Jierui Liang

Voltage-driven electrochemical cycling between fully oxygenated perovskite (P) and oxygen-vacancy-ordered brownmillerite (BM) phases is now well established in electrolyte-gated complex oxides such as perovskite cobaltites (e.g., La_0.5Sr_0.5CoO_3-δ (LSCO)), enabling exceptionally wide-range reversible modulation of electronic, magnetic, thermal, and optical properties. Moving toward applications of such topotactic electrochemical transistors, progress has recently been made with cycling endurance, but the limits on operating speed remain poorly understood. We address this here in ion-gel-gate transistors based on epitaxial LSCO films, using comprehensive source-drain and gate current measurements in both frequency and time domains to assess the impact of gate voltage, side- vs top-gate geometry, and ion gel and LSCO thickness. We first establish how to rigorously define switching times in such transistors, emphasizing the inherent trade-off between ON/OFF ratio and speed. We then show unambiguously that room-temperature switching of these devices is limited by oxygen diffusion in the LSCO, not electric double layer formation in the electrolyte. Under optimized conditions with 10-unit-cell-thick P LSCO films, we thus achieve 4 source-drain current ON/OFF ratio, orders of magnitude improved over prior work. These times scales are analyzed in terms of oxygen diffusivities in the P and BM phases of LSCO, highlighting the clear role of the phase transformation and generating critical insight into routes to improved switching speed.

Electrochemical gating of perovskite La_0.5Sr_0.5CoO_3-δ (LSCO) films yields large room-temperature, low-power, nonvolatile, complex refractive index changes across a metal-insulator transition. However, the underlying topotactic perovskite to brownmillerite phase transformation has demonstrated limited cycling endurance, and the role of H₂O in this electrochemical cycling is not well established. Here, we explore extended cycling endurance in ion-gel-gate LSCO electrochemical transistors through relative-humidity-dependent operando FTIR transmittance measurements, enabling correlation of the metal-insulator optical changes in LSCO to corresponding electrochemical features. We show that higher-humidity gating significantly lowers the threshold voltage for oxygen reinsertion into LSCO and thus improves optical property reversibility, but concurrently induces LSCO etching and Pt contact dewetting that limit prolonged cycling. With informed selection of a 15% relative humidity environment and Au device contacts, we then demonstrate sustained cycling of LSCO films between distinct metallic and insulating optical states for over 40 cycles, with sustained but dampened optical modulation to 100 cycles. These findings set a metal-insulator optical property endurance record for an electrochemically gated perovskite oxide undergoing a topotactic perovskite ↔ brownmillerite phase change, further motivate the use of ion-gel-gated LSCO films for tunable infrared photonic applications such as thermal camouflage and thermoregulation, and establish useful guiding principles to enhance cycling endurance in electrochemically tunable functional oxides.

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.

This article was originally published online on 18 April 2024 with an unnecessary period included in the chemical compounds La0.5Sr0.5CoO2.5 and La0.5Sr0.5CoO3− This article was originally published online on 18 April 2024 with an unnecessary period included in the chemical compounds La0.5Sr0.5CoO2.5 and La0.5Sr0.5CoO3−

Epitaxial cobaltites have emerged as exemplary materials for electrochemical gating, in large part due to their topotactic perovskite (P) ↔ brownmillerite (BM) transformations. SrCoO3−δ, for example, can be cycled between metallic ferromagnetic P SrCoO3 and insulating BM SrCoO2.5, realizing exceptional modulation of electronic, thermal, and optical properties. It is often presumed that such cycling also generates ferromagnetic–antiferromagnetic (F-AF) modulation due to the G-type AF order in bulk SrCoO2.5. Little is understood about magnetism in thin-film BM SrCoO2.5, however, meaning that the true magnetic property modulation is unclear. We address this here through a neutron diffraction study of BM La1−xSrxCoO2.5 films at x = 0.5 and 1.0. Lightly compressively strained SrCoO2.5 films are shown to retain G-type AF order, albeit with suppressed Néel temperature (∼340 K). Of high interest for AF spintronics, room-temperature F–AF cycling is thus possible across the SrCoO3-δ P ↔ BM transformation. At x = 0.5, however, BM La0.5Sr0.5CoO2.5 films are found to exhibit no detectable G-type AF order but instead weak F order (Curie temperature ∼115 K), unveiling a La0.5Sr0.5CoO3−δ phase diagram with two distinct F phases. These results thus uncover new, unanticipated magnetic phase behavior in these materials, in addition to being directly relevant to cobaltite-based magnetoionics.