Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetism owing to the uncompensated spins of dangling bonds (DBSs). We reveal that by optical spectroscopy in high magnetic fields up to 15 Tesla using the exciton spin as probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the DBS and exciton spin polarization as well as by the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction can be adjusted by the nanoplatelet growth conditions

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

Propriétés optiques de nanocristaux de CdSe/ZnS individuels à basse température

Les nanocristaux de CdSe font l'objet d'applications émergentes dans les domaines de la nanoélectronique, des technologies laser ou du marquage fluorescent de biomolécules. Pour ces applications, la détermination de la structure fine de l'exciton de bord de bande et des mécanismes de relaxation entre sous-niveaux est d'un intérêt majeur. Cette thèse a été consacrée à l'étude spectroscopique à basse température et sous champ magnétique de nanocristaux individuels de CdSe/ZnS. La remarquable photostabilité des nanocristaux étudiés a permis de caractériser les propriétés optiques des deux états excitoniques de plus basse énergie : l'état excitonique fondamental noir, et l'état excitonique brillant situé quelques meV plus haut en énergie. Ces études ont aussi permis d'identier un état excitonique chargé (trion) et de caractériser ses propriétés photophysiques. La possibilité de générer une cascade radiative biexciton-exciton a également été démontrée dans ces systèmes.CdSe nanocrystals are attractive for many applications such as nanoscale electronics, laser technology, and biological fluorescent labelling. A detailed understanding of the band-edge exciton fine structure and the relaxations pathways between sub-levels are crucial for these applications. During this project we have studied the optical properties of single CdSe/ZnS nanocrystals at cryogenical temperature and under magnetic eld. The dramatic photostability of the nanocrystals emission has allowed the optical study of the two lowest exciton states : the "dark" excitonic state and the "bright" excitonic state, lying few meV above. These studies have also enabled us to identify a charged excitonic state (negative trion) and to characterize its photophysical properties. Besides we have demonstrated that in these nanocrystals, radiative cascade biexciton-exciton might be generated

Propriétés optiques de nanocristaux de CdSe/ZnS individuels à basse température

Les nanocristaux de CdSe font l'objet d'applications émergentes dans les domaines de la nanoélectronique, des technologies laser ou du marquage fluorescent de biomolécules. Pour ces applications, la détermination de la structure fi ne de l'exciton de bord de bande et des mécanismes de relaxation entre sous-niveaux est d'un intérêt majeur. Cette thèse a été consacrée à l'étude spectroscopique à basse température et sous champ magnétique de nanocristaux individuels de CdSe/ZnS. La remarquable photostabilité des nanocristaux étudiés a permis de caractériser les propriétés optiques des deux états excitoniques de plus basse énergie : l'état excitonique fondamental noir, et l'état excitonique brillant situé quelques meV plus haut en énergie. Ces études ont aussi permis d'identi er un état excitonique chargé (trion) et de caractériser ses propriétés photophysiques. La possibilité de générer une cascade radiative biexciton-exciton a également été démontrée dans ces systèmes.CdSe nanocrystals are attractive for many applications such as nanoscale electronics, laser technology, and biological fluorescent labelling. A detailed understanding of the band-edge exciton fi ne structure and the relaxations pathways between sub-levels are crucial for these applications. During this project we have studied the optical properties of single CdSe/ZnS nanocrystals at cryogenical temperature and under magnetic eld. The dramatic photostability of the nanocrystals emission has allowed the optical study of the two lowest exciton states : the "dark" excitonic state and the "bright" excitonic state, lying few meV above. These studies have also enabled us to identify a charged excitonic state (negative trion) and to characterize its photophysical properties. Besides we have demonstrated that in these nanocrystals, radiative cascade biexciton-exciton might be generated

Propriétés optiques de nanocristaux de CdSe/ZnS individuels à basse température

Les nanocristaux de CdSe font l'objet d'applications émergentes dans les domaines de la nanoélectronique, des technologies laser ou du marquage fluorescent de biomolécules. Pour ces applications, la détermination de la structure fine de l'exciton de bord de bande et des mécanismes de relaxation entre sous-niveaux est d'un intérêt majeur. Cette thèse a été consacrée à l'étude spectroscopique à basse température et sous champ magnétique de nanocristaux individuels de CdSe/ZnS. La remarquable photostabilité des nanocristaux étudiés a permis de caractériser les propriétés optiques des deux états excitoniques de plus basse énergie : l'état excitonique fondamental noir, et l'état excitonique brillant situé quelques meV plus haut en énergie. Ces études ont aussi permis d'identier un état excitonique chargé (trion) et de caractériser ses propriétés photophysiques. La possibilité de générer une cascade radiative biexciton-exciton a également été démontrée dans ces systèmes.CdSe nanocrystals are attractive for many applications such as nanoscale electronics, laser technology, and biological fluorescent labelling. A detailed understanding of the band-edge exciton fine structure and the relaxations pathways between sub-levels are crucial for these applications. During this project we have studied the optical properties of single CdSe/ZnS nanocrystals at cryogenical temperature and under magnetic eld. The dramatic photostability of the nanocrystals emission has allowed the optical study of the two lowest exciton states : the "dark" excitonic state and the "bright" excitonic state, lying few meV above. These studies have also enabled us to identify a charged excitonic state (negative trion) and to characterize its photophysical properties. Besides we have demonstrated that in these nanocrystals, radiative cascade biexciton-exciton might be generated

Propriétés optiques de nanocristaux de CdSe/ZnS individuels à basse température

CdSe nanocrystals are attractive for many applications such as nanoscale electronics, laser technology, and biological fluorescent labelling. A detailed understanding of the band-edge exciton fine structure and the relaxations pathways between sub-levels are crucial for these applications. During this project we have studied the optical properties of single CdSe/ZnS nanocrystals at cryogenical temperature and under magnetic eld. The dramatic photostability of the nanocrystals emission has allowed the optical study of the two lowest exciton states : the "dark" excitonic state and the "bright" excitonic state, lying few meV above. These studies have also enabled us to identify a charged excitonic state (negative trion) and to characterize its photophysical properties. Besides we have demonstrated that in these nanocrystals, radiative cascade biexciton-exciton might be generated.Les nanocristaux de CdSe font l'objet d'applications émergentes dans les domaines de la nanoélectronique, des technologies laser ou du marquage fluorescent de biomolécules. Pour ces applications, la détermination de la structure fine de l'exciton de bord de bande et des mécanismes de relaxation entre sous-niveaux est d'un intérêt majeur. Cette thèse a été consacrée à l'étude spectroscopique à basse température et sous champ magnétique de nanocristaux individuels de CdSe/ZnS. La remarquable photostabilité des nanocristaux étudiés a permis de caractériser les propriétés optiques des deux états excitoniques de plus basse énergie : l'état excitonique fondamental noir, et l'état excitonique brillant situé quelques meV plus haut en énergie. Ces études ont aussi permis d'identier un état excitonique chargé (trion) et de caractériser ses propriétés photophysiques. La possibilité de générer une cascade radiative biexciton-exciton a également été démontrée dans ces systèmes

Spectroscopy of neutral and charged exciton states in single CdSe/ZnS nanocrystals

We report a low temperature spectroscopic study of the lowest-energy exciton states in single CdSe/ZnS colloidal nanocrystals (NCs). In their neutral charge state, the zero-phonon lines of the lowest two levels of the band-edge exciton fine structure are directly observed in the photoluminescence (PL) spectrum. The attribution of these lines is confirmed by polarization-resolved studies of the PL spectrum and decay. We also identified the emission from charged NCs which displays a single zero-phonon line ~17 meV redshifted with respect to the neutral NC emission lines. This bright trion state has a lifetime which ranges from 1.5 to 4.5 ns.PROtocoles quantiques utilisant des Sources à Photons unIQue

Direct Observation of the Two Lowest Exciton Zero-Phonon Lines in Single CdSe/ZnS Nanocrystals

We report a spectroscopic study of highly photostable individual CdSe-ZnS colloidal nanocrystals. At low temperature, photoluminescence spectra display two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest levels of the band-edge exciton fine structure. For the first time, resonant photoluminescence excitation spectra of these lines is performed, and spectral diffusion broadening of 10 µeV is measured over integration times of 100 ms, corresponding to an optical coherence lifetime longer than 100 ps.PROtocoles quantiques utilisant des Sources à Photons unIQue

Band-Edge Exciton Fine Structure of Single CdSe-ZnS Nanocrystals in External Magnetic Fields

We report a spectroscopic study of the two lowest-energy exciton levels of individual CdSe-ZnS nanocrystals under applied magnetic fields. Field-induced coupling between the bright and the dark excitonic states is directly observed in the low-temperature photoluminescence spectrum and decay and allows the determination of the angle between the nanocrystal c axis and the field. Orientation-dependent Zeeman splittings of the dark and bright exciton sublevels are measured and provide the corresponding exciton Landé factors, as well as spin-flip relaxation rates between Zeeman sublevels.PROtocoles quantiques utilisant des Sources à Photons unIQue