Vibronically Coupled Near-Infrared Emission and Excitation from d−d Transitions of Cs₂MX₆ (M = Mo/W, X = Cl/Br)

Abstract

In near-infrared (NIR)-emitting phosphors, the emission typically originates from forbidden d−d or f−f electronic transitions of metal ion dopants. But the excitation happens through higher-energy (UV or blue) allowed transitions, resulting in energy loss for this UV- or blue-to-NIR conversion. Here we report Cs2MX6 (M = Mo/W, X = Cl/Br) 0D perovskite derivatives with NIR excitation and emission arising from the same pair of d-electronic states, showing small Stokes shifts. The samples show significant optical absorption at 787 nm (12700 cm−1) and emission at 986 nm (10140 cm−1) because of the d 2 electronic configuration of M4+ in isolated [MX6]2− octahedra. Interestingly, isolated [MX6]2− also leads to vibrational fine structures in the electronic excitation and emission spectra at cryogenic temperatures, which are rare for undoped inorganic crystals. Finally, phosphor-converted light-emitting diodes are fabricated by coating Cs2MoCl6 on commercial 730 nm LED chips without requiring UV-blue chips.

Broad Dual Emission from Cs2Zr1–xMoxCl6: Enhancing the NIR Emission in Lead-free Vacancy Ordered Double Perovskites

“…This can be assigned to the normalE normalg 1 3 [ ( normald x y ) 1 ( normald x z , y z ) 1 ] → normalA normalg 1 1 [ ( normald x y ) 2 ] transition as previously reported for various transition metal (Mo 4+ , Tc 5+ , Os 6+ , and Re 5+ ) based complexes having d 2 configuration. − This explanation for NIR emission differs from that in previous work, which attribute these NIR emission to STEs with a large stoke shift . However, this broad NIR emission is assigned to vibronically coupled d–d transition of Mo 4+ in recently published reports by Nag and co-workers . The photoluminescence excitation (PLE) spectra for the dopant-induced emission of Mo:Cs 2 ZrCl 6 are shown in Figure b.…”