Ultrafine and uniform silicon nanowires (SiNWs), with a Si crystalline core of 1-5 nm (average 3 nm) in diameter and a SiO2 outer layer of 10-20 nm thick, were synthesized by the oxide-assisted growth method via the disproportionation of thermally evaporated SiO using zeolite as a template/precursor. From transmission and secondary electron microscopic characterizations, we deduced that the zeolite acted to limit the lateral growth of the Si crystalline core and supply the excess oxide to form the thick oxide outer layer. The ultrafine SiNWs exhibited strong photoluminescence that peaked at 720 nm. (C) 2002 Elsevier Science B.V. All rights reserved.