In this chapter we consider the optical processes in nanocrystals that can be interpreted in terms of creation and annihilation of a single electron-hole pair within a crystallite. Size-dependent absorption and emission spectra and their fine structures, as well as size-dependent radiative lifetimes, will be discussed for nanocrystals of II-VI, I-VII compounds and, where possible, for nanocrystals of III-V compounds and of group IV elements. Nontrivial aspects of excitonphonon interactions that manifest themselves in homogeneous linewidths and/or intraband relaxation processes will be outlined. Challenging experiments providing the optical information relevant to a single nanocrystal will be discussed as well. Most of these results have become possible because of a number of the spectrally and spatially selective techniques described in Chapter 4. An influence of a microcavity on spontaneous emission of nanocrystals, the competitive recombination mechanisms, and the electric field induced effects will be analyzed as well.

Size-dependent absorption spectra. Inhomogeneous broadening and homogeneous linewidths

Experimental evidence for quantum-size effects in real nanocrystals

In the early 1980s A. I. Ekimov and A. A. Onushchenko (Ekimov and Onushchenko 1982; Ekimov and Onushchenko 1984) and L. Brus with coworkers (Brus 1983; Rossetti, Nakahara, and Brus 1983) published pioneering articles in which size-dependent absorption spectra of semiconductor nanocrystals resulting from quantum confinement were demonstrated for the first time. During the same period S. V Gaponenko et al. reported on inhomogeneous broadening of the optical absorption spectra of glasses doped with semiconductor nanocrystals (Gaponenko, Zimin, and Nikeenko 1984).