Kinetic Control over CdS Nanocrystal Nucleation Using a Library of Thiocarbonates, Thiocarbamates, and Thioureas

Abstract

We report a family of substituted thiocarbonates, thiocarbamates, and thioureas and their reaction with cadmium oleate at 180–240 °C to form zincblende CdS nanocrystals (d = 2.2–5.9 nm). To monitor the kinetics of CdS formation with UV–vis spectroscopy, the size dependence of the extinction coefficient for λmax(1Se–1S1/2h) is determined. The precursor conversion reactivity spans 5 orders of magnitude depending on the precursor structure (2°-thioureas > 3°-thioureas ≥ 2°-thiocarbamates > 2°-thiocarbonates > 4°-thioureas ≥ 3°-thiocarbamates). The concentration of nanocrystals formed during nucleation increases when more reactive precursors are used, allowing the final size to be controlled by the precursor structure. 1H NMR spectroscopy is used to monitor the reaction of di-p-tolyl thiocarbonate and cadmium oleate where di-p-tolyl carbonate and oleic anhydride coproducts can be identified. These coproducts further decompose into p-tolyl oleate and p-cresol. The spectral features of CdS nanocrystals produced from thiocarbonates are exceptionally narrow (95–161 meV fwhm) as compared to those made from thioureas (137–174 meV fwhm) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others.