Peculiar Circularly Polarized Photoluminescence of CdSe Nanocrystals: the Role of Optical Phonons.

Aleksandr Golovatenko^a, Gang Qiang^b, Elena Shornikova^b, Dmitri Yakovlev^a^,^b, Anna Rodina^a, Mikhail Prosnikov^c, Alexey Onushchenko^d, Peter Christianen^c, Manfred Bayer^b

^aIoffe Institute, Saint Petersburg, Russia

^bExperimentelle Physik 2, Technische Universitӓt Dortmund, Dortmund, Germany

^cHigh Field Magnet Laboratory, HFML-EMFL, Radboud University, The Netherlands

^dITMO University, St. Petersburg, Russia, 49 Kronverkskii Avenue, St. Petersburg, Russian Federation

Proceedings of Internet Conference for Quantum Dots (iCQD)

Online, Spain, 2020 July 14th - 17th

Organizers: Quinten Akkerman, Raffaella Buonsanti, Zeger Hens and Maksym Kovalenko

Oral, Aleksandr Golovatenko, presentation 026
Publication date: 3rd July 2020

For state-of-the-art ensembles of colloidal CdSe quantum dots with size dispersion less than 5 % typical width of the photoluminescence (PL) spectrum under nonresonant excitation exceeds 100 meV. It results in spectral overlap between zero phonon (ZPL) and phonon-assisted (1PL) emission lines of dark excitons from quantum dots of different sizes within an ensemble. Recently it was shown theoretically that while ZPL emission is circularly polarized, 1PL emission is predominantly linearly polarized [1]. We propose a theoretical model which demonstrates that polarization properties of these recombination channels of the dark exciton are responsible for the long-standing contradiction between intensities and spectral positions of the σ⁺and σ^–polarized PL peaks in the external magnetic field [2-5].

The proposed theoretical model is used to describe the experimental data on circularly polarized PL in magnetic fields up to 30 Tesla for CdSe nanocrystals with diameter 3.3-6.1 nm embedded in silica glass matrix. It is shown that the saturation level of degree of circular polarization of PL (DCP) at high magnetic fields is controlled by the ratio between ZPL and 1PL intensities, which is found to be higher in small nanocrystals. We demonstrate that 1PL emission results in an increase of DCP towards the high energy side of PL spectra due to the gradual number decrease of NCs which can provide this contribution. Finally, we show that shifts of σ+ and σ– polarized PL peaks in the external magnetic field are also controlled by 1PL emission and not by the Zeeman shift of the dark exciton sublevels.

References:

[1] Rodina, A. V.; Efros Al. L. Radiative recombination from dark excitons in nanocrystals: Activation mechanisms and polarization properties. Phys. Rev. B 2016, 93, 155427.

[2] Johnston-Halperin, E.; Awschalom, D. D.; Crooker, S. A.; Efros, Al. L.; Rosen, M.; Peng, X.; Alivisatos, A. P. Spin spectroscopy of dark excitons in CdSe quantum dots to 60 T. Phys. Rev. B 2001, 63, 205309.

[3] Furis, M.; Hollingsworth, J. A.; Klimov, V. I.; Crooker, S. A. Time- and Polarization-Resolved Optical Spectroscopy of Colloidal CdSe Nanocrystal Quantum Dots in High Magnetic Fields. J. Phys. Chem. B 2005, 109, 15332-15338.

[4] Wijnen, F. J. P.; Blokland, J. H.; Chin, T. K.; Christianen, P. C. M.; Maan, J .C. Competition between zero-phonon and phonon-assisted luminescence in colloidal CdSe quantum dots. Phys. Rev. B 2008, 78, 235318.

[5] Liu, F.; Rodina, A. V.; Yakovlev, D. R.; Greilich, A.; Golovatenko, A. A.; Susha, A. S.; Rogach, A. L.; Kusrayev, Yu. G.; Bayer. M. Exciton spin dynamics of colloidal CdTe nanocrystals in magnetic fields. Phys. Rev. B 2014, 89, 115306.

Acknowledgements:

We acknowledge the financial support by the Deutsche Forschungsgemeinschaft through the International Collaborative Research Centre TRR160 (Project B1), the Russian Foundation for Basic Research (grant no. 19-52-12064 NNIO-a). A.A.G. acknowledges support of the Grants Council of the President of the Russian Federation.

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