Formation and photoluminescence of "cauliflower" silicon nanoparticles

The technological advantages of silicon make silicon nanoparticles, which can be used as quantum dots in a tandem configuration, highly relevant for photovoltaics. However, producing a silicon quantum dot solar cell structure remains a challenge. Here we use a gas aggregation cluster source to produce silicon nanoparticles. At a particular experimental condition, "cauliflower" silicon particles are formed as observed by high-resolution transmission electron microscopy. They are likely formed by aggregation of smaller silicon nanoparticles and are in an intermediate state in the evolution toward large crystalline particles. These "cauliflower" silicon particles consist of nanodomains of crystalline and amorphous silicon (a-Si) of which the latter exhibits photoluminescence. The luminescence decay times of the silicon "cauliflower" particles are increased a thousand times by isolating the particles. Here we present a detailed study of such "cauliflower" silicon nanoparticles which is part of the quest for using silicon quantum dots in solar cells.