Rare-earth Upconversion Nanocrystal-based Luminescence Color-tuning

In recent years,the unique optical properties of upconversion nanocrystals doped with rare earth elements,including large anti-Stokes shifts,sharp emission peaks,and low toxicity,have enabled them to show a wide range of applications in security labeling,3D displays,and in-vivo imaging of living organisms.In particular,the excellent engineering properties of rare earth-doped fluoride upconversion nanocrystals allow one to design specific energy transfer schemes in them,allowing different nanocrystals to emit different colors and providing a wealth of alternative colors.These rare-earth-doped fluoride nanocrystals with different colors of upconversion fluoresce nce are particularly suitable for multichannel detection and multicolor imaging.Therefore,the development of rare-earth upconversion nanocrystals with high brightness and rich colors is of great application.Lanthanide activators have a stepped arrangement of energy-levels.Lanthanide activators have stepped energy levels,so a single activator can usually produce a group of emission bands with wavelengths ranging from ultraviolet to near infrared.Rich luminescence colors can be achieved by adjusting the doping components to induce different luminescence paths.However,most of the color adjustments are multi-layer core-shell structure,and the manufacturing process is cumbersome and time-consuming.Therefore,the focus of this paper is to achieve a wide range of color-tuning of nanocrystals through a relatively simple core-shell structure.The research contents of this paper are as follows:（1）We prepared four kinds of heavily doped rare earth element Er/Yb nanocrystals with different proportions.Among the four groups of nanocrystals,the most appropriate proportion of two-color upconversion nanocrystals was optimized,which was 20Er/80Yb.Under the excitation of 980 and 1530 nm,red and green dominated upconversion luminescence could be generated respectively,the ⁴F_9/2→⁴I_15/2 transition corresponding to Er³⁺ion produces red emission of about 650 nm;the ²H_11/2,⁴S_3/2→⁴I_15/2 transition produces green emission of about540 nm.（2）A trace amount of Tm³⁺is further introduced into the above known 20Er/80Yb sample as the energy capture center,the energy transfer between Tm³⁺and Er³⁺inhibits the luminescence quenching caused by Er³⁺energy migration.In 20Er/80Yb system,980 nm excitation is a good excitation source for Yb³⁺.At the same time,found that the 1530 nm pump source showed great potential,and the luminescence color and brightness of pumped Er³⁺ion doped core-shell nanocrystals were improved.At the same time,the color tunability,temperature and power stability of the core-shell nanocrystals were tested through a series of experiments.（3）Finally,in the heavily doped Er/Yb system,it is also found that the inert layer can not only significantly improve the luminescent brightness of nanocrystals,but also affect their luminescent color.Therefore,based on Yb³⁺/Er³⁺heavily doped fluoride nanocrystals,the effects of inert layer on up conversion luminescence efficiency and color were studied.The influence mechanism of inert layer in two extreme cases of high Er low Yb and low Er high Yb is discussed through spectral data,and experimental verification is carried out,which provides a reference for the construction of high brightness polychromatic upconversion nanocrystals materials in the future.