Copper-sulfides, Cu2-xS nanocrystals (NCs) have been synthesized using the reaction between a Cu-alkyl amine complex and a sulfur/1-dodecanethiol. Here, 1-dodecanthiol plays an important role to reduce the reaction temperature of sulfuration. The Cu/S composition (the Cu-defect density) of Cu2-xS NCs depends on the stability of Cu-alkyl amine complex. XPS analyses reveal that the substance of Cu-defect is attributed to an introducing of Cu2+S22- species. There is a good correlation between an intensity of a near IR absorption peak and a Cu-defect density estimated by XPS analyses. These results demonstrate that optical properties of Cu2-xS NCs in a near IR region are tunable. (C) 2007 Elsevier Inc. All rights reserved.