Dielectric Dispersion Measurements of CdSe Nanocrystal Colloids: Observation of a Permanent Dipole Moment

We measure the dielectric dispersion of CdSe nanocrystal colloids and show the existence of large dipole moments of 25 and 47 debye for 34 and 46 Å diameter nanocrystals, respectively. The magnitude is consistent with the expected spontaneous polarization of the bulk wurtzite CdSe lattice and implies a potential drop of $\approx 0.25\mathrm{V}$ across the nanocrystal. This effect, which is intrinsic to the wurtzite structure but has been largely overlooked, should be incorporated in the description of the quantum confined electronic states.