jp963801q - Copyright 1997 by the American Chemical Society...

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LETTERS Fluorescence Spectroscopy on Single CdS Nanocrystals J. Tittel, ² W. Go 1 hde, F. Koberling, ² Th. Basche ´ ,* A. Kornowski, § H. Weller, § and A. Eychmu 1 ller* Institut fu ¨ r Physikalische Chemie, Uni V ersita ¨ tMu ¨ nchen, Sophienstrasse 11, 80333 Mu ¨ nchen, Germany, Physikalisches Institut, Uni V ersita ¨ tMu ¨ nster, Wilhelm-Klemm-Strasse 11, 48149 Mu ¨ nster, Germany, and Institut fu ¨ r Physikalische Chemie, Uni V ersita ¨ t Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany Recei V ed: No V ember 12, 1996; In Final Form: February 7, 1997 X By use of low-temperature (20 K) confocal microscopy, fluorescence spectra of isolated single CdS nanocrystals have been recorded. The narrowest measured line width of the main fluorescence band has been determined to be about 5 meV at the lowest excitation intensities. At higher intensities at least three additional observations are made: the main fluorescence band broadens, a line to the blue of the main peak evolves, and a peak due to the coupling to a longitudinal optical phonon appears. Introduction Research on nanocrystals, quantum dots, quantum wells, and atomic and molecular clusters has expanded dramatically over the past decade. A common interest in these fields is to study physical and chemical properties of a given substance as a function of size and dimensionality. A number of good review articles mirror this development. 1 - 7 In the subfield of wet- chemically prepared semiconductor nanocrystals, the sample quality expressed in terms of monodispersity, crystallinity, and surface control has reached a state unpredicted a few years ago. Leading groups are able to prepare gram amounts of CdSe 8 and CdS 9 particles with narrow size distributions in the desired size range from 1 to approximately 15 nm in diameter, which allow studies on their optical, electronic, catalytic, and field dependent properties as a function of size. Although by means of selective spectroscopies like hole burning 10 and fluorescence line nar- rowing 11 tremendous progress has been made to elucidate optical and electronic properties of particles within an inhomogeneous ensemble, questions as to the homogeneous line width, the broadening mechanism of the electronic transitions, and the polarization dependence of the absorption and fluorescence spectra (to name just a few) remain to be answered. To approach questions like these, one has to enter the domain of single-particle detection where measurements are not influenced by inhomogeneous contributions. In this field various spectro- scopic and microscopic techniques have been employed, which, e.g., allow the observation of the excitation and emission spectra of single molecules in solids 12,13 or on interfaces. 14,15
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This note was uploaded on 09/30/2010 for the course MANAGEMENT MG2432 taught by Professor Muhammadadeel during the Spring '10 term at École Normale Supérieure.

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jp963801q - Copyright 1997 by the American Chemical Society...

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