Web Release Date: July 18,
Size-Dependent Extinction Coefficients of PbS Quantum Dots
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Contribution from the Materials Chemistry Research Group, Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada, Dipartimento di Chimica GIAF, Università di Parma, Via G. P. Usberti 17/a, I-43100 Parma, Italy, Consiglio Nazionale delle Ricerche, CNR-IMM, Area della Ricerca di Bologna, Via Gobetti 101, I-40126 Bologna, Italy, and Consiglio Nazionale delle Ricerche, Istituto di Cristallografia (CNR-IC),Via Amendola 122/O, I-70126 Bari, Italy
Received May 12, 2006
Abstract:
We report here on a detailed study on PbS colloidal quantum dots. A characterization via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) allowed us to reliably determine the diameter and the shape of the nanocrystals. These data, together with second-derivative analysis of the absorption spectra, allowed us to determine the size dependence of seven transitions in the absorption spectrum; some of these transitions were identified on the basis of their normalized confinement energy. The size dependence of the first excitonic transition was best modeled by a four-band envelope approach which considers the anisotropy of the band edges (Andreev, A. D.; Lipovskii, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15402-15404). The extinction coefficients were calculated using concentrations obtained from inductively coupled plasma atomic emission spectrometry (ICP-AES), and their size dependence was found to follow a power law with exponent equal to ~2.5. In contrast with what was expected from the effective mass approximation, the per particle absorption cross section of the lowest transition was found to be strongly dependent on the particle size.
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