Uniform Thickness and Colloidal-Stable CdS Quantum Disks with Tunable Thickness: Synthesis and Properties

Uniform thickness and colloidal-stable CdS quantum disks have been reproducibly prepared using cadmium acetate, elemental sulfur, fatty acids and octadecene as the starting materials without any size/shape sorting. The thickness could be varied between 1.2 and 2.2 nm, i.e., 4.5, 5.5, 6.5 and 7.5 monolayers of CdS along the thickness direction. These single crystalline disks with lateral dimensions between 20 and 100 nm adopted the zinc blende crystal structure with < 100 > (possibly mixed with < 111 >) as the thickness direction. The basal planes and side facets were terminated with cadmium carboxylates, which dictated the thicknesses to be half a monolayer more than an integer number. Formation of CdS quantum disks probably occurs through a nucleation-growth mechanism, instead of aggregation of pre-formed magic clusters. Completion of a full monolayer along the lateral direction was found to be rather fast if two-dimensional nucleation was initiated on existing disks, which helped formation of atomically flat and thickness-controlled disks. As disk thickness decreased, the crystal lattice was found to dilate gradually, which has not been observed with CdS quantum dots. Compared with CdS quantum dots and rods, the disks displayed weakened quantum confinement and their photoluminescence lifetime (tens of picoseconds) was about two orders of magnitude shorter.