Comparison of Quantum Confinement Effects on the Electronic Absorption Spectra of Direct and Indirect Gap Semiconductor Nanocrystals

The size dependent electronic absorption spectra of CdSe nanocrystals have been measured in a diamond anvil cell. Under pressure, these nanocrystals are reversibly converted from a direct gap wurtzite structure to a rock salt structure which has an indirect gap in the bulk. It is thus possible to compare the influence of quantum confinement on direct and indirect transitions in nanocrystals of the same size. The ratio of oscillator strength between direct and indirect structures does not change with size, indicating that zero-phonon transitions are not occurring in the indirect nanocrystals.