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

- ZnS quantum dots with ultra small sizes were synthesized by decomposing zinc ethylxanthate single-source precursor.
- The lattice contraction with a decrease in the quantum dot sizes was demonstrated.
- 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 electron phonon coupling strength of ZnS quantum dots was studied quantitatively.

Cubic structured ZnS quantum dots (QDs) with sizes from 1.6 to 4.0 nm were grown at temperatures of 90-210 °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.

Variations of lattice parameter and contraction rate with the quantum dot sizes. a0 and a are the lattice parameters for ZnS bulk and quantum dot, respectively. Red sphere for the contraction rate obtained by Rietveld method; asterisk for η derived by Williamson-Hall relation.