skip to main content


Title: Optical Properties of Electrochemically Gated La 1− x Sr x CoO 3−δ as a Topotactic Phase‐Change Material
Abstract

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−xSrxCoO3−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.

 
more » « less
Award ID(s):
2011401
NSF-PAR ID:
10415091
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Optical Materials
Volume:
11
Issue:
16
ISSN:
2195-1071
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    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 tocontinuouslytune the thermal conductivity of nanoscale films of La0.5Sr0.5CoO3-δ(LSCO) by a factor of over 5, via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (withδ≈ 0.1) to an oxygen-vacancy-ordered brownmillerite phase (withδ= 0.5), accompanied by a metal-insulator transition. Combining time-domain thermoreflectance and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport equation, and structural characterization by X-ray diffraction, we uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. The wide-range continuous tunability of LSCO thermal conductivity enabled by low-voltage (below 4 V) room-temperature electrolyte gating opens the door to non-volatile dynamic control of thermal transport in perovskite-based functional materials, for thermal regulation and management in device applications.

     
    more » « less
  2. Abstract

    MXenes, a new class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted much attention due to their outstanding properties. Here, we report the broadband spatial self‐phase modulation of Ti2CTxMXene nanosheets dispersed in deionized water in the visible to near‐infrared regime, highlighting the broadband nonlinear optical (NLO) response of Ti2CTxMXene. Using ultrafast pulsed laser excitation, the nonlinear refractive indexn2and the third‐order nonlinear susceptibilityof Ti2CTxMXene were measured to be ∼10−13m2/W and ∼ 10−10esu, respectively. Leveraging the large optical nonlinearity of Ti2CTxMXene, an all‐optical modulator in the visible regime was fabricated based on the spatial cross‐phase modulation effect. This work suggests that 2D MXenes are ideal broadband NLO materials with excellent prospects in NLO applications.

    image

     
    more » « less
  3. Abstract

    Ultrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (ΛLSCO) 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‐temperatureΛLSCOof 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 measuredΛLSCOis 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 toΛLSCOalong the through‐plane direction for these ultrathin LSCO films on insulating substrates.

     
    more » « less
  4. Abstract

    Dynamic control of patterned properties in perovskite oxide films can enable new architectures for electronic, magnetic, and optical devices. In this study, it is shown that SrFeO3‐δ/SrFeO2F laterally‐heterostructured films enable voltage‐controlled tunable and reversible metal‐insulator patterned properties using room‐temperature ion gel gating. Specifically, SrFeO3‐δfilm regions can be toggled between insulating HxSrFeO2.5and metallic SrFeO3by electrochemical redox, while SrFeO2F regions remain robustly insulating and are unaffected by ion gel gating. Various gating architectures are also compared and establish the advantages of employing a conductive substrate as the contacting electrode, as opposed to at the film surface, thereby achieving complete and reversible reduction and oxidation among SrFeO3‐δ, HxSrFeO2.5, and SrFeO3. This approach to voltage‐modulated patterned electronic, optical, and magnetic properties should be broadly applicable to oxide materials amenable to fluoridation, and potentially other forms of anion substitution.

     
    more » « less
  5.  
    more » « less