Lattice and optical property evolution of ultra-small ZnS quantum dots grown from a single-source precursor

Cubic structured ZnS quantum dots (QDs) with sizes from 1.6 to 4.0 nm were grown at temperatures of 90-210 degrees C from zinc ethylxanthate single-source precursor. The quantum dots were characterized by X-ray diffraction, high resolution transmission electron microscope, UV-vis absorption spectroscope, photoluminescence spectroscope and micro-Raman spectroscope. Effects of the size on their lattice and optical properties were studied in detail. The lattices contract with a decrease in the quantum dot sizes, especially for quantum dots with sizes smaller than 2.9 nm. The contraction rate varies almost linearly with the quantum dot sizes and the maximum size for the contraction is derived to be 4.5 nm. The exciton ground state energy increases with a decrease in the quantum dot size. Weak UV emission and strong blue-green emission were observed and transition paths for luminescence were provided. Relative intensity of 1LO and 2LO modes of the quantum dots is sensitive to their sizes, while their frequencies are insensitive to the quantum dot sizes. (C) 2014 Elsevier B.V. All rights reserved.