In the early 2000s, low dimensional ferroelectric systems were predicted to have topologically nontrivial polar structures, such as vortices or skyrmions, depending on mechanical or electrical ...boundary conditions. A few variants of these structures have been experimentally observed in thin film model systems, where they are engineered by balancing electrostatic charge and elastic distortion energies. However, the measurement and classification of topological textures for general ferroelectric nanostructures have remained elusive, as it requires mapping the local polarization at the atomic scale in three dimensions. Here we unveil topological polar structures in ferroelectric BaTiO
nanoparticles via atomic electron tomography, which enables us to reconstruct the full three-dimensional arrangement of cation atoms at an individual atom level. Our three-dimensional polarization maps reveal clear topological orderings, along with evidence of size-dependent topological transitions from a single vortex structure to multiple vortices, consistent with theoretical predictions. The discovery of the predicted topological polar ordering in nanoscale ferroelectrics, independent of epitaxial strain, widens the research perspective and offers potential for practical applications utilizing contact-free switchable toroidal moments.
Abstract
Mott threshold switching, which is observed in quantum materials featuring an electrically fired insulator-to-metal transition, calls for delicate control of the percolative dynamics of ...electrically switchable domains on a nanoscale. Here, we demonstrate that embedded metallic nanoparticles (NP) dramatically promote metastability of switchable metallic domains in single-crystal-like VO
2
Mott switches. Using a model system of Pt-NP-VO
2
single-crystal-like films, interestingly, the embedded Pt NPs provide 33.3 times longer ‘memory’ of previous threshold metallic conduction by serving as pre-formed ‘stepping-stones’ in the switchable VO
2
matrix by consecutive electical pulse measurement; persistent memory of previous firing during the application of sub-threshold pulses was achieved on a six orders of magnitude longer timescale than the single-pulse recovery time of the insulating resistance in Pt-NP-VO
2
Mott switches. This discovery offers a fundamental strategy to exploit the geometric evolution of switchable domains in electrically fired transition and potential applications for non-Boolean computing using quantum materials.
NiO sub(6) octahedral tilts in the LaNiO sub(3)/SrTiO sub(3) superlattices are quantified using position averaged convergent beam electron diffraction in scanning transmission electron microscopy. It ...is shown that maintaining oxygen octahedra connectivity across the interface controls the octahedral tilts in the LaNiO sub(3) layers, their lattice parameters, and their transport properties. Unlike films and layers that are connected on one side to the substrate, subsequent LaNiO sub(3) layers in the superlattice exhibit a relaxation of octahedral tilts towards bulk values. This relaxation is facilitated by tilts in the SrTiO sub(3) layers and is correlated with the conductivity enhancement of the LaNiO sub(3) layers in the superlattices relative to individual films.
Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low ...temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO2.5 are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO2.5 thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.
Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic ...responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7–8.5 μC cm−2), which possibly reduces a rapid recombination rate of the photo-generated electron–hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.
Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion to reversibly provide a massive amount of hydrogen dopants in correlated oxides, but the mechanism ...at the surface of correlated oxides with metal catalyst are not well understood. Here we show that a significant amount of oxygen is released from the surface of correlated VO2 films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO2 films. By using ambient-pressure X-ray photoelectron spectroscopy, we prove that the formation of surface oxygen vacancies is a consequence of a favorable reaction for the generation of weakly adsorbed H2O from surface O atoms that have low coordination and weak binding strength. Our results reveal the importance of in situ characterization to prove the dynamic change during redox reaction and present an opportunity to control intrinsic defects at the surface.
Ionic–electronic coupling in a lattice strongly influences the memory and learning process by synaptic weight update in electrochemical synaptic transistors. In particular, anisotropic crystal ...symmetry offers a highly anisotropic diffusion process, which leads to facilitated ion migration and efficient coupling in synaptic devices. Here, we report all-solid-state VO2 synaptic transistors in which the proton diffusion under gate bias can be tuned by utilizing different crystal facets in anisotropic VO2 channels. Synaptic weight update (i.e., excitatory post-synaptic current) by a proton (H+) in the VO2 channel was sensitively tuned depending on the empty tunnel alignment of VO2 layers. By emulating synaptic functions using diffusion-pathway-controlled transistors, the alignment of a facile ionic pathway with gating direction increases the retention of H+ in VO2 lattices by locating H+ into the deep regions from the interfaces, and thus strengthens long-term memory in artificial synaptic devices. These results demonstrate that the control of field-driven ionic redistribution guided by crystal anisotropy provides an opportunity to manipulate the learning and forgetting behavior in artificial synaptic devices.
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