Infrared emitting properties and spherical symmetry of colloidal PbS quantum dots (QDs) chemically synthesized in organic solvent oleylamin as a ligand were investigated in terms of synthesis temperature, time, and precursor concentrations. The progress of the nucleation process and crystal growth of PbS nanocrystals can be interpreted using the LaMer mechanism. By decreasing the temperature or increasing the concentration of precursors, the degree of supersaturation becomes higher, resulting in precipitation of small nuclei. Meanwhile, in the case of the lower supersaturation, the particle size and the size distribution of nanocrystals become larger and narrower. Direct evaluations of the spherical symmetry of the synthesized QDs were carried out using symmetry indexes derived from the transmission electron microscopy images. These results clearly indicated that lower reaction temperature, higher supersaturation, and longer growing time are favorable for the synthesis of spherical QDs emitting at the wavelength of the 1.3 mu m region. (c) 2013 Elsevier B.V. All rights reserved.