Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Figure 11 Theoretically predicted pair states (solid lines) assigned to features (c),(e), and (g) in Fig. 7. The experimental data is shown for comparison (circles).(Adapted from Ref. 9.)assumptions, each of the electron–hole pair states is highly degenerate. Forexample, the first excited state (1S 3/2 1S e , which we refer to as the band-edgeexciton) is eightfold degenerate. However, in reality, these degeneracies willbe lifted by several second-order effects. First, as mentioned earlier, CdSenanocrystals have a unixial crystal lattice (wurtzite) which leads to asplitting of the valence subbands (Fig. 3b) [79]. Second, electron microscopyexperiments show that CdSe nanocrystals are not spherical, but ratherslightly prolate [7]. This shape anisotropy will split the electron–hole pairstates [80]. Finally, the electron–hole exchange interaction, which is negligiblein bulk CdSe, can lead to level splittings in nanocrystals due toenhanced overlap between the electron and hole [81–84]. Therefore, whenall of these effects are considered, the initially eightfold degenerate bandedgeexciton is split into five sublevels [10].This exciton fine structure is depicted in the energy level diagram ofFig. 13. To describe the structure, two limits are considered. On the left of Fig.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.