YBO3：2 at.% Eu3＋ was prepared by the solid state reaction and its temperature dependent luminescence was investigated for possible applications in temperature sensing. Phase composition of this material was confirmed by X-ray powder diffraction analysis and excitation and emission spectra were also provided. Under excitation of 355 nm, the fluorescence originating from 5D0 and 5D1 states varied as the temperature rose in a region from 333 to 773 K. The fluorescence intensity ratio （FIR） of SD0 and 5D1 was investigated which increased significantly with the rise of temperature. The maximal relative sensitivity in the whole temperature range was 1.8% K-1（at 333 K）. The results recommended YBO3：Eu3＋ as a new material of the FIR method for non-contact optical thermometry.
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.
Three types of β-NaYF_4nanoparticles, uncoated core（NaYF_4：Yb/Ho/Ce）, single-layer coated core@shell（NaYF_4：Yb/Ho/Ce@NaYF_4：Yb） and double-layer coated core@shell@shell（NaYF_4：Yb/Ho@NaYF_4：Yb@NaYF_4：Yb） with Ce^（3＋） doped in core, first and second shell, respectively, were synthesized through solvothermal method to investigate the cross-relaxation between Ho^（3＋） and Ce^（3＋） for the tunable upconversion luminescence. By doping Ce^（3＋） into different layers with different doping concentrations, a systematical investigation on the tunable upconversion luminescence from green to red was conducted. The results showed that a remarkable color tuning could be achieved from green to red when increasing the doping concentration of Ce^（3＋） in the same layer of Ho^（3＋）. And if Ce^（3＋） and Ho^（3＋） were separated in different layers, the color tuning would be depressed significantly due to the reduced cross-relaxation between Ho^（3＋） and Ce^（3＋）. Moreover, the UC emission intensity of core@shell and core@shell@shell was enhanced significantly compared with that of unmodified core nanoparticles.
An abnormal fluorescence intensity ratio （FIR） between two green emissions of Er3＋, at room temperature, which is larger than a normal value, emerged in many reported articles. However, up to now detailed work has seldom been done to clarify this abnormal phenomenon. In this paper, green upconversion luminescence of the β-NaLuF4：20%yb3＋,2%Er3＋ powder sample was investigated under 980 um excitation at different circumstances, different pump power densities and different temperatures as well as different air pressures. The corresponding local temperature calculated using FIR technique increased gradually with the enhancement of the pump power density. It was demonstrated that high pump power density of 980 nm laser led to the increase of local temperature of the luminescent material, which further gave the abnormal FIR.
In order to obtain a single-host white-light phosphor, a series of KCaPO4 powder samples tri-doped with Eu2＋, Tb3＋ and Mn2＋ were synthesized via high-temperature solid-state reaction method. Their structural and luminescence properties were investigated. Under proper ultraviolet excitation （255-405 urn）, white light was obtained, consisting of blue, green and red emissions stemming from Eu2＋, Th3＋, Mn2＋ ions respectively. The temperature stability of our sample was analyzed by studying the variation tendeney of CIE chromaticity coordinates at different temperatures. The results indicated that this phosphor could yield good color stability when utilized in WLED.
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.
Well dispersed orthorhombic structure KYb2F7：2 mol.%Er3＋ and KYb3F10：2 mol.%Er3＋nanoparticles were synthesized through solvothermal method. Measurements of X-ray diffi＇action （XRD）, transmission electron microscopy （TEM） and upconversion spectra showed excellent morphology and optical properties of the as-prepared samples. The intensity ratio of the red emission （650 nm） to the green emission （545 nm） in these two materials was much stronger than that in other Er3＋ doped materials. The transition mechanism of the processes was investigated and discussed. Due to their strong red emission, the samples are promising for applica- tions in display, color tuning, and biolabeling.
Based on the completely parametric crystal-field model, the energy level parameters, including free-ion parameters and crystal-field parameters, obtained by fitting the experimental energy level data sets of Ln^（3＋） in LiYF_4 were systematically analyzed. The results revealed that the regular variation trends of the major parameters at relatively low site symmetry still existed. The g factors of ground states were calculated using the parameters obtained from least-squares fitting. The results for Ce^（3＋）, Nd^（3＋）, Sm^（3＋）, Dy^（3＋） and Yb^（3＋） were in good agreement with experiment, while those of Er^（3＋） deviated from experiment dramatically. Further study showed that the g factors depended strongly on B_4~6, and a slightly different B_4~6 value of -580cm^（-1） led to g factors agreeing well with the experimental values.
An efficient near-infrared （NIR） downconversion （DC） by converting broadband ultraviolet （UV） into NIR was demon- strated in YVO4：Tma＋,yb3＋ phosphors. The phosphors were extensively characterized using various methods such as X-ray diffrac- tion, photoluminescence excitation, photoluminescence spectra and decay lifetime to provide supporting evidence for DC process. Upon UV light varying from 260 to 350 nm or blue light （473 nm） excitation, an intense NIR emission of Yb3＋ corresponding to tran- sition of 2Fs/2/5→2F7/2 peaking at 985 nm was generated. The visible emission, the NIR mission and the decay lifetime of the phosphors of various Yb3＋ concentrations were investigated. Experimental results showed that the energy transfer from vanadate group to Yb3＋ via Tm3＋ was very efficient. Application of the broadband DC YVO4：Tma＋,yb3＋ phosphors might greatly enhance response of sili- con-based solar cells.
