Charge Transition Band Research Articles

Role of energy band structure on the luminescence performance of double perovskite La2LiMO6:Eu3+ (M=Sb, Ta, Nb) red emitting phosphors

Abstract Monoclinic double perovskite La2LiMO6:Eu3+(M = Sb, Ta, Nb) phosphors were prepared by high temperature solid state reaction method in air. The material microstructures were characterized by XRD Rietveld refinement. All La2LiMO6:Eu3+ exhibited Eu3+ - emission in the red region under excitation by ultraviolet-blue light at room temperature. In the order of M = Sb, Ta, Nb, the emission of Eu3+ in La2LiMO6:Eu phosphor declined under charge transition band (CTB) excitation in turn, while enhanced gradually under 394 nm and 465 nm excitation. The optical band gap energy was calculated through the UV–Vis reflectance spectra of La2LiMO6:Eu indicating that photoionization is the quenching scheme in La2LiMO6:Eu. Co - doped with Bi3+ ions further reduces the energy band (i.e. forbidden bandwidth) of LLN:Eu, resulting in the significant enhancement of Eu3+- emission under 465 nm excitation, which demonstrates its potential application value as the red component of a blue chip based-white LED.

Build 3D Nanoparticles by Using Ultrathin 2D MOF Nanosheets for NIR Light-Triggered Molecular Switching.

The coordination interactions between transition-metal ions (Cu2+, Ag+) and sulfur atoms on ultrathin two-dimensional (2D) nanosheets of spin-crossover (SCO) metal-organic frameworks {[Fe(1,3-bpp)2(NCS)2]2}n (1,3-bpp = 1,3-di(4-pyridyl)propane), which constructed the ultrathin 2D nanosheets into three-dimensional (3D) nanoparticles, have made a profound effect on the SCO performance. Compared with 2D nanosheets, both the intraligand π-π* transition band and the metal-to-ligand charge transition band from the d(Fe) + π(NCS) to π*(1,3-bpp), for the 3D nanoparticles, have shown dramatic blue-shifts; meanwhile, the d-d transition band for the high-spin (HS) state Fe(II) ions has been generated, suggesting significantly the influence of 3D assemble-caused dimensional changes on the solid-state SCO performance of ultrathin 2D nanosheets. More importantly, by loading on the ytterbium ion (Yb3+)-sensitized hexagonal phase upconverting nanoparticles in the aqueous colloidal suspension, the near infrared (NIR) light (980 nm) triggered HS (high spin) to LS (low spin) state transitions have been observed, demonstrating the achievement of challenging target of NIR light-triggered molecular conversion under environment conditions.

The Synthesis and Photoluminescent Properties of CaMoO₄:Eu³⁺ Nanocrystals by a Soft Chemical Route.

In this paper, the CaMoO4:Eu3+ phosphors were prepared by a simple hydrothermal method assisted by the citric acid as the surfactant, and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and fluorescent spectrophotometry. The results of XRD show that the as-prepared samples are single phase. The process of the Ostwald ripening is controlled by the content of the citric acid in the hydrothermal reaction. The pH value of the precursor affects the shift of the charge transition band (CTB) in the excitation spectra. The reaction condition can strongly affect the luminescent intensity of the samples.

Structure evolution and tunable luminescence of (Sr0.98−mBamEu0.02)2Ca(Mo1−nWn)O6 phosphor with ultraviolet excitation for white LEDs

Double perovskite (Sr0.98−mBamEu0.02)2Ca(Mo1−nWn)O6 phosphors were successfully synthesized by an improved citrate–EDTA complexing method. The structure evolution and the resulting luminescence variation were systematically investigated by X-ray diffraction with Rietveld structure refinement, Fourier transform infrared spectra, Raman spectra, diffuse reflection spectra and photoluminescence spectra. The increased crystal symmetry and increased lattice parameters were observed from (Sr0.98−mBamEu0.02)2CaMoO6 phosphors. These series of phosphors could be effectively excited by near ultraviolet light and emitted a dominant 594nm (5D0–7F1). The hypersensitive 5D0–7F2 transition (615nm) varied with the changed structure symmetry. The CIE chromaticity coordinates were tuned from (0.642, 0.348) to (0.601, 0.389). The luminescence intensity of W6+ substituted powders was greatly enhanced by the blue shift of charge transition band of (Mo/W)O6 with stable emission color. The composition-optimized Sr2Ca(Mo0.5W0.5)O6:Eu phosphor had the highest luminescence intensity and quantum yield comparing to the commercial red phosphors.

Luminescence properties of Ca 10K(PO 4) 7:RE 3+ (RE = Ce, Tb, Dy, Tm and Sm) under vacuum ultraviolet excitation

A series of RE 3+ (RE = Ce, Tb, Dy, Tm and Sm) activated Ca 10K(PO 4) 7 were synthesized by conventional state reaction and their photoluminescence properties under vacuum ultraviolet excitation were investigated. The PO 4 3− absorption lies within the range from 125 to 180 nm in all the excitation spectra of Ca 10K(PO 4) 7:RE 3+. The first f–d transition of Ce 3+ is observed at 316 nm, and the Ce 3+ emission is located at about 350 nm. Both the first spin-allowed and spin-forbidden f–d transitions of Tb 3+ are situated at 232 and 263 nm, respectively. The emission spectrum of Ca 10K(PO 4) 7:Tb 3+ exhibits typical Tb 3+ emissions with the predominant peak at 544 nm. The O 2−–Dy 3+ charge transition band was calculated and identified around 173 nm, the CIE chromaticity coordinates of Dy 3+ were calculated to be 0.364 and 0.392. The Ca 10K(PO 4) 7:Tm 3+ demonstrates the strongest excitation at about 182 nm assigned to O 2−–Tm 3+, and gives the predominant emission at 453 nm. The 4G 5/2– 6H 7/2 transition of Sm 3+ at 601 nm is the most intensive in the emission spectrum.

The Luminescence of Nanoscale Y2Si2O7:Eu3+ Materials

The Y2Si2O7:Eu3+ sample was prepared with the sol-gel method. The Y2O3:EU3 was dispersed in SiO2, and the complex Y2Si2O7:Eu3+ particles were synthesized at high annealing temperature. The sample consisted of nearly spherical particles with an average size about 60 nm. The ultraviolet excitation spectra and emission spectra were measured. The sample excited by short ultraviolet light showed strongly red luminescence and fine monochromaticity. The luminescence was strongest from the 5D0 --> 7F2 electric dipole transition located at 611 nm. The excitation spectra of Y2Si2O7:Eu3+ excited with ultraviolet lights showed that the peak of the Eu(3+)-O2- charge transition band located at about 240 nm. During monitoring of different emission peaks of 5D0 --> 7F2, the charge transition band in the excitation spectra shifted, and the relative intensity of emission spectra changed obviously under the excitation of different ultraviolet wavelengths. These results confirmed that the Eu3+ could be excited with ultraviolet radiation of different wavelengths. At low temperature, using Eu3+ ions as fluorescence probes, we monitored the emission peaks of 5D0 --> 7F1 and 5D0 --> 7F2 transitions and measured the excitation spectra of 7F0 --> 5D0, 5D0 --> 7F1, and 5D0 --> 7F2 site-selective excitation spectra. These results indicated that Eu3+ ions are located in different local environments in the Y2Si2O7 host.

Spectroscope properties of KCaY(PO 4) 2:Eu 3+ in vacuum ultraviolet region

In order to investigate the energy migration mechanisms of Eu 3+ doped hexagonal KCaY(PO 4) 2 in vacuum ultraviolet (VUV) region, KCaY 1− x (PO 4) 2:Eu x 3+ (0 ≪ x < 0.1) are prepared and their photoluminescence (PL) properties are evaluated under 147 nm excitation. Monitored by 616 nm emission, the weak band around 136–162 nm in the excitation spectra of KCaY 0.95(PO 4) 20.05:Eu 3+ could be assigned to the absorption of PO 4 3−. The sharp peak at 213 nm which overlaps with the charge transition band (CTB) of Eu 3+–O 2− at about 177–280 nm should be assigned to the transition to 3H 4 levels of Eu 3+. Under 147 nm excitation the strongest emission peak is located at about 616 nm. When x reaches 0.05 in KCaY 1− x (PO 4) 2:Eu x 3+(0 < x < 0.1), the optimum emission intensity is obtained. A main reason for the low quenching concentration of Eu 3+ is that Eu 3+ occupies not only the Y 3+ position but also the K + and Ca 2+ sites, so the energy migration among Eu 3+ ions could be in three-dimensional.

Study on UV excitation properties of Eu 3+-doped rare-earth phosphates

In this paper, we report luminescence properties of La 3PO 7:Eu 3+. The emission spectrum is different from other doped Eu 3+ rare-earth phosphates or borates. The emission spectra of samples La 3PO 7:Eu 3+ excited with ultraviolet radiation shows that intensity of 5D 0– 7F 2 emission of Eu 3+ is stronger than 5D 0– 7F 1 emission of Eu 3+. The result of charge transition band shows that there are different local environments surrounding Eu 3+. The emission spectra at different wavelength UV excitation indicate that Eu 3 might occupy two types of sites in La 3PO 7 crystals.

Synthesis of Ca4GdO ( BO3 ) 3 : Eu3 + , Al3 + by Sol-Gel Processing and Its Luminescence Properties

Single phase of and codoped was successfully prepared by sol-gel processing at 1000°C, which is 400°C lower than that of solid-state reaction. Morphology analysis reveals that the synthesized phosphors have nonagglomerated, uniform, small particle size and sphere-like morphology without the grinding process. The photoluminescence properties of and activated phosphor prepared by sol-gel processing under UV and vacuum-UV excitation were studied and compared to the bulk synthesized by solid-state reaction. The broad bands centered at about 181 and 248 nm were obtained in their excitation spectra, which are assigned to the absorption of the host crystal and the charge transition (CT) band of . The broad band at about 135–160 nm was observed in this phosphor for the first time. By calculating, the broad band about 135–160 nm could be assigned to the CT band and f–d transition of . The excitation intensity was enhanced by codoping with into the lattices. Under 254- and 147-nm excitation, the intense pure red emission was observed in their emission spectra and the sample prepared by the sol-gel process had higher luminescence intensity than that derived from solid-state reaction.

Electronic-vibrational spectra of nitrobenzene in solutions, liquid phase and orientationally arranged wall-adjacent layers

The electronic-vibrational spectra of nitrobenzene in the frequency region v ≊50000−30000 cm−1 were investigated. The shape and main parameters of five fundamental absorption bands were determined. The absorption spectrum of ultrathin wall-adjacent orientationally ordered layers (with thicknessd≤100 nm) formed near a lyophilic quartz surface (so called epitropic liquid crystals — ELC) differs essentially from that of a bulk specimen and alters with decreasing thickness. In particular the half width of the intramolecular charge transition band decreased, and a bathochromic shift was observed. Parameters of other bands also change. It is probably connected with existence of ELC phase internal inhomogeneity.