Abstract
We report a method to control the composition and microstructure of CdSe1-x S x nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 °C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k E) spanning 1.3 × 10-5 s-1 to 2.0 × 10-1 s-1. Depending on the relative reactivity (k Se/k S), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe1-x S x alloys (k Se/k S = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed.
Highlights
All thiones in this study were prepared from thiocarbonyldiimidazole. 1H nuclear magnetic resonance (NMR) studies show that the second C–N bond forming step is inhibited by tert-butyl substituents, which can lead to oligomerization rather than cyclization
Assumptions: (1) the rate at which sul de or selenide deposit on the nanocrystal surface is equal to the instantaneous solute generation rate (M sÀ1), (2) solute generation exhibits rst order kinetics in the chalcogen precursor, i.e. the rate of solute generation is the product of the chalcogenourea concentration and the reactivity exponent, (3) the nanocrystals are spherical and grow isotropically without preference for one chalcogenide or the other
Many complicating scenarios could lead to deviations from this simpli ed behavior including: a surface reaction limited growth mechanism that selectively favors sul de or selenide deposition, or causes anisotropic growth, or the nite size of the nucleus, which could skew the composition of the core
Summary
Molecular precursors with tailored reactivity can precisely control the rate of solute supply and the extent of nucleation during colloidal crystallizations.[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.[9,10,11,12,13,14,15]. Raman spectroscopy can probe the composition of CdS1ÀxSex alloys, there are con icting claims regarding the number of characteristic Raman modes, and it is difficult to distinguish alloy and core/shell quantum dots on the basis of Raman spectroscopy alone.[39,50,51] X-ray photoelectron spectroscopy (XPS), photoemission spectroscopy, energy dispersive X-ray spectroscopy (EDX), Rutherford backscattering, and solid-state 113Cd and 77Se nuclear magnetic resonance (NMR) spectroscopy have all been used to monitor the radial evolution of nanocrystals during growth, but these techniques do not address effects from anisotropy.[29,39,40,51,52,53,54,55] These limitations obscure the study of alloy microstructure and how it is controlled by precursor reactivity. Our results demonstrate how the relative reactivity and the mole fraction of added precursor systematically in uence the elemental distribution of CdSe/CdS heterostructures and alloys, as well as their nal crystal size
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
