Growth kinetics and characterization of fluorescent CdS nanocrystals synthesized with different sulfur precursors in paraffin hot-matrix

The present paper describes the influence of the sulfur precursor on the nanocrystal growth rate and optical properties of cadmium sulfide nanocrystals in a hot-paraffin matrix. One of the precursors used is tributylphosphine sulfide; the other one is elemental sulfur. The growth kinetics is studied at various temperatures to estimate the activation energy for the formation of CdS nanocrystals from the temperature dependence of model kinetic parameters. The different sulfur precursors lead to different trends in the limiting nanocrystal radii and the time-constant of growth. The obtained nanocrystals are characterized by UV-absorption and photoluminescence spectroscopy, electrophoresis, X-ray powder diffraction and transmission electron microscopy. The nanocrystals prepared from tributylphosphine sulfide have fewer surface defects and their spectra exhibit a band-edge emission only. In this case the limiting radii of the nanocrystals increase with increasing the temperature of growth. The nanocrystals synthesized from elemental sulfur show in addition a trap-state emission due to much more surface defects. Here the limiting radii of nanocrystals decrease with increasing temperature in contrast to the previous case. In both cases, the semiconductor nanocrystals can be sterically and electrostatically stabilized in non-polar organic solvents by post-capping with tributylphosphine. Our findings are important for the controlled manufacturing of CdS nanocrystals with pre-determined properties and their utilization in a variety of optical devices.