Yanyan Bu

Optical temperature sensing of rare-earth ion doped phosphors

Accurate and reliable temperature measurement of many special inaccessible objects is a challenging task. Optical temperature sensing is a promising method to achieve it. The current status of optical thermometry of rare-earth ion doped phosphors is reviewed in detail. Based on the mechanisms of optical temperature sensing of different phosphors, temperature dependent luminescence spectra, the fluorescence intensity ratio technique in the data fitting process, and errors of the energy difference between thermally coupled levels, we describe the recent developments in the use of optical thermometry materials. The most important results obtained in each case are summarized, and the main challenges that we need to overcome are discussed. Research in the field of phosphor sensors has shown that they have significant advantages compared to conventional sensors in terms of their properties like greater sensitivity, freedom from electromagnetic interference, long path monitoring, and independence of compatibility with electronic devices.

Size and shape modifications, phase transition, and enhanced luminescence of fluoride nanocrystals induced by doping

The synthesis and growth mechanism of well-defined nanostructures are still challenging. In this study, we offer a novel soft chemistry route to modify the size and shape of irregular fluoride crystals by changing the surface charge distribution of the crystal nucleus by Bi3+, Al3+, Gd3+, Yb3+, Ba2+ and Ca2+ ion doping. As a result, irregular BaF2, SrF2, GdF3, and NaGdF4 alkaline-earth and lanthanide fluorides can be converted into nanoparticles with high yield and controlled size and shape, in which the phase transition from micro-crystals to single-crystalline is also achieved. Enhanced yellow and red emissions of Eu3+-doped BaF2 induced by doping are observed. In conjunction with density functional theory calculations, it shows that the shape and size modifications originate from the surface charge redistribution of the crystal nucleus induced by inner electron charge transfer between the doping ions and lattice cations.

Influence of Doping and Excitation Powers on Optical Thermometry in Yb 3+ -Er 3+ doped CaWO 4

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb3+-Er3+ co-doped CaWO4 phosphors by doping with Li+, Sr2+, and Mg2+ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (2H11/2/4S3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li+, Sr2+, and Mg2+ ions in the Yb3+-Er3+ co-doped CaWO4 system. Moreover, the relative sensitivity SR and the absolute sensitivity SA are proved to be dependent on the pump power of 980 nm laser. The sensitivities of SR and SA in Yb3+-Er3+ co-doped CaWO4 increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr2+.

Detecting the origin of luminescence in Er 3+ -doped hexagonal Na 1.5 Gd 1.5 F 6 phosphors

Understanding site-selective fluorescence is one of valuable importance for spectrum modulation. In this Letter, we observed the existence of two non-equivalent Gd-activated crystallographic sites in an Er3+-doped hexagonal Na1.5Gd1.5F6 phosphor. It is proved that two green emissions from the S3/24 level separately originate from the Gd1 (540 nm) and Na2/Gd2 (550-555 nm) crystallographic sites, and the 657 nm red emission from the F9/24 level only originates from Na2/Gd2 site through using the time-resolved luminescence spectra. The 142.2% absolute enhancement of the red emission is realized through the synergistic effect of ultraviolet downconversion and infrared upconversion induced by the 370 nm and 1.54 μm dual-mode excitation.

Giant enhancement of upconversion emission in NaYF4:Er3+@NaYF4:Yb3+ active-core/active-shell nanoparticles

Varying the type and the doping concentration of rare-earth ions was reported as a conventional method to tune upconversion emissions of rare-earth ions doped nanoparticles (NPs). In this work, an effective method to enhance greatly the infrared-visible upconversion emissions of Er3+ ions is demonstrated through coating an optimized active NaYF4:Yb3+ shell around NaYF4:Er3+ core nanoparticles. Phase, shape, and size of the resulting core–shell NPs are studied by X-ray diffraction and high-resolution transmission electron microscopy. Compared with the core-only NaYF4:Er3+ NPs, a maximum 97.56-fold overall enhancement in the emission intensity of Er3+ ions in the core–shell NPs is achieved under 980 nm infrared excitation, and a maximum 38.6-fold overall enhancement is achieved under 1540 nm infrared excitation. The luminescence enhancement effect and color output exhibits a strong dependence on the doping concentrations of Yb3+ in the NaYF4:Yb3+ active-shell. By analyzing the decay lifetimes of the emission bands, the giant luminescence enhancement is due to the significant increase in the near-infrared absorption and efficient energy transfer from Yb3+ primary-sensitizers to Er3+ activators via Yb3+ bridging sensitizers.

A novel optical thermometry based on the energy transfer from charge transfer band to Eu 3+ -Dy 3+ ions

Optical thermometry based on the up-conversion intensity ratio of thermally coupled levels of rare earth ions has been widely studied to achieve an inaccessible temperature measurement in submicron scale. In this work, a novel optical temperature sensing strategy based on the energy transfer from charge transfer bands of W-O and Eu-O to Eu3+-Dy3+ ions is proposed. A series of Eu3+/Dy3+ co-doped SrWO4 is synthesized by the conventional high-temperature solid-state method. It is found that the emission spectra, emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm), fluorescence color, lifetime decay curves of Dy3+ (572 nm) and Eu3+ (615 nm), and relative and absolute sensitivities of Eu3+/Dy3+ co-doped SrWO4 are temperature dependent under the 266 nm excitation in the temperature range from 11 K to 529 K. The emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm) ions exhibits exponentially relation to the temperature due to the different energy transfer from the charge transfer bands of W-O and Eu-O to Dy3+ and Eu3+ ions. In this host, the maximum relative sensitivity Sr can be reached at 1.71% K−1, being higher than those previously reported material. It opens a new route to obtain optical thermometry with high sensitivity through using down-conversion fluorescence under ultraviolet excitation.