Effect
of Structural Dynamics on the Opto-Electronic
Properties of Bare and Hydrated ZnS QDs
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Abstract
Quantum
mechanical calculations on the structural and optoelectronic
features of two realistic wurtzite-like ZnS quantum dot (QD) models,
namely, (ZnS)<sub>33</sub> and (ZnS)<sub>116</sub>, 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)<sub>33</sub> 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–OH<sub>2</sub> 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