Enhanced upconversion luminescence intensity of core-shell NaYF₄ nanocrystals guided by morphological control

How to further increase the upconversion luminescence (UCL) efficiency of core-shell upconversion nanoparticles (UCNPs) is highly desirable for their photoelectric and bio-logical applications. Herein, a novel but facile strategy is proposed to substantially enhance the UCL intensity of NaYF₄ based core-shell UCNPs by morphological control. The morphologies of core-shell UCNPs can be optimized from rod-like to spherical like by changing the ratio of oleic acid (OA) to 1-octadecene (ODE) during the shell growth process with other reaction conditions constant. The mechanism of shape control is further investigated based on the competitive absorption between OA molecules and lanthanide ions (Y³⁺, Yb³⁺, Er³⁺ or Tm³⁺) onto the different crystal axes (a, b and c) to guide their shell growth speed. The absolute quantum yields were up to 2.7% and 1.8% for spherical and rod like core-shell UCNPs under excitation of 980 nm laser (power density of 1.6 W cm^-2), respectively. Moreover, the UCL intensity and effective lifetime (τ _eff ) of Er³⁺ emission at 541 nm of spherical like core-shell UCNPs increased by 11.7 and 1.82 folds than rod like core-shell UCNPs. Therefore, our designed novel strategy can greatly improve the UCL efficiency of core-shell UCNPs and promote their development in diverse applications.