Intra-Configurational Spin-Flip d→$ \to $d Transition of Mo (III) Doped Perovskite for Ultra-Narrow Near Infrared-II Emission in Ambient Conditions

Abstract

Sharp near-infrared-II (NIR-II) emissions are typically achieved through electronic transitions of rare-earth ions, while transitions in transition metal ions are broad due to electron-ligand interactions. An exception is the intra-configurational spin-flip (ICSF) transition like t2g 3 t2g 3 of Mo3+ emitting sharp NIR-II emission, but only at cryogenic temperatures under vacuum. The high oxophilicity of Mo3+ created defects during the synthesis, quenching the emission at room temperature. Herein, we overcome this issue by synthesizing Mo3+- doped Cs2NaInCl6 double perovskites in a reducing H3PO2 environment. [MoCl6]3- octahedra are formed, exhibiting ultra-narrow ICSF (2T1g/2Eg 4A2g) NIR-II emission at 1095 nm in ambient conditions. In addition, a second ICSF 2T2g 4A2g emission is observed at 700 nm, violating the Kasha's rule. The intensity of ICSF emissions increase with increasing temperature (7-350 K) due to vibronic coupling relaxing the Laporte selection rule. The samples are stable for more than 6 months in ambient conditions, allowing for a detailed study of fundamental photophysics and fabrications of phosphor-converted light emitting diodes. This is the first Mo3+-based NIR-II optoelectronic device, opening opportunities for applications like optical fibers and lasing.