Room Temperature Colloidal Coating of II-VI Nanoplatelets with Quantum Dots

The low-temperature colloidal production of II-VI nanoplatelet heterostructures has stimulated the interest of researchers due to the possible uses of these materials in various optoelectronic devices. Here, we report a room temperature coating by CdS or ZnS dots of preprepared CdSe nanoplatelets. The dot coating process made use of a synthesis developed for the formation of freestanding CdS and ZnS species, involving injection of a metal precursor into reactive sulfur-amine solutions at room temperature. CdSe structured nanoplatelets with 1.75 nm thickness were used as the core constituent. The structural properties were investigated using Fourier transform infrared spectroscopy and advanced electron microscopy, while the elemental mapping was verified using high-angle annular dark field transmission electron microscopy. The results showed a dots-on-plate structure, resembling an intermediate configuration between core/crown or core/shell heterostructures. A thorough study of optical properties demonstrated a dramatic spectral shift of the absorption edge to lower energies, regardless of the uniformity of the coating layer, maintaining spectral stability over two months while stored at ambient conditions. Other optical measurements included examination of temperature-dependent photoluminescence and transient photoluminescence decay, from room temperature down to similar to 4 K. These measurements revealed excitons, trions, and trapped carrier recombination emission with variable relative intensities following the temperature change. The dots-on-plate structures studied displayed a short photoluminescence decay compatible with that of core/shell (crown) structures prepared at high temperatures. As a result, the presented techniques are alternative pathways that eliminate the requirement for excessive temperatures and/or multistep processes, instead providing rapid, inexpensive, and scalable procedures with practical advantages.