Engineering of the Electronic Structure of Core/Shell Quantum Dots by In-Situ Alloying
Klaus Boldt a b, Nicholas Kirkwood b, Kyra N. Schwarz b, Trevor A. Smith b, Paul Mulvaney b
a Technical University (TU) Dresden, Mommsenstr. 13, Dresden, 1062, Germany
b University of Melbourne, School of Chemistry & Bio21 Institute, Flemington Road, 30, Parkville, Australia
Oral, Klaus Boldt, presentation 011
Publication date: 27th June 2014

We present a facile and robust synthetic method for the formation of graded multi-shell semiconductor nanoparticles.1 Epitaxial growth of a shell from a different material has been used extensively to improve the properties of quantum dots. The most common strategy is the type-I configuration, which employs a large band gap material to confine the exciton into the core and drastically improve photoluminescence quantum yield (PL QY). The staggered band alignment of a type-II structure in contrast induces a spatial charge separation, which is of interest for charge carrier extraction and stimulated emission, but often suffers from low PL QY.2,3

We extend the CdSe/CdS synthesis published by Chen et al.4 to a range of core/shell and core/shell/shell structures. By adding cadmium or zinc oleate and octane thiol as precursors at elevated temperatures (260-310 °C) interface alloying between core and shell or between consecutive shells is induced. This process is used to engineer the electronic structure of the nanoparticles between the type-I and quasi type-II or type-II regime of charge carrier localisation.

Thus prepared CdSe/CdS/ZnS particles exhibit very high fluorescence quantum yields close to unity and photo-stability when exposed to electron or hole scavengers. A blue-shift of the absorption and fluorescence spectrum caused by the alloying of CdS and ZnS opens a new synthetic pathway to high quality green emitting quantum dots.

When starting with ZnSe cores the resultant ZnSe/CdS and ZnSe/CdS/ZnS particles have a type-II configuration. The PL can be tuned over a wide range of the visible spectrum from violet to orange, and closely packed films exhibit a low pump threshold for stimulated emission at room temperature. To our knowledge our method produces the highest published PL QY for such a structure, peaking at 75 % for ZnSe/CdS/ZnS. The charge carrier overlap, Stokes shift, and fluorescence lifetime can be controlled via the shelling temperature, and have been monitored by nanosecond transient absorption and emission spectroscopy techniques.5

1. Boldt, K.; Kirkwood, N.; Beane, G. A.; Mulvaney, P. Chem. Mater. 2013, 25, 4731.

2. De Mello Donegá, C. Chem. Soc. Rev. 2011, 40, 1512.

3. Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I. V.; Mcguire, J. A.; Piryatinski, A. Nature 2007, 447, 441.

4. Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.-S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Nat. Mater. 2013, 12, 445.

5. Boldt, K.; Schwarz, K. N.; Kirkwood, N.; Smith, T. A.; Mulvaney, P. J.Phys. Chm. C., submitted.



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