Perovskite Ferroelectric Films Research Articles

Passivation of 1D@3D perovskite ferroelectric film with hydrophobic molecules for ferro-pyro-phototronic effect-based self-driven photodetector.

High-performance, self-driven photodetectors in commercial and public applications show promising prospects. The pyro-phototronic effect is a promising method for building these detectors, but limitations in interfacial contact conditions hinder the use of the ferro-pyro-phototronic effect. By modifying the surface of 1D@3D perovskite ferroelectric film with tetra-ethyl ammonium (TEAI) molecules, the interfacial defect density is reduced, resulting in a high-performance, stable photodetector. Moreover, the passivation can greatly enhance the ferro-pyro-phototronic effect, which can be explained by the increased band bending and decreased trap states at the SnS/perovskite interface resulting in less re-distribution of charge carriers directly across the interface. Our work offers a feasible and effective method for producing pyro-phototronic responses in perovskite films-based devices, and thus presents a feasible solution for high-performance, self-driven and flexible photodetection.

A Self-Powered UV Photodetector With Ultrahigh Responsivity Based on 2D Perovskite Ferroelectric Films With Mixed Spacer Cations.

Self-powered photodetectors (PDs) have the advantages of no external power requirement, wireless operation, and long life. Spontaneous ferroelectric polarizations can significantly increase built-in electric field intensity, showing great potential in self-powered photodetection. Moreover, ferroelectrics possess pyroelectric and piezoelectric properties, beneficial for enhancing self-powered PDs. 2D metal halide perovskites (MHPs), which have ferroelectric properties, are suitable for fabricating high-performance self-powered PDs. However, the research on 2D metal halide perovskites ferroelectrics focuses on growing bulk crystals. Herein, 2D ferroelectric perovskite films with mixed spacer cations for self-powered PDs are demonstrated by mixing Ruddlesden-Popper (RP)-type and Dion-Jacobson (DJ)-type perovskite. The (BDA0.7 (BA2 )0.3 )(EA)2 Pb3 Br10 film possesses, overall, the best film qualities with the best crystalline quality, lowest trap density, good phase purity, and obvious ferroelectricity. Based on the ferro-pyro-phototronic effect, the PD at 360nm exhibits excellent photoelectric properties, with an ultrahigh peak responsivity greater than 93 A W-1 and a detectivity of 2.5 × 1015 Jones, together with excellent reproducibility and stability. The maximum responsivities can be modulated by piezo-phototronic effect with an effective enhancement ratio of 480%. This work will open up a new route of designing MHP ferroelectric films for high-performance PDs and offers the opportunity to utilize it for various optoelectronics applications.

Writing-Speed Dependent Thresholds of Ferroelectric Domain Switching in Monolayer α-In2 Se3.

An electrical-biased or mechanical-loaded scanning probe written on the ferroelectric surface can generate programmable domain nanopatterns for ultra-scaled and reconfigurable nanoscale electronics. Fabricating ferroelectric domain patterns by direct-writing as quickly as possible is highly desirable for high response rate devices. Using monolayer α-In2 Se3 ferroelectric with ≈1.2nm thickness and intrinsic out-of-plane polarization as an example, a writing-speed dependent effect on ferroelectric domain switching is discovered. The results indicate that the threshold voltages and threshold forces for domain switching can be increased from -4.2 to -5V and from 365 to 1216 nN, respectively, as the writing-speed increases from 2.2 to 10.6µm s-1 . The writing-speed dependent threshold voltages can be attributed to the nucleations of reoriented ferroelectric domains, in which sufficient time is needed for subsequent domain growth. The writing-speed dependent threshold forces can be attributed to the flexoelectric effect. Furthermore, the electrical-mechanical coupling can be employed to decrease the threshold force, achieving as low as ≈189±41 nN, a value smaller than those of perovskite ferroelectric films. Such findings reveal a critical issue of ferroelectric domain pattern engineering that should be carefully addressed for programmable direct-writing electronics applications.

2D Ruddlesden-Popper perovskite ferroelectric film for high-performance, self-powered and ultra-stable UV photodetector boosted by ferro-pyro-phototronic effect and surface passivation

2D Ruddlesden-Popper (RP) perovskites have attracted enormous attention in optoelectronic fields compared with their 3D counterparts. However, most of the reported perovskite ferroelectrics-based self-powered photodetectors (PDs) are fabricated from bulk single crystals rather than perovskite thin films with improved reproducibility, flexibility, and less preparation time. Nevertheless, the defect sites at surfaces and grain boundaries of 2D perovskites polycrystalline films make them vulnerable to humidity. Herein, the highly crystallized 2D RP perovskite ferroelectric films are successfully obtained. Then, a passivation method is applied on 2D perovskite film by an in-situ reaction with 4-fluoro-phenylethylamine iodide (4-FPEAI), which reduces the defect sites and endows the perovskite films with the outstanding hydrophobic property. Then, the as-prepared 2D-perovskite ferroelectric thin films were fabricated into self-powered PDs. Boosted by the ferro-pyro-phototronic effect, the self-powered PDs demonstrate excellent optoelectronic performances at 360 nm, with dark current less than 1 × 10−12 A, a peak responsivity over 1 A W−1, a detectivity of 1.5 × 1014 Jones and fast response/recovery speed. The photoresponsivity and specific detectivity are both enhanced by 67.8 times regarding the relative peak-to-peak current, compared with those only based on the photovoltaic induced photocurrent. Further, the PDs show high stability towards high humidity and thermal treatment.

Atomic Imaging of Superelasticity of 2-dimensional Freestanding Perovskite Ferroelectric Films

Atomic Imaging of Superelasticity of 2-dimensional Freestanding Perovskite Ferroelectric Films

Film thickness dependence of ferroelectric properties in polar-axis-oriented epitaxial tetragonal (Bi,K)TiO3 films prepared by hydrothermal method

Tetragonal (00l)-oriented epitaxial (Bi,K)TiO3 films were grown at 240 °C on (100)cSrRuO3//(100)SrTiO3 substrates by the hydrothermal method. KOH aqueous solutions and Bi(NO3)3 · 5H2O and TiO2 powders were used as the starting materials. Film thickness was controlled from 33 to 1200 nm by changing the deposition time, and the Bi/(Bi+K) ratio in the A-site of perovskite ABO3 was almost constant for all film thicknesses. Polar-axis (00l)-oriented epitaxial (Bi,K)TiO3 films were obtained without a secondary phase and/or other orientation for all thickness ranges. Large ferroelectricity with the remanent polarization (Pr) of about 84 µC/cm2, comparable to previously reported lead-based ferroelectric films, was observed for (Bi,K)TiO3 films down to 33 nm in thickness. On the other hand, Ec increased with decreasing film thickness, but did not show strong film thickness dependence like other perovskite ferroelectric films. These data are very useful for understanding the degradation mechanism of ferroelectric thin films.

Wake‐Up in Al1−xBxN Ferroelectric Films

AbstractThe polarization wake‐up process is demonstrated here for ferroelectric switching in epitaxial Al0.93B0.07N films on W coated c‐axis oriented Al2O3 (sapphire) substrates. During the wake‐up process, the remanent polarization grows from ≈0 to >100 µC cm−2. As it does so, both the reversible and irreversible Rayleigh coefficients rise substantially, suggesting that the concentration of mobile interfaces that separate regions of opposite dipole orientation is increasing. The irreversible Rayleigh coefficient is very small (≈3.5 × 10−4 cm kV−1), four to five orders of magnitude below those of perovskite ferroelectric films such as PbZr0.52Ti0.48O3. These small values are consistent with the high coercive fields observed in the nitride ferroelectrics. The temperature dependence of the Rayleigh coefficients suggests that the interface motion is thermally activated. On increasing frequency, the Rayleigh coefficients drop, suggesting time‐dependent pinning processes also occur in this family of materials. With information from anisotropic etching experiments upon field‐cycling, a self‐consistent model that describes a polar domain microstructure evolution process during wake‐up is proposed.

Ultrahigh energy storage performances derived from the relaxation behaviors and inhibition of the grain growth in La doped Bi5Ti3FeO15 films

Developing environment-friendly film capacitors, which possessing a high energy storage performance, an extra wide working temperature range and a longest lifetime are desirable in applications owning to energy shortage and environmental pollution. Here, lead-free Bi5-xLaxTi3FeO15 films with four-layer Aurivillius structure were fabricated by the sol–gel method. It is revealed that the introduction of lanthanum ions into Bi5Ti3FeO15 films induces relaxation behaviors and inhibits grain growth, thus improving the energy storage performances. Bi4LaTi3FeO15 films achieved a large recoverable energy storage density of 83.2 J/cm3 and a high efficiency of 79.1% under 3460 kV/cm. Moreover, Bi4LaTi3FeO15 films exhibited excellent energy storage performances in fatigue endurance and thermal stability. This work demonstrates that the route through rare-earth doped multilayered perovskite ferroelectric films is an effective approach to enhance energy storage properties.

Achieving an ultra-high capacitive energy density in ferroelectric films consisting of superfine columnar nanograins

Well-crystallized perovskite ferroelectric films usually display a bulk-like polarization response (P) under an external electric field (E), i.e., a large P-E hysteresis loop featuring a sizable remnant polarization and an early polarization saturation. Such characteristics are undesirable for capacitive energy storage applications. In this work, we demonstrate an optimal P-E behavior, i.e., a small remnant polarization and a delayed polarization saturation, in perovskite BaTiO3 films consisting of superfine columnar nanograins. In a low-temperature, nucleation-dominated sputtering deposition, an in-situ grown conductive buffer layer promotes the formation of these nanograins, which display a controllable diameter down to ~10 nm and extend throughout the film thickness. The deterioration of the remnant polarization and its delayed saturation under an electric field, can be attributed to a strong polarization-constraining effect from the densely-packed, non-ferroelectric grain boundaries, which is supported by a phase field modeling simulation. The resulted BaTiO3 film capacitors integrated on Si at 350°C display a high recyclable energy density (Wrec~135±10 J/cm3) and efficiency (η~80%±4%) which are thickness-scalable. An intrinsically high power density, a simple and stable chemical composition, and good thermal (-150°C ~ 170°C) and cycling stabilities (up to ~ 2 × 108 charge-discharge cycles) warrant a broad range of applications for these film capacitors.

A Highly Strained Phase in PbZr0.2Ti0.8O3 Films with Enhanced Ferroelectric Properties.

Although epitaxial strain imparted by lattice mismatch between a film and the underlying substrate has led to distinct structures and emergent functionalities, the discrete lattice parameters of limited substrates, combined with strain relaxations driven by film thickness, result in severe obstructions to subtly regulate electro‐elastic coupling properties in perovskite ferroelectric films. Here a practical and universal method to achieve highly strained phases with large tetragonal distortions in Pb‐based ferroelectric films through synergetic effects of moderately (≈1.0%) misfit strains and laser fluences during pulsed laser deposition process is demonstrated. The phase possesses unexpectedly large Poisson's ratio and negative thermal expansion, and concomitant enhancements of spontaneous polarization (≈100 µC cm−2) and Curie temperature (≈800 °C), 40% and 75% larger than that of bulk counterparts, respectively. This strategy efficiently circumvents the long‐standing issue of limited numbers of discrete substrates and enables continuous regulations of exploitable lattice states in functional oxide films with tightly elastic coupled performances beyond their present levels.

Modulating Photovoltaic Conversion Efficiency of BiFeO3-Based Ferroelectric Films by the Introduction of Electron Transport Layers

The sol–gel method exhibits the advantages of simple preparation, low cost, and easy control of stoichiometry, which is widely used for the fabrication of BiFeO3-based perovskite ferroelectric film...

Crystallographically engineered hierarchical polydomain nanostructures in perovskite ferroelectric films

Hierarchical polydomain nanostructures, usually formed in epitaxial films, serve to fully relax the misfits between the film and substrate, as well as the strains between the poly-twin variants of the film material. The minimization of the elastic energy endows this type of structure superior stability over other domain architectures. However, the existence of energetically degenerate polytwin variants and their intersecting domain boundaries often led to undesirable functional properties in non-engineered films. Here we show that such nanostructures in epitaxial perovskite ferroelectric films can be crystallographically engineered to eliminate intersecting domain boundaries, thereby achieving an improved ferroelectric performance. In addition to a reduced coercive field and increased dielectric and piezoelectric responses, this type of ferroelectric polydomain nanostructure shows an enhanced ferroelectric polarization with a substantially improved fatigue resistance, as opposed to non-engineered nanostructures with intersecting domain boundaries. These findings suggest an alternative route for the fabrication of highly-reliable nonvolatile ferroelectric memory devices, and may find broader applications in piezoelectrics and energy storage.

Anisotropic strain: A critical role in domain evolution in (111)- Oriented ferroelectric films

Domain behavior of (111)- oriented perovskite ferroelectric films is significantly different from (001)-/(101)- oriented ones, resulting in enhancing property responses such as a superior susceptibility and a reduced coercive field. However, the domain structures and evolutions with the thickness of (111)-oriented ferroelectric films, which are crucial to further understand the distinctive properties, are still obscure. In this study, the ferroelectric domains of (111)- oriented PbTiO3 films are investigated by transmission electronic microscopy (TEM) and piezoresponse force microscopy (PFM). We identify the domain evolution with the film thicknesses under anisotropic strains imposed by the orthorhombic GdScO3 (101)O substrates. Contrast analysis and electron diffraction patterns reveal that only four ferroelectric variants evolve in PTO films: d2+, d2-, d3+ and d3-, with the polarization directions along [010], [01¯0], [001] and [001¯], respectively. Two kinds of domain walls are formed: “inclined” (011) domain walls for d2-/d3+ (d2+/d3-) domains, “normal” (011¯) domain walls for d2-/d3+ (d2+/d3-) domains. The width of periodically distributed d2+/d3+ (d2-/d3-) domains increases with film thickness following the square root rule. Aberration-corrected scanning transmission electronic microscopy demonstrates the lattice characteristics of domains in (111)- oriented PbTiO3 films are consistent with tetragonal ferroelectric domains. PFM studies reveal that both the out-of-plane and in-plane polarization components of d2-/d3+ (d2+/d3-) domains are non-identical, whereas the d2+/d3+ (d2-/d3-) domains possess uniform out-of-plane polarization component and non-uniform in-plane polarization component. This study discloses the domain structure in (111)- oriented tetragonal ferroelectric films under anisotropic strains and the association with the ferroelectric properties.

Possible absence of critical thickness and size effect in ultrathin perovskite ferroelectric films

Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr0.2Ti0.8O3 films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films <10-unit cells thick (∼4 nm). However, approximately the polarization never vanishes. The residual polarization is ∼16 μCcm−2 (∼17%) at 1.5-unit cells (∼0.6 nm) thick film on bare SrTiO3 and ∼22 μCcm−2 at 2-unit cells thick film on SrTiO3 with SrRuO3 electrode. The residual polarization in these ultrathin films is mainly attributed to the robust covalent Pb–O bond. Our atomic study provides new insights into mechanistic understanding of nanoscale ferroelectricity and the size effects.

Reverse-directional explosive crystallization of microstructures in transparent film on absorbing substrate by a multipulse femtosecond radiation

Abstract The crystallization in a transparent precursor of a perovskite ferroelectric film deposited on an absorbing platinized silicon substrate initiated by multipulse femtosecond sharply focused laser beam of near-infrared spectral range is studied by transmission electron microscopy. Time dependences of the shapes of crystallized areas point to initiation of explosive crystallization with a seed on the opposite side of a heat source localized in a platinum interface layer. The radius of the crystalized semispheres varies from 150 to 900 nm, with maximal crystallization velocity up to 1.2 cm/s. Reverse direction of the spherical wave front propagation regarding to a heat source is explained in terms of the developed model based on thermal stress-induced modification of the activation energy.

A Molecular Ferroelectric Thin Film of Imidazolium Perchlorate That Shows Superior Electromechanical Coupling

Molecular ferroelectric thin films are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. A thin film of imidazolium perchlorate processed from aqueous solution is an excellent molecular ferroelectric with high spontaneous polarization, high Curie temperature, low coercivity, and superior electromechanical coupling. These attributes make it a molecular alternative to perovskite ferroelectric films in sensing, actuation, data storage, electro-optics, and molecular/flexible electronics.

A Molecular Ferroelectric Thin Film of Imidazolium Perchlorate That Shows Superior Electromechanical Coupling

AbstractMolecular ferroelectric thin films are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. A thin film of imidazolium perchlorate processed from aqueous solution is an excellent molecular ferroelectric with high spontaneous polarization, high Curie temperature, low coercivity, and superior electromechanical coupling. These attributes make it a molecular alternative to perovskite ferroelectric films in sensing, actuation, data storage, electro‐optics, and molecular/flexible electronics.

Low-temperature evolution of local polarization properties of PbZr0.65Ti0.35O3 thin films probed by piezoresponse force microscopy

The temperature evolution of local polarization properties in epitaxial PbZr0.65Ti0.35O3 films is studied by the low-temperature piezoresponse force microscopy (PFM). Pronounced changes in the film polarization state, including apparent polarization rotations and possible transitions between single-domain and polydomain states of individual ferroelectric nanocolumns, are revealed on cooling from the room temperature to 8 K using PFM imaging. More than two-fold increase in the coercive voltage extracted from the piezoresponse hysteresis loops is found on cooling from 240 to 8 K. The results are explained by the thermodynamic theory of strained epitaxial perovskite ferroelectric films.

Simulation of electric displacement hysteresis and strain butterfly loops in perovskite ferroelectric films

A model for the electric displacement hysteresis and strain butterfly loops of ferroelectric films under electrical loading is proposed based on an improved Preisach model for nonlinear remanent polarization. Our model shows improved displacement and strain versus electric field loops that agree reasonably well with the experimental data. Compared to the previous model, the current model, including the history-dependent electric field effect, which is always neglected in the conventional model, provides electric displacement and strain loops with a full and symmetric shape. In addition, both the loops of electric displacement and strain under intrinsic defects and injected charges have also been investigated by our model.

First-principles modeling of strain in perovskite ferroelectric thin films

We review the role that first-principles calculations have played in understanding the effects of substrate-imposed misfit strain on epitaxially grown perovskite ferroelectric films. We do so by analyzing the case of BaTiO3, complementing our previous publications on this subject with unpublished data to help explain in detail how these calculations are done. We also review similar studies in the literature for other perovskite ferroelectric-film materials.