Optical absorbance of doped Si quantum dots calculated by time-dependent density functional theory with partial...

The optical properties of Si quantum dots (QDs) with phosphorous and aluminum dopants have been calculated with the recently tested Heyd-Scuseria-Ernzerhof (HSE) density functionals to ascertain the effect of functional corrections to electronic self-interaction. New results have been obtained for 20 crystalline and amorphous structures of Si(29) and Si(35) quantum dots and are compared to our previous results obtained using the PW91∕PW91 functionals. The bandgaps are greater in magnitude and shifted to higher energies in HSE calculations compared to PW91 calculations, and the absorption spectrum is blueshifted in HSE. Trends in the shifts of absorbances due to doping are similar for both sets of calculations, with doped QDs absorbing at lower photon energies than undoped QDs. Consistent with previous results, the bandgaps of QDs are found to decrease as the size of the QD increases, and the absorption spectra of amorphous QDs are redshifted compared to those of crystalline structures. The molecular orbitals involved in the transitions with the largest oscillator strengths show that the electron density moves towards the surface of the quantum dot as the structure is excited. The lifetimes of photoexcited states were found to differ substantially between the two functionals due to their sensitivity to the overlaps of initial and final orbitals. Comparison with available experimental and independent theoretical results supports the conclusion that the HSE functional better matches experimental results due to the partial inclusion of Hartree-Fock exchange.