Ronghua Ma

Rapid growth of magnetite nanoplates by ultrasonic irradiation at low temperature

Two-dimensional plate-like Fe(3)O(4) nanocrystals were synthesized by a facile method using ultrasonic irradiation in aqueous solution at low temperature without protection from oxygen. The crystals were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier Transform infrared spectroscopy. The products subjected to ultrasound showed a two-dimensional morphology. The results obtained indicate that the morphologies of the magnetite crystals depend more on the ultrasonic irradiation than on the growth temperature. The thickness and width of the crystals increased with increasing temperature of the reaction medium. In addition, the magnetic hysteresis loop of the magnetite nanoplates was obtained at room temperature.

Shielding Upconversion by Surface Coating: A Study of the Emission Enhancement Factor.

Surface coating is a commonly used strategy to enhance upconversion emissions by shielding the luminescent core from surface quenching. In this work, we provide insights into the effect of surface coating on upconversion by investigating NaYF4 :Yb/Er nanoparticles and the corresponding NaYF4 :Yb/Er@NaYF4 core-shell nanoparticles, as a function of dopant concentration of Yb(3+) and excitation power. We observe declining emission enhancement factors with decreasing Yb(3+) concentration and increasing excitation power. Our mechanistic investigations suggest that the phenomenon originates from stepwise excitation in the upconversion process, as well as energy hopping among the Yb(3+) dopants. This increased understanding of the effect of surface coating on upconversion should be important towards the rational design of lanthanide-doped core-shell nanoparticles for various applications.

Dual mode temperature sensing through luminescence lifetimes of F- and O-coordinated Cr3+ sites in fluorosilicate glass-ceramics

Luminescence lifetime based temperature sensing has an intrinsic immunity to the influence of external conditions, and dual mode thermometry is highly accurate due to its “self-calibration” merit. To develop thermometry with both features, we investigated the phase and microstructural evolution of Cr3+-doped calcium-fluorosilicate glass and glass-ceramics, which revealed different luminescent behavior relating to the different Cr3+ sites in the materials. From the photoluminescence (PL) spectra, the emission at 717 nm was derived from the O-coordinated octahedral sites, while the 1 μm super-broad emission was assigned to the F-coordinated octahedral sites. After an annealing treatment, cubic CaF2 nanocrystals were homogeneously precipitated in the glass-ceramics; thus, both the O-coordination in the residual glass phase and F-coordination in the CaF2 crystalline phase were strengthened. This led to the enhancement of both the emissions at 717 nm and 1 μm. The O-coordinated sites were relatively strong-field sites in which the fluorescence of Cr3+ originated from the radiative transitions of the two thermally coupled energy levels, 2E and 4T2, while the F-coordinated sites were relatively weak-field sites. Hence, the Cr3+ exhibits only one excited state 4T2, which is inactivated by radiative transitions and non-radiative transitions from the thermal quench. Based on the obtained results, the maximum relative temperature sensitivity coefficients are 0.76% K−1 at 498 K for the 717 nm emission and 0.47% K−1 at 351 K for the 1 μm emission. This provides the possibility of developing a dual mode temperature sensor with high precision only using a single material.

High performance hierarchical nanoporous antireflective films by a facile sol–gel process

Single layer antireflection (SLAR) sol–gel films on super white glass substrates were prepared by hierarchically introducing hollow silica nano-spheres (HSNs) and F127 templated nano-pores. HSNs and F127 templated nano-pores were formed by base and acid catalyzed sol–gel processes, respectively. The dispersibility of HSNs was ameliorated by Ph-TES surface modifications. The porosity of the SLAR films was engineered by a step-by-step addition of nano-pores and HSNs so that the refractive index could be tuned to be as low as the desired value of 1.24. It accordingly improved the transmittance of the films with an optimized maximum transmittance of 98.21% in the visible spectral region. Furthermore, the concentration of modified HSNs was adjusted to modulate the peak transmittance wavelength from 550 nm to 680 nm. The nano-scale surface embossments due to the addition of HSNs was also contributory to the antireflective effect of the films, especially at 45° or 5° incident angle. As a result, the average transmittance of the double side coating super white glass was increased by 7.10% at 400–1000 nm with good wear resistance. The porosity directed mechanical durability varied from 6H to 3H (pencil hardness). It displays considerable AR application prospects for suitable optical and optoelectronic devices.

Facile synthesis of monodisperse SrAl2O4:Eu2+ cage-like microspheres with an excellent luminescence quantum yield

Commercial LED phosphors are usually prepared by a conventional solid-state reaction method and a subsequent ball milling process, which often results in non-uniform size distributions, irregular shape and impurity contamination of the phosphors. In this work, the SrAl2O4:Eu2+ cage-like microspheres with a uniform size distribution and excellent photoluminescence (PL) quantum yield (QY) are prepared by an epoxide-driven sol–gel route and subsequent heat treatment in a reducing atmosphere. This unique cage-like microstructure not only makes better use of the excited light but also significantly increases the PL quenching concentration of the Eu2+ ions. The optimum internal QY of the SrAl2O4:Eu2+ cage-like microspheres is measured to be up to 94.94%, much higher than the highest internal QY (55%) of the SrAl2O4:Eu2+ phosphors reported so far. The approach may be readily applied to prepare other lanthanide ions doped cage-like microspheres, implying a new route for preparing highly efficient PL phosphors with regular shape and uniform particle size.