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.