The Structure and Stokes Shift of Hydrogenated Silicon Nanoclusters
  We evaluate the optical gap and Stokes shift of several candidate 1 nm silicon nanocrystal structures using density functional and quantum Monte Carlo (QMC) methods. We find that the combination of absorption gap calculations and Stokes shift calculations may be used to determine structures. We find that although absorption gaps calculated within B3LYP and QMC agree for spherical, completely hydrogenated silicon nanocrystals, they disagree in clusters with different surface bonding networks. The nature of the Stokes shift of the ultrabright luminescence is examined by comparing possible relaxation mechanisms. We find that the exciton which reproduces the experimental value of the Stokes shift is most likely a state formed by a collective structural relaxation distributed over the entire cluster. 
Lucas Wagner, Aaron Puzder, Andrew Williamson, Zachary Helms, Jeffrey   C. Grossman, Lubos Mitas, Giulia Galli and Munir Nayfeh