L Biadala

Technische Universität Dortmund, Dortmund, North Rhine-Westphalia, Germany

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Publications (10)92.17 Total impact

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    ABSTRACT: We report a time-resolved study of the photoluminescence of CdSe colloidal nanoplatelets with two different thicknesses. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. The splitting between bright and dark excitons is enhanced compared to epitaxial quantum well structures as result of dielectric confinement. Despite of strong variations in the absolute magnitude, by comparison with literature data we find a relatively slightly varying bright-dark exciton lifetime ratio in very different CdSe-based colloidal nanostructures, regardless of growth technique and of core and shell properties such as materials, dimensions, etc. This finding points to a universal mechanism in the dark exciton recombination.
    Nano Letters 02/2014; · 13.03 Impact Factor
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    ABSTRACT: The fluorescence of single colloidal thick-shell CdSe/CdS nanocrystals (NCs), at cryogenic temperature (4 K) and room temperature (RT), is studied using the intensity autocorrelation function (ACF) and lifetime measurements. The radiative and Auger decay rates corresponding to the desexcitation of the charged biexcitonic state are determined through an original method of photon postselection. Especially, the charged biexciton quantum yield increases from about 15% at RT to 60% at 4 K. The high inhibition of Auger recombination already observed for the trion state of CdSe/CdS NCs at low temperature is also demonstrated for the charged biexcitonic state. At 4 K, the ACF is equal to 1 for time scales ranging from 50 ns to 200 ms. In contrast with RT operation, the intensity of the trion emission is then perfectly stable and no blinking is observed. All the results highlight the strong confinement of the charge carriers in the CdSe core.
    12/2013; 89(3).
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    ABSTRACT: The spin dynamics in chemically synthesized CdSe/CdS core/shell nanocrystals (NCs) are studied by polarization- and time-resolved photoluminescence (PL) techniques in high magnetic fields and at low temperatures. Analysis of the recombination dynamics shows that the emission of thin-shell NCs is contributed by neutral excitons, while for thick-shell NCs it is dominated by charged excitons (trions). The sign of the PL polarization unambiguously demonstrates that these trions are negatively charged. A theoretical model of the PL polarization in an ensemble of randomly oriented NCs describes well magnetic field and time dependences of the PL polarization degree and allows us to determine the hole g factor in CdSe/CdS NCs, gh=−0.54. From direct measurements of the spin relaxation rate dependences on magnetic field and temperature, we identify the mechanism of the negative trion spin relaxation as two-phonon-assisted Raman scattering between the hole spin sublevels mixed by the applied magnetic field.
    Physical Review B 07/2013; 88(3). · 3.66 Impact Factor
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    ABSTRACT: We show that colloidal nanoplatelets can self-assemble to form 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, not observed when the nanoplatelets are not self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton and its presence in self-assembled nanoplatelets only is explained using a model based on the efficient photons re-absorption between neighboring nanoplatelets. The presence of phonon replica at low temperature in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets.
    ACS Nano 03/2013; · 12.03 Impact Factor
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    ABSTRACT: Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that their fluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, because non-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesize colloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes at low temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy in the temperature range 30-300 K shows that the non-radiative Auger recombination is thermally activated around 200 K. Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trion electrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to a route for the design of core/shell structures with 100% quantum yield at room temperature.
    Nature Nanotechnology 02/2013; · 31.17 Impact Factor
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    ABSTRACT: We use a combination of low-temperature magneto-optical and lifetime spectroscopies to study the band-edge exciton fine structure of highly photostable single CdSe/ZnS nanocrystals (NCs). Neutral NCs displaying multiline emission spectra and multiexponential photoluminescence (PL) decays are studied as a function of temperature and external magnetic fields. Three different fine structure regimes are identified as a function of the NC aspect ratio. In particular, we identify an optically inactive ground exciton state, whose oscillator strength is tuned up under magnetic field coupling to bright exciton states, and attribute it to the zero angular momentum ground exciton state of elongated NCs. We also show evidence for highly efficient biexciton emission in these NCs, with radiative yields approaching unity in some cases.
    Nano Letters 09/2011; 11(10):4370-5. · 13.03 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 10/2010; 105(15):157402. · 7.73 Impact Factor
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    ABSTRACT: We study the optical properties of the lowest-energy exciton states in highly photostable individual CdSe/ZnS nanocrystals at low temperatures. We observe two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest-energy levels of the band-edge exciton fine structure. By using resonant laser excitation on these two lines we measure a spectral broadening of 10 μeV over integration times of 100 ms.
    Journal of Physics Conference Series 09/2010; 245(1):012057.
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    ABSTRACT: 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.
    Applied Physics Letters 01/2010; 96:203111. · 3.79 Impact Factor
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    ABSTRACT: 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 microeV is measured over integration times of 100 ms, corresponding to an optical coherence lifetime longer than 100 ps.
    Physical Review Letters 08/2009; 103(3):037404. · 7.73 Impact Factor

Publication Stats

51 Citations
92.17 Total Impact Points

Institutions

  • 2013
    • Technische Universität Dortmund
      Dortmund, North Rhine-Westphalia, Germany
    • Université de Versailles Saint-Quentin
      • Groupe d'Etude de la Matière Condensée (GEMaC)
      Versailles, Île-de-France, France
    • Pierre and Marie Curie University - Paris 6
      • Laboratoire de Physique et Etude des Matériaux (LPEM)
      Lutetia Parisorum, Île-de-France, France
  • 2011
    • Université Bordeaux 1
      • UMR LP2N - Laboratoire Photonique, Numérique et Nanosciences
      Talence, Aquitaine, France
  • 2010
    • University of Bordeaux
      Burdeos, Aquitaine, France
  • 2009–2010
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France