Excited-state relaxations in molecules are responsible for a red shift of the absorption peak with respect to the emission peak (Franck-Condon shift). The magnitude of this shift in semiconductor quantum dots is still unknown. Here we report first-principle calculations of excited-state relaxations in small (diameter < 2.2 nm) Si nanocrystals, showing that the Franck-Condon shift is surprisingly large (~60 meV for a 2.2 nm-diameter nanocrystal). The physical mechanism of the excited-state relaxations changes abruptly around 1 nanoeter in size, providing a clear demarcation between ``molecules'' and ``nanocrystals''.
@article{arxiv.cond-mat/0209626,
title = {Excited-state relaxations and Franck-Condon shift in Si quantum dots},
author = {A. Franceschetti and S. T. Pantelides},
journal= {arXiv preprint arXiv:cond-mat/0209626},
year = {2009}
}