Chunguang Liu

Single-Phased White-Emitting 12CaO·7Al2O3:Ce3+, Dy3+ Phosphors with Suitable Electrical Conductivity for Field Emission Displays

A novel white-light-emitting phosphor, 12CaO·7Al2O3:Ce3+, Dy3+with H− encaging, was prepared by the solid-state reaction in H2 atmosphere. Upon excitation at 362 nm, the phosphor shows intense white-light emission that combines the blue and yellow emissions at 476 and 576 nm, assigned to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+, respectively. A weak broad blue emission centered at 430 nm is also observed and attributed to the 5d–4f transitions of Ce3+. The photoluminescence intensity of Dy3+ increases greatly with increasing Ce3+ concentration, indicating that the effective energy transfer occurred from Ce3+ to Dy3+ in the phosphor. In particular, the phosphor can be converted to a persistent semiconductor upon ultraviolet irradiation due to the electrons released from the encaged H− ions, and the electrical conductivity is measured to be 10−2 S cm−1. The conductive phosphor exhibits excellent white-light emission (CIE coordinate of (0.324, 0.323)) under low-voltage (5 kV) electron beam excitation, suggesting that the phosphor is a potential candidate for applications in field emission displays.

A multiphase strategy for realizing green cathodoluminescence in 12CaO center dot 7Al(2)O(3)-CaCeAl3O7:Ce3+,Tb3+ conductive phosphor

A multiphase strategy is proposed and successfully applied to make the insulating green phosphor CaCeAl3O7:Tb(3+) conductive in the form of 12CaO·7Al2O3-CaCeAl3O7:Ce(3+),Tb(3+). The phosphor shows bright green-light emission with a short lifetime (2.51 ms) under low-voltage electron beam excitation (3 kV). The green photo- and cathodoluminescence from (5)D4-(7)F(J) (J = 6, 5, 4, 3) transitions of Tb(3+) are significantly enhanced in comparison with pure C12A7:Tb(3+). It was confirmed that this enhancement is the consequence of the joint effects of energy transfer from Ce(3+) to Tb(3+) and broadening of the absorption spectrum of Ce(3+) due to the existence of multiple phases. In particular, under 800 V electron beam excitation, cathodoluminescence is improved by the modified electrical conductivity of the phosphor. When compared to the commercial Zn2SiO4:Mn(2+) with a long luminescence lifetime of 11.9 ms, this conductive green phosphor has greater advantage for fast displays.

Spectral Modulation through Controlling Anions in Nanocaged Phosphors

A new approach has been proposed and validated to modulate the emission spectra of europium-doped 12CaO·7Al2O3 phosphors by tuning the nonradiative and radiative transition rates, realized by controlling the sort and amount of the encaged anions. A single wavelength at 255 nm can excite simultaneously Eu2+ and Eu3+ centres to yield blue and red emissions, respectively. The amount of the anions in the nanocages, like OH− and H−, can be controlled by heat treatment processes. The existence of encaged OH− accelerates the nonradiative process of the emission centres, while the presence of encaged H− induces a higher symmetry that decreases the radiative transition rate, confirmed by the analysis of decay processes. It is demonstrated that the emission colour is tuned finely from red to blue with the CIE chromaticity coordinates from (0.630, 0.352) to (0.216, 0.086) when the annealing time increases. This strategy can be readily extended to similar systems with other rare earth active centres.