On the origin of a permanent dipole moment in nanocrystals with a cubic crystal lattice: Effects of truncation, stabilizers, and medium for CdS tetrahedral homologues

A large anomalous dipole moment has previously been reported for nanocrystals with a cubic crystal lattice. By considering truncations of a regular tetrahedral CdS nanocrystal, the hypothesis that shape asymmetry is responsible for the observed dipole moment was tested and verified. The location and degree of the truncations were systematically varied, and corresponding dipole moments were calculated by using a PM3 semiempirical quantum mechanical algorithm. The calculated dipole moment of 50-100 D is in good agreement with a variety of experimental data. This approach also affords simple evaluation of the potential effect of the media for aqueous dispersions of nanocrystals. The substitution of the truncated corner(s) by molecules of H2O typically results in a substantial increase of the dipole moment, and often, in the reversal of its direction. The molecular modeling approach presented here is suitable for detailed theoretical studies of the dipole moments of II-VI and other nanoparticles and interparticle interactions in fluids. The data obtained from these calculations can be the starting point for modeling of agglomeration and self-organization behavior of large nanoparticle ensembles.