Bright and Water-Soluble Near IR-Emitting CdSe/CdTe/ZnSe Type-II/Type-I Nanocrystals, Tuning the Efficiency and Stability by Growth

Highly photoluminescent CdSe/CdTe/ZnSe type-II/type-I composite nanocrystals, both dot- and peanut-shaped, were prepared via the modified successive ionic layer adsorption and reaction (SILAR) techniques, straight SILAR for peanut-shaped ones and SILAR coupled with thermal-cycling (SILAR-TC) for dot-shaped ones. The CdSe/CdTe type-II heterojunction offered the nanocrystals with near-infrared emission and the CdTe/ZnSe type-I heterojunction helped to confine the photogenerated charges away from the ligands and solution environment. This structural feature makes the photoluminescence quantum yield of the CdSe/CdTe/ZnSe core/shell/shell type-II/type-I dots that have a uniformly grown ZnSe shell retain as high as 60% after replacing the original amine ligands with mercaptopropionic acid (MPA). Conversely, the emission of the corresponding CdSe/CdTe core/shell dots (CdSe/CdTe/ZnSe composite peanuts) was completely (almost completely) quenched by the same ligand treatment. The emission properties of the MPA-coated CdSe/CdTe/ZnSe core/shell/shell dots were stable in water in the buffer solutions with their pH in a range between about 5 and 9.