Enhanced red upconversion emission of Ho3+ in NaYF4 nanocrystals

Abstract Hexagonal-phase NaYF 4 :Yb 3+ /Ho 3+ nanocrystals (with size of 20 nm) were successfully prepared by a modified solution-based method. Under 980 nm near- infrared (NIR) excitation, the upconversion (UC) emission was tuned from green to red in NaYF 4 :Yb 3+ /Ho 3+ nanocrystals by introducing Ce 3+ ions. The corresponding red to green (R/G) emission ratio of Ho 3+ increased by approximately 10-fold as the Ce 3+ concentration increased to 14%. However, the overall UC emission intensity gradually decreased. A series of core–shell structures for NaYF 4 :Yb 3+ /Ho 3+ /Ce 3+ @NaYF 4 :Yb 3+ nanocrystals codoped with different Yb 3+ ion concentrations in the shell was successfully fabricated by a sequential growth process to further enhance the red UC emission intensity. When the Yb 3+ concentration from the shell of NaYF 4 :Yb 3+ was increased from 0% to 20%, the UC emission intensity of the core–shell nanocrystals first increased and then decreased. Moreover, the highest UC emission intensity was obtained in the core–shell nanocrystals with 10% Yb 3+ . Experimental results indicated red UC was enhanced due to the resonance energy transfer between adjacent Ho 3+ and Ce 3+ and Yb 3+ and Ho 3+ ions. These nanocrystals with red UC emission have great potential in the biological field and multicolor applications.