Operando Optical Probes of Ion-Insertion-Driven Phase Change in Electrochemically Gated La_1-xSr_xCoO_3-δ (LSCO) for Tunable Photonics

Electrolyte gating has enabled dramatic control of electronic, magnetic, and optical properties in materials <i>via</i> electrochemically driven ion insertion and extraction at the interface between electrolytes and active materials. In ion-gel-gated perovskite La_1-<i>x</i>Sr<i>_x</i>CoO_3-δ(LSCO) thin films with <i>x</i> = 0.50, strong interfacial electric fields can support sufficient oxygen vacancy formation/migration to drive a topotactic transformation from the as-grown metallic perovskite phase to an insulating, oxygen-vacancy-ordered brownmillerite phase that extends from the surface through the depth of the film. Our recent <i>ex situ</i> refractive index studies uncovered large optical contrast before and after this electrochemically induced oxygen extraction, framing LSCO as an attractive phase change material for tunable photonics.<br/><br/>Here, we apply <i>operando</i> FTIR spectroscopy to ion-gel-gated LSCO films to carefully probe the reversibility and cyclability of the metal-insulator optical changes induced by oxygen insertion/extraction at the ion gel-LSCO interface. Specifically, we will show FTIR transmission measurements of <i>x</i> = 0.50 LSCO under ion-gel electrochemical gating that reveal a repeated opening and closing of an electronic bandgap by switching between the as-grown metallic perovskite phase and the voltage-induced insulating brownmillerite phase, over many cycles and at room temperature. The <i>operando </i>FTIR measurements of this electrochemically-mediated photonic switching are correlated with insights from XRD, spectroscopic ellipsometry, and electronic transport.<br/><br/>We find that the topotactic phase changes in ion-gel-gated LSCO films with <i>x </i>= 0.50 are nearly fully recoverable, with positive gate voltage driving oxygen extraction from the nominally metallic perovskite phase to form the insulating brownmillerite phase, and negative voltage driving oxygen reinsertion through the gel-LSCO interface to re-form the metallic perovskite phase. This opening and closing of an electronic bandgap is observed over many cycles in <i>operando</i> FTIR transmission measurements, with some hysteresis due to imperfect reconstruction of the original perovskite phase and asymmetric redox kinetics. These differences in electrochemical reduction <i>vs.</i> oxidation are further explored through <i>operando </i>measurements under different environments. A low-moisture environment further drives oxygen extraction in the brownmillerite phase of LSCO, while an ambient environment is preferred in the reverse direction to reinsert more oxygen through the LSCO-gel interface and recover the original perovskite phase. The enhanced ionic movement under these respective environments corresponds to lower optical loss in the insulating phase and higher loss in the metallic phase, widening the optical contrast in LSCO. Together, these findings answer important questions about the reversibility and endurance of the electrochemically driven optical changes in ion-gel-gated LSCO films, provide insight on how different environments affect ion insertion dynamics, and, combined with the nonvolatile and low-power nature of these optical changes, further highlight the powerful role that electrochemically driven ion insertion and extraction can play in developing tunable photonic devices for adaptive sensing, optical memories, and more.