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ABSTRACT We report the synthesis of a series of nearly monodisperse ZnS-overcoated CdSe quantum dots whose room temperature photoluminescence quantum yield approaches 50%. This spectrally narrow (FWHM < 40nm) band edge luminescence spans the visible region from blue to red light. We use a two-step synthesis based on the high temperature decomposition of organometallic precursors in a coordinating solvent. We characterize our composite quantum dots using optical spectroscopies, wavelength dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, high resolution transmission electron microscopy, small angle x-ray scattering, and wide angle x-ray diffraction. The data indicate that samples displaying the highest quantum yield are those which have just achieved a closed shell of ZnS encapsulating the CdSe core. As more ZnS is added, the quantum yield decreases somewhat, possibly due to the many defects present in larger ZnS shells. INTRODUCTION Semiconductor nanocrystallites (quantum dots) whose radii are smaller than the bulk Quantum exciton Bohr radius exhibit interesting optical and structural properties[l]. confinement in all three dimensions of both the electron and hole leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the absorption and emission of these quantum dots shift to the blue (higher energies) as the size of the dots gets smaller. Numerous optical and structural studies have been carried out on such systems in order to better understand the evolution of a material from the molecular level to the bulk[2,3]. Core-shell type composite quantum dots, in which the interior core of the dots is composed of one material while the exterior shell is made up of another, exhibit properties which make them interesting from both an experimental and a practical point of view. While the optical and structural properties of quantum dots of uniform composition have been studied extensively, composite quantum dots are not as well characterized. Examples of core-shell type quantum dot structures in the literature include Cd(OH) 2 on CdS[4], ZnS on CdSe and the inverse structure[5], CdS/HgS/CdS quantum dot quantum wells[6] and ZnSe on CdSe[7]. Composite quantum dots can potentially be useful for the fabrication of electrooptic devies, since the core of the dots can be chosen for a specific optical property while the external shell can be selected to protect the internal dot from its surrounding environment. 359 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997Materials Research Society

Very recently Hines et al.[8] reported making (CdSe)ZnS nanocrystallites that photoluminesce with a quantum yield of 50% at 530nm. We use a modification of their synthetic procedure and that of Danek et al.[7] to synthesize high quantum yield (30%-50%) core-shell (CdSe)ZnS nanocrystallites of various sizes with narrow band edge luminescence spanning most of the visible spectrum from 470nm to 625nm. Such materials should improve the stability of elecroluminescent devices (LED's)