Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state

The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 Å in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d≳70 Å), the dipole moment decreases. The dipolar character is lost in clusters less than 25 Å. These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.