Photoexcitation of Si-Si surface states in nanocrystallites

Intrinsic localized radiative surface states belonging to Si-Si dimers on the surface of silicon nanocrystallites have been recently predicted. We examine the various photoexcitation pathways involved in populating these molecular states. We include both direct excitation from the ground state and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. We determine the absorption and excitation spectra and the quantum efficiency of the photoluminescence as a function of the crystallite size. Our calculation gives a dramatic enhancement in the efficiency for sizes below a critical size of about 1.4 nm.