Construction of active-inert core–shell structured nanocrystals for broad range multicolor upconversion luminescence

Rare earth doped up-conversion luminescent nano-materials exhibit abundant emission colors under suitable excitation condition. In this work, NaYF₄:Er/Ho@NaYF₄ and NaYbF₄:Tm@NaYF₄ nanoparticles were synthesized by co-precipitation method. The pure red emission can be realized by the designed NaYF₄:Er/Ho@NaYF₄ nanocrystals and the R/Gs reach 23.3 and 25 under excitations of 980 and 1550 nm lasers, respectively. The R/G declines as the power increasing with the emission color changing from red to yellow, which is due to the quick saturation of the energy levels, radiating red emissions. Meanwhile, the emission intensity of NaYbF₄:Tm@NaYF₄ nanocrystals increases by 58.3 folds after encasing the inert shell NaYF₄ and the CIE color coordinate reaches (0.1646, 0.0602) under 980 nm laser excitation. Furthermore, broad range multicolor from blue to red and yellow up-conversion emissions is achieved by mixing NaYF₄:Er/Ho@NaYF₄ and NaYbF₄:Tm@NaYF₄ nanocrystals, which could be applied to colorful displaying, security anti-counterfeiting and information coding.