Giant Enhancement of Upconversion Fluorescence of NaYF₄:Yb³⁺,Tm³⁺ Nanocrystals with Resonant Waveguide Grating Substrate

By embedding NaYF₄:Yb³⁺,Tm³⁺ nanocrystals into the top cladding layer of a resonant waveguide grating structure, we demonstrate that the upconversion fluorescence of Tm³⁺ ions can be greatly enhanced, by a factor of up to 10⁴. The resonant waveguide grating structure consists of an SU8 bottom layer with sinusoidal grating morphology coated with a thin TiO₂ waveguide layer and then covered with a poly­(methyl methacrylate) cladding layer doped with NaYF₄:Yb³⁺,Tm³⁺ nanocrystals. The giant enhancement of the upconversion fluorescence is achieved first by coupling the excitation light with a guided mode of the resonant waveguide grating structure and then the fluorescent light with a second guided mode. Our numerical simulation results obtained by rigorous coupled-wave analysis indicate that the electric field of the incident light is strongly enhanced near the interface of the TiO₂ layer and the poly­(methyl methacrylate) layer at guided mode resonance, and this is the major effect of the observed enhancement of the upconversion fluorescence of the nanocrystals. The resonance between the fluorescent emission and the waveguide structure further enhances the intensities of the fluorescent signal. We also find that the lifetime of upconversion fluorescence at 480 nm wavelength from the rare-earth nanocrystals is reduced about 1.34-fold when both excitation and extraction resonance occurs in the waveguide structure

Macroscopic, Freestanding, and Tubular Graphene Architectures Fabricated <i>via</i> Thermal Annealing

Manipulation of individual graphene sheets/films into specific architectures at macroscopic scales is crucially important for practical uses of graphene. We present herein a versatile and robust method based on annealing of solid carbon precursors on nickel templates and thermo-assisted removal of poly(methyl methacrylate) under low vacuum of ∼0.6 Pa for fabrication of macroscopic, freestanding, and tubular graphene (TG) architectures. Specifically, the TG architectures can be obtained as individual and woven tubes with a diameter of ∼50 μm, a wall thickness in the range of 2.1–2.9 nm, a density of ∼1.53 mg·cm^–3, a thermal stability up to 600 °C in air, an electrical conductivity of ∼1.48 × 10⁶ S·m^–1, and field emission current densities on the order of 10⁴ A·cm^–2 at low applied electrical fields of 0.6–0.7 V·μm^–1. These properties show great promise for applications in flexible and lightweight electronics, electron guns, or X-ray tube sources

Hollow Few-Layer Graphene-Based Structures from Parafilm Waste for Flexible Transparent Supercapacitors and Oil Spill Cleanup

We report a versatile strategy to exploit parafilm waste as a carbon precursor for fabrication of freestanding, hollow few-layer graphene fiber mesh (HFGM) structures without use of any gaseous carriers/promoters via an annealing route. The freestanding HFGMs possess good mechanical flexibility, tailorable transparency, and high electrical conductivity, consequently qualifying them as promising electrochemical electrodes. Because of the hollow spaces, electrolyte ions can easily access into and contact with interior surfaces of the graphene fibers, accordingly increasing electrode/electrolyte interfacial area. As expected, solid-state supercapacitors based on the HFGMs exhibit a considerable enhancement in specific capacitance (20–30 fold) as compared to those employing chemical vapor deposition compact graphene films. Moreover, the parafilm waste is found to be beneficial for one-step fabrication of nanocarbon/few-layer graphene composite meshes with superior electrochemical performance, outstanding superhydrophobic property, good self-cleaning ability, and great promise for oil spill cleanup

Hollow Few-Layer Graphene-Based Structures from Parafilm Waste for Flexible Transparent Supercapacitors and Oil Spill Cleanup

We report a versatile strategy to exploit parafilm waste as a carbon precursor for fabrication of freestanding, hollow few-layer graphene fiber mesh (HFGM) structures without use of any gaseous carriers/promoters via an annealing route. The freestanding HFGMs possess good mechanical flexibility, tailorable transparency, and high electrical conductivity, consequently qualifying them as promising electrochemical electrodes. Because of the hollow spaces, electrolyte ions can easily access into and contact with interior surfaces of the graphene fibers, accordingly increasing electrode/electrolyte interfacial area. As expected, solid-state supercapacitors based on the HFGMs exhibit a considerable enhancement in specific capacitance (20–30 fold) as compared to those employing chemical vapor deposition compact graphene films. Moreover, the parafilm waste is found to be beneficial for one-step fabrication of nanocarbon/few-layer graphene composite meshes with superior electrochemical performance, outstanding superhydrophobic property, good self-cleaning ability, and great promise for oil spill cleanup