A. S. Sigov

Temperature-driven phase transformation in self-assembled diphenylalanine peptide nanotubes

Diphenylalanine (FF) peptide nanotubes (PNTs) represent a unique class of self-assembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity and chemical stability. In addition, strong piezoelectric activity has recently been observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show a gradual decrease in polarization with increasing temperature accompanied by an irreversible phase transition into another crystalline phase at about 140‐150 ◦ C. The results are corroborated by the molecular dynamic simulations predicting an order‐disorder phase transition into a centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighbouring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to the high coercive field in FF PNTs. S Online supplementary data available from stacks.iop.org/JPhysD/43/462001/mmedia (Some figures in this article are in colour only in the electronic version)

Nonlinear-optical probing of nanosecond ferroelectric switching

It is demonstrated that the switching dynamics of the ferroelectric polarization in thin Ba(SrTi)O3 (BST) films can be followed by optical second harmonic generation with a time resolution that is only limited by the gating electronics. It is shown that the characteristic time of electric-field induced polarization switching in 70 nm BST films is less than τ=5 ns.

Domain orientation in ultrathin (Ba,Sr)TiO3 films measured by optical second harmonic generation

The analysis of polarization diagrams for specular and scattered second harmonic generation (SHG) was used for the structural characterization of submicron domain structures of thin (Ba,Sr)TiO3 (BST) films. It is shown that the lack of separation of these two contributions may lead to completely wrong conclusions about the domain orientation in these films. SHG studies of the thickness dependence of domain fractions (including 180° domains) reveal the presence of ferroelectric domains in ultrathin BST films (6 nm), although no domain structure was observed by atomic force microscopy. Thus the presence of ferroelectric ordering was demonstrated in perovskite films with a thickness down to 6 nm.

Crystallization behaviour of PZT in multilayer heterostructures

Microstructural and electrical properties of PZT (lead zirconate titanate) thin films prepared by sol-gel techniques at annealing temperatures in the range from 550°C to 900°C are studied. Perovskite (Pe) grain nucleation in PZT film starts but not completes at 550°C. Along with formation of round Pe (111) grains on the Pt (111) interface, the film contains small Pe and pyrochlore (Py) grains. Films annealed at the temperatures higher than 600°C demonstrate column structure of Pe grains, the amount of Py inclusions reduces with the annealing temperature and practically disappears at 700°C. An increase of annealing temperature leads to enhancement of (100) Pe orientation as a result of Ti diffusion on the Pt surface. Polarization decreases with the annealing temperature (maximum at 600°C), whereas permittivity increases up to the annealing temperature of 750°C.

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ∼130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.

Porous silicon-based ferroelectric nanostructures

A procedure is suggested for the preparation of porous silicon-based ferroelectric nanostructures. It is demonstrated that the method of chemical deposition from solutions provides for the penetration of the initial components of the solution into the matrix pores, and subsequent annealing leads to the crystallization of the ferroelectric phase. The diagnostics of the ferroelectric properties is performed using the method of generation of second optical harmonic. The spectral characteristics of the prepared ferroelectric nanostructures are investigated.

Role of precursors in the formation of lead zirconate titanate thin films

Thin ferroelectric Pb(Zr0.48Ti0.52)O3 lead zirconate titanate films have been prepared on platinized silicon substrates via chemical solution deposition. The starting film-forming solutions were prepared by reacting lead acetate with Ti and Zr alkoxides in 2-methoxyethanol. The electrical properties of the films were shown to depend on the Ti and Zr alkoxide precursors used in synthesis. The nature of the zirconium alkoxide had the strongest effect on the properties of the films. The best properties were obtained with zirconium n-propoxide.

Microelectronic applications of ferroelectric films

The promising application of ferroelectric films as a gate dielectric in some microelectronic devices has been assessed. It is shown that the high dielectric constant of ferroelectrics allows improvement of device performance (amplification and power characteristics of MIS transistors and information amount transferred through the register of CCD). The requirements towards ferroelectric material and ferroelectric-semiconductor interface performance in ferroelectric non-volatile memories have been discussed. On the base of fluctuation theory of surface states it is supposed that the spontaneous polarization may induce additional traps on the ferroelectric-semiconductor interface.

Strong Thermo-Induced Single And Two-Photon Green Luminescence In Self-Organized Peptide Microtubes

Diphenylalanine peptide nano- and microtubes formed by self-assembly demonstrate strongly enhanced and tunable single-photon and two-photon luminescence in the visible range, which appears after heat- or laser treatment of these self-organized peptide microtubes. This process significantly extends the functionality of these microstructures and can trigger a new interest in the optical properties of structures based on short peptides.

Crystallization of PZT in Porous Alumina Membrane Channels

Composite Pb(Zr,Ti)O 3 (PZT)/alumina nanostructures are fabricated using 2-dimensi- onal porous alumina as a template. Details of pores filling as well as of crystallization are studied by atomic force microscopy and transmission electron microscopy. The presence of a ferroelectric phase is confirmed by optical second harmonic generation experiments.