Formation of nanosized cap structures on a thermally treated 6H-SiC(000-1) substrate was investigated using atomic-resolution ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). After removal of surface oxides by heating under UHV, clusters of carbon particles 1-2 nanometer in diameter were formed on the SiC surface at 1150 degrees C. Upon heating above 1150 degrees C, the carbon nanoparticles in the clusters merged, forming nanosized cap structures, which subsequently transformed into carbon nanotubes (CNTs). The clusters of nanosized carbon particles are thought to be the precursors of carbon nanocaps, and their sizes are strongly related to the sizes of CNT caps forming on the SiC surface. (c) 2006 Elsevier B.V. All rights reserved.