Understanding colloidal semiconductor nanocrystals with computational chemistry: towards realistic models

Abstract
Colloidal semiconductor nanocrystals are characterized by a large surface-to-volume ratio that renders them extremely sensible to processes occurring on the surface. The surfactant ligands, that are used to stabilize the nanocrystals in an organic solvent, play thus an important role in influencing the structure and the optoelectronic properties of these materials.

Despite major progresses attained in the last years to characterize the chemical reactions occurring on the surface of cadmium and lead chalcogenides quantum dots (QDs), there are still several key questions to be answered on the nature of the QD-ligand interactions and how trap states, which are deleterious for charge transport, are formed on the surface.

A leap forward in solving the above issues is to analyze the surface using first principle simulations. Until now some of the major drawbacks of these simulations have been: (i) the size of the system that can be handled that in the best cases is restrained to a few hundreds atoms (i.e. a small sized QD surrounded by short ligands), and (ii) the description of static properties with the absence of dynamic effects.

Based on a combination of Density Functional Theory and Molecular Dynamics simulations, we present the first multiscale modeling of real sized CdSe QDs (about 4.0 nm) passivated with oleate ligands and immersed in an organic solvent like dichloromethane with a simulation box containing 25000 atoms. Molecular dynamics simulations, carried out up to the microsecond timescale, provide crucial insights on the surface dynamics, and the role of the ligands on the structural optoelectronic properties of these materials.