Aqueous route to color-tunable Mn-doped ZnS quantum dots

Mn-doped zinc sulfide (Mn:ZnS) quantum dots stabilized by 3-mercaptopropionic acid (MPA) were synthesized at 100 °C in basic aqueous solution using the nucleation-doping strategy. The optical properties and structure of the obtained Mn:ZnS QDs have been characterized by UV–vis, photoluminescence (PL) and time-resolved PL spectroscopies, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron spin resonance (ESR). The obtained nearly monodisperse Mn:ZnS@MPA QDs have an average diameter of ca. 2.5 nm and a zinc-blende crystal structure. By varying the base (LiOH, NaOH, KOH, CsOH) used to deprotonate the MPA ligand and adjust the pH of the aqueous solution to 11, the PL emission wavelengths can be tuned within a relatively large optical window, from 567 to 594 nm. The variations of charge density near the surface of the QDs obtained by changing the cation associated to the MPA ligand and of the dopant location in the ZnS host are at the origin of the PL shifts observed. In order to improve the PL emission efficiency, a ZnS shell was subsequently overcoated around the Mn:ZnS core nanocrystals. With ZnS shell growth, the PL emission wavelength was restricted between 570 and 583 nm but PL quantum efficiency of Mn:ZnS/ZnS core/shell QDs increased up to 18.4%.