On the Origin of a Permanent Dipole Moment in Cubic Nanocrystals

Sachin Shanbhag and Nicholas A. Kotov. Chemical Engineering, University of Michigan, 3074 H.H. Dow, 2300 Hayward st., Ann Arbor, MI 48109

Dipole moments in nanocrystals mediate their interaction with other nanoscalar objects. Due to the intrinsic polar character of the wurtzite lattice, nanocrystals such as CdSe are expected to be polar. In contrast, cubic nanocrystals such as ZnS, CdS, ZnSe, PbSe, CdTe etc. are expected to be nonpolar due to Td point group symmetry. However, large anomalous dipole moments have been reported for these nanocrystals. By considering truncations of a regular tetrahedral CdS nanocrystal, we systematically tested and verified the hypothesis that shape asymmetry is responsible for the observed dipole moment. We varied the location and degree of the truncations, and also considered, for aqueous dispersions of nanocrystals, the substitution of the truncated corner(s) by water. The calculated dipole moment of 50-100 D is in good agreement with experimental data. We also considered the effect of stabilizing group, nanocrystal size, and composition on the dipole moment.