Anomalous light polarization of semiconductor nanocrystals explained
Researchers of the TU Dortmund, the Ioffe Institute and ITMO University (both in St. Petersburg) and the High Field Magnet Laboratory have resolved a long-standing open question concerning the anomalous polarization of light emitted by colloidal semiconductor nanocrystals in a magnetic field. Their findings have recently been published in the journal Nanoscale.
Colloidal semiconductor Nanocrystals
For a few decades, colloidal semiconductor nanocrystals have been the focus of intensive research. Due to the continuous progress in technology, nanocrystals with different sizes, shapes, compositions, and surface properties have been synthesized. Understanding their optical, electrical and chemical properties has led to applications in various fields, such as light-emitting diodes, laser technology, field-effect transistors, solar cells and biological labels. In these efforts, external magnetic fields have been used as a powerful tool not only to address magneto-optical properties and spin-dependent phenomena, but also to determine the basic optical properties of the nanocrystals, which are dominated by absorption and emission of coupled electron-hole pairs (excitons).
Puzzling behaviour of the magneto-optical properties
Usually these experiments were performed on wet-chemically synthesized nanocrystals, showing a number of interesting magneto-optical effects: a field induced shortening of the exciton lifetime, circular polarization of the photoluminescence emission, a fine structure splitting of the exciton energy levels, including the Zeeman effect in single nanocrystals, an anisotropic electron-hole exchange interaction and electron spin coherence. However, these previous photoluminescence experiments also revealed several unusual appearances: (i) a spectral dependence of the photoluminescence circular polarization degree, (ii) its low saturation value, and (iii) a stronger intensity of the Zeeman component which is higher in energy. The latter feature is the most surprising being in contradiction with the thermal population of the exciton spin sublevels.
Solving the puzzle
To resolve these open questions a team of researchers of the TU Dortmund University, the Ioffe Institute in St. Petersburg, the ITMO University in St Petersburg and the High Field Magnet Laboratory (HFML-EMFL) in Nijmegen performed experiments on CdSe nanocrystals embedded in a glass matrix, a system that hadn’t been investigated before in high magnetic fields. They measured polarized photoluminescence in magnetic fields up to 30 T and observed the same puzzling behavior as the earlier reports described above. The team developed a model that takes into account the cumulative contribution of both zero-phonon and phonon-assisted recombination of dark excitons to the emission spectra of the nanocrystal ensemble. This model describes well all unusual experimental findings and can be readily extended to other colloidal nanocrystals, whose inhomogeneous broadening exceeds the optical phonon energy. These results demonstrate the promising role that colloidal nanocrystals could play for spintronics and quantum information applications based on spin-dependent phenomena.
The photoluminescence spectrum of an ensemble of colloidal CdSe nanocrystals is a combination of zero-phonon (ZPL) and phonon-assisted (1PL) emission of differently sized quantum dots (left panel). This results in an anomalous behaviour of the circularly polarized photoluminescence emission in high magnetic fields: the higher energy, s--polarized exciton level has a higher intensity (blue curves in right panel).
Polarized emission of CdSe nanocrystals in magnetic field: the role of phonon-assisted recombination of the dark exciton
Qiang, A. A. Golovatenko, E. V. Shornikova, D. R. Yakovlev, A. V. Rodina, E. A. Zhukov, I. V. Kalitukha, V. F. Sapega, V. Kh. Kaibyshev, M. A. Prosnikov, P. C. M. Christianen, A. A. Onushchenko and M. Bayer, Nanoscale 13, 790 (2021). http://dx.doi.org/10.1039/d0nr07117j