Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−
Solid-state control of the thermal conductivity of materials is of exceptional interest for novel devices such as thermal diodes and switches. Here, we demonstrate the ability to
- NSF-PAR ID:
- 10411859
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract x Srx CoO3−δ (LSCO) with 0.00 ≤x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators forx < 0.18 but lossy metals forx > 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors withx > 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n > 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at highx . Atx < 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn > 0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories. -
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Abstract Ultrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (
Λ LSCO) of epitaxial La0.5Sr0.5CoO3−δ (LSCO) of varying thickness (<20 nm) on LaAlO3and SrTiO3substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room‐temperatureΛ LSCOof LSCO on both substrates (1.7 W m−1K−1) are nearly a factor of four lower than that of bulk single‐crystal LSCO (6.2 W m−1K−1). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m−1K−1for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass‐like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measuredΛ LSCOis rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution toΛ LSCOalong the through‐plane direction for these ultrathin LSCO films on insulating substrates. -
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Abstract Achieving high oxygen evolution reaction (OER) activity while maintaining performance stability is a key challenge for designing perovskite structure oxide OER catalysts, which are often unstable in alkaline environments transforming into an amorphous phase. While the chemical and structural transformation occurring during electrolysis at the electrolyte–catalyst interface is now regarded as a crucial factor influencing OER activity, here, using La0.7Sr0.3CoO3−
δ (LSCO) as an active OER catalyst, the critical influence of buried layers on the oxidation current stability in nanoscopically thin, chemically and structurally evolving, catalyst layers is revealed. The use of epitaxial thin films is demonstrated to engineer both depletion layer widths and chemical stability of the catalyst support structure resulting in heterostructured anodes that maintain facile transport kinetics across the electrolyte–anode interface for atomically thin (2–3 unit cells) LSCO catalyst layers and greatly enhanced oxidation current stability as the perovskite structure OER catalysts chemically and structurally transform. This work opens up an approach to design robust and active heterostructured anodes with dynamically evolving ultrathin OER electrocatalyst layers for future green fuel technologies such as conformal coatings of high‐density 3D anode topologies for water splitting. -
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Abstract Nanostructuring to reduce thermal conductivity is among the most promising strategies for designing next‐generation, high‐performance thermoelectric materials. In practice, electrical grain boundary resistance can overwhelm the thermal conductivity reduction induced by nanostructuring, which results in worse overall performance. Since a large body of work has characterized the transport of both polycrystalline ceramics and single crystals of SrTiO3, it is an ideal material system for conducting a case study of electrical grain boundary resistance. An effective mass model is used to characterize the transport signatures of electrical grain boundary resistance and evaluate thermodynamic design principles for controlling that resistance. Treating the grain boundary as a secondary phase to the bulk crystallites explains the transport phenomena. Considering that the interface can be engineered by controlling oxygen partial pressure, temperature, and the addition of extrinsic elements into the grain boundary phase, the outlook for SrTiO3as a nanostructured thermoelectric is promising, and the
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Abstract Ternary metal‐oxide material systems commonly crystallize in the perovskite crystal structure, which is utilized in numerous electronic applications. In contrast to oxides, there are no known nitride perovskites, likely due to the competition with oxidation, which makes the formation of pure nitride materials difficult and synthesis of oxynitride materials more common. While deposition of oxynitride perovskite thin films is important for many electronic applications, it is difficult to control oxygen and nitrogen stoichiometry. Lanthanum tungsten oxynitride (LaWN3−
δ Oδ ) thin films with varying La:W ratio are synthesized by combinatorial sputtering and characterized for their chemical composition, crystal structure, and microstructure. A three‐step synthesis method, which involves co‐sputtering, capping layer deposition, and rapid thermal annealing, is established for producing crystalline thin films while minimizing the oxygen content. Elemental depth profiling results show that the cation‐stoichiometric films contain approximately one oxygen for every five nitrogen (δ = 0.5). Synchrotron‐based diffraction indicates a tetragonal perovskite crystal structure. These results are discussed in terms of the perovskite tolerance factors, octahedral tilting, and bond valence. Overall, this synthesis and characterization is expected to pave the way toward future thin film property measurements of lanthanum tungsten oxynitrides and eventual synthesis of oxygen‐free nitride perovskites.
