The synthesis and characterization of highly luminescent cadmium selenide, zinc selenide, and core/shell (cadmium selenide)zinc sulfide nanocrystal colloids

Semiconductor nanocrystals exhibit unique size tunable optical properties. Colloidal CdSe nanocrystals were synthesized in a range of sizes using established synthetic methods. These particles were studied to gain further insight into their electronic structure. Dielectric dispersion measurements on the materials provided information on their polar nature and established that the nanocrystals have a permanent ground state dipole moment. Two-photon spectroscopy determined that inversion symmetry is broken within the nanocrystal due to a mixing of odd and even spectroscopic states, which could be due to the dipole moment. Visible-induced infrared absorption to examine intraband transitions, which have very large cross sections, proved to be a viable technique to do separate electron and hole spectroscopies. A procedure was developed for the synthesis of ZnSe nanocrystal colloids. ZnSe has strong UV/blue luminescence and cubic crystal symmetry. They provide an extension to the spectral range available with CdSe and complementary information for nanocrystal electronic structure theories. Finally, CdSe nanocrystals were capped with ZnS to provide highly luminescent materials. The higher band gap ZnS shell serves to passivate the surface of the CdSe core and confine the electron hole pair. The resulting nanocrystals are more stable and robust materials with quantum yields as high as 50%, which makes possible applications such as single cluster spectroscopy and light emitting diodes.