Microscopic studies of polycrystalline nanoparticle growth in free space

•Microscopy techniques to study growth and crystallization of Si nanoparticles (NPs).•NPs classified into various phases based on a growth model.•Size, shape and crystallinity of NPs controlled by changing the plasma ON time.•Single nucleus hypothesis for cauliflower shaped NPs proposed.•This contradicts claims of particle agglomeration found in literature. We have extensively studied by multiple microscopic techniques the growth and crystallization of silicon nanoparticles in pulsed SiH4/Ar plasmas. We observe that the crystallinity of the particles can be tuned from amorphous to crystalline by altering the plasma ON time, tON. Three phases can be identified as a function of tON. Microscopic studies reveal that, in the initial gas phase (phase I) single particles of polycrystalline nature are formed which according to our hypothesis grow out of a single nucleus. The individual crystallites of the polycrystalline particles become bigger crystalline regions which marks the onset of cauliflower shaped particles (phase II). At longer tON (phase III) distinct cauliflower particles are formed by the growth of these crystalline regions by local epitaxy.