Abstract
Hexagonal-phase NaYF4:Yb3+/Ho3+ 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 NaYF4:Yb3+/Ho3+ nanocrystals by introducing Ce3+ ions. The corresponding red to green (R/G) emission ratio of Ho3+ increased by approximately 10-fold as the Ce3+ concentration increased to 14%. However, the overall UC emission intensity gradually decreased. A series of core–shell structures for NaYF4:Yb3+/Ho3+/Ce3+@NaYF4:Yb3+ nanocrystals codoped with different Yb3+ ion concentrations in the shell was successfully fabricated by a sequential growth process to further enhance the red UC emission intensity. When the Yb3+ concentration from the shell of NaYF4:Yb3+ 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% Yb3+. Experimental results indicated red UC was enhanced due to the resonance energy transfer between adjacent Ho3+ and Ce3+ and Yb3+ and Ho3+ ions. These nanocrystals with red UC emission have great potential in the biological field and multicolor applications.
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