Homogeneous precipitation of doped zinc sulfide nanocrystals for photonic applications
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A process was developed to prepare nanocrystalline and quantum-confined particles of manganese-doped zinc sulfide. By the reaction of diethylzinc with solubilized hydrogen sulfide, particle sizes of 30-36 A were achieved by control of reactant concentration, and size appeared to vary with the thermodynamic considerations indicative of homogeneous precipitation. Managanese doping required the development of an in situ chemical reaction compatible with the homogeneous precipitation reaction. To that end, ethylmagnesium chloride was reacted with manganese chloride to form the metastable diethylmanganese which acted as the dopant source. Quantum confinement of the particles was accomplished by using methacrylic acid and poly(methyl methacrylate) polymer of low molecular weights. These surfactants were transparent to the ultraviolet wavelengths of light which allowed luminescent excitation of the material and provided surface passivation which enhanced phosphor brightness. The surfactant adsorption and effect of ultraviolet curing of the surfactant on the luminescent efficiency of the doped nanocrystals was investigated by infrared spectroscopy. These results indicate that the chemisorption of the surfactants to the nanoparticle surface and oxidation followed by crosslinking during curing are responsible for the improvement in luminescent efficiency.
I. INTRODUCTION Nanocrystals of semiconductor compounds are interesting because the optical and electronic characteristics can undergo dramatic changes as a result of threedimensional quantum confinement. 15 Many practical uses of semiconductors as photonic materials would require the incorporation of a luminescent center in the semiconductor (i.e., a dopant) which efficiently emits light. We have created such a doped semiconductor quantum dot system and have recently reported that these materials have (i) high luminescent efficiencies,6 (ii) an efficiency increase with decreased size (counter to established theories), and (iii) a luminescence decay more than five orders of magnitude faster than the corresponding radiative transition in the bulk material.7 This paper will discuss the homogeneous precipitation of 30-100 A Mn 2+ doped zinc sulfide (ZnS) particles with methacrylic acid coatings to maintain the quantum confinement and the unique processing issues which result from working with nanocomposite systems. In semiconductor crystals the electronic excitation consists of a loosely bounded electron-hole pair (the Mott-Wannier exciton) which is usually spread over a dimension much larger than the lattice constant. As the a) Present
address: Nanocrystals Technology, P. O. Box 820, Briarcliff Manor, New York 10510.
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J. Mater. Res., Vol. 10, No. 4, Apr 1995
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diameter of the semiconductor crystallites approaches this exciton Bohr diameter, the electronic properties begin to change and quantum size effects set in. 35 ' 8 For ZnS nanocrystallites smaller than 50 A in diameter (i.e., the Bohr diameter), the shift of
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