| Lanthanides doping fluoride nanocrystals materials exhibit low phonon energy,high photo-stability and low biotoxicity and show excellent down-shifting and upconversion photoluminescence properties.Consequently,they have a good potential application value in fields of remote sensing,pollutant monitoring,solar cell,temperature sensing,biology and photochemistry and so on.For examples,Tm:?-NaYF4 microcrystals,with size of larger than 1?m,realize sequential three-step three-photon near-infrared quantum cutting for application in low-bandgap solar cells and thermo-photovoltaic energy conversion by 470 nm laser excitation;Yb/Tm:NaYF4 nanocrystals achieve bio-imaging in vitro and vivo through 975 nm laser excitation.Notwithstanding the excellent natures,lanthanides doping fluoride nanocrystals materials still have some aspects to develop and improve,as following:firstly,the optimum doping concentration of Tm3+ ions in fluorides nanocrystals are respectively about 0.5%due to concentration quenching when fixing Yb3+ doping concentration in 20%;Secondly,most of the upconversion nanothermometers show their temperature sensing through fluorescence,which jump out of the biological transparent window(650-1000 nm)and suffer from the interference of surrounding environment and shallow penetration depth;Thirdly,to the best our knowledge,the 1800 nm and 2300 nm emissionsgenerated from Tm3+ ions almost are actualized in bulks of materials,such as several kinds of glass(ceramics),single-crystal and micro/nano-sized(>70 nm)oxides.Aiming at these insufficiencies above,we carried outserials of experiments and got results as follows:1.In principle,the most effective approach to enhance upconversion intensity is increasing the number of activators.However,the upper limit of Tm3+ ions doping concentration is just 0.5%when fixing Yb3+ doping concentration in 20%.We synthesized uniform and monodesperse N aGdF4@ Yb/T m:NaGdF4 inert-core/active-shell nanocrystals by method of coprecipitation.The results demonstrate that the optimum doping concentration of Tm3+ ions increases from 0.5%to 3%and the upconversion intensity enhances about 6 times in comparison with Yb/Tm:NaGdF4 nanocrystals,due to the decreased defects and enhanced energy transfer efficiency from Yb3+ to Tm3+ ions through confining them in a two-dimensional space.2.Nano thermometers usually take short wavelength light as excitation or emit lights in visible region,and temperature measurement is not inbiological temperature belt.We report a monodispersed and ultrasmall size of nanoprobe synthesized by the method of coprecipitation and seed medium growth,i.e.,Yb/Tm:a-NaYF4?CaF2 core-shell nanocrystals.The sensitivity is theoretically evaluated in the temperature region of 313-373 K through the transition groups of 3H40??3H6 with 3H41??3H6,Tm3+:3H41??3H6 with 1G4?3H6,i.e.0.43-0.34%K-1 and 3.8%K-1,respectively.It indicates that this kind of nanocrystals is a promising candidate for biological temperature sensing.3.The 1800 nm emission mainly is realized in bulks which are not suitable forbiological and medical application becausesome rigorous requirements,such as nano size,uniform monodispersed and biocompatiblity.We synthesized uniform monodispersed Yb/Tm:NaYF4 nanocrystals,with size of 20-40 nm,which show strong 1830 nm emissions through 980 nm laser excitation.We further investigated the photoluminescence properties dependent on the concentration of Tm3+ ions,the shell thickness and composition.The results demonstrate that the luminescence of Yb/Tm:NaYF4@Yb:NaYF4 core/shell nanocrystals enhances about 160 times in comparsion with Yb/Tm:NaYF4nanocrystals when the Tm3+ doping concentration and shell thickness are 2 mol%,6.5 nm,respectively. |
PDF Full Text Request