Ion-redistribution induced efficient upconversion in β-NaYF:20%Yb,2%Er microcrystals with well controlled morphology and size.

We develop an efficient green upconversion (UC) β-NaYF:20%Yb,2%Er microcrystal with well controlled morphology and size by hydrothermal method using two different chelating agents of CIT and EDTA-2Na via a simple ion-exchange reaction. Importantly, the UC emission efficiency of newly developed CIT and EDTA-2Na β-NaYF:20%Yb,2%Er microcrystals is almost as strong as that of commercial counterpart by solid-state method. A proof-of-concept β-NaYF:20%Yb,2%Er microcrystal waveguide is demonstrated to extend their applications in modern micro-optoelectronics. The local ion-redistribution process during the ion-exchange reaction, which effectively disperses the locally clustered Yb, accounts for the enormously enhanced UC emission in β-NaYF:20%Yb,2%Er microcrystals.