Enhancement of Local Photovoltaic Current at Ferroelectric Domain Walls in BiFeO3

The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

Interplay of domain structure and phase transitions: theory, experiment and functionality

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices.

Enhancing the bulk photovoltaic effect by tuning domain walls in epitaxial BiFeO3films

In this letter, the role of domain wall (DW) on bulk photovoltaic effect (BPV) effect in BiFeO₃(BFO) films was studied by x-ray reciprocal space mapping and conductive atomic force microscope. It was found that the domain structure and DW can be tuned by controlling the epitaxial orientation of BFO film. Remarkably, under 1 sun AM 1.5 G illumination, the 109° DW enhances the transport of photogenerated carriers and simultaneously improves the conductivity and power conversion efficiency (PCE). The short-circuit current density and PCE can reach 171.15μA cm^-2and 0.1127%, respectively. Therefore, our study reveals the correlation between the DW and the BPV effect in BFO film and provides a new pathway to improve the PCE of BFO-based photovoltaic device.

Effect of polarization rotation on the optical and photovoltaic properties of BiFeO3thin films

Visible-light-active ferroelectric materials are gaining increasing attention due to the unique ferroelectric photovoltaic effect. To boost the light harvesting capability, vast research is devoted to band gap engineering by chemical substitutions, regardless of the side effect on ferroelectric polarization. Here, we focus on how polar order affects the optical and photovoltaic properties. Using BiFeO₃as the model system, we induce the polarization rotation by A-site La substitution, which results in continuous reduction of optical anisotropy of the samples, as revealed by the concerted optical characterizations. This further causes the decrease of angular dependence of ferroelectric photovoltaic effect on the light polarization. The results demonstrate the inner connection of the ferroelectric polarization and optical anisotropy via the lattice degree of freedom.

Nanocomposites with Three-Dimensional Architecture and Impact on Photovoltaic Effect

We demonstrate the synthesis of self-assembled three-dimensional nanocomposite thin films consisting of NiO nanocolumns in an layered Aurivillius phase matrix. The structures were grown on single-crystal SrTiO₃ substrates via pulsed laser deposition (PLD) with single ceramic (PbTiO₃)_x(BiNi_2/3Nb_1/3O₃)_1-x targets. The nanocolumns, which are about 10 nm in diameter each, extend over the entire film thickness of up to 225 nm. We reveal the difference in electrical conduction properties of the nanocolumns and the surrounding matrix on the nanoscale via conductive atomic force microscopy. The nanocomposite thin films exhibit improved photovoltaic performance compared to both pure PbTiO₃ and homogeneous Aurivillius phase thin films.

Optical studies of ferroelectric and ferroelastic domain walls

Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use.

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells

Designing solid-state perovskite oxide solar cells with large short circuit current (J_SC) and open circuit voltage (V_OC) has been a challenging problem. Epitaxial BiFeO₃ (BFO) films are known to exhibit large V_OC (>50 V). However, they exhibit low J_SC (≪μA/cm²) under 1 Sun illumination. In this work, taking polycrystalline BiFeO₃ thin films, we demonstrate that oxygen vacancies (V_O) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high J_SC and V_OC simultaneously. While aliovalent substitution (Ca²⁺ at Bi³⁺ site) is used to control the lattice V_O, Ca and Ti cosubstitution is used to bring out only GB-V_O. Fluorine-doped tin oxide (FTO)/Bi_1-xCa_xFe_1-yTi_yO_3-δ/Au devices are tested for photovoltaic characteristics. Introducing V_O increases the photocurrent by four orders (J_SC ∼ 3 mA/cm²). On the contrary, V_OC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO₃. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V_O. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in J_SC. The cosubstitution, following a charge compensation mechanism, decreases the lattice V_O concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve V_OC (3-8 V) much larger than that of BiFeO₃ (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V_O concentration dominates GB-V_O concentration. Notwithstanding, both lattice and GB-V_O contribute to the increase in J_SC; the former weakens ferroelectricity, and as a consequence, undesirably, V_OC is lowered well below 0.5 V. Using optimum J_SC and V_OC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Functional Ferroic Domain Walls for Nanoelectronics

A prominent challenge towards novel nanoelectronic technologies is to understand and control materials functionalities down to the smallest scale. Topological defects in ordered solid-state (multi-)ferroic materials, e.g., domain walls, are a promising gateway towards alternative sustainable technologies. In this article, we review advances in the field of domain walls in ferroic materials with a focus on ferroelectric and multiferroic systems and recent developments in prototype nanoelectronic devices.

Manipulation of Oxygen Vacancy for High Photovoltaic Output in Bismuth Ferrite Films

Very recently, the ferroelectric photovoltaic property of bismuth ferrite (BiFeO₃, BFO) has attracted much attention. However, the physical mechanisms for its anomalous photovoltaic effect and switchable photovoltaic effect are still largely unclear. Herein, a novel design was proposed to realize a high photovoltaic output in BiFeO₃ films by manipulating its oxygen vacancy concentration through the alteration of the Bi content. Subsequent results and analysis manifested that the highest photovoltaic output was achieved in Bi_1.05FeO₃ films, differing 1000 times from that of Bi_0.95FeO₃ films. Simultaneously, the origin of photovoltaic effect in all BiFeO₃ films was suggested as the bulk photovoltaic mechanism instead of the Schottky effect. Moreover, oxygen vacancy migration should be the dominant factor determining the switchable photovoltaic effect rather than the ferroelectric polarization. A switchable Schottky-to-Ohmic interfacial contact model was proposed to illustrate the observed switchable photovoltaic or diodelike effect. Therefore, the present work may open a new way to realize the high power output and controllable photovoltaic switching behavior for the photovoltaic applications of BiFeO₃ compounds.

Strain-gradient mediated local conduction in strained bismuth ferrite films

It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO₃ thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T\prime$$\end{document}T′-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices.

In semiconductors strain gradients can separate light induced electron-hole pairs via the flexo-photovoltaic effect. Here the authors show that this effect can also account for the enhancement of the photoconduction in certain phase regions at the morphotropic phase boundary in Bismuth Ferrite films.

Ferroelectric Oxides for Solar Energy Conversion, Multi-Source Energy Harvesting/Sensing, and Opto-Ferroelectric Applications

Photoferroelectrics belong to a unique material family that exhibits both photovoltaic and ferroelectric effects simultaneously. The photovoltaic effect is the only known direct method of converting light into electricity and is the basis of solar cells. The ferroelectric effect can induce piezoelectric and pyroelectric effects, which are the working principles of widely used kinetic and thermal sensors, transducers, actuators, and energy harvesters. For a long time, photoferroelectric research was restricted to theoretical investigations only because of either the wide band gap (Eg ), which is not able to effectively absorb visible light, or to the weak ferroelectricity caused by a narrow Eg . Recent scientific breakthroughs, however, have opened doors for the development of practical applications. In this article, emerging concepts of creating balanced photovoltaic and ferroelectric properties for photoferroelectrics, as well as those of novel applications in future devices, are presented.

Piezoresponse force microscopy and nanoferroic phenomena

Since its inception more than 25 years ago, Piezoresponse Force Microscopy (PFM) has become one of the mainstream techniques in the field of nanoferroic materials. This review describes the evolution of PFM from an imaging technique to a set of advanced methods, which have played a critical role in launching new areas of ferroic research, such as multiferroic devices and domain wall nanoelectronics. The paper reviews the impact of advanced PFM modes concerning the discovery and scientific understanding of novel nanoferroic phenomena and discusses challenges associated with the correct interpretation of PFM data. In conclusion, it offers an outlook for future trends and developments in PFM.

Increasing bulk photovoltaic current by strain tuning

Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO₃ bulk single crystal the existence of a purely intrinsic "piezophotovoltaic" effect that leads to a linear increase in photovoltaic current density. The increase reaches 75% under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented.

Ferroelastic domain identification in BiFeO3 crystals using Raman spectroscopy

Multiferroic BiFeO₃ crystals were investigated by means of micro-Raman spectroscopy using the laser wavelengths of 442 nm (resonant conditions) and 633 nm (non-resonant conditions). The azimuthal angle dependence of the intensity of the Raman modes allowed their symmetry assignment. The experimental data are consistent with a simulation based on Raman tensor formalism. Mixed symmetries were taken into account, considering the orientation of the crystal optic axis along a pseudo-cubic <111> direction. The strong anisotropic intensity variation of some of the polar Raman modes was used for line scans and mappings in order to identify ferroelastic domain patterns. The line scans performed with different excitation wavelengths and hence different information depths indicate a tilt of the domain walls with respect to the sample surface. The domain distribution found by Raman spectroscopy is in very good agreement with the finding of electron back scattering diffraction.

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO3 Thin Films

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO₃ (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO₃)_0.29(SrTa_1/2Al_1/2O₃)_0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c₁ domains and [001̅] for c₂ domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Energy Harvesting Research: The Road from Single Source to Multisource

Energy harvesting technology may be considered an ultimate solution to replace batteries and provide a long-term power supply for wireless sensor networks. Looking back into its research history, individual energy harvesters for the conversion of single energy sources into electricity are developed first, followed by hybrid counterparts designed for use with multiple energy sources. Very recently, the concept of a truly multisource energy harvester built from only a single piece of material as the energy conversion component is proposed. This review, from the aspect of materials and device configurations, explains in detail a wide scope to give an overview of energy harvesting research. It covers single-source devices including solar, thermal, kinetic and other types of energy harvesters, hybrid energy harvesting configurations for both single and multiple energy sources and single material, and multisource energy harvesters. It also includes the energy conversion principles of photovoltaic, electromagnetic, piezoelectric, triboelectric, electrostatic, electrostrictive, thermoelectric, pyroelectric, magnetostrictive, and dielectric devices. This is one of the most comprehensive reviews conducted to date, focusing on the entire energy harvesting research scene and providing a guide to seeking deeper and more specific research references and resources from every corner of the scientific community.

Light-Induced Reversible Control of Ferroelectric Polarization in BiFeO3

Manipulation of ferroic order parameters, namely (anti-)ferromagnetic, ferroelectric, and ferroelastic, by light at room temperature is a fascinating topic in modern solid-state physics due to potential cross-fertilization in research fields that are largely decoupled. Here, full optical control, that is, reversible switching, of the ferroelectric/ferroelastic domains in BiFeO₃ thin films at room temperature by the mediation of the tip-enhanced photovoltaic effect is demonstrated. The enhanced short-circuit photocurrent density at the tip contact area generates a local electric field well exceeding the coercive field, enabling ferroelectric polarization switching. Interestingly, by tailoring the photocurrent direction, via either tuning the illumination geometry or simply rotating the light polarization, full control of the ferroelectric polarization is achieved. The finding offers a new insight into the interactions between light and ferroic orders, enabling fully optical control of all the ferroic orders at room temperature and providing guidance to design novel optoferroic devices for data storage and sensing.

Controlling the ferroelectric and resistive switching properties of a BiFeO3 thin film prepared using sub-5 nm dimension nanoparticles

In recent years, BiFeO₃ has attracted significant attention as an interesting multiferroic material in the exploration of fundamental science and development of novel applications. Our previous study (Phys. Chem. Chem. Phys.18, 2016, 25409) highlighted the interesting physicochemical features of BiFeO₃ of sub-5 nm dimension. The study also accentuated the existence of weak ferroelectricity at sub-5 nm dimensions in BiFeO₃. Based on this feature, we have prepared thin films using sub-5 nm BiFeO₃ nanoparticles and explored various physicochemical properties of the thin film. We report that during the formation of the thin film, the nanoparticles aggregated; particularly, annihilation of their nanotwinning nature was observed. Qualitatively, the Gibbs free energy change ΔG governed the abovementioned processes. The thin film exhibited an R3c phase and enhanced Bi-O-Fe coordination as compared to the sub-5 nm nanoparticles. Raman spectroscopy under the influence of a magnetic field shows a magnetoelectric effect, spin phonon coupling, and magnetic anisotropy. We report room-temperature ferroelectric behavior in the thin film, which enhances with the application of a magnetic field; this confirms the multiferroic nature of the thin film. The thin film shows polarization switching ability at multiple voltages and read-write operation at low bias (±0.5 V). Furthermore, the thin film shows negative differential-complementary resistive switching behavior in the nano-microampere current range. We report nearly stable 1-bit operation for 10² cycles, 10⁵ voltage pulses, and 10⁵ s, demonstrating the paradigm device applications. The observed results thus show that the thin films prepared using sub-5 nm BiFeO₃ nanoparticles are a promising candidate for future spintronics and memory applications. The reported approach can also be pertinent to explore the physicochemical properties and develop potential applications of several other nanoparticles.

Stable charged antiparallel domain walls in hyperferroelectrics

Charge-neutral 180° domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180° domain walls running perpendicular to the polarization directions are highly energetically unfavorable because of the depolarization field and are difficult to stabilize. We explore both neutral and charged 180° domain walls in hyperferroelectrics, a class of proper ferroelectrics with persistent polarization in the presence of a depolarization field, using density functional theory. We obtain zero temperature equilibrium structures of head-to-head and tail-to-tail walls in recently discovered ABC-type hexagonal hyperferroelectrics. Charged domain walls can also be stabilized in canonical ferroelectrics represented by LiNbO₃ without any dopants, defects or mechanical clamping. First-principles electronic structure calculations show that charged domain walls can reduce and even close the band gap of host materials and support quasi-two-dimensional electron(hole) gas with enhanced electrical conductivity.