Optical properties of nanosized gold particles
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Optical properties of nanosized gold particles Robert H. Doremus and Pratima Rao Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received 31 July 1995; accepted 19 June 1996)
Gold particles smaller than 2 nm in diameter were grown in glass. Their optical absorption spectrum did not show the usual plasma absorption band for gold particles; this band was spread by the small particle size. The absorption spectrum was calculated from the bulk optical properties of gold, modified for the small particle size; there was good agreement between calculated and measured spectra. The smallest particles containing from 20 to 40 gold atoms showed a l24 dependence of absorption on wavelength l. This result implies that the absorption in these particles was entirely from free electrons at wavelengths above 0.3 mm.
I. INTRODUCTION
Small metallic particles have attractive colors and are of scientific and technological interest. They can test measurements of optical properties of metals,1,2 and have potential uses as nonlinear devices.3,4 They also exemplify the transition from metal atoms to bulk metal. In recent years there has been much interest in the optical absorption of small metal clusters containing from five to fifty atoms. Experimental work on metal clusters is reviewed in Ref. 5, and theoretical work in Refs. 6, 7, and 8; in these papers the emphasis is on small clusters of alkali metals in the vapor. Earlier reviews are in Refs. 9 and 10; in the latter the emphasis is on silver particles. New work continues to be reported, for example temperature effects on optical absorption of clusters (experiment, Ref. 11; theory, Ref. 12) and theory of shape isomerism in sodium clusters.13 The conclusions of these papers are discussed in the section on particle size below. In the present work small gold particles were grown in a matrix of potassium borosilicate glass. Glass is an excellent matrix for growing small metallic particles, because they can be nucleated by heat or radiation, and then grown at a higher temperature, resulting in a very narrow size distribution. Since optical properties depend strongly on particle size, this method allows one to study these properties as a function of particle size, without uncertainties caused by a size distribution. The uniformity of particle sizes grown in glass has been demonstrated by electron microscopy.14,15 The outline of this paper is as follows: II. A brief description of the calculation of optical absorption for spherical particles from their optical proprieties, and the choice of the most suitable measurements of the optical properties of gold. III. A discussion of previous experimental and theoretical work on the influence of particle size on the optical properties of metals. IV. The methods of calculating absorption spectra of very small 2834
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 11, Nov 1996
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gold particles for comparison with the experim
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