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.