Design And Applicartions Of Rare Earth Doped Core-shell Upconversion Nanoparticles

Rare earth doped upconversion nanoparticles have drawn much attention in optical field due to their large anti-Stokes' shift,small emission bandwidth,long luminescence lifetimes,high chemical stability,photostability,and low toxicity.However,their practical applications have been largely limited by the low quantum efficiency.Core-shell structures which can eliminate quenching from surrounding environments by spatial separation are used to suppress surface-related deactivations,and to change the energy transfer process.So far,few reports have thoroughly discussed the working mechanisms of core-shell structures.Here in this paper,the mechanism and application of core-shell structures are studied in the following three aspects:First,we prepared NaYF4:20%Yb3+,2%Er3+@NaYF4 with varying shell thickness and discussed the effect of shell on the upconversion properties of the particles.With the increase of shell thickness,the quantum efficiency becomes higher.According to the power-dependence measurement,it was found that transition from 4F9/2 energy level corresponding to red emission was populated after relaxation from 4G11/2/2H9/2 rather than 2Hn11/2/4S3/2.Second,NaYF4:20%Yb3+,x%Er3+@NaYF4(x=2,4,6,8)nanocrystals were synthesized to examine the dependence of upconversion properties on the dopant concentration in core-shell structure.The NaYF4 shell layer protects the lanthanide ions against quenching by surface groups,leading to an increase of Er3+ quenching concentration in P-NaYF4 core-shell nanocrystals from 2 mol.%to 6 mol.%.This result was also confirmed by the single particle measurement.Finally,NaYF4:20%Yb3+,2%Er3+ and NaYF4:20%Yb3+,2%Er3+@NaYF4 nanocrystals were prepared and used for ratio-metric temperature sensing.The results show that shell protection increases the thermal stability without changing the temperature sensitivity.