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 Angstrom 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 Delta mu 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 Angstrom), the dipole moment decreases. The dipolar character is lost in clusters less than 25 Angstrom. 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.