Single-source precursor route for overcoating CdS and ZnS shells around CdSe core nanocrystals

Abstract

We reported a facile route for overcoating CdS and ZnS shells around colloidal CdSe core nanocrystals. To synthesize such double shelled core/shell nanocrystals, first, CdSe core nanocrystals were prepared in a much "greener" and cheap route, which did not involve the use of hazardous and expensive trioctylphosphine. Then, a low-cost and labor-saving route was adopted for the CdS and ZnS shell growth with the use of thermal decomposition of commercial available air stable single-source precursors cadmium diethyldithiocarbamate and zinc diethyldithiocarbamate in a noncoordinating solvent at intermediate temperatures.Powder X-ray diffraction patterns and transmission electron microscopy images confirm the epitaxial growth of the shell in the core/shell nanocrystals. The photoluminescence quantum yield of the resulting CdSe/CdS/ZnS core/shell nanocrystals can be as high as 90% in organic media and up to 60% after phase transfer into aqueous media. By varying the size of CdSe cores, the emission wavelength of the obtained core/shell nanostructures can span from 554 to 636 nm.

Crystal Structure Control of Zinc-Blende CdSe/CdS Core/Shell Nanocrystals: Synthesis and Structure-Dependent Optical Properties

“…During the revision of this manuscript, one referee reminded us of a report on growth of CdSe/CdS core/shell nanocrystals using Cd(DDTC) 2 as the single precursor. 18 However, the final reaction temperature in that report was set as 200 °C, which was found to be too high to maintain a zinc-blende structure in our experiments.…”

“…Results revealed that it was difficult to maintain a zinc-blende structure under the traditional reaction temperature which was typically above 200 °C (see details below). During the revision of this manuscript, one referee reminded us of a report on growth of CdSe/CdS core/shell nanocrystals using Cd(DDTC) 2 as the single precursor . However, the final reaction temperature in that report was set as 200 °C, which was found to be too high to maintain a zinc-blende structure in our experiments.…”

“…In a semiconductor nanocrystal, the wave function of the exciton is delocalized and can thus be affected by possible structure defects. As a result, core/shell quantum dots very likely need to be made with precise control by epitaxial growth, which unfortunately is still mostly empirical. − For instance, although mostly studied CdSe nanocrystals could be made in both wurtzite (hexagonal) and zinc-blende (cubic) crystal structures with high quality, epitaxial growth of other semiconductor materials onto zinc-blende CdSe core nanocrystals has only achieved limited success. − For instance, it has been noticed that, starting from zinc-blende CdSe core nanocrystals, the resulting core/shell nanocrystals often could not retain the original crystal structure. ,, This and other types of poor control of crystal structure during epitaxial growth might yield core/shell nanocrystals without desired optical properties, especially emissive properties. For example, the Krauss research group reported that blinking behavior of single-dot PL of CdSe-based core/shell nanocrystals was strongly affected by the control of shell composition.…”

“…In classic crystallization literature, it is well-known that, for CdSe and CdS, zinc-blende structure is more stable at low temperatures. This reminded us that the reaction temperatures used in the attempts at growing zinc-blende CdSe/CdS core/shell nanocrystals, usually at 200 °C or above, − might be too high. Two issues must be considered if one wants to reduce epitaxial growth temperature.…”

Synthesis of Highly Luminescent InP/ZnS Quantum Dots with Suppressed Thermal Quenching

“…In order to further enhance the emission, we adopted a single source precursor Zn(DDTC) 2 to grow a second layer of ZnS shell. The Zn(DDTC) 2 was shown to decompose at relatively low temperatures and form ZnS shells [20,21]. Here, the use of Zn(DDTC) 2 was shown to be critical for highly luminescent InP QDs.…”