| Quantum-cutting(QC) comprises the absorption of a high energy photon and the subsequent emission of two or more lower energy photons, thus the quantum efficiency(QE) can be over 100%. QC was theoretically proposed by Dexter in 1957, and was first reported experimentally by Sommerdijk and Piper in 1970. Trupke reported the potential application of near-infrared(NIR) QC materials in overcoming the classical efficiency limit of Si solar cells in 2002. After that, Considerable effort has been devoted to develop NIR QC luminescent materials with high efficiency. This thesis mainly investigates the NIR QC process of RE ion singly doped system and broadband sensitization for the NIR QC emssion of a single RE ion. As the host material, Ca3(PO4)2 can provide suitable crystal field environment for rare earth ions. Therefore, selected RE ions doped phosphors Ca3(PO4)2: Tm3+, Ca3(PO4)2: Ho3+, Ca3(PO4)2: Tm3+, Ce3+, and Ca3(PO4)2: Ho3+, Ce3+ were synthesized.The three-photon near-infrared quantum-cutting luminescence of Tm3+ assigned to the electronic transitions of 1G4 â†' 3H4(1181 nm), 3H4 â†' 3F4(1461 nm), 3F4 â†' 3H6(1800 nm) in Ca3(PO4)2: Tm3+ phosphors under the excitation of a blue photon was reported, and we attributed the 1800 nm emission to 3F4 â†' 3H6 electronic transitions of Tm3+ based on the energy level structure and photoluminescence spectra of Tm3+ under 808 nm excitation. Besides, two kinds of near-infrared quantum-cutting luminescence in Ca3(PO4)2: Tm3+ accurred at the same time with different rates based on the cross relaxation(CR) of 1D2 + 3H6 â†' 1G4 + 3F4 and 1D2 + 3H6 â†' 3H4 + 3F2,3 under the excitation of a ultraviolet photon. And also, the effect of cross relaxation 1G4 + 3H6 â†' 3H4 + 3H5(Tm3+) on the relative intensity of near infrared emissions was studied.The enhancement of Tm3+ near-infrared luminescence in Ca3(PO4)2: Tm3+, Ce3+ phosphors by the broadband absorption of Ce3+ and the ET from Ce3+ to Tm3+ was verified. The ET from Ce3+ to Tm3+ mainly occured via cross-relaxation Ce3+(5d) + Tm3+(3H6) â†' Ce3+(2F5/2) + Tm3+(1D2). The ET efficiency from Ce3+ to Tm3+ was calculated and went up to 34.5% for the Ca2.86-2xCe0.07TmxNa0.07+x(PO4)2 with x = 0.15.The two-photon near-infrared quantum-cutting luminescence of Ho3+ assigned to the electronic transitions of 5F4, 5S2 â†' 5I6(1015 nm) and 5I6 â†' 5I8(1190 nm) in Ca3(PO4)2: Ho3+ phosphors was reported. We supposed that the non-radiative relaxation from 5F4, 5S2 to 5F5 level(energy gap between 5F4, 5S2 and 5F5 is 2139 cm–1) contributed to the relatively weak 1015 nm emission. And higher optimal doping amount of Ho 3+ than that of Tm3+ in Ca3(PO4)2 was due to more interaction among Tm3+ ions that caused by similar energy gap in Tm3+. Besides, the enhancement of Ho3+ near-infrared luminescence in Ca3(PO4)2: Ho 3+, Ce3+ by Ce3+ was confirmed. The energy transfer from Ce3+ to Ho3+ mainly occurred via cross-relaxation Ce3+(5d) + Ho3+(5I8) â†' Ce3+(2F7/2) + Ho3+(3K7, 5G4). The relatively small energy mismatch in this cross-relaxation resulted in more efficient ET process from Ce3+ to Ho3+ than that from Ce3+ to Tm3+ in Ca3(PO4)2 host material. Energy-transfer efficiency from Ce3+ to Ho3+ went up to 56.7% for the Ca2.56Ce0.07Ho0.15Na0.22(PO4)2. |
PDF Full Text Request