| White light-emitting diodes (w-LEDs) have been expected to be the next generation light source due to their high luminous efficiency, long lifespan, energy conservation, high reliability, environmental friendliness and easy assembly. Typically, w-LEDs can be made by combining InGaN-based blue chips with a yellow-emitting phosphor (YAG:Ce+) or coupling a near-UV LED with tricolor (red, green, and blue) phosphors. Unfortunately, the commercial w-LEDs obtained from both approaches exhibit a low color rendering index due to the scarcity of red light component in the emission spectra. Therefore, it is of great significance to search for novel red phosphors.Tungstates and molybdates are considered recently to be a promising luminescent host candidate due to their excellent thermal and chemical stability. An enormous amount of investigations focuses on Eu3+-substituted phosphors. In this work, a series of the red phosphors AY(MoO4)2:Eu3+, Sm3+(A= Na+, K+) and Ba2MW(Mo)O6:Eu3+(M= Ba2+, Zn2+) were synthesized via different methods including convenient solid-state method, microwave assisted hydrothermal method and sol-gel method. The luminescence properties of the obtained phosphors are characterized by the XRD, SEM, luminescent spectra, UV-vis absorption spectra and Raman spectra and the results are as follows:1) NaYo.9(MoO4)2:0.1Eu3+ phosphors were first synthesized using the microwave assisted hydrothermal process without further sintering treatment. XRD patterns with different heating temperatures indicate that the improvement of crystallinity with increasing the microwave heating temperature. The time-dependent shape evolution experiment indicates the swift particle growth. The products prepared at 0.5 and 1 min are mainly composed of monodisperse rice-like micro-particles with the average dimension of 0.5×1μm. While for the reaction times of 5 and 10 min, the products show monodispersed micro-architectures with uniform cubic morphology and an average diameter of about 2μmn. The morphology evolution of the NaYo.9(Mo04)2:0.1Eu3+ micro-particles is closely related to the process of crystal growth. Initially, direct mixing of three solutions containing Na2Mo04-2H20, Y(NO3)3-6H2O and Eu(NO3)3-6H2O generate a great amount of Na+, Y3+ and MoO42- ions and these ions in the solution react to form NaYo.9(Mo04)2:0.1Eu3+nuclei. With prolonging the reaction time, the remaining reactant of Na2Mo04·2H20, Y(NO3)3-6H2O are gradually dissolved and released Na+, Y3+, MoO42- ions and the released ions continue to form NaYo,9(Mo04)2:0.1Eu3+. Finally, these crystal nuclei grow into particles at the cost of the small ones and form NaY(MoO4)2 micro-particles with different morphologies.2) KY(MoO4)2 phosphors with orthorhombic structure were synthesized by the convenient solid-state method. The results indicate that the optimal doping concentration of Sm3+ in KY(MoO4)2:xSm3+ phosphor is about 3 mol%. Initially, the emission intensity increases, reaches a maximum at 3 mol% Sm3+, and then decreases above 3 mol% with increased Sm3+ doping concentration. This phenomenon illustrates the occurrence of energy migration between Sm3+ in different sites, resulting concentration quenching. With the analysis of the luminescent spectrum, the results exhibit that with Sm3+ and Eu3+ ions co-doped into the host, the excitations around 394 nm and 465 nm become broader compared with that of the single Eu3+ doped sample. Meanwhile, the luminescence intensity is prominently enhanced, which is due to the energy transfer between Sm3+ and Eu3+ and Sm3+ ions can transfer a part of its absorbed energy to Eu3+ ions.3) Novel Ba3WO6:Eu3+ nanowire phosphors were first synthesized by solid state reaction method. In Ba3WO6 host, the cations locate at two different sites, viz. Ba2+(A-site) at the 8c sites with Td symmetry, Ba2+(B-site) at the 4a sites with Oh symmetry, respectively. The results indicate that Eu3+ ions are substitute for Ba2+(B-site, six-coordination) ions in Ba3WO6 host. And the phosphors emit strong red light centered at 595 nm corresponding to 5D0→7F1 transition of Eu3+ ion under CT band excitation (310,313,315,318,321 nm). With the analysis of the excitation spectra, it is noted that the CT position of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. Using Li+, Na+, K+ to offset the charge imbalance, the K+ ion is the optimal compensator charge among Li+, Na+, K+ ions, which is due to the smaller change of crystal structure when two Ba2+ ions are substituted by Eu3+ ion and K+ ion.4) Ba2Zn1-xMoO6:xEu3+ orange-red emitting phosphors were synthesized by using the sol-gel method. The crystalline structure and photoluminescence properties of the phosphors were investigated. The X-ray diffraction (XRD) patterns indicate that the structure of matrix Ba2ZnMoO6 is cubic double-perovskite with space group Fm-3m. The Ba2ZnMoO6:Eu3+ phosphors present an intense broad charge transfer (CT) band absorption in near UV range (370-410 nm), which matches well with the emission wavelength of near-UV LEDs chips, and performs orange-red emission of Eu3+(5D0→7F1 transition) at around 596 nm. The Ba2ZnMoO6:Eu3+ phosphors are considered to be a promising orange-red emitting phosphor for near ultraviolet GaN-based white light emitting diode. |
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