Abstract The energy of dislocation bonds to paramagnetic obstacles depends on the spin multiplicity of the radical pairs formed by the orbitals of these entities. The radical pairs may be either in the singlet or in triplet spin states with strongly different binding energies. A magnetic field induces transitions between the singlet and triplet states. These transitions may result in an additional population of the triplet states with a lower binding energy than that of the singlet states. Depinning of the dislocations from obstacles is facilitated and, hence, the plasticity in a magnetic field increases. This mechanism allows one to explain the principal features of the magnetoplastic effect. The notions about spin-dependent transitions in the dislocation–obstacle systems were also used for an explanation of the nature of the electroplastic effect. We consider the electroplastic effect as a result of the magnetic field, induced by the electric current, acting on the dislocation depinning from paramagnetic obstacles.

Nanoscale piezoelectric coefficient measurements in ionic conducting ferroelectrics

In this work the piezoresponse mode of the atomic force microscope has been applied for piezoelectric coefficient measurements in nanometer scale in high conductive RbTiOPO4 and KTiOPO4 ferroelectric crystals with specifically tailored domain configurations. A strong dependence of the amplitude and phase contrast between oppositely polarized domains on the frequency of the measuring alternate voltage was observed, and allowed the finding of the optimal conditions for piezoelectric coefficient measurements. A theoretical method, taking into account the inhomogeneity of the electric field under the atomic force microscope tip apex, the screening of the applied electric field, and the elastic clamping of the piezoelectrically excited region by the surrounding matrix has been developed for obtaining d33 in ferroelectrics with high ionic conductivity.

Charge-induced wettability modification

Electric charges deposited on a liquid droplet, located on a solid surface, strengthen the wetting effect. Here, the authors report on the opposite phenomenon—a decrease of wettability induced by a low-energy electron irradiation of solids. They provide evidence that the electron-induced surface charge decreases solid/liquid and solid/vapor interfacial energies when reduction of the latter is always higher. This explains the observed dependence of the droplet shape on the incident electron charge and energy, as well as on a liquid origin. This phenomenon is reversible when the charged material is subjected to ultraviolet illumination, which restores its initial state.

Generation of ferroelectric domains in films using atomic force microscope

Kinetics of domain formation in ferroelectric films subjected to electric field of atomic force microscope (AFM) is considered for a case of low reversal voltage. Dependence of equilibrium domain sizes on AFM and film parameters is defined. It is shown that formation of domains is possible if the applied voltage exceeds some threshold value. Above this threshold lateral sizes of the domains increase proportionally to the voltage. Dynamic equations of the domain wall motion during the domain formation in films are constructed and solved. For films having high activation fields the domain radius grows logarithmically with time. The time of the domain formation is defined. The calculated results are in agreement with experiments on lead zirconate titanate, lithium tantalate, and lithium niobate films. Kinetics of the domain growth in films with low activation fields is predicted.