Non-quantum-confinement optics in (CdS)n clusters

Geometry optimizations and the calculation of properties of (CdS)n clusters for n = 2-12 are performed using density functional theory (DFT) with generalized gradient approximation. The all-electron basis is used for sulfur atoms while cadmium atoms are treated using relativistic effective core potentials. It is confirmed that the lowest energy state of (CdS)n has a ring structure for n = 3-5 and a cage geometry for n ⩾ 6. Optical absorption spectra are simulated using the energies and oscillator strengths of the first 20 singlet-singlet transitions computed for each cluster by time-dependent DFT. It is found that the energies of the lowest optical transitions do oscillate around the bulk gap value, which contradicts the concept of quantum confinement. The congender (CdSe)12 and (CdTe)12 clusters are found to show similar optical properties as the (CdS)12 cluster.