Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
The strong dimensionality-dependence of the opto-electronic properties of nanostructured materials has stimulated a widespread investigation into the size and shape control of fluorescent colloidal nanocrystals (NCs). This has resulted in the synthesis of 0D quantum dots, 1D quantum rods and most recently 2D quantum wells. Additional functionality can be introduced by the growth of core/shell nanostructures, which creates the possibility to control the electron-hole overlap via a type-I, type-II and quasi-type II band alignment in heteroNCs 1. A further degree of freedom to wave function engineering is given by strain and piezo-electric fields in wurtzite CdSe/CdS heteroNCs.
In this presentation, we discuss the fabrication and detailed structural characterization of these nanomaterials. We synthesized CdSe/CdS rod-in-rod NCs with a large anistropic CdSe core embedded in a thick CdS shell, using a high-temperature reaction in the presence of CdCl2. X-ray diffraction confirmed that both the CdSe core and CdS shell have a wurtzite crystal structure. Bright-field transmission electron microscopy (TEM) revealed a particular diffraction contrast around the CdSe core, and analysis of the local lattice constant with high-resolution TEM strain maps yielded an area of coherent crystal lattice significantly larger than the CdSe dimensions itself. Both clearly show the presence of strain. With Raman spectroscopy we investigated the core/shell interface. Next to the different surface and longitudinal optical phonon modes, we observed additional peaks that could be attributed to the formation of a CdSexS1-x alloy layer. The alloyed interface likely results from the high temperature used during synthesis (380°C), and could explain why, despite the significant strain, the NCs maintain an acceptable PL QE of 10-25%, while the lifetime is extended by two orders of magnitude.
In conclusion, with strained wurtzite CdSe/CdS heteroNCs we created a particular type of quantum dots, where confinement is governed by internal piezo-electric fields. In these systems, electron and holes become localized at opposite ends of the CdSe core, and opto-electronic properties can by tuned continuously by carefully adjusting the nanocrystal core and shell dimension.
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