| White light-emitting diodes (LEDs) are considered as the new generation of illumination source owing to their numerous advantages such as long lifetime, energy saving, mercury-free, etc. The most well known white LED is achieved by combination of a blue emitting diode with a yellow phosphor Y3Al5O12:Ce3+, which was fabricated firstly by Nichia Chemical Co. Y3Al5O12:Ce3+ has a high quantum efficiency and high thermal stability, however, the lack of red-emitting component leads to a low color rendering index (CRI) of the white LEDs (< 80). There are several ways to obtain a high CRI value, for example, the combination of a near-UV chip with tricolor phosphors, or a blue-emitting chip combined with green/yellow and red phosphors, etc. Therefore, it is of great importance in searching for efficient phosphors with strong absorption in the near-UV and/or blue region.Ce+-activated phosphors usually show intense absorptions and emissions due to spin-and orbital-allowed electron transitions. Since the 5d orbits of Ce3+ ions are on the outer shell, the positions of the excitation and emission bands are sensitive to the host lattice, to be precise, the surroundings of the crystalline site for Ce3+ ions. As a result, the emission color of Ce3+-activated phosphors can be tuned from blue to red.Due to the six different Ca2+ crystalline sites in Ca3Al2O6 single crystal, there should be different luminescent properties when Ce3+ ions are doped into different Ca2+ sites in Ca3Al2O6 host. A single-phase multi-color emitting phosphor, Ca3Al2O6:Ce3+,Li+, was prepared by a solid-state reaction. When the Ce3+ concentration is lower than 0.030 (molar ratio in CasAl2O6), yellow and greenish blue emissions can be observed under the excitation by a blue and a near UV light, respectively. The yellow-emitting phosphor possesses an internal quantum efficiency of 89%. Additional purplish blue emission turns up when Ce3+ concentrations are higher than 0.040. By changing the Ce3+ doping concentration, emissions with several colors can be realized under different excitation wavelengths in the aluminate phosphor. The relationship between the emission wavelengths and site occupancies of Ce3+ ions was investigated. Tunable emission bands are originated from Ce3+ ions on different Ca sites in Ca3Al2O6 Although the emission band of purplish blue or greenish blue overlaps the excitation band of yellow emission, and the distances between the unlike Ce3+ ions are in the range of electric dipole-dipole interaction, no energy transfer is observed. Furthermore, emission wavelengths for the yellow, greenish blue and purplish blue emission show little change upon increasing Ce3+ concentrations.Ce3+/Eu2+, Tb3+ and Mn2+ co-doping in single-phase hosts is a common strategy to achieve white-light phosphors through energy transfer, which provides high color rendering index (CRI) value and good color stability. However, not all of the hosts are suitable for white-light phosphors due to inefficient energy transfer. In this study, site-sensitive energy transfer from different crystallographic sites of Ce3+. to Tb3+/Mn2+ in CasAl2O6 has been investigated in detail. The energy transfer from purplish-blue Ce3+ to Tb3+ is an electric dipole-dipole mode, and the calculated critical distance (Rc) suggests the existence of purplish-blue Ce3+-Tb3+ clusters. No energy transfer is observed from purplish-blue Ce3+ to Mn2+. In co-doped phosphors based on greenish-blue Ce3+, however, radiative mode dominates the energy transfer from Ce3+ to Tb3+, and an electric dipole-quadrupole interaction is responsible for the energy transfer from Ce3+ to Mn2+. The detailed discussion in site-sensitive energy transfer modes might provide a new aspect to discuss and understand the possibilities and mechanisms of energy transfer according to certain crystallographic sites in a complex host with different cation sites as well as a possible approach in searching single-phase white-light-emitting phosphors. |
PDF Full Text Request