Figure 2

SEM images of Y³⁺-absent NaLuF₄:Yb³⁺, Tm³⁺ nano/micro-crystals prepared at different reaction temperatures for 12 h. (a–e) Refer to 110 °C, 130 °C, 150 °C, 180 °C, and 200 °C, respectively. Scale bars are 200 nm for (a), and 5 μm for (b–e).