Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects

Size/shape-controlled colloidal CdSe quantum disks with zinc-blende (cubic) crystal structure were synthesized using air-stable and generic starting materials. The colloidal CdSe quantum disks were approximately square, and their lateral dimensions were varied between 20 and 100 nm with the thickness controlled between 1 and 3 nm, which resulted in sharp and blue-shifted UV-vis and PL peaks due to one-dimensional quantum confinement. The quantum disks were grown with either <001> or <111> direction, polar directions in the single crystalline disks, as the short axis, and both basal planes were terminated with Cd ions. These surface Cd ions were passivated with negatively charged fatty acid ligands to neutralize the net positive charges caused by the excess monolayer of Cd ions. The coordination of the Cd ions and carboxylate groups further enabled the close-packing monolayer of fatty acid ligands on each basal plane. The close packing of the hydrocarbon chains of fatty acids dictated the up temperature limit for synthesis of the colloidal quantum disks, and the low temperature limit was found to be related to the reactivity of the starting materials. Overall, a high Cd to Se precursor ratio, negative-charged fatty acid ligands with a long hydrocarbon chain, and a proper temperature range (approximately between 140 and 250 °C) were found to be needed for successful synthesis of the colloidal CdSe quantum disks.