Homo–hetero/core–shell structure design strategy of NaYF4 nanocrystals for superior upconversion luminescence

Abstract

A comprehensive strategy has been developed to construct nano-sized homogeneous and heterogeneous core/shell structures of NaYF₄ host. Synthesis conditions of cubic phase/α-NaYF₄ and hexagonal phase/β-NaYF₄ are discussed. Pure cubic NaYF₄:Yb,Er nanocrystals were synthesized with different average sizes extending from 7 nm to 15 nm by varying the reaction time. Temperature and time thresholds of hexagonal nucleation were determined and utilized for controlled core/shell structures of different phases. α-NaYF₄:Yb,Er@α-NaYF₄, α-NaYF₄:Yb,Er@β-NaYF₄, β-NaYF₄:Yb,Er@α-NaYF₄, and β-NaYF₄:Yb,Er@β-NaYF₄ core/shell structures were prepared by adopting the required conditions to achieve the desired phase. Excess sodium was used to grow hexagonal shell over metastable cubic core under controlled conditions of reaction time and temperature to prevent the structural transition of the core. Upconversion emission spectra have also been obtained. UCL integrated intensities demonstrated about 5-fold enhancement for α-shell over α-core as compared to the core alone and 22-fold enhancement with β-shell. On the other hand, α-shell over β-core exhibited 5-fold enhancement and β-shell over β-core exhibited 6-fold enhancement.