Photoexcitation of Si-Si radiative surface states in nanocrystallites

Recently, intrinsic localized excitonic radiative surface states belonging to interacting dangling bonds of expanded, unstrained Si-Si dimers on the surface of nanocrystallites have been predicted. Those are connected, via a potential barrier, to the non-radiative delocalized excitonic states on the unexpanded, strained dimer. We present a theoretical analysis of the photoexcitation pathways involved in populating the radiative states. We include both direct above barrier, and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. Our calculation gives an enhancement in the efficiency for sizes below a critical size of 1.4 nm, the size for which the outer trapped states become stable against tunneling and thermal activation. It is as if the material were changed from an indirect gap to a somewhat direct gap material. The results show that the photoluminescence exhibits a large Stokes shift resulting from the expansion of the radiative dimers. Moreover, the emission bandwidth is found to be quite wide especially for the ultra small crystallites (0.8 nm) where it encompasses nearly all of the visible spectrum, and the near infrared. Spectra simulated over size distributions are presented.