Ferroelectric Surface Research Articles

Piezoelectric response enhancement in the proximity of grain boundaries of relaxor-ferroelectric thin films

The influence of surface morphology on the local piezoelectric response of highly (100)-textured 0.70PbMg2/3Nb1/3O3-0.30PbTiO3 thin films is studied using piezoresponse force microscopy in band-excitation mode. The local electromechanical response is mostly suppressed in direct proximity of the grain boundaries. However, within 100–200 nm of the grain boundary, the piezoresponse is substantially enhanced, before decaying again within a region at the center of the grain itself. Nested piezoresponse hysteresis curves confirm the influence of topography descriptors on parameters affecting the hysteresis loop shape. The enhancement of the electromechanical response is rationalized through reduced lateral clamping in the grains with deep trenched boundaries, as well as an expected lower energy for complex domain wall structures, due to curved ferroelectric surfaces. The lower piezoresponse at the center of the grain is assigned to the lateral clamping by the surrounding piezoelectric material.

Atomic mechanism of polarization-controlled surface reconstruction in ferroelectric thin films.

At the ferroelectric surface, the broken translational symmetry induced bound charge should significantly alter the local atomic configurations. Experimentally revealing the atomic structure of ferroelectric surface, however, is very challenging due to the strong spatial variety between nano-sized domains, and strong interactions between the polarization and other structural parameters. Here, we study surface structures of Pb(Zr0.2Ti0.8)O3 thin film by using the annular bright-field imaging. We find that six atomic layers with suppressed polarization and a charged 180° domain wall are at negatively poled surfaces, no reconstruction exists at positively poled surfaces, and seven atomic layers with suppressed polarization and a charged 90° domain wall exist at nominally neutral surfaces in ferroelastic domains. Our results provide critical insights into engineering ferroelectric thin films, fine grain ceramics and surface chemistry devices. The state-of-the-art methodology demonstrated here can greatly advance our understanding of surface science for oxides.

Controllable Charge Transfer by Ferroelectric Polarization Mediated Triboelectricity

Next‐generation memory and energy harvesting devices require a higher output performance for charging lectric devices. Generally, it is very limited to control charge transfer through triboelectricity in triboelectric materials. Here, using ferroelectric polarization, it is found that both the amount and direction of charge transfer in triboelectric materials can be controlled. The ferroelectric‐dependent triboelectricity in a ferroelectric co‐polymer film is explored using atomic force microscopy (AFM). Ferroelectric surfaces are rubbed with the AFM tip after poling with positive and negative bias voltages to achieve a triboelectric effect. The surface potential of the positively (negatively) poled area becomes smaller (larger) after rubbing the film surface with the AFM tip. Furthermore, the power output from the triboelectric nanogenerator is dependent on the ferroelectric polarization state. The results indicate that the amount and direction of the charge transfer in triboelectricity can be controlled by the ferroelectric polarization state.

Polarization-driven catalysis via ferroelectric oxide surfaces.

The surface chemistry and physics of oxide ferroelectric surfaces with a fixed polarization state have been studied experimentally for some time. Here, we discuss the possibility of using these materials in a different mode, namely under cyclically changing polarization conditions achievable via periodic perturbations by external fields (e.g., temperature, strain or electric field). We use Density Functional Theory (DFT) and electronic structure analysis to understand the polarization-dependent surface physics and chemistry of ferroelectric oxide PbTiO3 as an example of this class of materials. This knowledge is then applied to design catalytic cycles for industrially important reactions including NOx direct decomposition and SO2 oxidation into SO3. The possibility of catalyzing direct partial oxidation of methane to methanol is also investigated. More generally, we discuss how using ferroelectrics under cyclically changing polarization conditions can help overcome some of the fundamental challenges facing the catalysis community such as the limitations imposed by the Sabatier principle and scaling relations.

Ferroelectric oxide surface chemistry: water splitting via pyroelectricity

We propose a cyclic catalytic system that splits water by harnessing the pyroelectric effect in ferroelectric oxides.

Thickness dependence of La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 magnetoelectric interfaces

Magnetoelectric materials have great potential to revolutionize electronic devices due to the coupling of their electric and magnetic properties. Thickness varying La0.7Sr0.3MnO3 (LSMO)/PbZr0.2Ti0.8O3 (PZT) heterostructures were built and measured in this article by valence sensitive x-ray absorption spectroscopy. The sizing effects of the heterostructures on the LSMO/PZT magnetoelectric interfaces were investigated through the behavior of Mn valence, a property associated with the LSMO magnetization. We found that Mn valence increases with both LSMO and PZT thickness. Piezoresponse force microscopy revealed a transition from monodomain to polydomain structure along the PZT thickness gradient. The ferroelectric surface charge may change with domain structure and its effects on Mn valence were simulated using a two-orbital double-exchange model. The screening of ferroelectric surface charge increases the electron charges in the interface region, and greatly changes the interfacial Mn valence, which likely plays a leading role in the interfacial magnetoelectric coupling. The LSMO thickness dependence was examined through the combination of two detection modes with drastically different attenuation depths. The different length scales of these techniques' sensitivity to the atomic valence were used to estimate the depth dependence Mn valence. A smaller interfacial Mn valence than the bulk was found by globally fitting the experimental results.

Laser Fabrication of Polymer Ferroelectric Nanostructures for Nonvolatile Organic Memory Devices.

Polymer ferroelectric laser-induced periodic surface structures (LIPSS) have been prepared on ferroelectric thin films of a poly(vinylidene fluoride-trifluoroethylene) copolymer. Although this copolymer does not absorb light at the laser wavelength, LIPSS on the copolymer can be obtained by forming a bilayer with other light-absorbing polymers. The ferroelectric nature of the structured bilayer was proven by piezoresponse force microscopy measurements. Ferroelectric hysteresis was found on both the bilayer and the laser-structured bilayer. We show that it is possible to write ferroelectric information at the nanoscale. The laser-structured ferroelectric bilayer showed an increase in the information storage density of an order of magnitude, in comparison to the original bilayer.

Surface Shearing Effects on Langmuir–Blodgett Thin Films of P(VDF-TrFE) Ferroelectric Surface

Polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymers, also called Nobel polymers due to their ferroelectric, piezoelectric, and pyroelectric properties, possess non-lineal optical properties and low acoustic impedance that pair with water and living tissue (i.e. are biocompatible). These qualities make them promising for multiple electronic applications. In this paper we explore the effects of applying a shearing force on copolymer of Polyvinylidene Fluoride with Trifluoroethylene (P(VDF-TrFE) thin film has been studied. A shearing technique was used to induce stress on the film, which produced an improvement in crystallization and grain size.

Tip-induced domain growth on the non-polar cuts of lithium niobate single-crystals

Currently, ferroelectric materials with designed domain structures are considered as a perspective material for new generation of photonic, data storage, and data processing devices. Application of external electric field is the most convenient way of the domain structure formation. Lots of papers are devoted to the investigation of domain kinetics on polar surface of crystals while the forward growth remains one of the most mysterious stages due to lack of experimental methods allowing to study it. Here, we performed tip-induced polarization reversal on X- and Y-non-polar cuts in single-crystal of congruent lithium niobate which allows us to study the forward growth with high spatial resolution. The revealed difference in the shape and length of domains induced on X- and Y-cuts is beyond previously developed theoretical approaches used for the theoretical consideration of the domains growth at non-polar ferroelectric surfaces. To explain experimental results, we used kinetic approach with anisotropy of screening efficiency along different crystallographic directions.

Finite size effects in ferroelectric-semiconductor thin films under open-circuit electric boundary conditions

General features of finite size effects in the ferroelectric-semiconductor film under open-circuit electric boundary conditions are analyzed using Landau-Ginzburg-Devonshire theory and continuum media electrostatics. The temperature dependence of the film critical thickness, spontaneous polarization, and depolarization field profiles of the open-circuited films are found to be significantly different from the characteristics of short-circuited ones. In particular, we predict the re-entrant type transition boundary between the mono-domain and poly-domain ferroelectric states due to reduced internal screening efficiency and analyzed possible experimental scenarios created by this mechanism. Performed analysis is relevant for the quantitative description of free-standing ferroelectric films phase diagrams and polar properties. Also our results can be useful for the explanation of the scanning-probe microscopy experiments on free ferroelectric surfaces.

Mechanical Removal and Rescreening of Local Screening Charges at Ferroelectric Surfaces

Charge screening on a polar surface under ambient conditions is difficult to investigate, because the surface is immediately affected by adsorbed polar molecules and ions. The authors control the degree of screening on a ferroelectric surface using charge gradient microscopy, and find that the kinetics is governed by an exponential recovery with a universal time constant, independent of the degree of screening. This study demonstrates that the degree of screening at a ferroelectric surface can be controlled mechanically, without disturbing the polarization beneath.

Patterned growth of Au nanoparticles on polycrystalline lead zirconate titanate surfaces.

We report the patterned growth of Au nanoparticles (NPs) on polarity-patterned polycrystalline lead zirconate titanate (PZT) as a template through photochemical reaction. The photochemical deposition of the Au NPs includes ultraviolet (UV) light illumination of the patterned PZT while immersed in a HAuCl4 solution. In particular, the influence of the UV wavelength, and the influence of the solution with the stabilizer and reducer on the growth selectivity of the Au NPs on the polarity-patterned regions was investigated. For a UV light of 365 nm wavelength, corresponding to a band-gap excitation of the PZT, more Au NPs were deposited on the + z polar region than on the other non-polar regions. However, no deposition of the Au NPs was observed for UV light wave-lengths longer than - 365 nm. When ascorbic acid (AA) as a reducer and cetyl-trimethylammonium bromide (CTAB) as a stabilizer were added to the HAuCl4 solution, the Au NPs on the + z polar region were observed to be deposited with a UV light of 435 nm, which is larger than the optical band gap wavelength of the PZT. Also, the growth selectivity and size uniformity on the + z region was significantly improved. These results could be due to the defect-induced photo-excitation of electrons and enhanced reduction process of Au+ ions by adding the reducer and the stabilizer in the photo-chemical process. This study suggests the possibility of the patterned growth of Au NPs on a ferroelectric surface through polarity patterning and photochemical reaction by optimizing the UV wavelength and employing reduction potential agents in a metal salt solution.

Probing of Surface Potential Using Atomic Force Microscopy

Surface electrical properties are being interest for many different applications including solar cell (Coffey & Ginger, 2006; Yoo et al., 2014) and information technologies (Vasudevan et al., 2013). Recently, as decreasing device size, nanoscale surface properties become more significant. In particular, since surface potential is directly relevant to work function of materials and/or surface charge, nanoscale probing of surface potential has paid much attention for understanding various surface phenomena such as photovoltaic phenomena, screening behavior of ferroelectric surfaces, and work function and surface states of semiconducting materials (Takahashi et al., 2000; Kalinin & Bonnell, 2001, 2004; Hong et al., 2009). The nanoscale probing of surface potential is still challengeable. In fact, atomic force microscopy (AFM) is a well know technique which allows exploring nanoscale material properties. Among various AFM modes, electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) allow us probing surface potential in many different types of materials including metal, ceramic, and organic materials at the nanoscale (Ellison et al., 2011). We review operational mechanisms of EFM and KPFM which allow exploring surface potential at the nanoscale. Also, we discuss advantages and disadvantages of each technique and show some of experimental examples of these AFM modes.

Extrinsic and intrinsic charge trapping at the graphene/ferroelectric interface.

The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is studied. Tuning the transition temperature through the PTO/STO volume fraction minimizes the adorbates at the graphene/ferroelectric interface, allowing robust ferroelectric hysteresis to be demonstrated. "Intrinsic" charge traps from the ferroelectric surface defects can adversely affect the graphene channel hysteresis and can be controlled by careful sample processing, enabling systematic study of the charge trapping mechanism.

Ferroelectric domain triggers the charge modulation in semiconductors (invited)

We consider a typical heterostructure “domain patterned ferroelectric film—ultra-thin dielectric layer—semiconductor,” where the semiconductor can be an electrolyte, paraelectric or multi-layered graphene. Unexpectedly, we have found that the space charge modulation profile and amplitude in the semiconductor, that screens the spontaneous polarization of a 180-deg domain structure of ferroelectric, depends on the domain structure period, dielectric layer thickness and semiconductor screening radius in a rather non-trivial nonlinear way. Multiple size effects appearance and manifestation are defined by the relationship between these three parameters. In addition, we show that the concept of effective gap can be introduced in a simple way only for a single-domain limit. Obtained analytical results open the way for understanding of current-AFM maps of contaminated ferroelectric surfaces in ambient atmosphere as well as explore the possibilities of conductivity control in ultra-thin semiconductor layers.

Cellular Response to Ferroelectric PVDF-TrFE Nanoscale Surfaces Formed by Varying Polymer Concentration

Poly[(vinylidenefluoride-co-trifluoroethylene] [PVDF(TrFE)], a ferroelectric polymer capable of undergoing strain of up to 7% is both highly chemically resistant and has the advantage of being cytocompatible. Here we present a preliminary study into the response of mesenchymal stem cells to spin-coated PVDF(TrFE) films and examine how the polymer concentration influences electrophysical film properties and cell adhesion. Surface characterization was quantitatively assayed in terms of topography, roughness, wettability and chemistry, while the ferroelectric response of the films was confirmed via electrical probing of the polarization-field hysteresis. At the micron- and nanoscale scale, AFM analysis revealed that PVDF(TrFE) exhibited increased roughness and topographical features with increasing PVDF(TrFE) concentration. PVDF(TrFE) films displayed higher contact angles compared with control glass substrates, as indicated by wettability assay, yet elemental composition was unchanged. Electric field/polarization response analysis revealed films required bias voltages of up 100 v to undergo saturated polarisation hysteresis. Mesenchymal stem cells were cultured on PVDF(TrFE) films for up to 7 days, and assessed by morphometric analysis, focal adhesion quantification and real-time PCR. Cells grown on films formed from 3% w/v PVDF(TrFE) solutions exhibited significantly higher expression of FAK and focal adhesion reinforcement following 1 day of culture, indicating nanoscale roughness in PVDF(TrFE) films as a potent modulator of MSC adhesion to ferroelectric polymers. Keywords: Actuator, ferroelectric, focal adhesions, PVDF-TrFE, stem cell, topography.

Anomalous Photodeposition of Ag on Ferroelectric Surfaces with Below‐Bandgap Excitation

Ferroelectric lithography, a recent method of fabricating functional interfaces, traditionally involves photoreduction on polarized ferroelectric surfaces at optical energies above the bandgap of the ferroelectric. In this work, for the first time, photochemical deposition of elemental Ag nanoparticles on a specifically poled lithium niobate substrate is reported, with a broad white‐light spectrum transmitted through the crystal. The transmitted light has energies only below the bandgap, leading to the conclusion that the Ag reduction proceeds through non‐linear effects, specifically, second harmonic generation.

Piezoelectric and ferroelectric materials and structures for energy harvesting applications

This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

Chaotic memory

Controlled switching of interacting ferroelectric surface domains leads to a variety of regular and chaotic patterns, and could provide a physical platform for performing calculations.

One-dimensional confinement in crystallization of P(VDF-TrFE) thin films with transfer-printed metal electrode

The crystallization of thin polymer film depends on surface/interface properties, due to the fact that molecular chain motion is affected by the presence of the surface. In this work, we measured the ferroelectric properties, crystallinity, chain conformation and surface morphologies of one-dimensionally confined P(VDF-TrFE) thin films using transfer-printed Au film, annealed at elevated temperatures, from just below melting temperature up to 200 °C. Crystallization at low temperature, i.e., below melting temperature, the confinement effect has been found to be negligible. At high temperatures, however, confined crystallization has led to superior ferroelectric properties, compared to samples annealed without confinement. These observations have led to two- or three-layer model for those crystallized thin films with or without confinement, respectively. Further, the transfer-printing of metal as a confining surface has been found to be beneficial, compared to vacuum evaporation, due to deposition-induced damages on organic polymer. This confinement-induced retention of ferroelectricity in P(VDF-TrFE) thin films above its melting temperature can extend processing temperature in organic devices using the ferroelectric polymer, such as non-volatile organic memory devices.