High purity green upconversion luminescence and temperature sensing in Bi${}_2$Mo${}_2$O${}_9$: Yb${}^{3+}$/Er${}^{3+}$ phosphors

Using a solid-state technique under low temperature conditions, Er${}^{3+}$/Yb${}^{3+}$ co-doped Bi${}_2$Mo${}_2$O${}_9$ phosphors with different concentrations were prepared, exhibiting excellent green up-conversion luminescence property. A thorough investigation was conducted on the phase structure, optical transition, up-conversion luminescence, and down-shifting luminescence properties of the prepared samples. Bi${}_2$Mo${}_2$O${}_9$: Er${}^{3+}$/Yb${}^{3+}$ phosphors excited at 381, 980, and 1550 nm showed strong green fluorescence together with very faint red emission. The mechanism of up-conversion luminescence and its capacity for temperature sensing were examined under the excitation of various wavelengths. Under 980 nm excitation, the maximal relative and absolute sensitivities were determined to be 1.107%K${}^{-1}$ and 0.756%K${}^{-1}$ at 303 K and 513 K, respectively. Furthermore, the Judd-Ofelt intensity parameters of Er${}^{3+}$ in Bi${}_2$Mo${}_2$O${}_9$: Er${}^{3+}$/Yb${}^{3+}$ phosphors were calculated using the diffuse reflectance spectra and fluorescence lifetime of Er${}^{3+}$. In addition, material was combined with polyacrylic acid to make an anti-counterfeiting pattern. The results indicate that Bi${}_2$Mo${}_2$O${}_9$: Er${}^{3+}$/Yb${}^{3+}$ phosphor is an excellent green phosphor with optical temperature measurement function, and it also has certain applications in the field of anti-counterfeiting.