Yongjin Shin

Topotactic phase transitions in perovskite cobaltites (R1–xAxCoO3−δ; R = rare earth, A = alkaline-earth ion) play a key role in the operation of neuromorphic devices, where oxygen vacancy concentration (δ) modulates resistive switching. However, the emergence of an insulating intermediate Grenier phase (δ = 0.33) often creates an energetic barrier for electronic transport, leading to a nonmonotonic increase in the threshold voltage. We employ first-principles calculations to examine how different combinations of lanthanoids (R = La, Nd) and alkaline-earth metals (A = Ca, Sr, Ba) influence the stability of intermediate phases. Our results reveal that while smaller cation combinations such as (R, A) = (La, Sr) and (La, Ca) stabilize the insulating Grenier phase with tetrahedral CoO4 coordination, specific cation substitutions (R, A) = (Nd, Sr) and (La, Ba) shift the thermodynamic stability toward a staggered square pyramidal phase (δ = 0.25). The latter is metallic and may facilitate topotactic phase transitions from the perovskite to brownmillerite structure. Overall, our findings provide a design strategy to achieve energy-efficient, low-voltage switching in cobaltite-based memristive systems.

Halide perovskite heterostructures offer promising interfacial interactions for energy conversion, yet challenges in synthesizing structurally well-defined systems limit detailed investigations into structure-property relationships. Here, we report the synthesis of compositionally controlled 3D/3D and 3D/2D halide perovskite heterostructures using evaporation crystallization-polymer pen lithography (EC-PPL) and single-particle analysis of their properties. By systematically varying A-site-cation combinations and crystal dimensions, we show that heterointerfaces induce local lattice distortions that modulate vibrational dynamics and electron-phonon coupling. These interfacial effects result in significantly extended carrier lifetimes compared to compositionally similar pure phases. Raman spectroscopy, temperature-dependent photoluminescence, and power-dependent emission analysis reveal that localized structural modulations at the interface govern exciton-phonon interactions. These effects are magnified in smaller crystals due to increased interfacial contributions. Our findings highlight the critical role of interface-driven lattice control in tuning the optoelectronic properties of halide perovskites and provide design principles for engineering heterostructures in next-generation optoelectronic devices.

The perovskite cobaltites La_1-xSr_xCoO_3-δ have been proposed as promising candidates for neuromorphic devices, since in this class of materials a topotactically driven metal-insulator transition (MIT) can be triggered by a moderate applied voltage. The control and minimization of such voltages is important to optimize the efficiency of neuromorphic devices. By using a combination of density functional theory calculations and operando X-ray diffraction measurements on electrolyte-gated epitaxial films, here we investigate the impact of hidden Grenier phases on the nonmonotonic change in threshold voltage for varying Sr concentrations. We show that the threshold voltage for the reduction of La_1-xSr_xCoO₃ to brownmillerite La_1-xSr_xCoO_2.5 is influenced by the presence of intermediate La_1-xSr_xCoO_2.67 Grenier phases, which are challenging to detect. We discuss how the stability of these Grenier phases depends on the Sr concentration, cation ordering and the epitaxial strain applied to La_1-xSr_xCoO_3-δ films. In particular, our calculations show that by applying a biaxial strain of varied strength, one may obtain either metallic or insulating intermediate Grenier phases, which in turn may be controlled by the choice of the substrate. Our findings provide fresh insights into the importance of atomic scale control of topotactic transitions in La_1-xSr_xCoO_3-δ films for neuromorphic computing applications.

We demonstrate a quantitative framework to evaluate a nonradiative capture process with multiphonon emission in a semiconductor/dielectric heterostructure, mediated by a tunneling process. In addition to evanescent tunneling decay, the presence of an internal electric field modifies the relative energy depth of the carrier trap depending on the distance, modulating the overall capture behavior. We quantitatively derive the capture rate based on the parameters of the device structure (gate voltage, threshold voltage, dielectric material thickness, and temperature). We benchmark the formalism for the case of a Si/SiO2 heterostructure, using an isolated Si dangling bond in α-quartz as a prototype defect in silica. We find that the calculated capture coefficients show a nonmonotonic trend as a function of distance of the defect from the interface, due to countervailing trends in capture barrier and wave-function decay. The predicted capture rates are in reasonable agreement with experimentally measured capture time constants, showing promise for the application of this compact model.

Ordered oxygen vacancies (OOVs) in perovskites can exhibit long-range order and may be used to direct materials properties through modifications in electronic structures and broken symmetries. Based on the various vacancy patterns observed in previously known compounds, we explore the ordering principles of oxygen-deficient perovskite oxides with ABO_2.5 stoichiometry to identify other OOV variants. We performed first-principles calculations to assess the OOV stability on a data set of 50 OOV structures generated from our bespoke algorithm. The algorithm employs uniform planar vacancy patterns on (111) pseudocubic perovskite layers and the approach proves effective for generating stable OOV patterns with minimal computational loads. We find as expected that the major factors determining the stability of OOV structures include coordination preferences of transition metals and elastic penalties resulting from the assemblies of polyhedra. Cooperative rotational modes of polyhedra within the OOV structures reduce elastic instabilities by optimizing the bond valence of A- and B cations. This finding explains the observed formation of vacancy channels along low-index crystallographic directions in prototypical OOV phases. The identified ordering principles enable us to devise other stable vacancy patterns with longer periodicity for targeted property design in yet to be synthesized compounds.

Using first-principles calculations, we predict that tunable ferroelectricity can be realized in oxide perovskites with the Grenier structure and ordered oxygen vacancies. Specifically, we show that R_1/3A_2/3FeO_2.67 solids (where R is a rare-earth ion and A an alkaline-earth cation) exhibit polar phases, with a spontaneous polarization tunable by an appropriate choice of R and A. We find that larger cations combined with small R elements lead to a maximum in the polarization and to a minimum in the energy barriers required to switch the sign of the polarization. Ferroelectricity arises from cooperative distortions of octahedral and tetrahedral units, where a combination of rotational and sliding modes controls the emergence of polarization within three-dimensional connected layers. Our results indicate that polar Grenier phases of oxide perovskites are promising materials for microelectronic applications and, in general, for the study of phenomena emerging from breaking inversion symmetry in solids.

The stability, reliability, and performance of halide‐perovskite‐based devices depend upon the structure, composition, and particle size of the device‐enabling materials. Indeed, the degree of ion mixing in multicomponent perovskite crystals, although challenging to control, is a key factor in determining properties. Herein, an emerging method termed evaporation–crystallization polymer pen lithography is used to synthesize and systematically study the degree of ionic mixing of Cs_0.5FA_0.5PbX₃ (FA = formamidinium; X = halide anion, ABX₃) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A‐site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs–FA ion segregation. This preference for segregation can be overcome to form a solid‐solution by decreasing crystal size (0.5FA_0.5Pb(Br/I)₃ that significantly suppress photoinduced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.