Excited-state relaxations and Franck-Condon shift in Si quantum dots

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 nanometer in size, providing a clear demarcation between “molecules” and “nanocrystals”.