Single-step synthesis of quantum dots with chemical composition gradients

We demonstrate a single-step synthetic method for highly luminescent (i.e., quantum yield up to 80%) and stable quantum dots (QDs) by using the reactivity difference between Cd and Zn precursors and that between Se and S precursors. A wide range of emission wavelengths (500-610 nm) with a narrow fwhm (< 35 nm) is obtained by changing the ratios of the precursors. Under the reaction conditions selected, Cd- and Se (with a bit of S)-based cores are formed first and Zn- and S-based shells are formed successively; therefore, the QDs have a core/shell structure with composition gradients, which relieve the lattice mismatch between core and shells. The QDs are characterized using the combined techniques of HR-TEM, UV-vis, PL spectroscopy, and ICP-AES. The QDs also have energy gradients depending on their compositions in a radial direction, which energetically confine carriers (electrons and holes) to the cores. This leads to the stability of QDs during their surface passivation from oleic acid to mercaptopropionic acid and ensures their processibility for further purposes such as optoelectronic and biological applications.