Temperature-dependent photoluminescent behavior of millimeter-scale Cs4PbBr6/CsPbBr3 bulk crystals and their application to white light-emitting diodes

The zero-dimensional perovskite composite Cs4PbBr6/CsPbBr3 has attracted significant attention for its remarkable photoluminescence(PL),which remains highly effective even in solid state.This work presents a detailed analysis of the steady-state and time-resolved PL(TRPL)behavior of millimeter-scale Cs4PbBr6/CsPbBr3 crystals over a temperature range of 80 to 360 K,which covers exciton binding en-ergy,phonon energy,and PL peak energy shifting with increasing temperature.According to the results,Cs4PbBr6/CsPbBr3 exhibits high exciton binding energy and phonon energy,with calculated values of 358.7 and 94.8 meV,respectively.Specifically,when the temperature is below～235 K,thermal expan-sion dominates to influence the TRPL and peak energy,whereas electron-phonon interaction becomes the dominant factor as temperature rises from 235 to 325 K.It is found that Cs4PbBr6/CsPbBr3 has a PL behavior similar to CsPbBr3,and characterization and TRPL results demonstrate that nanometer-scale CsPbBr3 crystals embed in the Cs4PbBr6 bulk matrix.Meanwhile,a white light-emitting diode(WLED)device based on Cs4PbBr6/CsPbBr3 with luminous efficiency of 64.56 Im/W is fabricated,and its color coordinate is measured as(0.34,0.31)under 20 mA,which is in close proximity to the standard white color coordinate.Moreover,the color gamut of the device is measured as 128.66％of the National Televi-sion Systems Committee(NTSC).The WLED electroluminescence(EL)spectra show high Correlated Color Temperature(CCT)stability for the working current varying from 5 to 100 mA,and after continuous oper-ation for 12 h,the EL intensity decreases and stabilizes at～70％of the initial EL intensity.These findings suggest that Cs4PbBr6/CsPbBr3 crystals are a promising candidate for WLEDs.     

Conversion of green emission into white light in Gd2O3 nanophosphors

Gd₂O₃ nanophosphors were prepared by combustion synthesis with and without doping of Dy³⁺ ions. The X-ray powder diffraction patterns indicate that as-prepared Gd₂O₃ and 0.1 mol% Dy₂O₃ doped Gd₂O₃ nanophosphors have monoclinic structures. The transmission electron microscope (TEM) studies revealed that the as-prepared phosphors had an average crystallite sizes around 37 nm. The excitation and emission properties have been investigated for Dy³⁺ doped and undoped Gd₂O₃ nanophosphors. New emission bands were observed in the visible region for Gd₂O₃ nanophosphors without any rare earth ion doping under different excitations. A tentative mechanism for the origin of luminescence from Gd₂O₃ host was discussed. Emission properties also measured for 0.1 mol% Dy³⁺ doped Gd₂O₃ nanophosphors and found the characteristic Dy³⁺ visible emissions at 489 and 580 nm due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively. The chromaticity coordinates were calculated based on the emission spectra of Dy³⁺ doped and undoped Gd₂O₃ nanophosphors and analyzed with Commission Internationale de l'Eclairage (CIE) chromaticity diagram. These nanophosphors exhibit green color in undoped Gd₂O₃ and white color after adding 0.1 mol% Dy₂O₃ to Gd₂O₃ nanophosphors under UV excitation. These phosphors could be a promising phosphor for applications in flat panel displays.     

Controllable synthesis and optical properties of NdOHCO3 dodecahedral microcrystals

NdOHCO₃ dodecahedral microcrystals with an orthorhombic structure have been successfully synthesized by the hydrothermal method used urea as the precipitator. Experimental parameters, such as the reaction temperature, the reaction time, and the molar ratio of the starting reagents were examined. The as-synthesized products were characterized by powder X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and photoluminescence. The possible growth process of NdOHCO₃ dodecahedral microcrystals was discussed.     

Facile preparation of Ag3PO4 rhombic dodecahedron microcrystals with enhanced catalytic activities under visible light irradiation

Rhombic dodecahedron shaped Ag₃PO₄ microcrystals were prepared by hydrothermal treatment of as-precipitated Ag₃PO₄ products. The phase formation and morphology of the synthesized products were characterized by powder X-ray diffraction and field emission scanning electron microscopy. Experimental results revealed that hydrothermal reaction time can effectively influence the formation of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals. The possible formation mechanism for rhombic dodecahedrons morphology was studied. Moreover, the rhombic dodecahedron-shaped Ag₃PO₄ microcrystals such as 48 h hydrothermally treated sample exhibited higher catalytic activity than as-precipitated and 12 h hydrothermally treated samples under visible light irradiation for the degradation of methylene blue. The enhanced photocatalytic activities of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals is attributed to the existence of most exposed {110} facets.     

Room temperature synthesis and characterization of different morphological TbF3 nano/microcrystals

At room temperature, a simple solution-based procedure employing NaBF₄ as the fluoride source has been developed to selectively prepare different morphological TbF₃ nano/microcrystals (disks, peanuts and spindles) with an orthorhombic structure. The crystal structure, morphology and photoluminescence properties of the as-prepared TbF₃ crystals were investigated by x-ray powder diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectrophotometer. The results revealed that large-scale and uniform disks with a mean thickness of 20 nm can be easily synthesized, and the spindle-like and peanut-like TbF₃ crystals were assembled from nanodisks. The effects of synthesis parameters such as NaBF₄, reaction time and molar ratio of the reactants were systematically investigated. The room temperature PL properties of these different morphological TbF₃ microcrystals were measured.     

Hydrothermal synthesis and luminescence properties of octahedral LiYbF4: Er microcrystals

LiYbF₄: Er³⁺ octahedral microcrystals have been successfully prepared through a facile hydrothermal method assisted with EDTA (ethylenediaminetetraacetic acid). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric and differential scanning calorimeters (TG-DSC), photoluminescence (PL) spectra are used to characterize the samples. Under 976 nm excitation, the upconversion (UC) luminescence emission spectra of LiYbF₄: Er³⁺ microcrystals show the characteristic Er³⁺ emissions. The results show that the infrared light emissions at 792 nm of ⁴I_9/2 → ⁴I_15/2 are dominantly strong unusually, while the green emissions at 526 and 545 nm assigned to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2, respectively, and the red emission at 667 nm of ⁴F_9/2→⁴I_15/2 are relatively weaker. Most importantly, the samples show more efficient luminescence with further heat treatment.     

Fabrication and characterization of high quality n-ZnO/p-GaN heterojunction light emission diodes

High quality single crystalline n-type ZnO film was grown on p-type GaN substrate using molecular beam epitaxy. Transmission electron microscopy reveals a sharp ZnO/GaN interface. Light-emitting diode was fabricated from this heterostructure, and a turn-on voltage of ~ 3.4 V was demonstrated. We found that the emission peak shifts from violet (430 nm) to near-ultraviolet (375 nm) when the driving current increases from 0.38 mA to 3.08 mA. This intriguing phenomenon can be understood by charged carrier's radical recombination occurring at both sides of the device, and the current enhancement of ZnO emission efficiency.     

Novel synthesis and characterization of bismuth nano/microcrystals with sodium hypophosphite as reductant

In this paper, we report on the low temperature solution reduction method employed in the synthesis of large quantities of nano/micro-sized bismuth (Bi) crystals with sodium hypophosphite (NaH₂PO₂·H₂O) as reductant in acidic solutions. The achieved Bi crystals exhibited plate-like (100 nm in size and few nanometers in thickness) or polyhedral (500 nm in size) shapes. Bi nanocrystals transformed to octahedron-like Bi microcrystals only by prolonging the reaction time. The assembly and oriented growth should be the reason. To understand the growth mechanism, we also discussed the possible growth of the Bi nanocrystals exhibits influence of experimental parameters such as reaction time, NaH₂PO₂·H₂O concentration, and pH value. The resulting Bi crystals were characterized by using scanning electron microscopy, X-ray powder diffraction and differential thermal analysis and thermogravimetry. Optical properties of the samples were studied by ultraviolet–visible spectroscopy.     

Facile synthesis of monodisperse superparamagnetic Fe3O4/PMMA composite nanospheres with high magnetization

Monodisperse superparamagnetic Fe(3)O(4)/polymethyl methacrylate (PMMA) composite nanospheres with high saturation magnetization were successfully prepared by a facile novel miniemulsion polymerization method. The ferrofluid, MMA monomer and surfactants were co-sonicated and emulsified to form stable miniemulsion for polymerization. The samples were characterized by DLS, TEM, FTIR, XRD, TGA and VSM. The diameter of the Fe(3)O(4)/PMMA composite nanospheres by DLS was close to 90 nm with corresponding polydispersity index (PDI) as small as 0.099, which indicated that the nanospheres have excellent homogeneity in aqueous medium. The TEM results implied that the Fe(3)O(4)/PMMA composite nanospheres had a perfect core-shell structure with about 3 nm thin PMMA shells, and the core was composed of many homogeneous and closely packed Fe(3)O(4) nanoparticles. VSM and TGA showed that the Fe(3)O(4)/PMMA composite nanospheres with at least 65% high magnetite content were superparamagnetic, and the saturation magnetization was as high as around 39 emu g(-1) (total mass), which was only decreased by 17% compared with the initial bare Fe(3)O(4) nanoparticles.     

All fluorescent and high color rendering index white organic light-emitting devices with improved color stability at high brightness

We have demonstrated high color rendering index (CRI) white organic light-emitting devices using a multilayer structure based on all fluorescent emitters. The CRI of white light can be easily tuned by adjusting the thickness of the blue emitting layer (B-EML). In particular, the device with 8-nm-thick B-EML obtains very high CRI values (91-92) over a wide range of luminance (4000-23,000 cd/m²). Furthermore, the Commission Internationale De L'Eclairage coordinates of the device is rather stable and the variation is only (± 0.005, ± 0.003) over 4000-23,000 cd/m². The values are very close to white light equivalent energy point of (0.333, 0.333). We attribute these unique performances to the competition of the two mechanisms of the carrier trapping of the red dopant and efficient förster energy transfer in the blue emitting layer.     

One-step hydrothermal synthesis of carbon@Fe3O4 nanoparticles with high adsorption capacity

The magnetic carbon@Fe₃O₄ nanoparticles have been fabricated by a simple one-step hydrothermal method and applied as adsorbents for removal of organic dyes such as Congo red from aqueous solution. The prepared products were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectra, thermo-gravimetric analysis, nitrogen adsorption–desorption isotherms, UV–vis spectrum, and vibrating sample magnetometer. The adsorption performances of the products were tested with removal of Congo red from aqueous solution. The carbon@Fe₃O₄ nanoparticles possess high adsorption capacity and excellent magnetic separability by an external magnetic field, which is attributed to its high specific surface area providing more surface active sites.     

Synthesis and characterization of Ag3PO4/multiwalled carbon nanotube composite photocatalyst with enhanced photocatalytic activity and stability under visible light

A novel Ag₃PO₄/multiwalled carbon nanotube (Ag₃PO₄/MWCNT) composite has been prepared by a simple precipitant approach. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, UV–Vis diffuse reflectance, and Raman spectroscopy. It is found that the MWCNTs are well deposited on Ag₃PO₄ nanoparticles. The Ag₃PO₄/MWCNT composite with MWCNT content of 1.4 wt% exhibits optimal photocatalytic activity under visible light. The rate constant of MO degradation over Ag₃PO₄/1.4 wt% MWCNT is 5.84 times that of pure Ag₃PO₄. Compared with pure Ag₃PO₄, the incorporation of MWCNTs not only significantly enhances the photocatalytic activity but also improves the structural stability of Ag₃PO₄, suggesting that the synergistic effects take place in the composite. The improvement is attributed to the conductive structure supported by MWCNTs, which favors electron–hole separation and the removal of photogenerated electrons from the decorated Ag₃PO₄. A possible photocatalytic mechanism of the Ag₃PO₄/MWCNT photocatalyst was assumed as well.     

Fabrication and characterization of high strength Nylon-6/Si3N4 polymer nanocomposite fibers

Alignment of silicon nitride nanorods in Nylon-6 polymeric matrix has been achieved through melt extrusion process. Tensile properties of as-fabricated single fibers show that the strength and modulus of a nanocomposite fibers increased by 273% and 610%, respectively, as compared to the neat Nylon-6 fibers. Field emission scanning electron microscopy (FE-SEM) studies have confirmed the successful alignment of nanorods along the filament length. Larger increase in modulus of the nanorods reinforced Nylon-6 was at a cost of about 35% decrease (reduced from 65% to 30%) in fracture strain. In an attempt to retain the ultimate strain, nanorods were replaced by spherical Si₃N₄ (30 nm dia) particles and it was demonstrated that the improvement in tensile properties is still by 179% for strength and 276% for modulus practically without any loss of fracture strain.     

Synthesis and luminescent properties: Mn4+-doped Sr3NaNbO6 phosphor with far-red emission

Journal of Materials Science: Materials in Electronics - The unique luminescence properties of far-red emitting materials make them important components in the phosphor-converted LEDs (pc-LEDs) for...     

Rapid synthesis of single-crystalline TbF3 with novel nanostructure via ultrasound irradiation

Terbium fluoride (TbF₃) nanopeanut has been successfully synthesized via a mild sonochemical route from an aqueous solution of terbium nitrate and fluoroborate without any template or organic additive. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) spectra were utilized to characterize the synthesized samples. The morphologies and optical properties of the obtained TbF₃ nanopeanut can be tuned by ultrasound irradiation as well as the fluoride source. The prepared TbF₃ nanopeanut shows extraordinarily high room temperature photoluminescence intensity comparing to the products prepared by stirring. The possible formation mechanism is proposed in this paper.     

Hydrothermal synthesis and photoluminescence of SrWO4:Tb novel green phosphor

Tb³⁺-doped SrWO₄ phosphors with a scheelite structure have been prepared by hydrothermal reaction. X-ray powder diffraction, field-emission scanning electron microscopy, photoluminescence excitation and emission spectra and decay curve were used to characterize the resulting samples. Scanning electron microscopy image showed that the obtained SrWO₄:Tb³⁺ phosphors appeared to be nearly spherical and their sizes ranged from 1 to 3 μm. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under ultraviolet light excitation. Because 12 at.% SWO₄:Tb³⁺ phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), the excellent luminescence properties make it a new promising green phosphor for fluorescent lamps application.     

Hydrothermal synthesis and characterization of single-crystalline Fe3O4 nanowires with high aspect ratio and uniformity

Uniform, ultra-thin, single-crystalline Fe₃O₄ nanowires with narrow diameter distribution centered at 15 nm and length up to several microns, were synthesized by a simple hydrothermal route with the assistance of polyethylene glycol (PEG) 400. The morphologies and structure of the obtained nanowires were examined by means of X-ray diffraction (XRD), Mössbauer spectrum, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The effect of PEG content and molecular weight on the yield and morphology of Fe₃O₄ nanowires was investigated. Saturation magnetization of Fe₃O₄ nanowires was determined to be 23.0 emu/g, which is distinctly lower than that of Fe₃O₄ nanoparticles.