Density functional studies of the luminescence Of Si29H36

The molecular structures of the ground and the lowest excited state of a Si29H36 cluster have been optimized at the density-functional-theory (DFT) level using the time-dependent perturbation-theory (TDDFT) approach for the excited state. The electronic absorption and emission energies for the fully optimized molecular structures have been calculated at the TDDFT level. The excitation energy calculated for the equilibrium structure of the excited state using the Becke-Perdew (BP) functional was found to be 0.72 eV smaller than the vertical excitation energy for the ground-state structure. The calculated wavelength of the emitted light is 396 nm which agrees well with experiment as the silicon cluster emits blue light. Small structural changes caused by the relaxation of the excited state introduce strains in the ground-state structure. The strain and the relaxation are found to contribute equally to the large red shift of the emitted light. Thus, the present calculations do not support the notion that the potential-energy surface of the excited state has a double-well structure.