Figure 4.

HRTEM images (×500 K) of (a) LiYF₄:Yb³⁺_0.25,Tm³⁺_0.01 core UCNP and (b) LiYF₄:Yb³⁺_0.25,Tm³⁺_0.01@LiYF₄:Yb³⁺_0.2 core/shell UCNP. (c) HRTEM image (×300 K) of the single-crystalline structure of the LiYF₄:Yb³⁺_0.25,Tm³⁺_0.01@LiYF₄:Yb³⁺_0.2 core/shell UCNP. (d) SAED pattern of the LiYF₄:Yb³⁺_0.25,Tm³⁺_0.01@LiYF₄:Yb³⁺_0.2 core/shell UCNP.