β-NaYF4:Yb, Er at β-NaYF4 core/shell nanocrystals with significantly enhanced upconversion fluorescence by a successive two-step hot-injection approach

Surface passivation is a general approach to enhance the luminescence of small-scaled size upconversion nanomaterials. In this reported work, a two-step hot-injection method was developed to synthesise core/shell structured NaYF₄. A transmission electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy were employed to prove the successful fabrication of β-NaYF₄:Yb, Er at β-NaYF₄. Core size and shell thickness were controlled by the amount of stabiliser and precursors addition, respectively. The upconversion efficiency of the resultant core/shell structured nanocrystals could be improved more than a 100-fold by comparison with the original core nanocrystals. Significant fluorescence enhancement was ascribed to the successive injection of precursors at elevated temperature.