Salt-assisted rapid transformation of NaYF4:Yb3+,Er3+ nanocrystals from cubic to hexagonal

Abstract

High-quality hexagonal upconversion nanocrystals of β-NaYF₄:Yb³⁺,Er³⁺ were prepared through a one-pot mild solvothermal synthesis. The crystal structure of NaYF₄:Yb³⁺,Er³⁺ can rapidly transform from cubic (α-) to hexagonal (β-)phase in the presence of salts, such as sodium hydrogen phosphate (Na₂HPO₄). It was found that the size and structure of as-prepared NaYF₄:Yb³⁺,Er³⁺ can be controlled by changing the molar ratio of phosphate to Ln³⁺ (Ln³⁺ represents the total amount of Y³⁺ and the doped rare earth elements such as Yb³⁺, Er³⁺). A possible formation mechanism was proposed on the basis of XRD analyses and TEM observations of the products under different reaction conditions. The upconversion fluorescence at 550 nm of β-NaYF₄:Yb³⁺,Er³⁺ nanocrystals with hexagonal structure and narrow size distribution was enhanced up to 22.5-fold compared to the cubic nanocrystals (α-NaYF₄:Yb³⁺,Er³⁺). This study is also expected to provide a new synthetic strategy for the exploration of other complexes with controllable structures and optical properties.