Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size Michael J. Hostetler, † Julia E. Wingate, † Chuan-Jian Zhong, ‡ Jay E. Harris, † Richard W. Vachet, † Michael R. Clark, † J. David Londono, § Stephen J. Green, † Jennifer J. Stokes, † George D. Wignall, § Gary L. Glish, † Marc D. Porter, ‡ Neal D. Evans, | and Royce W. Murray* ,† Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, and Oak Ridge Institute for Science and Education, P.O. Box 117, Oak Ridge, Tennessee 37831 Received June 5, 1997. In Final Form: October 23, 1997 X The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by 1 H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (∼110-4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ∼53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, differential scanning calorimetry, contact-angle measurements, and thermal desorption mass spectrometry. The dodecanethiolate monolayer on small and large core clusters exhibits discernable differences; the line dividing “3-dimensional” monolayers and those resembling self-assembled monolayers on flat Au (2-dimensional monolayers) occurs at clusters with ∼4.4 nm core diameters. The ability to selectively synthesize metal nanoparticles of any desired size or shape would generate significant opportunities in chemistry, because catalytic, optical, magnetic, and electronic activities can be dimensionally sensitive. 1-3 Very small clusters (<∼50 metal atoms) act like large molecules, whereas large ones (>∼300 atoms) exhibit characteristics of a bulk sample of those atoms. Between these extremes lie materials with intermediate chemical and physical properties that are largely un- known; gaining access to and evidence of such materials is one of the themes of this paper. As an example, the optical properties of a metal cluster, including the intensity and energy of its surface plasmon bands, have been strongly correlated to its size. 4 The smallest clusters of some [including gold (Au)], but not all, metals exhibit electronic spectra with molecular transitions; as the number of atoms increases, over a range that depends on the particular metal in the cluster, the plasmon band intensifies until the optical spectrum resembles that of the bulk metal. Clearly, investigations of additional cluster properties over a broad range of sizes would be valuable. What is needed is the ability to selectively synthesize large quantities of these materials with ease. In this regard, recent reports on the synthesis and characterization of relatively monodisperse Au nanopar- ticles are noteworthy. 5-7 These clusters of Au atoms are stabilized to a remarkable degree by a monolayer of chemisorbed alkanethiolate ligands and are readily * Author to whom correspondence should be addressed. † Kenan Laboratories of Chemistry, University of North Carolina. ‡ Department of Chemistry, Iowa State University. § Oak Ridge National Laboratory. | Oak Ridge Institute for Science and Education. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Matijevic ´, E. Curr. Opin. Coll. Interface Sci. 1996, 1, 176-183. (b) Belloni, J. Curr. Opin. Coll. Interface Sci. 1996, 1, 184-196. (c) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. J. Phys. Chem. 1996, 100, 12142- 12153. (d) Haberland, H., Ed. Clusters of Atoms and Molecules; Springer- Verlag: New York, 1994. (e) Clusters and Colloids. 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