Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Colloidal nanostructures are taking off in many field of application such as in display screen, sensing, LED, or biolabelling. In addition, colloidal quantum dots are promising in various fields from laser technology, as low threshold amplification medium, to quantum optics as single photons sources or polarization-entangled photon pair source. Over the past decades research have been mainly conducted on CdSe-based heteronanostructures (HNS). Unfortunately Cd-based materials are not desirable due to the high toxicity of cadmium and are doomed to be abandoned. Significant effort have been made in chemical synthesis to find heavy metal-free material that would have the same optical properties than CdSe. Among the potential materials, InP appears to be the ideal candidate since it has optical properties close to CdSe as well as a reduced toxicity. Nonetheless the widespread use of InP-based HNS implies a comprehensive knowledge of the size dependence on the fundamental physical properties. Despite decades of studies on InP NCs, only a little is known on the physics of these NCs especially on the band edge exciton fine structure and its recombination dynamics.
Here we report the first systematic study of the band edge exciton fine structure on InP/ZnS NCs with diameter varying from 2.4 to 3.3 nm. We show that at cryogenic temperature, the PL stems from the recombination of the two lowest fine structure levels that are thermally populated. These states often referred to as the bright and the dark states are split by a fine structure splitting (FSS) that strongly depends on the NCs size as a result of the electron-hole exchange interaction. From the temperature dependence of the PL decay we have determined for the first time in InP/ZnS NCs, the bright and the dark exciton radiative rate as well as their size dependence. While the bright exciton lifetime increases with the size due to a reduce electron-hole wavefunction overlap, the dark exciton lifetime shortens. Interestingly, the dark exciton lifetime follows a linear trend with the bright-dark FSS similarly to CdSe/CdS dot-in-Rod. This surprising result highlights an universal mechanism in the dark exciton radiative recombination and re-opens the debate on its origin.
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