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

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 λ_max(1S_e-1S_1/2h) 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.