Controlled impurity doping of wide-bandgap semiconductors can be used to introduce color centers, which are point defects that activate sub-bandgap, optical photoluminescence (PL). Color centers can serve as sources of tunable PL for bioimaging and optoelectronic applications, as well as localized, optically addressable, electronic spin states for applications in quantum information science. For all ofthese applications, colloidal nanocrystals (NCs) can provide advantages over analogous, bulk materials because they can be processed using wet-chemical methods, and their large surface areas and effects of quantum confinement allow for highly tunable optical and electronic properties.

Copper-doped zinc sulfide (ZnS:Cu) exhibits down-conversion luminescence in the UV, visible, and IR; the visible red, green, and blue emission is referred to as R-Cu, G-Cu, and B-Cu, respectively. The sub-bandgap emission arises from optical transitions between localized electronic states created by point defects, making ZnS:Cu a prolific phosphor material and an intriguing candidate material for quantum information science, where point defects excel as single-photon sources and spin qubits. Colloidal nanocrystals (NCs) of ZnS:Cu are particularly interesting as hosts for the creation, isolation, and measurement of quantum defects, since their size, composition, and surface chemistry can be precisely tailored for biosensing and optoelectronic applications.

Here, a method is presented for synthesizing colloidal ZnS:Cu NCs that emit primarily R-Cu, which has been proposed to arise from the Cu_Zn-V_S complex, an impurity vacancy point defect structure  analogous to well-known quantum defects in other materials that produce favorable optical and spin dynamics. Understanding of R-Cu emission dynamics, combined with a controlled synthesis method for obtaining R-Cu centers in colloidal NC hosts, will greatly facilitate the development of Cu_Zn-V_S and related complexes as quantum point defects in ZnS.