V3+ Doping in Cs2NaInCl6 Double Perovskite for Near-Infrared-II Luminescence

Abstract

V3+ with 3d2 electrons is a promising near-infrared (NIR) emitter because of their d–d electronic transitions. But V3+ is prone to further oxidation in ambient conditions, complicating the spectral features. Here we report a simple hydrothermal synthesis of V-doped Cs2NaInCl6 in the reducing environment of H3PO2, resulting into ambient-stable V3+ dopants. Optical excitation and emission spectra are explained by using the Tanabe–Sugano diagram of d2 electrons in [VCl6]3– octahedral unit. At 300 K, a broad NIR-II emission at ∼1225 nm, with a lifetime of 195 μs, is observed owing to the spin-allowed but Laporte-forbidden 3T2g → 3T1gd–d transition. The spectral features change dramatically below 125 K, when a sharp emission at ∼1063 nm with a long lifetime of 19.26 ms is observed. The sharp emission originates from 1Eg/1T2g → 3T1g intraconfiguration spin-flip transition. The presented synthesis and spectroscopic insights will aid future development of V3+ as an NIR-II optoelectronic material.