Surface Of Ferroelectrics Research Articles

Oxygen vacancies assisted ferroelectric heterojunction for enhanced photocatalytic activity

Designing an architecture that ferroelectric materials are introduced oxygen vacancies (OVs) without weakening the ferroelectric polarization has a positive effect on promoting the separation and inhibiting recombination of photogenerated carriers. Here, an indirect strategy that BiOCl rich in OVs were introduced into the positive surface of ferroelectrics to form PTO@BiOCl-OVs heterojunction nanoplates by a simple hydrothermal method and UV light irradiation were reported. The 3% PTO@BiOCl-OVs heterojunction nanoplates displayed an excellent photocatalytic activity for RhB degradation under visible light. ESR signals confirmed that ·O2−, ·OH and 1O2 were the main primary reactive species. TPV response demonstrated that 3% PTO@BiOCl-OVs possessed a negative TPV signal and had the smallest L value of 1.910 × 10−4, which meant the photogenerated carriers had fastest transfer and longest lifetime. Experimental and theoretical analysis collectively demonstrated that the synergistic interaction between ferroelectric polarization, heterojunctions and OVs can promote the separation efficiency, lower the recombination rate of photogenerated carriers and enhance the photocatalytic reactivity.

Charging and domain switching in ferroelectrics LiNbO3 by electron beam

Comprehensive experimental results on the kinetic characteristics of LiNbO3 charging process are reported. Secondary electron emission (SEE) yield, surface potential, and the sum of leakage and displacement currents were experimentally obtained during electron beam irradiation in the energy range from 200 eV to 10 keV for both sides of the Z-cut. We found that at the initial stage of charging, the SEE current that were collected by a hemispherical electrode changes its sign. It was explained by the emergence of a large displacement current on the collector. The direction of this current is opposite to the directions of SEE and backscattered electron currents. If a large positive charge arises on the surface of ferroelectrics, the positive displacement current dominates the secondary emission current; this fact was confirmed experimentally. Also, a high value of the negative surface potential in the electron energy range 200–700 eV for −Z cut of lithium niobate was found, though in this energy region, positive charging is observed for nonpolar dielectrics. The influence of domain repolarization currents on the measured emitted electrons and displacement current signals is shown.

Dual Extraction of Photogenerated Electrons and Holes from a Ferroelectric Sr0.5Ba0.5Nb2O6 Semiconductor.

The separation of photogenerated charges is a critical factor in photocatalysis. Recently, anomalous photovoltaic (APV) field effects (Voc ∼ 10(3) V/cm) in ferroelectrics, with their strong driving force for charge separation, have attracted much attention in photocatalysis and photoelectrocatalysis. However, it is still unknown whether photogenerated electrons and holes can be simultaneously extracted by the strong driving force toward the surface of ferroelectrics and can become available for surface reactions. This issue becomes critically important in photocatalysis because the surface reaction utilizes both the electrons and holes that reach the surface. In this work, a model lateral symmetric structure, metal/Sr0.5Ba0.5Nb2O6/metal (metal = Ag or Pt), as an electrode was fabricated. The dual extractions of photogenerated electrons and holes on the two opposite metal electrodes were achieved, as revealed by photovoltaic and ferroelectrical hysteresis measurements and photoassisted Kelvin probe force microscopy (KPFM). It was found that the high Schottky barriers of the two opposite Sr0.5Ba0.5Nb2O6-Pt electrodes are key factors that alter the two space charge regions (SCRs) by a poling effect. The resulting built-in electrical fields with parallel directions near both electrodes significantly enhance the charge separation ability. Our model unravels the driving force of charge separation in ferroelectric semiconductors, thus demonstrating the potential for highly efficient charge separation in photocatalysis.

Novel Electrical Conduction of Insulators under Examination of Defects and Injection and Relationship to Theories of Ferroelectric Domains

ABSTRACTElectrical conductions in insulators such as resistance switching, conduction at interfaces, and conduction at domain boundaries and free surface of ferroelectrics are of interest. These conductions are often attributed to novel mechanism such as ferroelectric polarization. On the other hand, these interpretations appear not fully accepted, because the recent advanced theories of ferroelectric domains disregard screening indicated by these conduction phenomena. That is, these conduction phenomena are quietly regarded as the classical conduction originating from defects. In this paper, we examine these conductions in pure wide bandgap insulators in view of defects, using the direct-accessibility (tangibility) of conduction at free surfaces. Although most of these conductions in ferroelectrics may not be useful in large-scale applications, we show that they have fundamental implications on renovations of ferroelectric basics.

Electron emission from charged surfaces of ferroelectrics-electrets: Part 2. Fundamentals underlying interpretation of spectra of anomalous electronic emission (AEE)

It has been demonstrated, both theoretically and experimentally, that specificities in the distribution of field and polarization over a sample's surface condition the form of electronic emission spectra obtained through irradiation of the negatively charged surface of poled ferroelectrics-electrets with the X-ray emission of the Al Kα 1,2—line of weak intensity. Techniques are proposed allowing one to derive from the form of spectra information about the potential relief on the surface of poled ferroelectrics. These techniques are explained by invoking concrete examples.

Electron/ion emission from the plasma formed on the surface of ferroelectrics. II. Studies of electron diode operation with a ferroelectric plasma cathode

We present experimental results of electron diode operation with an active plasma cathode. The plasma was formed by a noncomplete discharge on the surface of different unpoled ferroelectric samples having the form of disks or tubes made of BaTiO3 or Pb(Zr,Ti)O3. To produce the discharge, a positive or a negative driving pulse was applied to the front or to the rear electrode of the sample. We studied the operation of planar and coaxial electron diodes under the application of an accelerating high-voltage pulse ⩽45 kV with repetition rate ⩽5 Hz or ⩽250 kV in a single-mode operation. Electron beams with a current density of several hundreds of A/cm2 and a time duration of several hundreds of ns were generated. It was shown that the parameters of the electron beam as well as the operation of the electron diode depend strongly on the method of the plasma formation and the time delay between the beginning of the plasma formation and the application of the high-voltage pulse.

Electron/ion emission from the plasma formed on the surface of ferroelectrics. I. Studies of plasma parameters without applying an extracting voltage

We present experimental results of plasma formation on the surface of ferroelectric samples. Different poled and unpoled ferroelectric samples having a disk or tube form and made of Pb(Zr, Ti)O3 or BaTiO3 were tested. Using fast framing photography and different electric probes it was found that the application of a high-voltage driving pulse to the ferroelectric sample causes a fast surface plasma formation. This plasma formation occurs within a few nanoseconds from the start of the driving pulse for all the tested ferroelectric samples and the methods of applying the driving pulse. It was found that reversing the polarization of a ferroelectric does not play a significant role in the process of the plasma formation. Parameters of the plasma and of the neutral flow formed during the plasma formation versus the polarity and the amplitude of the driving pulse are presented.

Formation of discharge plasma on the surface of cathodes with different dielectric constants

We present a detailed study of the spatial and temporal evolution of visible light emission from plasma formed on the surface of ferroelectrics [Pb(Zr,Ti)O3 and BaTiO3] and a printed-circuit board under the application of a high-voltage pulse. For all samples studied, plasma formation occurs within the first 5 ns from the start of the high-voltage pulse. It is shown that the plasma appears at the edges of the front strip electrodes. Further, the plasma spreads along the dielectric surface covering it partially. It is found that the uniformity of the plasma and the distance of its propagation along the surface depend on the properties of the dielectric, the polarity and amplitude of the high-voltage pulse, and the vacuum conditions. A qualitative model which explains the observed experimental data is suggested.

Surface inhomogeneities and coercive field of thin ferroelectric films

Abstract We calculate the coercive field of thin ferroelectric films caused by the domain wall pinning by surface inhomogeneities. The Joanny-De Gennes formula is generalized to the elastic energy of the line of intersection of the domain wall with the surface of ferroelectrics. The coercive field induced by the pinning is shown to be inversely related to the film thickness and to have a magnitude of the order of that observed in experiments.