Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

Abstract

Mixed ionic–electronic conductors offer chemical and electrical means for active tuning of their optical constants, e.g., with variations in oxygen non‐stoichiometry in Pr0.1Ce0.9O2–δ, enabling implementation of adaptive thin film optical devices. In situ chemo‐tuning of the extinction coefficient in Pr0.1Ce0.9O2–δ at elevated temperatures is demonstrated and a tuning model is provided that treats the interdependence of mobile oxygen vacancies and small polarons coupled to variations in optically active praseodymium ions. Furthermore, a new means for electro‐tuning of the optical constants of mixed ionic–electronic conductors is demonstrated experimentally and modeled for Pr0.1Ce0.9O2–δ thin films deposited on grid‐like electrode structures. Modeling of non‐steady‐state optical transmittance modulations in the latter allows for estimation of oxygen vacancy mobility that determines the switching speed of the device. Quenched‐in values of nr and k to room temperature become nonvolatile, providing a modulation range in the extinction coefficient of Δk ≈ 0.1 (change of ≈800%) and in the refractive index of Δnr ≈ 0.1 (relative to initial nr of ≈2.35). Key figures of merit, including transmission optical modulation of ≈0.04 per 1 mV nm–1, switching energy per area of 1.9 nJ µm–2, and switching times of seconds, are demonstrated, with further improvements possible.

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

“…The OAR technique measures the oxygen exchange kinetics by probing the time relaxation in the light intensity transmitted through the STF film as it becomes more oxidized (more strongly absorbing in the visible due to an increase in Fe 4+ concentration) or reduced (decrease in Fe 4+ concentration) in response to step changes in oxygen partial pressure ( p O 2 ), according to the defect model . The OAR measurements were conducted using a custom-made near infrared–VIS–UV transmission spectrophotometer, capable of in situ measurements at elevated temperature and in a controlled gas environment, as described in detail elsewhere . The gas composition (and p O 2 ) of the atmosphere inside the optical flow cell was controlled by a constant flow (at rate of 140 sccm) of premixed pure O 2 and Ar gases.…”

“…58 The OAR measurements were conducted using a custom-made near infrared−VIS−UV transmission spectrophotometer, capable of in situ measurements at elevated temperature and in a controlled gas environment, as described in detail elsewhere. 67 The gas composition (and pO 2 ) of the atmosphere inside the optical flow cell was controlled by a constant flow (at rate of 140 sccm) of premixed pure O 2 and Ar gases. The pO 2 steps were created by automated changes in the O 2 /Ar ratio, keeping the flow rate constant.…”

Strain-Dependent Surface Defect Equilibria of Mixed Ionic-Electronic Conducting Perovskites

“…To measure the total transmittance at controlled temperatures and atmospheres, we used a custom-made spectrophotometer which was described in detail previously. 66 For the optical measurements, we grew by PLD a 120 nm LSF film onto the two-side polished MgO (001) substrate of 10 mm × 5 mm × 0.5 mm dimensions (MTI corporation), using the same growth parameters as other samples. The gas composition of the atmosphere inside the flow cell was controlled by a constant flow (at rate of 140 sccm) of premixed gases.…”

Simultaneous electrical impedance and optical absorption spectroscopy for rapid characterization of oxygen vacancies and small polarons in doped ceria

“…20,24 Additionally, PCO is a tunable optical material as Pr 4+ dopants form in-gap states 25 that strongly absorb in the visible spectrum (above B2 eV) and upon complete reduction of the Pr dopants from 4+ to 3+ valence state, become nearly transparent. 10,11,22 The reduction state of PCO can be controlled by varying the deviation from oxygen stoichiometry, d, providing means for active tuning of the optical (and electrical) properties of PCO by either exposure to reactive gases or by applying an electrical bias. 10,11,26 To fully reveal the mechanisms underlying PCO's ionic and small polaron transport and optical properties, it needs to be studied as a function of the oxygen non-stoichiometry, d, that at equilibrium depends on both temperature and oxygen partial pressure (pO 2 ).…”

“…20,24 Additionally, PCO is a tunable optical material as Pr 4+ dopants form in-gap states 25 that strongly absorb in the visible spectrum (above ∼2 eV) and upon complete reduction of the Pr dopants from 4+ to 3+ valence state, become nearly transparent. 10,11,22 The reduction state of PCO can be controlled by varying the deviation from oxygen stoichiometry, δ , providing means for active tuning of the optical (and electrical) properties of PCO by either exposure to reactive gases or by applying an electrical bias. 10,11,26…”