Zero Phonon Line with Enhanced Debye–Waller Factor and Förster Resonance Energy Transfer Assisted Tunable Broadband NIR Emissions in (Cr3+, Mo4+):Cs4CdSb2Cl12

Abstract Near-infrared-II emitting materials are of great interest for optoelectronic and photonic technologies due to their superior tissue penetration and low photon scattering. Here, we report photoluminescence studies of Cr3+-doped, and (Cr3+, Mo4+)-co-doped Cs4CdSb2Cl12 layered halide double perovskites. At 93 K, both systems exhibited a sharp zero phonon line (ZPL) from 4T2 → 4A2 transition of Cr3+ center. Mo4+ codoping reduces the full width at half-maximum (fwhm) of ZPL from 1.9 to 1.2 nm, while enhancing Debye–Waller factor (FDW) from 0.11 to 0.29. At room temperature, Cr3+:Cs4CdSb2Cl12 shows broadband NIR-I emission at 757 nm, while Mo4+:Cs4CdSb2Cl12 exhibits weak NIR-II emission at 905 nm. In the codoped system, efficient Cr3+→Mo4+ Förster resonance energy transfer significantly enhances NIR-II emission, thermally stable up to 483 K. Furthermore, a sharp ZPL at 728 nm (below 273 K), with a large FDW and a multimodal emission spanning the visible to NIR-II region at 300 K, was observed.