Optical properties of zinc oxide (ZnO) nanoparticles prepared in supercritical methanol and supercritical water were investigated. Interstitial zinc and oxygen vacancy were the major defects in the ZnO nanoparticles synthesized in supercritical methanol. The addition of oleic acid as a surface modifier resulted in the isolation of the nanoparticles and the removal of interstitial zinc. Lasing was observed with sharp longitudinal modes and a superlinear increase in the emission intensity of the ZnO nanowires synthesized in supercritical water. The band gap renormalization due to the electron-hole plasma induced a red-shift in the emission band, whereas the individual longitudinal modes were blue-shifted due to a change in refractive index. (C) 2011 Elsevier B.V. All rights reserved.