Quantum Size Effect in Metal Coated Nanoparticles
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QUANTUM SIZE EFFECT IN METAL COATED NANOPARTICLES
H.S. Zhout, I. Honmat, H. Komiyamat and J.W. Haust t Dept. of Chemical Engineering, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, JAPAN t Physics Dept., Rensselaer Polytechnic Institute, Troy, NY 12180-3590
ABSTRACT In this paper we report the synthesis of Au 2S nanoparticles with a gold coating on the surface; the particles are characterized by transmission electron microscopy (TEM), electron diffraction and absorption data. The absorption spectrum is recorded during the gold coating growth process; this data demonstrates a large resonance shift during the growth process. The results are consistent with the growth of a metal shell around the particles; the theoretical model includes electron quantum confinement in the metal shell. The size dependence of the frequency shift, as well as the width of the resonance are consistent with the model. INTRODUCTION Quantum properties of carriers in materials is the subject of ongoing, intensive research and the goal of designing and improving the physical properties of materials for specific tasks continues to challenge researchers. With the desirability of great control over the electronic properties, the confinement of carriers in three dimensions has developed as an important field of research over the past decade.' Semiconductor materials have demonstrated enormous changes in their electronic and optical properties. Metals too have demonstrated features that are attributed to quantum confinement of the conduction electrons. However, until now an unambiguous determination of absorption peak shifts, which are so common for semiconductor nanoparticles, have not been reported for metal particles; metals have displayed broadening of the resonance structure, which is attributed to the confinement of the electrons2 , but quantum shifts of the resonance have been small and controversial. PREPARATION The particles were prepared by a two-step process. First, we dissolved chloroauric acid (HAuC144H 20) and sodium sulphide Na 2S9H 2 0 into superpure water at room temperature. We put 12 ml 1.0 mM Na 2 S solution into 10 ml 2.0 mM HAuCl 4 solution to get the unstable gold sulphide Au 2S (if the quanity of Na 2S is more than the quantity of HAuCl 4, we can get the stable Au2S3) 3. The color of the Au 2S solution is shallow brown. Second, we inject 2 ml 1.0mM Na2S into the above Au2 S solution. The color changes from shallow brown to gray, shallow red, red and dark red. In the reaction the gold atom in the Au - S bond on the surface of Au2 S was reduced by the S2- ion 4 and the surface can become gold coated. This process is supported by TEM images that show spherical particles with facets whose average radius is about 2.5 nm coexisting with particles whose average radius is 10 nm; also, the radii of the Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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Au2 S particles before precipitating the gold is approximately the same. The electron diffraction pattern of the particles after gold
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