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-Na Lu F4:20%Yb3+,2%Er3+ powder sample was investigated under 980 nm 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 KCaP O4 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 nm), white light was obtained, consisting of blue, green and red emissions stemming from Eu2+, Tb3+, Mn2+ ions respectively. The temperature stability of our sample was analyzed by studying the variation tendency of CIE chromaticity coordinates at different temperatures. The results indicated that this phosphor could yield good color stability when utilized in WLED.
Yb^(3+)-Er^(3+) co-doped K_2GdF_5 up-conversion phosphor was successfully synthesized by a solid-state reaction method. The phase purity and structure of the sample were characterized by powder X-ray diffraction. The sample emitted orange light at room temperature and its up-conversion spectra at different temperatures were recorded under the excitation of a 980 nm diode laser. The energy transfer from Yb^(3+) to Er^(3+) notably enhanced the up-conversion luminescence intensity. The possible up-conversion mechanisms and processes were proposed based on the power dependence of the luminescence intensities. The temperature-dependent up-conversion luminescence and temperature sensing performances of the sample were discussed according to the fluorescence intensity ratio of green emissions originating from ~2H_(11/2)/~4S_(3/2)→~4I_(15/2) transitions of Er^(3+) in the range from 307 K to 570 K under the excitation of 980 nm laser with power of 260 mW. The dependence of the fluorescence intensity ratio on temperature was fitted with an exponential function and the effective energy difference obtained was 690 cm^(–1), which further gave a relative temperature sensitivity of 1.1%/K at 307 K. The results suggested that the Yb^(3+)-Er^(3+) co-doped K_2GdF_5 sample is a promising candidate for optical temperature sensor.
Samarium doped pyrovanadate Ba_2V_2O_7:Sm^(3+) phosphors were synthesized by traditional high-temperature solid-state reaction method. The phase and the structure of the samples were characterized by powder X-ray diffraction(XRD), and the luminescent properties and the energy transfer mechanism of the material were investigated using quantitative photoluminescence(PL) spectroscopy. The excitation spectrum of the sample exhibited a broad ultraviolet(UV) band with maximum at around 341 nm due to V–O charge transfer transition of the host. The emission spectrum displayed a yellow-greenish broadband(peaking at around 498 nm) coming from the host Ba_2V_2O_7 and three narrow peaks(at 561, 599 and 646 nm) attributed to the dopant Sm^(3+) ions. The PL spectra revealed the energy transfer from the host to the Sm^(3+) ions. In addition, the color coordinates and the color temperature of the phosphor Ba_(1.95)V_2O_7:5%Sm^(3+) were(0.314, 0.365) and 6135 K, respectively, under 365 nm excitation, suggesting it to be a candidate of single-phase converting phosphors for white-light-emitting diodes(WLEDs) with near-UV chips.
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.
YBO_3:2 at.% Eu^(3+) 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 ~5D_0 and ~5D_1 states varied as the temperature rose in a region from 333 to 773 K. The fluorescence intensity ratio(FIR) of ~5D_0 and ~5D_1 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 YBO_3:Eu^(3+) as a new material of the FIR method for non-contact optical thermometry.
Tb^(3+)/Eu^(3+) co-doped transparent glass ceramics containing CaF_2 nanocrystals were successfully synthesized by high temperature melt-quenching method and subsequent heating. The structure and morphology of the samples were investigated by X-ray diffraction(XRD), transmittance electron microscopy(TEM), high resolution TEM(HRTEM) and selected area electron diffraction(SAED). The photoluminescence properties and energy transfer process from Tb^(3+) to Eu^(3+) of CaF_2:Tb^(3+),Eu^(3+) phosphors were also investigated through excitation spectra and decay curves. In addition, the emission spectra of the glass ceramics in a wide temperature range from 21 to 320 K were recorded under the excitation of 485 nm. It was found that the fluorescence intensity ratios of Tb^(3+) at 545 nm(~5D_4→~7F_5) to Eu^(3+) at 615 nm(~5D_0→~7F_2) was highly temperature-dependent with an approximate linear relationship, and the temperature sensitivity was about 0.4%/K. It is expected that the investigated Tb^(3+)/Eu^(3+) co-doped CaF_2 glass ceramics may have prospective application in optical thermometry.
The SrB_4O_7:Sm^(2+) phosphor was synthesized by high temperature solid state reaction method. The phosphor-in-glass was prepared by mixing and firing the phosphor and TeO_2-ZnO glass precursor. The XRD results showed that the phosphor-in-glass was in amorphous phase because of the tiny mass fraction of phosphor in glass. The emission spectrum under the 355 nm excitation was measured, and it exhibited the same characteristic emission peaks as the phosphor sample. The temperature characteristic of the SrB_4O_7:Sm^(2+) phosphor-in-glass was discussed by analyzing the lifetime of 684 nm emission at various temperatures. The relative temperature sensitivity of 5%/K around 573 K was obtained for the temperature dependent lifetime, suggesting that the material in this study is a promising candidate for temperature sensor application.
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.
Well dispersed orthorhombic structure KYb2F7:2 mol.%Er3+ and KYb3F10:2 mol.%Er3+ nanoparticles were synthesized through solvothermal method. Measurements of X-ray diffraction(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 applications in display, color tuning, and biolabeling.
