Systematic and Extensive Emission Tuning of Highly Efficient Cu–In–S-Based Quantum Dots from Visible to Near Infrared

Group I–III–VI chalcogenides are emerging candidates for the synthesis of efficient quantum dot (QD) emitters, particularly since they are free from environmentally harmful substances such as Cd, Pb, and As. Among them, Cu–In–S (CIS) and Cu–In–Se1–x–Sx (CISeS) are the most common compositions as visible and near-infrared (NIR) QD emitters, respectively. We herein explore efficient synthetic pathways to demonstrate extensively emission-tuned CIS QDs from visible to NIR with high photoluminescence quantum yields (PL QYs) of over 70%. To systematically tune PL, synthetic parameters of CIS core QDs are varied such as Cu/In molar ratio, core growth condition, Ag alloying, and In precursor change, whereas a highly reactive elemental sulfur is commonly adopted for core growth. Starting from visible CIS/ZnS QDs, whose emission is tuned in green (534 nm) to red (625 nm), depending on the Cu/In ratio, their emissions gradually shift in the PL peak to 744 nm by controlling the core growth condition to 806 nm by alloying with Ag and further to 868 nm by switching an In salt precursor from In acetate to In iodide. These NIR-emitting QDs, particularly those having PL peaks longer than 800 nm, possess excellent QYs of 81–91%, which are the record values among deep NIR-emitting I–III–VI QDs to date. To enhance the QD stability against environmental stimuli, Al doping into Zn shell is implemented on 868 nm emitting CIS/ZnS QDs, resulting in exceptional photostability under prolonged UV irradiation exposure. These highly luminescent, photostable NIR-emitting CIS/ZnS QDs will be attractive candidates for further application as fluorophores in luminescent solar concentrator and in vivo bioimaging.