Publication date: 27th June 2014
Core shell quantum dots possess size-dependent (tunable) electronic and optical properties together with large dipole moments. Their colloidal characteristics are ideal for experiments in the domain of quantum optics [1]. For our recent studies, we embed single CdSe/CdS core/shell quantum dots into PMMA particles via miniemulsion polymerization. These hybrid particles exhibit high quantum yields and single photon emission together with long-term photochemical stabilities [2-4].
At cryogenic temperatures, we observe fine-structure emission originating from the two band edge excitons (i.e. the low energy “dark” and the high energy bright exciton [5]) of the CdSe/CdS system. The fluorescence decay time consists of a long and a short component. These components are dependent on the relaxation rate between the two fine-structure states and the relaxation of each of them to the ground-state, at a given temperature. By employing a three level temperature-dependent rate-equation model [5], we fit the relaxation rate between the fine-structure states. This rate strongly depends on the existence of acoustic phonon modes. In nanoscale objects, acoustic phonons are confined modes with eigen-energies in the range of a few meV. The examination of hybrid particles with similar size and different degrees of fine-structure splitting allows us to map the energy spectrum of the confined acoustic phonon modes. In addition, we are able to observe these modes as pronounced peaks in the photoluminescence spectra. Our theoretical analysis of the vibrational eigen-modes of an elastic sphere is in excellent agreement with the measurements.
[1] F. Sotier, T. Thomay, T. Hanke, J. Korger, S. Mahapatra, A. Frey, K. Brunner, R. Bratschitsch, and A. Leitenstorfer, Femtosecond fewfermion dynamics and deterministic single-photon gain in a quantum dot, Nature Phys. 5, 352-356 (2009).
[2] C. Negele, J. Haase, A. Leitenstorfer, and S. Mecking, Polyfluorene Nanoparticles and Quantum Dot Hybrids via Miniemulsion Polymerization, ACS Macro Lett. 1, 1343-1346 (2012).
[3] C. Negele, J. Haase, A. Budweg, A. Leitenstorfer, and S. Mecking, Stable Single-Photon Emission by Quantum Dot/Polymer Hybrid Particles, Macromol. Rapid Commun. 34, 1145-1150 (2013).
[4] T. de Roo, J. Haase, J. Keller, C. Hinz, M. Schmid, D. V. Seletskiy, H. Cölfen, A. Leitenstorfer, and S. Mecking, A Direct Approach to Organic/Inorganic Semiconductor Hybrid Particles via Functionalized Polyfluorene Ligands, Adv. Funct. Mater. 24, 2714-2719 (2014).
[5] L. Biadala, Y. Louyer, P. Tamarat, and B. Lounis, Direct Observation of the Two Lowest Exciton Zero-Phonon Lines in Single CdSe/ZnS Nanocrystals, Phys. Rev. Lett. 103, 037404(2009).
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