Atomic scale imaging of competing polar states in a Ruddlesden-Popper layered oxide

Large Electromechanical Response in a Polycrystalline Alkali-Deficient (K,Na)NbO3 Thin Film on Silicon

The demand for large electromechanical performance in lead-free polycrystalline piezoelectric thin films is driven by the need for compact, high-performance microelectromechanical systems (MEMS) based devices operating at low voltages. Here we significantly enhance the electromechanical response in a polycrystalline lead-free oxide thin film by utilizing lattice-defect-induced structural inhomogeneities. Unlike prior observations in mismatched epitaxial films with limited low-frequency enhancements, we achieve large electromechanical strain in a polycrystalline (K,Na)NbO3 film integrated on silicon. This is achieved by inducing self-assembled Nb-rich planar faults with a nonstoichiometric composition. The film exhibits an effective piezoelectric coefficient of 565 pm V-1 at 1 kHz, surpassing those of lead-based counterparts. Notably, lattice defect growth is substrate-independent, and the large electromechanical response is extended to even higher frequencies in a polycrystalline film. Improved properties arise from unique lattice defect morphology and frequency-dependent relaxation behavior, offering a new route to remarkable electromechanical response in polycrystalline thin films.

Feasibility and stability in large Lotka Volterra systems with interaction structure

Complex system stability can be studied via linear stability analysis using random matrix theory (RMT) or via feasibility (requiring positive equilibrium abundances). Both approaches highlight the importance of interaction structure. Here we show, analytically and numerically, how RMT and feasibility approaches can be complementary. In generalized Lotka-Volterra (GLV) models with random interaction matrices, feasibility increases when predator-prey interactions increase; increasing competition/mutualism has the opposite effect. These changes have crucial impact on the stability of the GLV model.

Diverse Defects in Alkali Niobate Thin Films: Understanding at Atomic Scales and Their Implications on Properties

Defects in ferroelectric materials have many implications on the material properties which, in most cases, are detrimental. However, engineering these defects can also create opportunities for property enhancement as well as for tailoring novel functionalities. To purposely manipulate these defects, a thorough knowledge of their spatial atomic arrangement, as well as elastic and electrostatic interactions with the surrounding lattice, is highly crucial. In this work, analytical scanning transmission electron microscopy (STEM) is used to reveal a diverse range of multidimensional crystalline defects (point, line, planar, and secondary phase) in (K,Na)NbO₃ (KNN) ferroelectric thin films. The atomic-scale analyses of the defect-lattice interactions suggest strong elastic and electrostatic couplings which vary among the individual defects and correspondingly affect the electric polarization. In particular, the observed polarization orientations are correlated with lattice relaxations as well as strain gradients and can strongly impact the properties of the ferroelectric films. The knowledge and understanding obtained in this study open a new avenue for the improvement of properties as well as the discovery of defect-based functionalities in alkali niobate thin films.

Structural Dimensionality Dependence of the Band Gap in An+1BnX3n+1 Ruddlesden-Popper Perovskites: A Global Picture

Dimensionality engineering in A_n+1B_nX_3n+1 Ruddlesden-Popper (RP) perovskites has recently emerged as a promising tool for tuning the band gap to improve optoelectronic properties. However, the evolution of the band gap is dependent on the material; distinguishing the effects of different factors is urgently needed to guide the rational design of high-performance materials. Through first-principles calculations, we perform a systematic investigation of RP oxide, chalcogenide, and halide perovskites. The results reveal that in addition to the confinement effect and the change in octahedral rotation motions and/or amplitudes, interfacial rumpling and a change in the A-site cation coordination number also determine the evolution of the band gap. More importantly, we emphasize that the evolution of the band gap in RP perovskites is not dependent on the material family. Instead, the B-site frontier orbital type (s, p, and d) and bandwidth, A-site cation, interfacial rumpling, and structural distortions simultaneously determine the evolution of the band gap. These insights enable a complete and deeper understanding of various experimental observations.

Double-Bilayer polar nanoregions and Mn antisites in (Ca, Sr)3Mn2O7

The layered perovskite Ca₃Mn₂O₇ (CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases. However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in Ca_2.9Sr_0.1Mn₂O₇ using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °C. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric.

The competition between the polar and nonpolar phase in the prototypical hybrid improper ferroelectric crystal Ca₃Mn₂O₇ leads to exotic properties. Here, the authors directly imaged the crystal at atomic resolution to understand its nanostructure and discovered the double bilayer polar nanoregion.

Migration Kinetics of Surface Ions in Oxygen-Deficient Perovskite During Topotactic Transitions

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variation. In this study, the evolution of oxygen content is identified in real-time during the progress of a topotactic phase transition in La_0.7 Sr_0.3 MnO_3-δ epitaxial thin films, both at the surface and throughout the bulk. Using polarized neutron reflectometry, a quantitative depth profile of the oxygen content gradient is achieved, which, alongside atomic-resolution scanning transmission electron microscopy, uniquely reveals the formation of a novel structural phase near the surface. Surface-sensitive X-ray spectroscopies further confirm a significant change of the electronic structure accompanying the transition. The anisotropic features of this novel phase enable a distinct oxygen diffusion pathway in contrast to conventional observation of oxygen motion at moderate temperatures. The results provide insights furthering the design of solid oxygen ion conductors within the framework of topotactic phase transitions.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi₂TiO₄F₂ depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

Aurivillius oxyfluoride Bi₂TiO₄F₂ shows a promising photocatalytic behavior, which has been related to its polar structure, but polar properties have been difficult to reproduce in bulk samples. We explore a possible anion order and how this could give polar structures when combined with other distortions. We highlight the influence of strain on anion order and therefore the properties in these systems.

Alkali-deficiency driven charged out-of-phase boundaries for giant electromechanical response

Traditional strategies for improving piezoelectric properties have focused on phase boundary engineering through complex chemical alloying and phase control. Although they have been successfully employed in bulk materials, they have not been effective in thin films due to the severe deterioration in epitaxy, which is critical to film properties. Contending with the opposing effects of alloying and epitaxy in thin films has been a long-standing issue. Herein we demonstrate a new strategy in alkali niobate epitaxial films, utilizing alkali vacancies without alloying to form nanopillars enclosed with out-of-phase boundaries that can give rise to a giant electromechanical response. Both atomically resolved polarization mapping and phase field simulations show that the boundaries are strained and charged, manifesting as head-head and tail-tail polarization bound charges. Such charged boundaries produce a giant local depolarization field, which facilitates a steady polarization rotation between the matrix and nanopillars. The local elastic strain and charge manipulation at out-of-phase boundaries, demonstrated here, can be used as an effective pathway to obtain large electromechanical response with good temperature stability in similar perovskite oxides.

Phase boundary engineering through chemical alloying and phase control is a traditional approach to enhancing piezoelectric properties. Here, the authors design a strategy in alkali niobate films, utilizing alkali vacancies without alloying to form nanopillars enclosed.

Rewritable High-Mobility Electrons in Oxide Heterostructure of Layered Perovskite/Perovskite

Perovskite oxide SrTiO₃ can be electron-doped and exhibits high mobility by introducing oxygen vacancies or dopants such as Nb or La. A reversible after-growth tuning of high mobility carriers in SrTiO₃ is highly desired for the applications in high-speed electronic devices. Here, we report the observation of tunable high-mobility electrons in layered perovskite/perovskite (Sr_n+1Ti_nO_3n+1/SrTiO₃) heterostructure. By use of Sr_n+1Ti_nO_3n+1 as the oxygen diffusion barrier, the oxygen vacancy concentration near the interface can be reversibly engineered by high-temperature annealing or infrared laser heating. Because of the identical elemental compositions (Sr, Ti, and O) throughout the whole heterostructure, interfacial ionic intermixing is absent, giving rise to an extremely high mobility (exceeding 55000 cm² V^-1 s^-1 at 2 K) in this type of oxide heterostructure. This layered perovskite/perovskite heterostructure provides a promising platform for reconfigurable high-speed electronic devices.

Metallic line defect in wide-bandgap transparent perovskite BaSnO3

A line defect with metallic characteristics has been found in optically transparent BaSnO₃ perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level-crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO₃ films.

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn 2Ti2NbO10 Dion-Jacobson Phases

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n = 2 systems and from proper mechanisms in n = 3 CsBi2Ti2NbO10. We report here a combined experimental and computational study on analogous n = 3 CsLn 2Ti2NbO10 (Ln = La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.

Temperature-Driven Transformation of CsPbBr3 Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light-emitting applications. Recent studies have focused on nanoplatelets that are able to self-assemble and transform on solid substrates. However, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present a detailed analysis of the transformation of self-assembled stacks of CsPbBr₃ nanoplatelets in solution over a period of a few months by using ex situ transmission electron microscopy and surface analysis. We demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: (i) desorption of the ligands from the surfaces of the particles, causing the seamless atomic merging of nanoplatelet stacks into nanobelts; (ii) merging of neighboring nanobelts that form more extended nanoplates; and (iii) attachment of nanobelts and nanoplates, forming objects with an atomic structure that resembles a mosaic made of broken nanotiles. We reveal that aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr, with the atomic columns of neighboring perovskite lattices shifted by a half-unit-cell, forming Ruddlesden-Popper planar faults. We also show, via in situ monitoring of the nanocrystal photoluminescence combined with transmission electron microscopy analysis, that the transformation is temperature driven and that it can take place within tens of minutes in solution and in spin-coated films. Understanding this process gives crucial information for the design and fabrication of perovskite materials, where control over the type and density of defects is desired, stimulating the development of perovskite nanocrystal structures with tailored electronic properties.

A streaming multi-GPU implementation of image simulation algorithms for scanning transmission electron microscopy

Simulation of atomic-resolution image formation in scanning transmission electron microscopy can require significant computation times using traditional methods. A recently developed method, termed plane-wave reciprocal-space interpolated scattering matrix (PRISM), demonstrates potential for significant acceleration of such simulations with negligible loss of accuracy. Here, we present a software package called Prismatic for parallelized simulation of image formation in scanning transmission electron microscopy (STEM) using both the PRISM and multislice methods. By distributing the workload between multiple CUDA-enabled GPUs and multicore processors, accelerations as high as 1000 × for PRISM and 15 × for multislice are achieved relative to traditional multislice implementations using a single 4-GPU machine. We demonstrate a potentially important application of Prismatic, using it to compute images for atomic electron tomography at sufficient speeds to include in the reconstruction pipeline. Prismatic is freely available both as an open-source CUDA/C++ package with a graphical user interface and as a Python package, PyPrismatic.