We report a method to control the composition and microstructure of CdSe 1−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. CdSe 1−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.
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Solution-phase synthesis and thermal conductivity of nanostructured CdSe, In 2 Se 3 , and composites thereof
The use of nanoparticle-in-matrix composites is a common motif among a broad range of nanoscience applications and is of particular interest to the thermal sciences community. To explore this morphological theme, we create crystalline inorganic composites with nanoparticle volume fractions ranging from 0 to ∼100% using solution-phase processing. We synthesize these composites by mixing colloidal CdSe nanocrystals and In 2 Se 3 metal–chalcogenide complex (MCC) precursor in the solution-phase and then thermally transform the MCC precursor into a crystalline In 2 Se 3 matrix. We find rich structural and chemical interactions between the CdSe nanocrystals and the In 2 Se 3 matrix, including alterations in In 2 Se 3 grain size and orientation as well as the formation of a ternary phase, CdIn 2 Se 4 . The average thermal conductivities of the 100% In 2 Se 3 and ∼100% CdSe composites are 0.32 and 0.53 W m −1 K −1 , respectively. These thermal conductivities are remarkably low for inorganic crystalline materials and are comparable to amorphous polymers. With the exception of the ∼100% CdSe samples, the thermal conductivities of these nanocomposites are insensitive to CdSe volume fraction and are ∼0.3 W m −1 K −1 in all cases. We attribute this insensitivity to competing effects that arise from structural morphology changes during composite formation. This insensitivity to CdSe volume fraction also suggests that very low thermal conductivities can be reliably achieved using this solution-phase route to nanocomposites.
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- Award ID(s):
- 1227979
- NSF-PAR ID:
- 10188008
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 3
- Issue:
- 25
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 13483 to 13491
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c5ta02755a
