Photoluminescence of size-separated silicon nanocrystals: Confirmation of quantum confinement

Silicon nanocrystals with diameters between 2.5 and 8 nm were prepared by pulsed ${\mathrm{CO}}_2$ laser pyrolysis of silane in a gas flow reactor and expanded through a conical nozzle into a high vacuum. Using a fast-spinning molecular-beam chopper, they were size-selectively deposited on dedicated quartz substrates. Finally, the photoluminescence of the silicon nanocrystals and their yield were measured as a function of their size. It was found that the photoluminescence follows very closely the quantum-confinement model. The yield shows a pronounced maximum for sizes between 3 and 4 nm.