Using Fe as catalyser, Si nanowires (SiNWs) with diameters of 5-15 nm and lengths of similar to mu m were synthesized via vapor-liquid-solid growth method at high temperature. The as-prepared SiNWs with the Raman peak positions of <= 511 cm(-1) show a strong photoluminescence (PL) peak centered at 745 nm. The position of the PL peak exhibits no dependence on the SiNW diameters, but its intensity increases with decreasing the SiNW diameters. When the as prepared SiNWs were gradually oxidized, the 745 nm PL peak reduces its intensity and meantime a new PL peak appears at 620 nm. When the SiNWs were completely oxidized, the 745 nm PL vanishes and the 620 nm PL peak reaches its maximal intensity. Spectral analyses and microstructural observations suggest that the 745 nm PL arises from optical transition in the interfacial defect states between the SiNW core and the surface Si oxide, whereas the photogeneration of carriers takes place in the quantum confined SiNW core. Based on the electron paramagnetic resonance result, we correlate the 620 nm PL with the nonbridging oxygen hole centers at the surfaces of the SiNWs. (c) 2005 Elsevier B.V. All rights reserved.