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  1. 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.

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  2. Abstract

    With a high NIR reflection, a transparent radiative cooling photonic structure consisting of 2D silica gratings atop ZnO/Ag/ZnO is conceived and demonstrated. With 77% visible light transmitted, 57% NIR solar radiation reflected and 91% thermal infrared radiation emitted, a synthetical cooling is realized by this photonic structure. The theoretical total cooling power of this structure is more than double that of a planar silica and is 63.3% higher than that of a typical NIR reflecting filter, that is, ZnO/Ag/ZnO film. The field test facing the sunlight shows that the air temperature inside a chamber sealed with this structure is 12.5 and 2.5 °C lower than that sealed with planar silica and ZnO/Ag/ZnO, respectively. This work shows that the concept of daytime radiative cooling can be applied in combination with the utilization of visible light and the proposed ultrathin photonic structure shows potentials for passive radiative cooling of transparent applications.

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  3. Abstract

    Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

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  4. Abstract

    Although processing via external stimuli is a promising technique to tune the structure and properties of polymeric materials, the impact of magnetic fields on phase transitions in thermoresponsive polymer solutions is not well‐understood. As nanoparticle (NP) addition is also known to impact these thermodynamic and optical properties, synergistic effects from combining magnetic fields with NP incorporation provide a novel route for tuning material properties. Here, the thermodynamic, optical, and rheological properties of aqueous poly(N‐isopropyl acrylamide) (PNIPAM) solutions are examined in the presence of hydrophilic silica NPs and magnetic fields, individually and jointly, via Fourier‐transform infrared spectroscopy (FTIR), magneto‐turbidimetry, differential scanning calorimetry (DSC), and magneto‐rheology. While NPs and magnetic fields both reduce the phase separation energy barrier and lower optical transition temperatures by altering hydrogen bonding (H‐bonding), infrared spectra demonstrate that the mechanism by which these changes occur is distinct. Magnetic fields primarily alter solvent polarization while NPs provide PNIPAM–NP H‐bonding sites. Combining NP addition with field application uniquely alters the solution environment and results in field‐dependent rheological behavior that is unseen in polymer‐only solutions. These investigations provide fundamental understanding on the interplay of magnetic fields and NP addition on PNIPAM thermoresponsivity which can be harnessed for increasingly complex stimuli‐responsive materials.

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  5. 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.

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  6. Abstract

    Flexible biosensors exhibit great potential for the detection of various biomarkers with the ability to adapt to different surface textures. Here, a lab‐on‐a‐needle biosensing platform based on ultra‐flexible giant magnetoresistance (GMR) biosensors is developed. The fabricated flexible GMR sensors exhibit a MR ratio of 5.2% and a sensitivity of 0.13%/Oe in the linear region, which are comparable to their rigid counterparts. It is found that the magnetic properties of the flexible GMR sensors remain unchanged after 500 cycles of compressive and tensile stress, indicating strong robustness even when applied to a surface that is constantly in motion. The developed platform is then employed for the detection of different concentrations of canine osteosarcoma (OSCA‐8) cells with a limit of detection (LOD) of 200 cells in 20 µL sample (104cells per mL), which demonstrate the ability to perform real‐time, sensitive, and quantitative cell detection.

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  7. Abstract

    Gyroid optical metamaterials consist of triply periodic chiral networks that are attractive photonic structures due to the combination of intriguing optical properties and spontaneous self‐assembly‐based fabrication routes using materials such as block copolymers. A previous experimental investigation found that gyroid metamaterials support strong circular dichroism, beyond what simulations only considering bulk interactions predict. In this work, simulations are used to unravel the contributions of bulk and surface interactions on the circular dichroism spectra of silver‐infilled gyroid metamaterial films. It is found that surface interactions have a significant, often dominating, contribution to circular dichroism. The relative strength of bulk and surface contributions can be tuned by controlling the crystallographic orientation, termination plane of the film, thickness, metal volume fraction, and defect density. Importantly, the dominance of surface interactions allows double gyroids, which are achiral in the bulk, to support strong circular dichroism responses withg‐factor magnitudes as large as 0.25.

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  8. Abstract

    The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe2) and a topological semimetal (ZrTe2). We find that one unit-cell (u.c.) thick 1T-CrTe2grown epitaxially on ZrTe2is a 2D ferromagnet with a clear anomalous Hall effect. In thicker samples (12 u.c. thick CrTe2), the anomalous Hall effect has characteristics that may arise from real-space Berry curvature. Finally, in ultrathin CrTe2(3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device.

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  9. Abstract

    Fabricating flexible electronics on plastic is often limited by the poor dimensional stability of polymer substrates. To mitigate, glass carriers are used during fabrication, but removing the plastic substrate from a carrier without damaging the electronics remains challenging. Here we utilize a large-area, high-throughput photonic lift-off (PLO) process to rapidly separate polymer films from rigid carriers. PLO uses a 150 µs pulse of broadband light from flashlamps to lift-off functional thin films from glass carrier substrates coated with a light absorber layer (LAL). Modeling indicates that the polymer/LAL interface reaches above 800 °C during PLO, but the top surface of the PI remains below 120 °C. An array of indium zinc oxide (IZO) thin-film transistors (TFTs) was fabricated on a polyimide substrate and photonically lifted off from the glass carrier. The TFT mobility was unchanged by PLO. The flexible TFTs were mechanically robust, with no reduction in mobility while flexed.

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  10. Abstract

    Engineering a material's work function is of central importance for many technologies and in particular electron emitters used in high‐power vacuum electronics and thermionic energy converters. A low work function surface is typically achieved through unstable surface functional species, especially in high power thermionic electron emitter applications. Discovering and engineering new materials with intrinsic, stable low work functions obtainable without volatile surface species would mark a definitive advancement in the design of electron emitters. This work reports evidence for the existence of a low work function surface on a bulk, monolithic, electrically conductive perovskite oxide: SrVO3. After considering the patch field effect on the heterogeneous emitting surface of the bulk polycrystalline samples, this study suggests the presence of low work function (≈2 eV) emissive grains on SrVO3surface. Emission current densities of 10–100 mA cm–2at ≈1000 °C, comparable to commercial LaB6thermionic cathodes, indicative of an overall effective thermionic work function of 2.3–2.7 eV are obtained. This study demonstrates that perovskites like SrVO3may have intrinsically low work functions comparable to commercialized W‐based dispenser cathodes and suggests that, with further engineering, perovskites may represent a new class of low work function electron emitters.

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