Defect-enhanced energy storage
Dielectric capacitors are vital components of electronics and power systems. The thin-film materials of which capacitors are composed are usually optimized by changing ...the material composition. However, Kim
et al.
found that postprocessing an already effective thin-film dielectric by high-energy ion bombardment further improved the material because of the introduction of specific types of defects that ultimately improved the energy storage performance. The results suggest that postprocessing may be important for developing the next generation of capacitors.
Science
, this issue p.
81
High-energy ion bombardment considerably improves the dielectric properties of a relaxor ferroelectric thin film.
Dielectric capacitors can store and release electric energy at ultrafast rates and are extensively studied for applications in electronics and electric power systems. Among various candidates, thin films based on relaxor ferroelectrics, a special kind of ferroelectric with nanometer-sized domains, have attracted special attention because of their high energy densities and efficiencies. We show that high-energy ion bombardment improves the energy storage performance of relaxor ferroelectric thin films. Intrinsic point defects created by ion bombardment reduce leakage, delay low-field polarization saturation, enhance high-field polarizability, and improve breakdown strength. We demonstrate energy storage densities as high as ~133 joules per cubic centimeter with efficiencies exceeding 75%. Deterministic control of defects by means of postsynthesis processing methods such as ion bombardment can be used to overcome the trade-off between high polarizability and breakdown strength.
The bending-induced polarization of barium titanate single crystals has been measured with an aim to elucidate the origin of the large difference between theoretically predicted and experimentally ...measured flexoelectricity in this material. The results indicate that part of the difference is due to polar regions (short-range order) that exist above T(C) and up to T*≈200-225 °C. Above T*, however, the flexovoltage coefficient still shows an unexpectedly large anisotropy for a cubic material, with (001)-oriented crystals displaying 10 times more flexoelectricity than (111)-oriented crystals. Theoretical analysis shows that this anisotropy cannot be a bulk property, and we therefore interpret it as indirect evidence for the theoretically predicted but experimentally elusive contribution of surface piezoelectricity to macroscopic bending-induced polarization.
Ferroelastic switching in ferroelectric/multiferroic oxides plays a crucial role in determining their dielectric, piezoelectric, and magnetoelectric properties. In thin films of these materials, ...however, substrate clamping is generally thought to limit the electric-field- or mechanical-force-driven responses to the local scale. Here, we report mechanical-force-induced large-area, non-local, collective ferroelastic domain switching in PbTiO
epitaxial thin films by tuning the misfit-strain to be near a phase boundary wherein c/a and a
/a
nanodomains coexist. Phenomenological models suggest that the collective, c-a-c-a ferroelastic switching arises from the small potential barrier between the degenerate domain structures, and the large anisotropy of a and c domains, which collectively generates much larger response and large-area domain propagation. Large-area, non-local response under small stimuli, unlike traditional local response to external field, provides an opportunity of unique response to local stimuli, which has potential for use in high-sensitivity pressure sensors and switches.
Leveraging competition between energetically degenerate states to achieve large field‐driven responses is a hallmark of functional materials, but routes to such competition are limited. Here, a new ...route to such effects involving domain‐structure competition is demonstrated, which arises from strain‐induced spontaneous partitioning of PbTiO3 thin films into nearly energetically degenerate, hierarchical domain architectures of coexisting c/a and a1/a2 domain structures. Using band‐excitation piezoresponse force microscopy, this study manipulates and acoustically detects a facile interconversion of different ferroelastic variants via a two‐step, three‐state ferroelastic switching process (out‐of‐plane polarized c+ → in‐plane polarized a → out‐of‐plane polarized c− state), which is concomitant with large nonvolatile electromechanical strains (≈1.25%) and tunability of the local piezoresponse and elastic modulus (>23%). It is further demonstrated that deterministic, nonvolatile writing/erasure of large‐area patterns of this electromechanical response is possible, thus showing a new pathway to improved function and properties.
Strain‐induced structural competition between domain‐structure variants of tetragonal PbTiO3 drives the formation of exotic hierarchical domain patterns, as indicated by the complex topography observed in scanning‐probe‐based studies. This competition also promotes facile interconversion between in‐ and out‐of‐plane polarized ferroelastic variants, which is concomitant with large nonvolatile electromechanical strains, and tunability of local piezoresponse and elastic modulus. There is rich potential for on‐demand electrical writing of ferroelastic mesostructures.
We explore the effect of growth conditions on the cation and anion chemistry, electrical leakage, conduction mechanisms, and ferroelectric and dielectric behavior of BiFeO3. Although it is possible ...to produce single-phase, coherently strained films in all cases, small variations in the pulsed-laser deposition growth process, specifically the laser repetition rate and target composition, result in films with chemistries ranging from 10% Bi-deficiency to 4% Bi-excess and films possessing Bi gradients as large a 6% across the film thickness. Corresponding variations and gradients in the O chemistry are also observed. As a result of the varying film chemistry, marked differences in surface and domain morphology are observed wherein Bi-deficiency stabilizes atomically smooth surfaces and ordered stripe domains. Subsequent investigation of the current–voltage response reveals large differences in leakage current density arising from changes in both the overall stoichiometry and gradients. In turn, the film stoichiometry drives variations in the dominant conduction mechanism including examples of Schottky, Poole–Frenkel, and modified Poole–Frenkel emission depending on the film chemistry. Finally, slightly Bi-excess films are found to exhibit the best low-frequency ferroelectric and dielectric response while increasing Bi-deficiency worsens the low-frequency ferroelectric performance and reduces the dielectric permittivity.
Solid‐oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure–property relationships that would enable the ...rational design of better materials. Here, using epitaxial thin‐film growth, synchrotron radiation, impedance spectroscopy, and density‐functional theory, the impact of structural parameters (i.e., unit‐cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9Sr0.1Ga0.95Mg0.05O3–δ. As compared to the zero‐strain state, compressive strain reduces the unit‐cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit‐cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit‐cell volumes and octahedral rotations decrease migration barriers and create low‐energy migration pathways, respectively. The desired combination of large unit‐cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit‐cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion‐conducting perovskite electrolytes.
Ionic conduction in the perovskite oxide La0.9Sr0.1Ga0.95Mg0.05O3–δ (LSGM) is found to be strongly correlated with crystal structure. A structural design with simultaneously large unit‐cell volume and octahedral rotations for fast ionic conduction is proposed and realized in LSGM superlattice thin films, where the ionic conductivity is tuned with structure alone by a factor of ≈2.5 at 600 °C.
Results of switching behavior of the improper ferroelectric LuFeO3 are presented. Using a model set of films prepared under controlled chemical and growth‐rate conditions, it is shown that defects ...can reduce the quasi‐static switching voltage by up to 40% in qualitative agreement with first‐principles calculations. Switching studies show that the coercive field has a stronger frequency dispersion for the improper ferroelectrics compared to a proper ferroelectric such as PbTiO3. It is concluded that the primary structural order parameter controls the switching dynamics of such improper ferroelectrics.
Polarization switching in epitaxial thin films of the improper ferroelectric LuFeO3 shows that the inclusion of point defects can reduce the coercive electric field. Frequency dispersion of the switching field is larger when the thickness is reduced, suggesting that the mechanical constraint induced by the substrates may play a key role in the switching of the polarization state.
Hydrostatic pressure applied using diamond anvil cells (DAC) has been widely explored to modulate physical properties of materials by tuning their lattice degree of freedom. Independently, electrical ...field is able to tune the electronic degree of freedom of functional materials via, for example, the field-effect transistor (FET) configuration. Combining these two orthogonal approaches would allow discovery of new physical properties and phases going beyond the known phase space. Such experiments are, however, technically challenging and have not been demonstrated. Herein, we report a feasible strategy to prepare and measure FETs in a DAC by lithographically patterning the nanodevices onto the diamond culet. Multiple-terminal FETs were fabricated in the DAC using few-layer MoS2 and BN as the channel semiconductor and dielectric layer, respectively. It is found that the mobility, conductance, carrier concentration, and contact conductance of MoS2 can all be significantly enhanced with pressure. We expect that the approach could enable unprecedented ways to explore new phases and properties of materials under coupled mechano-electrostatic modulation.
Spin‐state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low‐spin to high‐spin transition in LaCoO3 thin films has drawn ...a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen‐vacancy formation, spin‐state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin‐film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen‐vacancy and spin‐ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low‐spin to high‐spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation‐off‐stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering.
This work provides compelling evidence that tensile‐strain‐induced tetragonal distortion significantly modifies the orbital occupancy in LaCoO3, leading to an emergent ferromagnetism through the high spin state, while samples grown at lower pressure demonstrate a suppressed magnetization due to the cation‐off‐stoichiometry induced lattice expansion. This result identifies a promising strategy to design an electronic state in complex oxides through cation‐stoichiometry engineering.
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