The data included here contain the information necessary to recreate the figures in a manuscript titled "Optical Properties of Electrochemically Gated La_1-xSr_xCoO_3-δ as a Topotactic Phase-Change Material". The data files include scanning transmission electron microscopy (STEM) images of electrochemically gated La_1-xSr_xCoO_3-δ (LSCO) films, finite-difference time-domain (FDTD)-simulated electric field and reflectance data for LSCO-based metasurfaces, transfer-matrix model reflectance data for LSCO films on gold substrates, complex refractive index data for LSCO films before and after electrochemical gating, electronic resistivity data for LSCO films before and after electrochemical gating, source-drain current measurements of LSCO films during electrochemical gating, and X-ray diffraction data for LSCO films before and after electrochemical gating.

Methods for collection/generation of data:

Growth and Fabrication of LSCO Transistors: La_1-xSr_xCoO_3-δ (LSCO) films with 0 ≤ x ≤ 0.70, along with brownmillerite-phase SrCoO_2.5 (BM SCO) films (x = 1.00), were deposited on (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 (001) (LSAT) substrates using high-pressure-oxygen sputtering using previously-optimized conditions. First, 10 mm x 10 mm x 0.5 mm commercial LSAT(001) substrates from MTI were annealed at 900 °C under 1 Torr of ultra-high-purity (99.998%) O₂ for 15 minutes. LSCO films with 0 ≤ x ≤ 0.70 were then sputtered from polycrystalline ceramic LSCO targets at 600 °C substrate temperature (700 °C in the case of x = 0), 45-65 W of DC power, and 1.5 Torr O₂ pressure, after which the films were cooled to room temperature in 600 Torr O₂ pressure. BM SCO films were grown at a substrate temperature of 700 °C using 40 W of DC power and cooled to room temperature at the growth pressure of 1.5 Torr O₂. The resulting LSCO films ranged from 12-22 nm in thickness across all compositions, as determined from wide-angle XRD Laue fringes. All LSCO films at a particular composition were grown under identical conditions and therefore have the same thickness within ~0.5 nm. LSCO films were grown on both one- and two-side polished LSAT substrates to enable subsequent spectroscopic ellipsometry measurements and intensity reflection/transmission measurements, respectively. 

Electric double-layer transistors (EDLTs) based on LSCO were fabricated following established methods. 3.5 x 4.0 mm² center channels in the LSCO film were first defined with Ar ion milling using a steel mask. Mg (5 nm)/Pt (50 nm) films were then deposited through a second mask to form the source, drain, and gate electrodes, and subsequently rapid-thermal-annealed at 450 °C for 10 minutes in flowing O₂. The EDLTs were fabricated in a side-gate geometry so that the LSCO channel could be probed by XRD and various optical spectroscopy techniques. The ion gel electrolytes used for gating experiments consisted of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI:TFSI) embedded in poly(vinylidene fluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)). Using acetone as the solvent, a 1:4:7 by-weight mixture of polymer : ionic liquid : solvent was spin-coated onto glass wafers, then treated under vacuum at 70 °C to remove the solvent. The “cut-and-stick” nature of these ion gels allows one to easily cut sections of the dried gels with a blade and use tweezers to apply them directly onto the device, completing the EDLTs.

Electrolyte Gating, Source-Drain Measurements, and Resistivity Measurements: Electrolyte gating experiments and temperature-dependent resistivity measurements of LSCO EDLTs were performed in a Quantum Design Physical Property Measurement System and a closed-cycle He refrigerator. Keithley 2400 source-measure units were used to measure channel resistances and apply gate voltages (V_g). All gating experiments were performed under the identical conditions of 300 K and vacuum (< 10^-5 Torr). The gating was accomplished by sweeping Vg from 0 V to +3.5 V at a sweep rate of 0.5 mV s^-1, during which in-situ source-drain current measurements were also obtained with a constant source-drain voltage (V_SD) of 0.2 V. Using the same system, temperature-dependent resistivity measurements were performed by cooling down the sample to low temperature, then taking measurements at each temperature as the sample was gradually warmed to room temperature.

X-ray Diffraction Measurements: Ex-situ high-resolution wide-angle X-ray diffraction was performed using a Rigaku Smartlab XE diffractometer with a 5-axis goniometer, HyPix-3000 high energy resolution detector, ~1.54 Å (Cu Kα) incident wavelength, and a spot size of 2 x 2 mm².

Scanning Transmission Electron Microscopy (STEM) Measurements: STEM samples were prepared using an FEI Helios Nanolab G4 dual-beam focused ion beam (FIB) microscope. The sample was coated with amorphous carbon prior to ion beam exposure in the FIB, to prevent damage to the film surface. The sample was first thinned using 30 keV Ga ion beams followed by a 2 keV ion beam shower to remove damaged surface layers. High-angle annular dark field (HAADF) STEM imaging was performed on an aberration-corrected FEI Titan G2 60-300 (S)TEM which is equipped with a monochromator and a CEOS-DCOR probe corrector. The microscope was operated at 200 keV, with a probe current of 100 pA and probe convergence angle of 18.2 mrad. The HAADF detector inner and outer collection angles used were 58.5 and 200 mrad, respectively.

Refractive Index Measurements: All optical measurements were performed ex-situ under ambient conditions. Spectroscopic ellipsometry was performed on LSCO EDLTs grown on one-side polished substrates using a J.A. Woollam VASE ellipsometer in the wavelength range of 350–1100 nm. All ellipsometric measurements were performed at a 40° incident angle, which produces a spot width small enough (< 4 mm) to probe only the LSCO channel and not the surrounding electrodes. Separately, optical transmittance measurements were performed on LSCO EDLTs grown on two-side polished substrates. Transmittance data from 350–2500 nm were collected using a Cary 7000 UV/Vis/NIR spectrophotometer, where samples were mounted on an opaque holder with an opening smaller than 3.5 x 4.0 mm² in area such that only the LSCO channel was illuminated. Transmittance data from 2500–5000 nm were collected using a Bruker Hyperion 2000 FTIR microscope with liquid N₂-cooled MCT detector coupled to a Bruker Invenio-R FTIR spectrometer. All transmittance spectra were collected at normal incidence using unpolarized light and were normalized to the transmittance of air. For each composition of LSCO, the corresponding ellipsometric and transmittance data were then fit together to Kramers-Kronig-consistent refractive index models of LSCO ranging from 350–5000 nm. To enable this fitting process for LSCO, the refractive index of each two-side-polished LSAT substrate used for this study was separately modeled in a similar manner. 

LSCO Metasurface Simulations: FDTD simulations of LSCO plasmonic metasurfaces were performed using Ansys Lumerical software. The simulated structure consisted of a planar LSCO layer of variable height sandwiched between a Au nanodisk metasurface on top and a 100-nm thick planar Au layer on the bottom. Each Au nanodisk had 400 nm diameter and 90 nm height, and the cylinder array had a pitch of 800 nm. A broadband plane wave source was injected in the -z direction, periodic boundary conditions were used in the x- and y-directions, and perfectly matched layers were used as boundary conditions in the z-direction. The refractive index of LSCO was directly taken from the data gathered in this study, and the refractive index of Au was taken from the following literature source: https://doi.org/10.1364/AO.37.005271.

LSCO Thin Film Simulations: The reflectance spectra of LSCO/Au films at different composition (x) and thickness were calculated with open-source MATLAB code designed for transfer-matrix modeling: https://github.com/ulfgri/tftb-toolbox.

Methods for processing the data:

Processing of FDTD Simulation Data: The FDTD simulation results obtained with Ansys Lumerical software were post-processed in order to obtain the wavelength-dependent reflectance data and wavelength-specific electric field data included here. Reflectance data were derived from a 2D power monitor placed above the simulated structure, and the transmission() script command in Lumerical was applied to the power monitor to extract the wavelength-dependent reflectance. Further information regarding the transmission() command can be found here: https://optics.ansys.com/hc/en-us/articles/360034405354-transmission-Script-command. For a specific wavelength, electric field data were obtained from a separate 2D power monitor placed in the xz plane of the structure (located at y=0). At each (x,z) coordinate of this monitor, the x, y, and z components of the electric field were obtained using the getresult() script command. For each (x,z) coordinate, the squared amplitude of the electric field was then calculated as |E|² = |E_x|² + |E_y|² + |E_z|². Because the incident electric field is defined by Lumerical to have an amplitude of 1, this |E|² quantity also represents the local electric field intensity enhancement relative to the incident field, or |E_local|²/|E_incident|². The reflectance and electric field data were then exported into MATLAB and plotted for the associated publication. In MATLAB, the electric field data were plotted as imagesc(x,y,E).

Processing of the Resistivity Data: The resistivity data included here were obtained from 4-wire resistance measurements and corresponding analyses via the van der Pauw method. At each temperature, two resistance measurements (R₁ and R₂) were performed, representing the two different current-voltage wiring configurations for the 4-wire model. Using R₁ and R₂, the following van der Pauw equation was numerically solved to obtain the sheet resistance, R_s:

exp(-πR₁/R_s) + exp(-πR₂/R_s) = 1

Finally, R_s was multiplied by the LSCO film thickness to obtain the reported resistivity values (with units of Ohm cm).