Electron and hole g-factors and spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble

Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton slitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques

Intervalley Scattering of Interlayer Excitons in a MoS2_2/MoSe2_2/MoS2_2 Heterostructure in High Magnetic Field

Degenerate extrema in the energy dispersion of charge carriers in solids, also referred to as valleys, can be regarded as a binary quantum degree of freedom, which can potentially be used to implement valleytronic concepts in van der Waals heterostructures based on transition metal dichalcogenides. Using magneto-photoluminescence spectroscopy, we achieve a deeper insight into the valley polarization and depolarization mechanisms of interlayer excitons formed across a MoS$_2$/MoSe$_2$/MoS$_2$ heterostructure. We account for the non-trivial behavior of the valley polarization as a function of the magnetic field by considering the interplay between exchange interaction and phonon mediated intervalley scattering in a system consisting of Zeeman-split energy levels. Our results represent a crucial step towards the understanding of the properties of interlayer excitons, with strong implications for the implementation of atomically thin valleytronic devices.Comment: just accepted in Nano Letters, DOI: 10.1021/acs.nanolett.8b0148

Zeeman Splitting and Inverted Polarization of Biexciton Emission in Monolayer WS2

Atomically thin semiconductors provide an ideal testbed to investigate the physics of Coulomb-bound many-body states. We shed light on the intricate structure of such complexes by studying the magnetic-field-induced splitting of biexcitons in monolayer WS2 using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30 T. The observed g factor of the biexciton amounts to about -3.9, closely matching the g factor of the neutral exciton. The biexciton emission shows an inverted circular field-induced polarization upon linearly polarized excitation; i.e., it exhibits preferential emission from the high-energy peak in a magnetic field. This phenomenon is explained by taking into account the hybrid configuration of the biexciton constituents in momentum space and their respective energetic behavior in magnetic fields. Our findings reveal the critical role of dark excitons in the composition of this many-body state

Spatial extent of the excited exciton states in WS2 monolayers from diamagnetic shifts

We experimentally study the radii of excitons in hBN-encapsulated WS(2 )monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the g factor of the first excited state of -4.2 +/- 0.1 to be essentially equal to that of the ground state of -4.35 +/- 0.1. From diamagnetic shifts, we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5-8 nm and that of the ground state around 2 nm Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems

Magnetoexcitons in large area CVD-grown monolayer MoS2 and MoSe2 on sapphire

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Magnetoexcitons in large area CVD-grown monolayer MoS2 and MoSe2 on sapphire

International audienc

Excitonic Luminescence, X-ray Analysis and Local Band Structure of Chlorine Intercalated 2H- and 3R-MoS2 Polytypes

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Excitonic Luminescence, X-ray Analysis and Local Band Structure of Chlorine Intercalated 2H- and 3R-MoS2 Polytypes

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High Magnetic Field Reveals the Nature of Excitons in a Single GaAs/AlAs Core/Shell Nanowire

International audienceMagneto-photoluminescence measurements of individual zinc-blende GaAs/AlAs core/shell nanowires are reported. At low temperature, a strong emission line at 1.507 eV is observed under low power (nW) excitation. Measurements performed in high magnetic field allowed us to detect in this emission several lines associated with excitons bound to defect pairs. Such lines were observed before in epitaxial GaAs of very high quality, as reported by Kunzel and Ploog. This demonstrates that the optical quality of our GaAs/AlAs core/shell nanowires is comparable to the best GaAs layers grown by molecular beam epitaxy. Moreover, strong free exciton emission is observed even at room temperature. The bright optical emission of our nanowires in room temperature should open the way for numerous optoelectronic device applications