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Register a new userPolarimetry-based analysis of dipolar transitions of single colloidal CdSe/CdS dot-in-rods
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We prove experimentally, upon polarization analysis performed on a large statistic of single nanoemitters, that high quality core/shell CdSe/CdS dot-in-rods behave as linear dipoles. Moreover, the dipole in-plane and out-of-plane orientations could be assessed. We demonstrate in particular that, contrary to expectations, the emitting dipole is not aligned with the elongated axis of the dot-in-rod. Besides, the polarimetric measurements prove that the excitation transition cannot be approximated by a single linear dipole, contrary to the emission transition. Finally, we highlight that non-radiative channels of charge carrier recombination do not affect the dipolar nature of the radiative transitions.
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The photon statistics of CdSe/CdS dot-in-rods nanocrystals is studied with a method involving post-selection of the photon detection events based on the photoluminescence count rate. We show that flickering between two states needs to be taken into account to interpret the single-photon emission properties. With post-selection we are able to identify two emitting states: the exciton and the charged exciton (trion), characterized by different lifetimes and different second order correlation functions. Measurements of the second order autocorrelation function at zero delay with post- selection shows a degradation of the single photon emission for CdSe/CdS dot-in-rods in a charged state that we explain by deriving the neutral and charged biexciton quantum yields.
The blinking dynamics of colloidal core-shell CdSe/CdS dot-in-rods is studied in detail at the single particle level. Analyzing the autocorrelation function of the fluorescence intensity, we demonstrate that these nanoemitters are characterized by a short value of the mean duration of bright periods (ten to a few hundreds of microseconds). The comparison of the results obtained for samples with different geometries shows that not only the shell thickness is crucial but also the shape of the dot- in-rods. Increasing the shell aspect ratio results in shorter bright periods suggesting that surface traps impact the stability of the fluorescence intensity.
Colloidal quantum dots (cQDs) are now a mature nanomaterial with optical properties customizable through varying size and composition. However, their use in optical devices is limited as they are not widely available in convenient forms such as optical fibers. With advances in polymerization methods incorporating nanocrystals, nanocomposite materials suitable for processing into high quality hybrid active fibers can be achieved. We demonstrate a plastic optical fiber fabrication method which ensures homogeneous dispersion of cQDs within a polymer core matrix. Loading concentrations between 10$^{11}$-10$^{13}$ CdSe/CdS cQDs per cm$^{3}$ in polystyrene were electronically imaged, confirming only sporadic sub-wavelength aggregates. Rayleigh scattering losses are therefore dominant at energies below the semiconductors band gap, but are overtaken by a sharp CdS-related absorption onset around 525 nm facilitating cQD excitation. The redshifted photoluminescence emission is then minimally reabsorbed along the fiber with a spectrum barely affected by the polymerization and a quantum yield staying at $sim$65$%$ of its initial value. The latter, along with the glass transition temperature and refractive index, is independent of the cQD concentration hence yielding a proportionally increasing light output. Our cQD-doped fibers are photostable to within 5$%$ over days showing great promise for functional material applications.
We present a method to realize active optical tips for use in near-field optics that can operate at room temperature. A metal-coated optical tip is covered with a thin polymer layer stained with CdSe nanocrystals or nanorods at low density. The time analysis of the emission rate and emission spectra of the active tips reveal that a very small number of particles - possibly down to only one - can be made active at the tip apex. This opens the way to near-field optics with a single inorganic nanoparticle as a light source.
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.
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