Lanthanide ions (Ln3+) doping provides a potential strategy to control over the luminescent properties of lead-free halide double perovskite nanocrystals (DP NCs). However, due to the low energy transfer efficiency between self-trapped exciton (STE) and Ln3+ ions, the characteristic emissions of Ln3+ ions are not prominent. Furthermore, the energy transfer mechanism between STE and Ln3+ ions is also elusive and requires in-depth study. We chose trace Bi3+-doped Cs2Ag0.6Na0.4InCl6–xBrx as a representative DP matrix to demonstrate that by tuning the bromide concentration, the Ln3+ emission can be greatly enhanced. Such enhanced STE and Ln3+ ions energy transfer originates from the high covalency of Ln–Br bond, which contributes to improvement of the characteristic emission of Ln3+ ions. Furthermore, optical spectroscopy reveals that the energy transfer mechanism from DP to Eu3+ ions is different from all the other doped Ln3+ ions. The energy transfer from DP to Eu3+ ions is mostly through Eu–Br charge transfer while the other Ln3+ ions are excited by energy transfer from STE. The distinct energy transfer mechanism has resulted from the energy separation between the excited energy level of Ln3+ ions and the bottom of conduction band of DP. With increasing the energy separation, the energy transfer from STE to Ln3+ ions is less efficient because of the generation of a larger number of phonons and finally becomes impossible for Eu3+ ions. Our results provide new insight into tuning the energy transfer of Ln3+-doped DP NCs.

中文翻译：

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通过混合卤化物提高 Cs2Ag0.6Na0.4InCl6–xBrx 双钙钛矿中镧系元素的特征发射


镧系元素离子 （Ln3+） 掺杂提供了一种潜在的策略来控制无铅卤化物双钙钛矿纳米晶体 （DP NCs） 的发光特性。然而，由于自捕获激子 （STE） 和 Ln3+ 离子之间的能量转移效率低，因此 Ln3+ 离子的特征发射并不突出。此外，STE 和 Ln3+ 离子之间的能量转移机制也是难以捉摸的，需要深入研究。我们选择痕量 Bi3 + 掺杂的 Cs2Ag0.6Na0.4InCl 6 – xBrx 作为代表性的 DP 基质，以证明通过调整溴化物浓度，可以大大增强 Ln 3 + 发射。这种增强的 STE 和 Ln3+ 离子能量转移源于 Ln-Br 键的高共价性，这有助于改善 Ln3+ 离子的特征发射。此外，光谱学揭示了从 DP 到 Eu3+ 离子的能量转移机制与所有其他掺杂的 Ln3+ 离子不同。从 DP 到 Eu3+ 离子的能量转移主要通过 Eu-Br 电荷转移，而其他 Ln3+ 离子则被来自 STE 的能量转移激发。独特的能量转移机制是由 Ln3+ 离子的激发能级与 DP 的导带底部之间的能量分离产生的。随着能量分离的增加，由于 STE 产生更多的声子，从 STE 到 Ln3+ 离子的能量转移效率降低，最终对 Eu3+ 离子来说变得不可能。我们的结果为调整 Ln3+ 掺杂 DP NCs 的能量转移提供了新的见解。