Photoluminescence quantum yields of amorphous and crystalline silicon nanoparticles

While nanocrystalline silicon is known to be an efficient optical emitter, there have been few and sometimes contradictory reports of emission from amorphous silicon nanoparticles. This paper presents a study of the optical properties of amorphous and crystalline silicon nanoparticles synthesized by a nonthermal plasma reactor. By tuning the power delivered to the reactor, the particle structure was adjusted from amorphous to crystalline without otherwise changing the particle properties, such as nanoparticle size, in a significant manner. Two different kinds of surface passivation of nanoparticles are studied: the surface functionalization with organic ligands in a scheme known as hydrosilylation and the passivation with a native surface oxide. We observe a clear trend of the photoluminescence quantum yield increasing with the increasing degree of crystallinity of samples with largely amorphous samples, exhibiting almost no luminescence. Measurements suggest that the upper bound for the quantum yield of amorphous nanoparticles is 2%, while the quantum yield of silicon nanocrystals is routinely found to exceed 40%.