Excitonic Character in Optical Properties of Tetrahedral CdX (X = S, Se, Te) Clusters

Cadmium-based quantum dots incorporating group 16 elements show promise of superior optical properties. The excitonic properties of three small size CdX (X = S, Se, Te) tetrahedral clusters with structures mirroring the real chemical systems (Cd₁₇X₄(XH)₂₈Na₂, Cd₃₂X₁₄(XH)₄₀Na₄, Cd₅₄X₃₂(XH)₅₂Na₈) were investigated by density functional theory (DFT) in conjunction with quasi-particle corrections. Hallmarks of these systems' specific excitation properties were resolved with the Bethe-Salpeter equation (BSE) approach. Results showed that the strong electron-hole coupling leads to the exciton state, which strongly modulates the optical properties of CdX clusters. The lowest excitonic excitations observed involve the mixing of multiple single-level transitions, while the size-dependent exciton binding energy exhibits power-law scaling characteristics. The absolute value of its exponent is much larger than those in both 0D and 1D nanostructures, as manifested by stronger screening in these clusters, emphasizing the 3D character of the cluster cores.