Intrinsic Chiroptical Activity and Excitonic Properties of Group II-VI Magic-Size Clusters.

Semiconductor magic-size clusters (MSCs), lying in the local minima of the potential landscape, are important intermediates that emerge during the synthesis of colloidal quantum dots. They have definite geometrical and electronic structures, thus serving as atomically precise building blocks for assembling supramolecular structures and devices with unprecedented functionalities. Here we report the intrinsic chiroptical activity in the magic-size cadmium and zinc chalcogenide clusters with magic numbers of 13, 33, and 34 possessing unique core-shell structures. They are responsive to circularly polarized light from the ultraviolet to visible region, with size-tunable energy gap, absorption wavelength, and excitonic characteristics. The origin of the chiroptical activity and the evolution of excitonic states with magic size are disclosed by time-dependent density functional theory calculations within a correlated electron-hole picture. This molecular-level understanding of the photophysical properties of group II-VI MSCs provides essential guidelines for utilizing them for chiral optoelectronics and photonics.