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Creators/Authors contains: "Phuah, Xin Li"

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  1. Abstract

    Mott insulator VO2exhibits an ultrafast and reversible semiconductor‐to‐metal transition (SMT) near 340 K (67 °C). In order to fulfill the multifunctional device applications, effective transition temperature (Tc) tuning as well as integrated functionality in VO2is desired. In this study, multifunctionalities including tailorable SMT characteristics, ferromagnetic (FM) integration, and magneto‐optical (MO) coupling, have been demonstrated via metal/VO2nanocomposite designs with controlled morphology, i.e., a two‐phase Ni/VO2pillar‐in‐matrix geometry and a three‐phase Au/Ni/VO2particle‐in‐matrix geometry. EvidentTcreduction of 20.4 to 54.9 K has been achieved by morphology engineering. Interestingly, the Au/Ni/VO2film achieves a record‐lowTcof 295.2 K (22.2 °C), slightly below room temperature (25 °C). The change in film morphology is also correlated with unique property tuning. Highly anisotropic magnetic and optical properties have been demonstrated in Ni/VO2film, whereas Au/Ni/VO2film exhibits isotropic properties because of the uniform distribution of Au/Ni nanoparticles. Furthermore, a strong MO coupling with enhanced magnetic coercivity and anisotropy is demonstrated for both films, indicating great potential for optically active property tuning. This demonstration opens exciting opportunities for the VO2‐based device implementation towards smart windows, next‐generation optical‐coupled switches, and spintronic devices.

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  2. null (Ed.)
    Self-assembled oxide–metallic alloy nanopillars as hybrid plasmonic metamaterials ( e.g. , ZnO–Ag x Au 1−x ) in a thin film form have been grown using a pulsed laser deposition method. The hybrid films were demonstrated to be highly tunable via systematic tuning of the oxygen background pressure during deposition. The pressure effects on morphology and optical properties have been investigated and found to be critical to the overall properties of the hybrid films. Specifically, low background pressure results in the vertically aligned nanocomposite (VAN) form while the high-pressure results in more lateral growth of the nanoalloys. Strong surface plasmon resonance was observed in the UV-vis region and a hyperbolic dielectric function was achieved due to the anisotropic morphology. The oxide–nanoalloy hybrid material grown in this work presents a highly effective approach for tuning the binary nanoalloy morphology and properties through systematic parametric changes, important for their potential applications in integrated photonics and plasmonics such as sensors, energy harvesting devices, and beyond. 
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  3. Microwave radiation (MWR), a type of electromagnetic excitation source, reduces the synthesis temperature and processing time for chemical reactions compared to traditional synthesis methods. Recently, we demonstrated that MWR can engineer ceramics with different crystal phases compared to traditional methods [Journal of Materials Chemistry A 5, 35 (2017)]. In this study, we further apply the MWR-assisted technique to improve the electrochemical performance of LiCoO2 cathodes by engineering TiO2 and ZrO2 ceramic coatings. Electrochemical tests suggest that the TiO2 coating improves the rate capability of the LiCoO2 electrode. Both TiO2 and ZrO2 coatings improve the high-voltage (4.5 V) cycling stability of LiCoO2. The capacity remaining is improved from 52.8 to 84.4% and 81.9% by the TiO2 coating and the ZrO2 coating, respectively, after 40 cycles. We compare these results with existing studies that apply traditional methods to engineer TiO2/ZrO2 on LiCoO2, and find that the MWR-assisted method shows better performance improvement. X-ray photoelectron spectroscopy measurements suggest that the improved cycling stability arises from the formation of metal fluorides that protect the electrode from side reactions with electrolytes. This mechanism is further supported by the reduced Co dissolution from TiO2/ZrO2-coated LiCoO2 electrode after cycling. This study provides a new toolbox facilitating the integration of many delicate, low melting point materials like polymers into battery electrodes. 
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