Permanent dipole moment and charges in colloidal semiconductor quantum dots

The presence of a large permanent dipole moment has important implications on our understanding of nanocrystalline materials. We report the results of dielectric dispersion studies of CdSe, ZnSe, and metal nanocrystals. Due to the polar nature of the wurtzite lattice, a permanent dipole moment may be expected for CdSe nanocrystals. However, dielectric dispersion studies reveal a similar magnitude of the dipole moment, as well as its dependence on size, in zinc-blende ZnSe nanocrystals. These dipole moments may be intrinsic attributes to all nonmetal nanoparticles with surface localized charges. We show evidence for thermally induced charging of both semiconductor and metal nanocrystals and present a simple picture to describe the linear dependence of dipole moment on the size in semiconductor nanocrystals.