Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction

Abstract

Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.

Double-heterojunction nanorods

“…4a second spectrum from top, FWHM is B29 nm or 96 meV). As a comparison, the best CdSe/CdS core/ shell QDs of similar emission wavelength show 20-30 nm PL FWHM 42,43 . The convergence of the two CdSe PL peaks may be a result of enhanced growth of ZnSe on the smaller CdSe tip, which in turn would lead to a larger redshift than the PL of the larger CdSe tip on ZnSe shell growth 44 .…”

Interfacial engineering of core/shell heterostructured nanocrystal quantum dots for light-emitting applications

“…The ultimate method for the stepwise multilayer growth is SILAR (a successive ionic layer adsorption and reaction). The first demonstration of SILAR was demonstrated by alternating the ion adsorption on nanocrystals in the aqueous phase [1], and it was later further developed by Peng et al for adaptation to oil phase chemistry [20]. In principle, SILAR enables the alteration of the chemical compositions of heterostructured NQDs in an atomic scale.…”

“…In the last two decades, the structures of NQDs evolved into sophisticated heterostructures, boosting their optical performances and photophysical stabilities [19][20][21][22][23][24][25][26][27]. As a result of the multilateral efforts, core/shell heterostructured NQDs displaying almost 100% efficiency upon the excitation of both optical [24] or electrical pumping have been successfully realized.…”

Engineering imaging probes and molecular machines for nanomedicine

“…For example, iron oxide nanoparticles and quantum dots can be synthesized by thermal decomposition of precursor compounds in organic solvents [94,111,112].…”