Large-scale desired silicon nanowires without amorphous silicon oxide sheath have been synthesized by thermal chemical vapor deposition using SiH4 gas at 650degreesC in a flow mixture of H-2 and N-2, compared with the short and thick Si nanowires with amorphous SiOx coating obtained in N-2. Scanning electron microscopy (SEM), Energy dispersive X-ray spectrometry (EDX) analysis, and high-resolution transmission electron microscopy (HRTEM) have been employed to characterize the Si nanowires. The effects of H-2 gas on the catalytic particle size and on the formation of Si nanowires are discussed in detail. Photoluminescence (PL) characteristics further demonstrate the large differences between the H-2-assisted grown Si nanowires and the Si nanowires grown in N-2. (C) 2003 Elsevier B.V. All rights reserved.