Second order resonant Raman scattering in single layer tungsten disulfide (WS2_{2})

Resonant Raman spectra of single layer WS$_{2}$ flakes are presented. A second order Raman peak (2LA) appears under resonant excitation with a separation from the E$^{1}_{2g}$ mode of only $4$cm$^{-1}$. Depending on the intensity ratio and the respective line widths of these two peaks, any analysis which neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the E$^{1}_{2g}$ mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter which is neglected in many Raman studies.Comment: 6 pages, 4 figure

Etude des propriétés électroniques de monocristaux massifs et monocouches de dichalcogénures de tungstène par magnéto-spectroscopie

In this thesis, we have studied tungsten dichalcogenides (WS2 and WSe2) by meansof steady-state µ-photoluminescence (µ-PL) and Raman spectroscopy, optical interbandabsorption and time-resolved µ-PL techniques in the visible spectral rangecombined with high magnetic fields.We demonstrate that the ratio between the trion and exciton emission can be tunedby varying the power of the laser used for excitation of the µ-PL in ungated monolayerWS2 and WSe2 samples. Moreover, the intensity of the trion emission can beindependently tuned using additional sub band gap illumination. This is a directevidence that we can control the density of carriers in a 2D system.We have investigated the resonant Raman scattering in a WS2 monolayer. We observea second order longitudinal acoustic mode (2LA) at only 4cm-1 below the firstorder E12g mode. We demonstrate, that depending on the intensity ratio and therespective line widths of these two peaks, any analysis which neglects the presenceof the 2LA mode can lead to a potentially incorrect assignment for the number oflayers.The valley dynamics in monolayer WSe2 has been probed by monitoring the emissionand polarization dynamics of neutral and charged excitons in µ-PL. We demonstratethat the exciton inter valley scattering between the K+ and K- valleys is in theorder of several picoseconds.Finally, using magneto-spectroscopy studies, we reveal the very different nature ofcarriers in monolayer and bulk dichalcogenides. We demonstrate that in monolayerWSe2, the carriers behave as massive Dirac fermions, while in bulk WSe2 we observea distinctly excitonic behavior which is best described within the hydrogen model.Dans cette thèse, nous avons étudié les propriétés électroniques de WS2 et WSe2 parµ-PL, spectroscopie Raman, absorption optique inter bandeet µ-PL résolue en temps combinées avec des champs magnétiques intenses.Nous montrons que l'émission de l'exciton par rapport au trion dans les monocouchesde WS2 et WSe2 est fonction de la puissance du laser utilisé pour l'excitation dela µ-PL. De plus, nous montrons que l'intensité de l'émission du trion peut êtrecontrôlée indépendamment en utilisant une énergie d'excitation plus basse que labande interdite. Il s'agit d'une preuve du contrôle de la densité de porteurs dansces systèmes 2D.Nous avons également étudié la diffusion Raman en résonance dans une monocouchede WS2. Nous observons un mode acoustique (2LA), seulement 4cm-1en-dessous du mode E12g. Nous montrons qu'en fonction du rapport des intensité etla largeur de ligne de chacun de ces deux pics, toute analyse qui néglige la présencede la mode 2LA peut conduire à une estimation incorrecte du nombre de couche.Les propriétés électroniques de chaque vallée d'une monocouche de WSe2 ont étésondées par µ-PL via l'étude de l'émission et de la polarisation des excitons neutreset chargés. Nous montrons que le temps de diffusion de l'exciton entre les vallées deK+ et K- est de l'ordre de plusieurs ps.Enfin, grâce à la magnéto-spectroscopie, nous mettons en évidence différents types deporteurs de charges entre la monocouche et le cristal massif. Nousmontrons que dans la monocouche, les porteurs de charge se comportent comme desfermions massifs Dirac, tandis que dans le monocristal de WSe2 nous observons uncomportement excitonique, décrit par le modèle de l'atome d'hydrogène

Site-selective measurement of coupled spin pairs in an organic semiconductor

From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet (S=0) and dark triplet quintet (S=1,2) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3–5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems

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