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 λ max (1S e-1S 1/2h) is determined. The precursor conversion kinetics span five 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 by the nucleation reaction increases with increasing precursor conversion reactivity, allowing the final size to be controlled by the precursor structure. 1 H NMR spectroscopy is used to monitor the reaction of dip -tolyl thiocarbonate and cadmium oleate where dip -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) compared to those made from thioureas (137-174 meV FWHM) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others. Additional nanocrystal synthesis and precursor coproduct identification Figures S1-S23 and 1 H, 13 C{ 1 H}, and 19 F{ 1 H} NMR characterization of molecules. (PDF)

Precursor reaction kinetics control compositional grading and size of CdSe1−xSx nanocrystal heterostructures

“…All of these substituent effects can be explained by a mechanism involving Lewis acidic activation of the chalcogenourea prior to cleavage of the C]E bond, a mechanism analogous to the one proposed for phosphine chalcogenides and thiocarbonates. 7,63 If the C]E bond cleavage reaction is preceded by a fast preequilibrium Lewis acid activation, one precursor could, in principle, reduce the available cadmium and the conversion kinetics of the other. In one example studied here the conversion kinetics of a mixed precursor synthesis was monitored using 1 H NMR spectroscopy and compared to the kinetics of the single component reactions (see ESI †).…”

“…Faster conversion reactivity nucleates greater numbers of nanocrystals as has been described earlier. 2,[4][5][6][7]68 Interestingly, the nucleation of CdS and CdSe exhibit different dependences on the conversion reactivity, that may arise from differences in the growth kinetics and/or nucleus size. 1 Compared to CdSe, greater numbers of CdS nanocrystals nucleate at a given solute supply rate, and in several cases, nucleation of CdSe nanocrystals does not follow a simple trend.…”

“…[1][2][3][4] Recent examples illustrate this principle using libraries of chalcogenone derivatives to synthesize CdS, CdSe, PbS, and PbSe nanocrystals. [5][6][7][8] By choosing different substitution patterns, one can control the reactivity, the extent of nucleation, and the size following quantitative precursor conversion. [5][6][7] Such a method avoids the need to terminate the precursor reaction prematurely, modify the surfactant mixture, or adjust the reaction temperature to prepare a desired size, which can inadvertently change the surface chemistry and composition of the nal product.…”

“…[5][6][7][8] By choosing different substitution patterns, one can control the reactivity, the extent of nucleation, and the size following quantitative precursor conversion. [5][6][7] Such a method avoids the need to terminate the precursor reaction prematurely, modify the surfactant mixture, or adjust the reaction temperature to prepare a desired size, which can inadvertently change the surface chemistry and composition of the nal product. [9][10][11][12][13][14][15] By injecting a mixture of sulde and selenide precursors with carefully selected conversion reactivity, one can, in principle, adjust the solute evolution during growth and the resultant alloy microstructure (Scheme 1).…”

Quantum Dot Solar Cells: Small Beginnings Have Large Impacts

“…During this time however, the high surface energy of small QDs drives their dissolution and precipitation onto larger QDs leading to an increase in the size distribution-A process called Ostwald's ripening [5,11]. Influencing these processes by reaction temperature, precursor reactivity, and/or reaction kinetics allows for control over the size and size-distribution [12][13][14][15][16][17][18][19][20][21][22][23]. Once all the reactants are used and the concentration approaches zero, the growth regime ends.…”