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Accueil du site > Vie du laboratoire > Congrès / Colloques / Conférences > Optical Properties of Individual Nanowires and Quantum Dots in High Magnetic Field, Septembre 2014 > Programme du workshop > Spin dynamics of negative trions in ensemble of colloidal CdSe/CdS core/shell nanocrystals

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Spin dynamics of negative trions in ensemble of colloidal CdSe/CdS core/shell nanocrystals

Spin dynamics of negative trions in ensemble of colloidal CdSe/CdS core/shell nanocrystals 

 D. R. Yakovlev1,2, F. Liu1, A. V. Rodina2, L. Biadala1,3, D. Dunker1, C. Javaux4,

J.-P. Hermier3, Al. L. Efros5, B. Dubertret4 and M. Bayer1

 1TU Dortmund University, 44221 Dortmund, Germany

 2Ioffe Physical Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia

 3Universitie de Versailles-Saint-Quentin-en-Yvelines, 78035 Versailles, France

 4Laboratoire de Physique et d’Etude des Materiaux, CNRS, 75231 Paris, France

 5Naval Research Laboratory, Washington, DC 20375, USA

 

We have shown recently that photo-excitation of core/shell CdSe/CdS nanocrystals (NCs), which shell thickness exceeds 4 nm, leads to a single electron charging of NCs [1,2]. This is consequence of the charge separation of photogenerated in NC an electron-hole pair : the hole is captured by a surface state and the long-lived electron is created in the CdSe core. Photogeneration of any consequent electron-hole pair in this singly-charged NC results in formation of the negatively charged exciton (trion), consisting of two electrons in the singlet state and a hole. The type of the trion (i.e. negatively or positively charged trion) and it distinguishing from the exciton can be unambiguously provided by magneto-optical experiments. 

Here we report on experimental and theoretical studies of the trion and exciton spin dynamics in core/thick shellCdSe/CdS NCs. Time-resolved photoluminescence measurements were performed at low temperatures and in high magnetic fields up to 15 Tesla. From the decay of the photoluminescence intensity the trion radiative time  ns was measured. It is independent of the magnetic field reflecting the fact that the trion ground state is always optically bright (i.e. allowed in electric-dipole approximation). This is in strong contrast to the exciton states in NCs which dynamics is controlled by a competition of the bright and dark states, which can be mixed either by magnetic fields or thermally.

Spin relaxation times were measured from dynamics of the magnetic-field-induced circular polarization degree, controlled by the trion or exciton thermalization on the Zeeman sublevels split by magnetic field. For excitons these times are shorter than a nanosecond and are limited by time-resolution of the used setup. While for the trions very long spin relaxation time up to 60 ns was measured. It decreases by about two orders of magnitude down to 1 ns in strong magnetic field of 15 Tesla.

Theoretical description of the polarization dynamics takes into account random orientation of NCs hexagonal axises to the magnetic field direction and the anisotropy of the heavy-hole g-factor. The longitudinal component of the hole g factor of -0.54 was received from the modeling of experimental data. We conclude that the spin relaxation incore/thick-shellCdSe/CdS nanocrystalsis two-phonon assisted and is accelerated in external magnetic field due to the admixture of the light-hole to heavy-hole states. 

References

[1] C. Javaux, B. Mahler, B. Dubertret, A. Shabaev, A.V. Rodina, Al. L. Efros, D. R. Yakovlev, F. Liu, M. Bayer, G.Camps, L. Biadala, S. Buil, X. Quelin, and J.P. Hermier,Nature Nanotechnology8,206 (2013).

[2] F.Liu, L. Biadala, A.V. Rodina, D.R. Yakovlev, D. Dunker, C. Javaux, J.P. Hermier, Al.L. Efros, B. Dubertret, and M. Bayer, Phys Rev B88,035302 (2013).