Effect of Structural Dynamics on the Opto-Electronic Properties of Bare and Hydrated ZnS QDs

Quantum mechanical calculations on the structural and optoelectronic features of two realistic wurtzite-like ZnS quantum dot (QD) models, namely, (ZnS)₃₃ and (ZnS)₁₁₆, are presented both in vacuo and in an explicit water solution environment. Car–Parrinello molecular dynamics (CPMD) simulation and excited-state, Time-Dependent Density Functional Theory (DFT/TDDFT) calculations on extended models are combined to unravel hitherto inaccessible atomistic features of the investigated systems. Ultrasmall QDs are predicted to exhibit strong dynamical fluctuations. Accordingly, the bare (ZnS)₃₃ model undergoes a drastic structural rearrangement and evolves from the starting bulk-like structure to an amorphous phase. The geometrical changes occurring over the time are reflected on the opto-electronic properties. The band-edge states and the optical absorption onset both sizably vary along the CPMD trajectory. Eventually, the optical gap decreases due to the emergence of high-lying occupied orbitals. These midgap states are mainly localized in undercoordinated S sites and could act as trap states for the photogenerated holes. Water molecules are predicted to form strong Zn–OH₂ bonds with the surface Zn atoms. Hydration seems to lower the surface energy, stabilize the wurtzite polymorph, hinder the Zn–S bond breaking, and largely prevent the appearance of trap states. Besides, adsorbed water molecules produce a notable blue-shift of the optical gap. The electrostatic field induced by the solvent shell and the electron-donor properties of the water molecules are supposed to be responsible for the opening of the gap. Moreover, capping the QDs with water molecules increases the intensity of the lowest-lying electronic excitations. This study sheds light on the important opto-electronic modifications occurring for realistic QD in water solution and offers at the same time the methodological framework to investigate photocatalytic reactions mediated by ZnS