Chunhua Lu

Controllable self-assembly of NaREF4 upconversion nanoparticles and their distinctive fluorescence properties

The paper presents the growth of hexagonal NaYF4:Yb(3+), Tm(3+) nanocrystals with tunable sizes induced by different contents of doped Yb(3+) ions (10%-99.5%) using the thermal decomposition method. These nanoparticles, which have different sizes, are then self-assembled at the interface of cyclohexane and ethylene and transferred onto a normal glass slide. It is found that the size of nanoparticles directs their self-assembly. Due to the appropriate size of 40.5 nm, 15% Yb(3+) ions doped nanoparticles are able to be self-assembled into an ordered inorganic monolayer membrane with a large area of about 10 × 10 μm(2). More importantly, the obvious short-wave (300-500 nm) fluorescence improvement of the ordered 2D self-assembly structure is observed to be relative to disordered nanoparticles, which is because intrinsic absorption and scattering of upconversion nanoparticles leads to the self-loss of fluorescence, especially the short-wave fluorescence inside the disordered structure, and the relative emission of short-wave fluorescence is reduced. The construction of a 2D self-assembly structure can effectively avoid this and improve the radiated short-wave fluorescence, especially UV photons, and is able to direct the design of new types of solid-state optical materials in many fields.

Luminescence characteristics of the Ce 3+ -doped garnets: the case of Gd-admixed Y 3 Al 5 O 12 transparent ceramics

The Ce-doped (Gd, Y)3Al5O12 transparent ceramics with Gd3+ content of 0 to 80 mol% were fabricated using a solid-state sintering method under vacuum. Their phase compositions and structure parameters were checked by X-ray diffraction (XRD). Optical and luminescence characteristics were investigated and thoroughly discussed by transmittance, absorption spectra and luminescence examination. The Gd3+ ions were found of great importance to the band structure of crystal and energy state of Ce3+ in the garnet ceramics. Balanced Gd3+ admixture into the YAG lattice provided excellent color coordinates and CRI for the phosphor for warm white light illumination. This study provides an efficient approach to tailor the luminescence of the garnet phosphors.

Crystal structures and luminescence properties of NaYF4 microcrystals at different pH values

Luminescence properties of materials are closely related to their crystal structures and growth characteristics. In this study, a series of β-NaYF4:20% Yb3+, 2% Er3+ (20 and 2% represent the reactant mole percentage) microcrystals with different morphologies and sizes were synthesized using a hydrothermal route. The crystal phase, morphology, size, and upconversion luminescence properties of the as-prepared samples were characterized by x-ray diffraction, field emission scanning electron microscopy and photoluminescence analysis, respectively. The effects of the precursor solution pH value on the upconversion luminescence properties of the β-NaYF4:Yb3+, Er3+ crystals were investigated in detail. The results indicate that the preferential growth plane of the β-NaYF4 crystals was significantly influenced by the precursor pH values. Crystal plane characteristics affected the luminescence behavior of β-NaYF4 in the red and green spectral range. In addition, reasonable control of the β-NaYF4 crystal growth has a significant effect on the fluorescence lifetime.

Nd3+/Yb3+ cascade-sensitized single-band red upconversion emission in active-core/active-shell nanocrystals

Lanthanide-doped upconversion nanomaterials (UCNMs) have promoted extensive interest for its biological research and biomedical applications, benefiting from low autofluorescence background, deep light penetration depth, and minimal photo-damage to biological tissues. However, owing to the 980 nm laser-induced overheating issue and the attenuation effect associated with conventional multi-peak emissions, the usage of UCNMs as fluorescent bioprobes is still limited. To address these issues, an effective strategy has been proposed to tune both the excitation and emission peaks of UCNMs into the first biological window (650xa0∼xa0900 nm), where the light absorption by water and hemoglobin in biological tissues is minimal. Based on the Nd3+/Yb3+ cascade-sensitized upconversion process and efficient exchange-energy transfer between Mn2+ and Er3+ in conjunction with the active-core@active-shell nanostructured design, we have developed a new class of upconversion nanoparticles (UCNPs) that exhibit strong single-band red emission upon excitation of an 808 nm near-infrared laser. Hopefully, the well-designed KMnF3:Yb/Er/Nd@ KMnF3:Yb/Nd core-shell nanocrystals will be considered a promising alternative to conventionally used UCNPs for biolabeling applications without the concern of the overheating issue and the attenuation constraints.

Enhancement of fluorescent emission in photonic crystal film and application in photocatalysis

Fluorescent photonic crystal films composed of monodisperse NaYF4:15Yb,0.5Tm@SiO2 (where 15 and 0.5 represent the mole percentage of reactants) core-shell spheres were successfully fabricated and applied in photocatalysis. The core-shell spheres were prepared using a modified Stober method, and fluorescent photonic crystal films were fabricated via a simple self-assembly method. The morphologies, structures and upconversion fluorescent properties of the fluorescent photonic crystal films with different photonic band gaps were characterized. Moreover, their photocatalytic capability in decomposing rhodamine B using near-infrared light was studied. Results indicate that the band edge effect plays a critical role in the enhancement of short wave emission intensity of fluorescent photonic crystal films. Specifically, in comparison to the reference sample without a band edge effect, the 363 nm emission intensity was enhanced by 5.97 times, while the percentage of UV upconversion emission was improved by 6.23%. In addition, the 451 nm emission intensity was enhanced by 5.81 times, and the percentage of visible upconversion emission was improved by 8.88%. Furthermore, fluorescent photonic crystal films with enhanced short wave emission exhibited great photocatalytic performance in the degradation of rhodamine B aqueous solutions under near-infrared light.