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Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots

(2011) ACS NANO. 5(10). p.8033-8039
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Center for nano- and biophotonics (NB-Photonics)
Abstract
The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been Investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark bright energy level splitting of 2.4 - 5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, In the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level In small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark +/-2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range.
Keywords
semiconductor nanocrystals, II-VI quantum dots, cadmium chalcogenide, optical properties, SEMICONDUCTOR NANOCRYSTALS, ASSIGNMENT, SPECTRUM, STATES

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Citation

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Chicago
Moreels, Iwan, Gabriele Raino, Raquel Filipa Gomes Pinto Fernandes, Zeger Hens, Thilo Stoferle, and Rainer F Mahrt. 2011. “Band-edge Exciton Fine Structure of Small, Nearly Spherical Colloidal CdSe/ZnS Quantum Dots.” Acs Nano 5 (10): 8033–8039.
APA
Moreels, I., Raino, G., Gomes Pinto Fernandes, R. F., Hens, Z., Stoferle, T., & Mahrt, R. F. (2011). Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots. ACS NANO, 5(10), 8033–8039.
Vancouver
1.
Moreels I, Raino G, Gomes Pinto Fernandes RF, Hens Z, Stoferle T, Mahrt RF. Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots. ACS NANO. 2011;5(10):8033–9.
MLA
Moreels, Iwan, Gabriele Raino, Raquel Filipa Gomes Pinto Fernandes, et al. “Band-edge Exciton Fine Structure of Small, Nearly Spherical Colloidal CdSe/ZnS Quantum Dots.” ACS NANO 5.10 (2011): 8033–8039. Print.
@article{1973940,
  abstract     = {The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been Investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark bright energy level splitting of 2.4 - 5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, In the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level In small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark +/-2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range.},
  author       = {Moreels, Iwan and Raino, Gabriele and Gomes Pinto Fernandes, Raquel Filipa and Hens, Zeger and Stoferle, Thilo and Mahrt, Rainer F},
  issn         = {1936-0851},
  journal      = {ACS NANO},
  keyword      = {semiconductor nanocrystals,II-VI quantum dots,cadmium chalcogenide,optical properties,SEMICONDUCTOR NANOCRYSTALS,ASSIGNMENT,SPECTRUM,STATES},
  language     = {eng},
  number       = {10},
  pages        = {8033--8039},
  title        = {Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots},
  url          = {http://dx.doi.org/10.1021/nn202604z},
  volume       = {5},
  year         = {2011},
}

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