Li Li~(1,2),Wei Xiantao~1,Chen Yonghu~1,Guo Changxin~1,Yin Min~(1*) (1.Department of Physics,University of Science and Technology of China,Hefei 230026, P R China) (2.College of Mathematics and Physics,Chongqing University of Posts and Telecommunications, Chongqing 400065,P R China)
BaMgAl10Ol7：Eu^2＋,Yb^3＋ was investigated as a possible quantum cutting system to enhance solar cells efficiency. Phosphors were synthesized by combustion method and composed of nanorods. Photoluminescence spectra showed that Eu in the sample was reduced to bivalence while Yb remained trivalence. Through a cooperative energy transfer process, the obtained powders exhibited both blue emission of Eu^2＋ （around 450 nm） and near infrared emission of Yb^3＋ （around 1020 nm） under broad band excitation （250-410 nm） originating from 4f→5d transition of Eu2＋. Energy transfer phenomenon between the sensitizer Eu2＋ and the activator Yb3＋ was investigated via the luminescent spectra and the decay curves of Eu2＋ with different Yb3＋ concentrations. Results indicated that energy transfer efficiency from Eu2＋ to Yb3＋ was not high. The poor efficiency can be explained by the long distance between rare earth ions.
The local coordination structures around the doping Yb2＋ ions in sodium and potassium halides were calculated by using the first-principles supercell model. Both the cases with and without the charge compensation vacancy in the local environment of the doping Yb2＋ were calculated to study the effect of the doping on the local coordination structures of Yb2＋. Using the calculated local structures, we obtained the crystal-field parameters for the Yb2＋ ions doped in sodium and potassium halides by a method based on the combination of the quantum-chemical calculations and the effective Hamiltonian method. The calculated crystal-field parameters were analyzed and compared with the fitted results.
Under the excitation of 980 nm diode laser, intense green emission （5F4＋5S2–5I8） of Ho3＋ was observed in Ho3＋ and Yb3＋ co-doped cubic Y2O3. The doping concentration and laser power dependence of the upconverted emission were studied. The decay curves of 5F4＋5S2 emission of Ho3＋ under the excitation of 355 nm pulse laser were measured to investigate the energy transfer process between Ho3＋ and Yb3＋. The results indicated that two-photon process was responsible for the upconversion （UC） emission. The Ho3＋ concentration of 0.04 mol.% and the Yb3＋ concentration of 5 mol.% were determined to be the best value for the strongest Ho3＋ emission under the excitation of 980 nm light. The cross-relaxation between two neighboring Ho3＋ ions and the back energy transfer from Ho3＋ to Yb3＋ were important factors for determin- ing the optimal doping concentration. This material was a promising candidate for the application in biomedical fluorescent labels for the intense green emission upon excitation of near-infrared （NIR） light.
In order to obtain a single-host-white-light phosphor, a series of Bal.8 -x-y-zSrwLi0.4xCexEuyMnzSi04 （BSLS：Ce3＋,Eu2＋, Mn2＋） powder samples were synthesized via high temperature solid-state reaction. The structure and photoluminescence properties were investigated. Under ultraviolet excitation, the emission spectra contained three bands： the 370-470 nm blue band, the 470-570 nm green band and the 570-700 nm red band, which arose from the 5d---4f transitions of Ce3＋ and Eu2＋, and the 4TI---6A1 transition of Mn2＋, respectively. The excitation spectra of the emissions of Ce3＋ and Mn2＋ ions showed the energy transfer from Ce3＋ to Mn2＋. White light emission was obtained from the tri-doped samples of appropriate doping concentration under 31 0-360 nm excitation.
Tb3＋ and Yb3＋ codoped Lu2O3 nanophosphors were synthesized by the reverse-strike co-precipitation method. The obtained Lu2O3：Tb3＋,Yb3＋ nanophosphors were characterized by X-ray diffraction （XRD） and photoluminescence （PL） spectra. The XRD results showed that all the prepared nanophosphors could be readily indexed to pure cubic phase of Lu2O3 and indicated good crystallinity. The Tb3＋→Yb3＋ energy transfer mechanisms in the UV-blue region in Lu2O3 nanophosphors were investigated. The experimental results showed that the strong visible emission around 543 nm from Tb3＋ （5D4→7F5） and near-infrared （NIR） emission around 973 nm from Yb3＋ （2F5/2→2F7/2） of Lu2O3：Tb3＋,Yb3＋ nanophosphors were observed under ultraviolet light excitation, respectively. Tb3＋ could be effectively excited up to its 4f75d1 state and relaxed down to the 5D4 level, from which the energy was transferred cooperatively to two neighboring Yb3＋. The Yb3＋ concentration dependent luminescent properties and lifetimes of both the visible and NIR emissions were also studied. The lifetime of the visible emission decreased with the increase of Yb3＋ concentration, verifying the efficient energy transfer from the Tb3＋ to the Yb3＋. Cooperative energy transfer （CET） from Tb3＋ to Yb3＋ was discussed as a possible mechanism for the near-infrared emission. When doped concentrations were 1 mol.% Tb3＋ and 2 mol.% Yb3＋, the intensity of NIR emission was the strongest.
