Mingmei Wu

Formation Mechanism of CaTiO3 Hollow Crystals with Different Microstructures

The crystal growth of CaTiO(3) hollow crystals with different microstructures has been investigated. In a water-free poly(ethylene glycol) 200 (PEG-200) solution, CaTiO(3) nanocubes formed first. The nanocubes underwent an oriented self-assembly into spherical particles, enhanced by the surface-adsorbed polymer molecules. Since the growth of nanocubes and their aggregation took place simultaneously, the nanocubes in the outer shells were larger than those in the cores. Disappearance of the small nanocubes in the cores of the spheres during an Ostwald ripening process led to spherical hollow crystals. Addition of a small amount of water (1.25 vol %) in the polymer solution enhanced surface recrystallization of the aggregated spheres, forming a cubic morphology. The orthorhombic distortion of the perovskite CaTiO(3) structure did not have a significant effect on the nanocube aggregation, resulting in a domain structure in the shells. Single-crystalline hollow cubes were produced with a slightly higher water content, e.g., 5 vol %. This process of (1) aggregation of nanocubes and (2) surface crystallization followed by (3) surface-to-core extension of recrystallization gives a good example of the reversed crystal growth route in ceramic materials. The proposed formation mechanism of the hollow CaTiO(3) crystals would enable us to control the microstructures of these materials and to explain the formation of many other hollow crystals.

Continuous shape- and spectroscopy-tuning of hematite nanocrystals.

Uniform hexagonal hematite (α-Fe(2)O(3)) nanoplates have been synthesized by a facile alcohol-thermal reaction, and a new nanostructure of α-Fe(2)O(3) has been proposed. Each nanoplate is enclosed by (0001) basal planes and {1012} side surfaces. The phase, size, shape, and growth orientation of these nanocrystals were characterized by powder X-ray diffraction and electron microscopy. The thickness and diameter of these nanocrystals could be finely tuned by the selective use of alcohol solvent with increasing carbon atom number in the linear alkyl chain. A variety of nanocrystals with systemically changeable shapes from nanoplates to nanograins have been obtained. Specific adsorption of alcohol molecules on polar (0001) facets is proposed to be the main issue to modify the growth behavior of hematite nanocrystals. The presence of distilled water and the addition of sodium acetate have also been investigated. Either of them has a great influence on the growth of hematite nanocrystals, and shape-controlled growth can be rationally achieved. In addition, the post-aging of as-grown hematite nanocrystals in alcohol and distilled water has also been described. Both vibration spectroscopy (i.e., FTIR and Raman) and electronic spectra (diffused reflectance spectra) of these nanocrystals with a continuing shape change show a highly shape-dependent nature.

Highly Thermally Stable Single-Component White-Emitting Silicate Glass for Organic-Resin-Free White-Light-Emitting Diodes

Thermal management is still a great challenge for high-power phosphor-converted white-light-emitting diodes (pc-WLEDs) intended for future general lighting. In this paper, a series of single-component white-emitting silicate SiO2-Li2O-SrO-Al2O3-K2O-P2O5: Ce(3+), Tb(3+), Mn(2+) (SLSAKP: Ce(3+), Tb(3+), Mn(2+)) glasses that simultaneously play key roles as a luminescent convertor and an encapsulating material for WLEDs were prepared via the conventional melt-quenching method, and systematically studied using their absorption spectra, transmittance spectra, photoluminescence excitation and emission spectra in the temperature range 296-498 K, decay curves, and quantum efficiency. The glasses show strong and broad absorption in 250-380 nm region and exhibit intense white emission, produced by in situ mixing of blue-violet, green, and orange-red light from Ce(3+), Tb(3+), and Mn(2+) ions, respectively, in a single glass component. The quantum efficiency of SLSAKP: 0.3%Ce(3+), 2.0%Tb(3+), 2.0%Mn(2+) glass is determined to be 19%. More importantly, this glass shows good thermal stability, exhibiting at 373 and 423 K about 84.56 and 71.02%, respectively, of the observed room temperature (298 K) emission intensity. The chromaticity shift of SLSAKP: 0.3%Ce(3+), 2.0%Tb(3+), 2.0%Mn(2+) is 2.94 × 10(-2) at 498 K, only 57% of the commercial triple-color white-emitting phosphor mixture. Additionally, this glass shows no transmittance loss at the 370 nm emission of a UV-Chip-On-Board (UV-COB) after thermal aging for 240 h, compared with the 82% transmittance loss of epoxy resin. The thermal conductivity of the glass is about 1.07 W/mK, much larger than the 0.17 W/mK of epoxy resin. An organic-resin-free WLEDs device based on SLSAKP: 0.3%Ce(3+), 2.0%Tb(3+), 2.0%Mn(2+) glass and UV-COB is successfully demonstrated. All of our results demonstrate that the presented Ce(3+)/Tb(3+)/Mn(2+) tridoped lithium-strontium-silicate glass may serve as a promising candidate for high-power WLEDs.

Microcavity lasing behavior of oriented hexagonal ZnO nanowhiskers grown by hydrothermal oxidation

The highly aligned ZnO whiskers, which are grown by hydrothermal oxidation of metallic zinc plate in the presence of ethylenediamine molecules, exhibit single and/or a few modes of supernarrow spectral emissions (∼0.7 nm) at near 378 nm and negligible deep level defects emissions. Time-integrated and time-resolved photoluminescence show that the supernarrow spectral peaks are due to the laser action with an excitation threshold about 70 μJ/cm2 and emission lifetime of <30 ps. The lasing emission is highly polarized along the excitation laser polarization direction. The low lasing threshold, stable and regular supernarrow longitudinal modes, and strong lasing polarization effects can be well explained by the model of microcavity laser where the two end facets of whisker form the microcavity.

Advanced red phosphors for white light-emitting diodes

White light-emitting diodes (WLEDs) with high luminous brightness, low energy consumption, long lifetime and environmental friendliness can be applied in various fields. However, low colour rendering index and high correlated colour temperature have seriously limited the quality of white light, resulting from the deficiency of the red light component in commercial phosphors. New phosphors that can emit suitable red light are needed. The improvement of red emitters has gradually become a hot topic in WLED applications such as good colour rendering lighting and full colour displays. This review summarizes recent achievements concerning red phosphors mainly from three aspects. They are: intensifying absorption bands and reducing lattice defects to increase quantum efficiency; selecting an appropriate coordination environment and altering the lattice symmetry to fine-tune the luminescence spectra; and reducing the nonradiative transition rate and preventing charge imbalance of a luminescence centre to enhance thermal stability.

K2Ln(PO4)(WO4):Tb3+,Eu3+ (Ln = Y, Gd and Lu) phosphors: highly efficient pure red and tuneable emission for white light-emitting diodes

A novel phosphate/tungstate family, K2Ln(PO4)(WO4) (Ln = Y, Gd and Lu) doped with Tb3+ and Eu3+ is synthesized via a conventional high-temperature solid-state reaction to explore new pure red phosphors with high critical concentration for white light-emitting diodes (WLEDs). The results from the Rietveld method show that the crystal structures of the hosts are composed of phosphate layers and tungstate zigzags, and the Ln3+–Ln3+-units are isolated by the [PO4]3− groups in phosphate layers. The critical concentration of Tb3+ and Eu3+ is up to 40–50% in the singly doped phosphors, which is ascribed to the interaction of the isolated Ln3+ ions being mitigated by [PO4]3− and [WO4]2− groups, such that the special structure of K2Ln(PO4)(WO4) helps the interaction of luminescence centres. The energy transfer from Tb3+ to Eu3+ in K2Ln(PO4)(WO4) is demonstrated by fluorescence decay times. By adjusting the ratio of Eu3+ and Tb3+, we can tune the emission colour of K2Ln(PO4)(WO4):Tb3+,Eu3+ from green to yellow, orange and pure red. For K2Tb0.5Eu0.5(PO4)(WO4), the internal quantum efficiency is as high as 76.45% under an excitation of 394 nm, and the emission intensity at 150 °C is 92.2% of that at 25 °C.

Tunable Luminescent Properties and Concentration-Dependent, Site-Preferable Distribution of Eu2+ Ions in Silicate Glass for White LEDs Applications

The design of luminescent materials with widely and continuously tunable excitation and emission is still a challenge in the field of advanced optical applications. In this paper, we reported a Eu(2+)-doped SiO2-Li2O-SrO-Al2O3-K2O-P2O5 (abbreviated as SLSAKP:Eu(2+)) silicate luminescent glass. Interestingly, it can give an intense tunable emission from cyan (474 nm) to yellowish-green (538 nm) simply by changing excitation wavelength and adjusting the concentration of Eu(2+) ions. The absorption spectra, photoluminescence excitation (PLE) and emission (PL) spectra, and decay curves reveal that there are rich and distinguishable local cation sites in SLSAKP glasses and that Eu(2+) ions show preferable site distribution at different concentrations, which offer the possibility to engineer the local site environment available for Eu(2+) ions. Luminescent glasses based color and white LED devices were successfully fabricated by combining the as-synthesized glass and a 385 nm n-UV LED or 450 nm blue LED chip, which demonstrates the potential application of the site engineering of luminescent glasses in advanced solid-state lighting in the future.

Hierarchically Nanostructured Rutile Arrays: Acid Vapor Oxidation Growth and Tunable Morphologies

A general acid vapor oxidation (AVO) strategy has been developed to grow highly oriented hierarchically structured rutile TiO(2) nanoarrays with tunable morphologies from titanium thin films. This is a simple one-pot synthesis approach involving the reaction of a titanium surface with the vapor generated from a hydrochloric acid solution in a Teflon lined autoclave. To the best of our knowledge, this is the first successful attempt to grow ordered tree-like titania nanoarrays. A possible formation mechanism for the interesting architectures has been proposed based on series of time-dependent experiments. By adjusting the initial HCl concentration, films of different rutile structures including nanotrees, dendritic nanobundles, and nanorods can be selectively obtained. Subsequently, the surface morphologies and wettability can be readily tuned.

Size- and shape-tailored hydrothermal synthesis of YVO4 crystals in ultra-wide pH range conditions

Yttrium orthovanadate (YVO4) crystals have been prepared hydrothermally in both strongly acidic and basic media. The effects of vanadium source, acid, addition of sodium hydroxide, chelating reagent, and even solvent on the crystalline size and shape were explored using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Models for the generation of YVO4 crystals in both acidic and basic media are proposed from the experimental results. The influence of alkaline concentration on the sizes and shapes of YVO4 crystals has been investigated, and it has been found that the addition of sodium hydroxide can reduce the crystal sizes of YVO4. Well-defined YVO4 microcrystals with clear facets have been prepared, and dispersed YVO4 nanograins and nanoflakes with dimensions of 5–50 nm have been obtained. Both the size and shape of YVO4 crystals can be tailored by changing the reaction conditions even though YVO4 crystals generally show a tetragonal growth habit due to its tetragonal crystallographic symmetry.

A new red phosphor BaGeF6:Mn4+: hydrothermal synthesis, photo-luminescence properties, and its application in warm white LED devices

In this work, we report a new and efficient red phosphor BaGeF6:Mn4+ (denoted as BGFM) by hydrothermally etching BaCO3 and GeO2 in HF solution with an optimized KMnO4 concentration. The crystal structure and morphology were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) in detail. The influence of synthesis conditions on its photo-luminescent (PL) properties has been investigated comprehensively. It can present broad adsorption and sharp emissions in blue and red ranges respectively. The white LED device made of a blue GaN chip merged with a YAG:Ce–BGFM mixture presents warmer white light than that merged with only one YAG:Ce component.