Multi-mode up-conversion emission of Tm@Yb@Er phosphors for color modulation and thermometry

Rare earth doped up-conversion materials play an important role in information security, colorful display and temperature sensing. In this work, the up-conversion luminescence of NaYF ₄:0.2Tm/20Yb@NaYF ₄:20Yb@NaYF ₄:2Er/20Yb phosphors, synthesized by typical co-precipitation method, are investigated under different excitation condition. The emission color of sample can be tuned from yellow to blue-green through changing pump laser from 1550 to 980 nm because the Tm ³⁺ ions in the core cannot be excited by 1550 nm laser. More interesting, the value of red-to-green (R/G) and blue-to-green (B/G) can be finely manipulated by tuning the excitation pulse width and frequency. This phenomenon can be interpreted by different rising processes of the green, red and blue up-conversion emission. In addition, the core-shell structured sample displays a temperature-dependent color change, ranging from blue-green to purple. The green emissions, radiated from thermal coupled levels ( ²H _11/2 ⁴S _3/2), are used in thermometry and the max relatively sensing sensitivity reaches 0.01 K ⁻¹ at 303 K. The findings indicate that the core-shell Tm@Yb@Er phosphors, with excitation-dependent and temperature-dependent up-conversion luminescence, have potential applications in information security and temperature sensing.