US nanocrystals with an average diameter of 16 nm were synthesized in the CTAB/n-C5H11OH/n-C6H14/water quaternary microemulsions by a two-step hydrothermal process at 90 and 130 degreesC. The reaction of carbamide and carbon disulfide was employed as the sulfur source for the preparation of US nanocrystals. The resulting crystals were characterized with powder X-ray diffraction, transmission electron microscopy, UV-vis absorption spectroscopy, and photoluminescence spectroscopy. A unique core/shell structure of US nanocrystals was suggested for the explanation of the interesting phenomenon. (C) 2003 Elsevier Inc. All rights reserved.