Relatively Bright Intraband 5 μm Photoluminescence from HgSe/CdS Quantum Dot Films after Lead Halide Treatment

The 5 μm intraband photoluminescence (PL) from 200 nm thin films of n-doped mercury selenide/cadmium sulfide (HgSe/CdS) core/shell colloidal quantum dots reaches 0.2% apparent external efficiency, after solid-state ligand exchange with a solution of lead iodide (PbI₂) and annealing at 140 °C. The PbI₂ treatment removes 95% of the C-H vibration absorption, but the PL increase is correlated with improved n-doping and not with organic ligand removal. The integrated emission intensity increases only by 20% upon cooling from 423 to 85 K. This suggests that multiphonon relaxation is not limiting the PL and that brighter emission should still be possible. PbI₂ treatment gave the brightest films, but PbBr₂ and PbCl₂ have similar effects.