Phase transformation and morphology tuning of β-NaYF₄: Yb³⁺, Er³⁺ nanocrystals through K⁺ ions codoping

In this work, a simple method to modulate the crystal phase and morphology with a large amount of K⁺ ions codoping is proposed. The phase changes to the mixture of β-NaYF₄ and β -KYF₄ with increasing the content of K⁺ ions to 80 mol%. When it exceeds 80 mol%, β -NaYF₄ disappears gradually and β -KYF₄ dominates with a poor crystalline. In addition, the morphology changes from nanosphere to nanoplate, and then to nanoprism, which indicates that a higher content of K⁺ ions favors the growth rates along 0001 than the 10-10 of the nanocrystals. Additionally, the upconversion (UC) luminescence properties and the ratio of red/green (R/G) UC intensity of samples with different phases and morphologies are detected, which makes it possible to tune the UC fluorescence by varying the concentration of K⁺ ions.