The Impact of Alloying or Giant Size in Core/Shell Colloidal Quantum Dots on Their Excitonic Properties and Photo-stability

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2015 (NFM15)

Invited Speaker, Lifshitz Efrat, presentation 057
Publication date: 8th June 2015

Colloidal semiconductor quantum dots (CQDs) are characterized by tunable electronic properties with variation of size, shape and composition. Colloidal techniques facilitate the formation of high-quality CQDs with surface passivation by molecular ligands or/and hetero-structuring, showing potential application as opto-electronic devices and biological tagging. Despite the numerous investigations of the optical and electrical properties of those materials, a few fundamental issues concerning commonly observed fluorescence blinking, spectral diffusion, as well as carrier intra-band and inter-band relaxations, remained as open questions. Related studies have demonstrated that the issues mentioned are associated with intrinsic properties of the inorganic moiety, such as exciton charging, Auger relaxation and phonon assisted cooling. Influence of extrinsic factors, such as surface-mediated charge trapping orenergy transfer to ligand moietieshave not been fully explored. The present work includes the investigation of single- and multiple-excitons in CQDs that offer blinking-free behavior, comprised of core/shell heterostructures with two different internal designs: (1) CdTe/CdTexSe1-x CQDs with alloyed shell, offering close crystallographic and dielectric match at the core/shell interface,1and moreover, permit the suppression of an Auger relaxation by the smooth boundary;2 (2) CdTe/CdSeCQDs with giant core size (~ 20 nm) and a thin shell. Schematic scheme of the examined samples are shown in Fig. 1. The excitonic properties were investigated by monitoring the micro-photoluminescence (µ-PL) spectra of individual CQDs under continuous-wave excitation, with linear/circular polarization detection and in the presence of an external magnetic field. We performed experiments at cryogenic temperature and a varying magnetic field up to 7 Tesla. The core/alloyed-shell CQDs had an average size around 4 nm, while the giant-core/shell CQDs diameter varied from 9.5 to 25.5 nm, slightly above the bulk Bohr exciton radius (9 nm). The last represent structures with a moderate to a weak or even marginal degree of quantum confinement. In all samples, we distinguished the regimes of a single, bi- and even tri-excitons, which developed with the increase of the excitation power (see Fig. 2(a)). Furthermore, the linear and circular polarization component have been distinguished upon observation under the presence of the magnetic field (see example in Fig.2(b)), from which, we determined the diamagnetic shift and the Landé g-factors. Plots of those parameters versus the CQDs diameter is shown in Fig.3, suggesting approach to a bulk values with the increase of the CQDs' diameter. In particular, in giant-core/shell structures, the change in the diamagnetic shift coefficient revealed an exciton radius close to that of the bulk exciton, thus, the electron–hole pair is merely bound by Coulomb attraction and not by the spatial confinement. Hence, theexcitonis localized in effective volume that is smaller than the actual CQDs size, avoiding interaction with exterior surface or ligands, which might be source for spectral instabilities.

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