Multiexcitons in a Single Core-Shell Colloidal Quantum dots
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1207-N05-10
Multiexcitons in a Single Core-Shell Colloidal Quantum dots Dima Cheskis, Ruth Osovsky, Viki Kloper, Aldona Sashchiuk, and Efrat Lifshitz* Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute and Solid State Institute, Technion, Haifa 32000, Israel, * [email protected] ABSTRACT The micro-photoluminescence (µ-PL) spectra of a single CdTe/CdSe quasi- type II core-shell colloidal quantum dots (CQDs) unambiguously reveal an emission of a biexciton (2X), a triexciton (3X) and a quadraexciton (4X) under continuous-wave excitation. The CQDs were characterized by optical measurements and theoretical consideration of carrier's wavefunctions distribution of a quasi-type II core-shell structure. INTRODUCTION Semiconductor colloidal quantum dots (CQDs) exhibit unique properties with special scientific and technological importance [1-3]. Multiexcitons generation (e.g., biexcitons [2X], triexcitons [3X]), quadra excitons [4X]) or their charged analogies (e.g. 2X+), have recently been the focus of special interest [1], as the groundwork of gain devices, photovoltaic cells and single photon light sources. Multiexciton generation has been indirectly observed in colloidal quantum dots, both in solution and the solid state, however, this phenomena in a single CQD, which exhibit luminescence intermittency (blinking), produced due to Auger fast relaxation (~ps) of multiexcitons generated in the CQDs is only in the beginning of the investigation [4,5]. In the recent publication [6] we showed, for the first time, well-resolved 2X, 3X and 4X multiexciton emission bands in a single core-shell CdTe/CdSe CQD with high luminescent quantum efficiency (QE) [88%], which show a suppressed blinking behavior as observed in the continuouswavelength (CW) micro-photoluminescence (µ-PL) spectrum. The present work reports the descriptions of the samples’ preparation, their composition, optical characterization, and existence of the mentioned multiple excitons. Identification of these multiexcitons was based on the evolution of the µ-PL spectrum with a change in the pumping power. The binding energy of a multiexciton was determined by a standard second order perturbation theory, showing close agreement with the relevant experimental multiexciton emission energy. EXPERIMENTAL The investigated CdTe core and CdTe/CdSe core-shell CQDs were prepared by colloidal chemistry, according to the procedures given in Ref. [7]. This unique colloidal procedure allows the existence of Cd 0 ⇔ Cd +2 equilibrium during the reaction process and regulates precursors supply; emission QE of the produced CdTe CQDs was up to 80%. This method allowed the production of high quality CdTe/CdSe core-shell
structures (emission QE up to 88%). The samples are characterized by transmission electron microscopy (TEM), Raman and optical (absorption, PL and µ-PL) measurements, as discussed in our previous works [7, 8]. RESULTS and DISCUSSION TEM images of CdTe/CdSe CQDs are shown in Fig. 1(a). The inset in Fig. 1(a) corresponds to a fast Fourier transf
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