Web Release Date: September 27,
Shape Dependence of Band-Edge Exciton Fine Structure in CdSe Nanocrystals
and
Scientific Computing Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, and Computational Research Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received June 1, 2007
Revised August 30, 2007
Abstract:
The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a "dark" ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a "quasi-bright" ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length.
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