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Dispersed nickel sulfate (NiSO4) microclusters on Si substrates were fragmented by pulsed excimer laser irradiation to serve as catalysts for carbon nanotube/nanofiber (CNT/CNF) growth. At proper fluences, NiSO4 clusters were pulverized into nanoparticles. The sizes of clusters/nanoparticles were found to be dependent on laser fluence and laser pulse number. By increasing the laser fluence from 100 to 300 mJ/cm2, the size of disintegrated particles decreased drastically from several micrometers to several nanometers. It was found that laser-induced disintegration of as-dispersed NiSO4 clusters was mainly due to physical fragmentation by transient thermal expansion/ contraction. Thermal melting of nanoparticles in a multipulse regime was also suggested. Hot-filament chemical vapor deposition (HFCVD) was used for growth of CNTs from the pulsed-laser treated catalysts. For samples irradiated at 100 and 200 mJ/cm2, CNFs were dominant products. These CNFs grew radially out of big NiSO4 clusters, forming dendritic CNF bunches. For samples irradiated at 300 mJ/cm2, dense multiwalled carbon nanotubes (MWCNFs) with uniform diameters were obtained. It is suggested that elemental Ni was formed through thermal decomposition of NiSO4 clusters/nanoparticles during HFCVD. The size and the shape of the Ni aggregation, which were determined by the initial size of NiSO4 clusters/nanoparticles, might affect the preference in the synthesis of CNTs or CNFs.