Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
CdSe/CdS nanocrystals (NCs) are at present among the most studied colloidal core/shell nanomaterials. The lattice mismatch between CdSe and CdS equals 4%, small enough to allow the growth of up to 20-25 ML thick shells without appreciable defects. This yields so-called giant-shell nanocrystals –of about 20 nm diameter–, with high photoluminescence quantum efficiency. Both wurtzite1 and zinc blende2 CdSe cores can be coated. Starting from a wurtzite CdSe core, when adding phosphonic acid ligands to the reaction medium, growth can also be predominantly unidirectional, leading to the formation of a rod-like CdS shell.3 This synthesis leads to highly anisotropic, somewhat thinner NCs, with a length of up to 100 nm and an overall diameter below 9-10 nm. In contrast with spherical NCs, they show polarized emission,3 and the rod length can be used to control excited state carrier dynamics. In this presentation we show how we can combine the concepts of giant-shell NCs –e.g. efficient passivation of the CdSe surface by a thick shell- with those of dot-in-rod NCs. To synthesize a shell with a volume that exceeds the core volume by at least 2 orders of magnitude, we used a seeded growth approach and added metal halides. The shape could be controlled by careful adjustment of the reaction conditions (precursor composition and concentration, reaction temperature and time). The final CdSe/CdS nanocrystals were about 15 nm by 50 nm, and could be prepared using core diameters between 3.5 nm and 6 nm. Furthermore, by doping the shell with zinc cations, we could keep the exciton recombination time below 150-200 ns. Considering that the typical exciton lifetime in quasi-type II CdSe/CdS g-NCs can reach 300-650ns,1,4 we ascribe the reduced lifetime to an increase of the core/shell conduction band offset by the incorporation of zinc. In addition to already known procedures, we are now also able to obtain highly monodisperse dot-in-giant-rod nanocrystals, which offer the potential to optimize both shape and shell thickness. The large absorbance cross section and tunable emission properties (spectrum, lifetime, multiexciton emission…) may lead to applications in solar concentrators, lasing or LEDs.
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