Designing Er³⁺ Single-doped ternary sulfide for highly efficient upconversion luminescence under 1550 nm excitation

Optimizing the host lattice is the most efficient strategy to break through the intrinsic characteristic of low efficiency up-conversion luminescence (UCL). In principle, ternary sulfide, e.g. NaYS2, is an ideal UCL host matrix due to its low phonon energy and highly asymmetrical crystal structure. However, it was found that the low lying S2--> Yb3+ charge transfer state (CTS) existing in the ternary sulfide material severely quenches the UCL under 980 nm excitation, which generally employs Yb3+ ion as the conventional sensitizer. Herein, we design Er3+ single-doped NaYS2 phosphors for highly efficient UCL under 1550 nm excitation. This novel NaYS2:Er3+ phosphor excited at 1550 nm exhibits a 94-fold enhancement of UCL intensities compared to that of conventional NaYS2:Yb3+,Er3+ under 980 nm excitation. More encouragingly, compared to the commercial ss-NaYF4:Yb3+,Er3+ phosphors, the UCL efficiency and brightness of NaYS2:Er3+ excited at 1550 nm are twice that of ss-NaYF4:Yb3+, Er3+ excited at 980 nm. Such extraordinary efficient UCL is accomplished by taking advantages of the intrinsic low phonon energy (279 cm 1) of the NaYS2 host and the highly asymmetrical trigonal crystal system. Furthermore, in the Er3+ single-doped NaYS2 system, the S2-.Er3+ CTS lies at a high state of approximate to 34500 cm(-1), which will not quench the efficient UCL from Er3+ ions. The large absorption cross-section at I-4(13/2) energy level (high response to 1550 nm radiation) and long lifetime of the metastable state of I-4(9/2) also contribute to the population of the emission levels (H-2(11/2)/S-4(3/2)). These findings provide a new class of host materials for achieving highly efficient UCL.