Preparation And Luminescent Properties Of Novel Rare Earth Doped Phosphors

Phosphor-converted white light-emitting diodes are emerging as an indispensable solid state light source for the next generation lighting industry and display systems due to their uniqueproperties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Currently, there are mainly two methods to obtain a white light. One of the approaches is solely based on the combination of a blue LED and a yellow-emitting phosphor (YAG:Ce3+). However, it exhibits poor color rendering index (CRI) and high correlated color temperature due to the deficiency of red emission. One solution to this problem is to make LEDs by coating a near-ultraviolet (n-UV) emitting LED with a mixture of blue, green, and red emitting phosphors, which exhibits smoother spectral distribution over the whole visible range and therefore can obtain high quality white light. Accordingly, w-LEDs fabricated with n-UV LED chips and tricolor (red, green and blue) phosphors are expected to dominate the market in the near future.In this work, we reported several novel phosphors which can be excited by near-ultraviolet light via solid state reaction method or molten salt synthesis. In the meantime, we investigated the synthesis process, mophologies, crystal structures, ion station, photoluminescence and energy transfer mechnism in detials. The main content are listed as follows:(1)(Ce0.67Tb0.33)Mg1-xAl11O19:xMn2+and (Ce0.67,Tb0.33)Mg0.875-xAl11-2xSix019:0.125Mn2+phosphors were synthsized via solid state reaction method. The emission spectra exhibit not only the characteristic emission peaks of Tb3+while compared to (Ce0.67Tb0.33)MgAl11O19, but also a new broad emission band around516nm, which is assigned to the4T1g(G)→6A1g(S) electron transition of Mn2+. In the (Ce0.67Tb0.33)Mg1-xAl11O19:xMn2+phosphors, the integral area of the emission spectrum increases while doped with Mn2+as a co-activator, and the integral area is about226%of CMAT as x=0.15; after co-doping with Si4+-Mg2+pairs, the integral area increases to298%, indicating that those phosphor has higher luminous efficiency.(2) A novel charge transfer blue-emitting phosphor BaY2Si3O10has been synthesized by the conventional solid state reaction method. The excitation spectrum of BaY2Si3O10consists of two bands centered at294and333nm, which are associated with the1A1→1T2and1A1→1T1charge transfer between oxygen2p orbital and the3d orbital of Si4+ions, respectively. Strong emission with the peak position at426nm and with the decay time in ns range is revealed. The blue emission is corresponding to the superposition of3T2→1A1and3T1→1A1charge transfer under the near-ultraviolet excitation of333nm. While Eu2+doped BaY2Si3O10phosphor also exhibits broad blue emission band range from390to550nm under the excitation of323nm light, which is associated to the4f65d1→4f7transition of Eu2+.The critical concentration of Eu2+is0.08and electric quadrupole-quadrupole interaction is responsible to the concentration quench of BaY2Si3O10:Eu2+phosphor.(3) A series BaY2Si3O10:Ce T,Tb3+phosphors were synthsized by the solid state reaction method. The excitation spectra of BaY2Si3O10:Ce3+,Tb3+exhibit hree humps at298,337and373nm, which is contributed to the4f-5d transition of Ce3+, the emission spectra display a series of emissions at485,541,583, and621nm, due to the5D4→7FJ (J=6,5,4, and3) characteristic transitions of Tb3+ions. The decay mode of Ce3+confirms that the energy transfer occurred in BaY2Si3O10:Ce3+,Tb3+phosphors, and the intense green emission is realized in BaY2Si3O10:0.05Ce3+,0.85Tb3+on the base of high efficient energy transfer from Ce3+to Tb3+with an efficiency of70%. The CIE1931chromaticity coordinates of BaY2Si3O10:0.05Ce3+,Tb3+phosphors could be tuned from blue (0.156,0.098) to greenish yellow (0.294,0.562) position by gradually increasing the Tb+concentration.(4) The Ba5(VO4)3Cl:Eu3+,K+phosphors have been synthesized by the molten salt synthesis method. All the samples have nonaggregated smooth spherical morphology, and the partical size rises with the increasing calcined tempreture. Doping with Eu3+do not cause any detectable change in the host structure. However, the positions of the diffraction peaks ((211),(112) and (300)) are observed gradually move to the high degree with the increase of Eu3+contents (x) from0.30to1.60. The excitation spectrum monitored at614nm consists of a broad excitation band in the vicinity of220-350nm and five sharp4f transition lines of Eu3+, which cover the ranges from long-wavelength UV to visible blue-light region (350-500nm). The broad band centered at313nm is the charge transfer band (CTB) of Eu3+-O2+interaction. In addition to the CTB, five more sharp excitation peaks at363,382,395,416and466nm are also realized, which are attributed to the direct absorption of the Eu3+ions assigned to transitions of7F0→5D4,7F0→5L7,7F0→5L6,7F0→5D3and7F0→5D2, respectively. The emission spectra achieved by313,395and466nm excitation exhibit similarities by comparing the relative intensities of the emission lines from Eu3+4f-4f transitions corresponding to the transitions from the excited5D0level to the7FJ (J=0-4) levels of4f6configuration; other two emission peaks at about535and554nm corresponding to the5D1→7F1and5D1→7F2transitions are very weak due to the high energy phonons. The critical concentration of Eu3+is1.10and electric dipole-dipole interaction is responsible to the concentration quench of Ba5(VO4)3Cl:Eu3+,K+phosphor.