Nanoparticles and nanoparticle-based devices are of interest in numerous industrial applications due to their unique and often advantageous properties. However, a major obstacle to the full exploitation of nanoscale materials is the lack of production processes that yield specific materials reproducibly. A fundamental understanding of nanoparticle formation in candidate production systems is critical to achieving both product specificity and uniformity. The present work uses observations from transmission electron microscopy to elucidate the formation processes occurring for silicon and titanium nanoparticles produced from a thermal plasma process. The structure, chemistry, and defect morphology of the nanoparticles are consistent with a rapid cooling and crystallization from a liquid droplet. The observation of surface faceting on some nanoparticles suggests a transition between different crystalline growth regimes. (c) 2006 Springer Science + Business Media, Inc.