Revealing the role of a unique local structure in lanthanide-doped Cs2LiInCl6 in boosting visible and NIR-II luminescence

Abstract

Local structure engineering is one of the most useful strategies for tunable down-shifting and upconversion emissions in halide double perovskites (DPs). However, the roles of the local structure, including local symmetry, coordination structure, and active sites, in these DPs remain largely unexplored. Herein, we studied the local structure-correlated up/down-conversion luminescence in Yb³⁺/Er³⁺/Ho³⁺ co/tri-doped Cs₂LiInCl₆ with a unique local structure, which is different from its typical Cs₂NaInCl₆ counterpart. At optimal concentrations, the emission intensities of Cs₂LiInCl₆:Yb³⁺,Er³⁺ were 6 and 110 times higher than those of Cs₂NaInCl₆ at 552 and 1540 nm, respectively. The evolution of the local site symmetry calculated by the first-principles density functional theory demonstrates that the lower local symmetry of Cs₂LiInCl₆:Yb³⁺,Er³⁺ is responsible for luminescence enhancement at a low-doping level of Er³⁺. In addition, we verified that the concentration quenching effect can be effectively suppressed by two independent In³⁺ sites in Cs₂LiInCl₆, leading to increased visible and near-infrared (NIR-II) emissions close to the optimal concentration with higher Er³⁺ contents. Moreover, time-resolved photoluminescence (PL) measurements and steady-state rate equations are used to thoroughly understand the mechanism underlying the local-structure-dependent PL properties. Our results demonstrate that both the local symmetry breaking and presence of multiple independent active sites are beneficial for luminescence enhancement, and accordingly provide meaningful guidance for designing highly efficient visible and NIR-II emitters.