The Long-lasting Phosphors Based On Zinc Phosphate

The long-lasting phosphorescent(LLP) phosphors, green technical materials as light saving, have found various application in many fields. Zinc orthophosphates are highly stable in chemistry, easily synthesized, low cost, white, environment friendly, and so on. In this work, LLP phosphors based on Zn3(PO4)2 with pure defects or Mn2+ ions as luminescence centers were synthesized using solid state reaction, sol-gel method, and coprecipitation method, respectively. Various effects of doping ions on luminescence properties, and the trap defects were investigated. The possible work mechanisms were proposed according to the experimental results.The bluish green phosphors of MxZn3-0.5x(PO4)2(M=Li+, Na+, K+) with pure defects as luminescence centers have a broad emission band from 420 nm to 550 nm,centered at about 460 nm. The best long-lasting phosphorescence was observed with naked eyes in the dark for 4 h after the irradiation light sources have been removed. It has been observed that there are obvious effects of sintering environment and doping ions on the luminescence properties. The highest concentration of oxygen defects can be obtained in the reducing atmosphere. It was proposed that two possible luminescence mechanisms exist in phosphors. On the one aspect, the excited electrons by UV light from the valence band were captured by oxygen defects. Then, under thermal disturbance, the captured electrons released from the defects and then returned to the ground state with the emission of light. On the other way, the luminescent emission also resulted from the recombination between NaZn’ defects with negative charge and oxygen vacancies with positive charge. The suitable tunnel is beneficial for the shifting of Na+ to meet oxygen vacancies. The too big or small or collapsed tunnels in vitrification of materials are harmful for shifting of Na+, resulting in bad luminescence.The emission peak of the new green afterglow phosphor α-Zn3(PO4)2: Mn2+, M+(M=Na, K) was located at 548 nm, assigned to the 4T1g-6A1 g transition of Mn2+ in tetrahedral coordination. The LLP was observed with naked eyes in the dark for 2 h after the irradiation light sources have been removed. its luminescence performance. The increase of oxygen vacancies can be significantly improved the dopant ions of Na+/K+ ions. It was suggested that excited electrons in high energy levels were relaxed to low energy levels, and then most of them immediately returned to the ground state with emission of light. Still some few excited electrons which are captured by and saved in oxygen vacancies through the tunneling effect were released and jumped to ground states under thermal disturbing with the emission of light.The emission peak of the red afterglow phosphor β-Zn3(PO4)2: Mn2+,RE3+(RE = Pr, Dy) was found at 620 nm, assigned to the 4T1g-6A1 g transition of Mn2+ in octahedral coordination. The LLP was observed with naked eyes in the dark for 4 h after the irradiation light sources have been removed. The incorporation of Pr3+ and Dy3+ ions largely improved the red LPP performance of β-Zn3(PO4)2: Mn2+ phosphor. There exists the overlap of energy band between Pr3+(1D2, 3PJ, 1I6)-3H4 and Mn2+ 6A1-4T1. The luminescence properties of Mn2+ have been improved by the dopant of sensitized Pr3+ ions. The energy transfer from Pr3+ to Mn2+ occurred when the excited electrons in Pr3+(1D2, 3PJ, 1I6) relaxed to the excited states of Mn2+ 4T1. Electron-hole pairs are created in the phosphors β-Zn3(PO4)2: Mn2+, Dy3+ with the exposure of UV light. Some excited electrons immediately jump to the ground states with the emission of light. Other electrons and holes are captured by various kinds of defects, and then release to under thermal disturbing. The released holes and electrons may meet together to release photons or keep moving in the lattice or may be trapped again by other defect centers. The decay time of LLP phosphors depends on the life span of electron-hole pairs in the lattice.