Influence of the Jahn-Teller effect on absorption and photoluminescence spectra of Si nanocrystals

The optical properties of H‐passivated silicon nanocrystals are determined by the energetics of the frontier orbitals and their dependence on the deformation in the relaxed excited state. In the present work, the geometry of H‐passivated tetrahedral silicon nanocrystals is optimized with density functional methods, and the deformation in the relaxed excited geometries is obtained from time‐dependent density functional theory. The Jahn‐Teller effect in the lowest excited state results in a distortion towards tetragonal symmetry, contributing significantly to the red shift of the photoluminescence spectra. For nanocrystals up to a diameter of 1.5 nm, we project the deformations at the minima of the excited state potential surface onto the different symmetries, allowing for a discrimination of the respective contributions to the total Stokes shift. For selected systems, the influence of the Jahn‐Teller effect can exceed the red shift related to a symmetryconserving deformation in the relaxed excited state. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)