Optical Absorption Spectra of Nanocrystal Gold Molecules Marcos M. Alvarez, Joseph T. Khoury, T. Gregory Schaaff, Marat N. Shafigullin, Igor Vezmar, and Robert L. Whetten* Schools of Physics and Chemistry and Microelectronics Research Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0430 ReceiVed: September 23, 1996; In Final Form: March 4, 1997 X The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)sshave been measured across the electronic range (1.1-4.0 eV) in dilute solution at ordinary temperature. Each of the ∼20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (∼70 to ∼800 Au atoms), has been purified by fractional crystallization and has undergone a separate compositional and structural characterization by mass spectrometry and X-ray diffraction. With decreasing core mass (crystallite size) the spectra uniformly show a systematic evolution, specifically (i) a broadening of the so-called surface-plasmon band until it is essentially unidentifiable for crystallites of less than 2.0 nm effective diameter, (ii) the emergence of a distinct onset for strong absorption near the energy (∼1.7 eV) of the interbandgap (5d f 6sp), and (iii) the appearance in the smallest crystallites of a weak steplike structure above this onset, which is interpreted as arising from a series of transitions from the continuum d-band to the discrete level structure of the conduction band just above the Fermi level. The classical electrodynamic (Mie) theory, based on bulk optical properties, can reproduce this spectral evolutionsand thereby yield a consistent core-sizingsonly by making a strong assumption about the surface chemical interaction. Quantitative agreement with the spectral line shape requires a size-dependent offset of the frequency-dependent dielectric function, which may be explained by a transition in electronic structure just below 2.0 nm (∼200 atoms), as proposed earlier. I. Introduction Nanometer-scale metal particles exhibit optical properties of great aesthetic, technological, and intellectual value. These properties are conveniently elucidated through conventional optical spectroscopic methods. 1 At a fundamental level, optical absorption spectra provide information on the electronic struc- ture of small metallic particles. To understand how these and other properties evolve from an atomic to a macroscopic state continues to be the most emotive cry in research on metal clusters and small particles. 2 On a more practical level, the unique optical properties of small metallic particles are exploited in the manufacturing of optical filters as labels for biomacro- molecules, in reversible photosensitive monochromatic glasses, 3 for intensity enhancement in Raman spectroscopy (SER effect), 4 for optical switching based on their large, ultrafast nonlinear optical response, 5 and for optical trapping (or “tweezers”), based on their high polarizability. 6 For their beauty and resilience, colloidal gold suspensions have found numerous decorative applications, such as in purple of Cassius and in the ruby glasses dating to the Middle Ages. 7 In fact, it was the color variation of colloidal gold with size that motivated Mie to apply the general theory of light extinction to small particles. 8 The absorption spectra of many metallic nanoparticles are characterized by a strong broad absorption band that is absent in the bulk spectra. Classically, this giant dipole (or surface- plasmon) band is ascribed to a collective oscillation of the conduction electrons in response to optical excitation. 9 The presence of this band in the visible region of the spectrum is responsible for the striking colors of dilute colloidal solutions of noble, alkali, alkaline earth (Ca, Sr, Ba) and rare-earth (Eu, Yb) nanoparticles. 10 Mie’s theory predicts that below a certain size, less than one-tenth of the optical wavelength, the position and width of this band should remain constant, independent of size. 7 Experimental evidence, however, indicates a slight but significant shift to lower energy accompanied by a dramatic increase in width with decreasing size. 11 For free-electron metals, Fragstein and Kreibig and others 12-14 advanced the theory that when the particle diameter becomes smaller than the electronic mean-free path in the bulk metal (ca. 20 nm for gold), the scattering of free electrons with the particle surface begins to affect their response to optical excitation. Such a simple and practical theory succeeds in explaining spectra of relatively large particles (>3 nm for gold) but stands on loose ground when applied to the smaller sizes. It is anticipated that at one point the phenomenological description of free electrons, as well as inherent fundamental assumptions of infinite lattice periodicity and a continuous energy-level spectrum, must fail. Thus far, however, it has not been possible to unequivocally identify quantum size effects in the optical spectra of metal nanoparticles prepared in macroscopic quantities, although such effects are well-known from experiments on metal-cluster beams 2 and from conductance measurements on single-metal nanostructures. 15 One problem has been that experimental measurements for the small nanoparticles are often degraded by a lack of size and shape uniformity that renders comparison with theory questionable. 16 This obstacle has been largely overcome by the discovery of methods for preparing gold:alkylthiolate assemblies that are the nanometer-scale analogue to the well-studied surface system. Specifically, on extended Au surfaces, one finds an extraordinary example 17 of the uniform protection of a surface without modification of its essential structural and electronic properties. n-Alkylthiolates and their derivatives form compact, ordered monolayers in which thiolates (RS-) or dialkyl disulfides (RSSR) reversibly attach to various Au surfaces. Their spontaneous assembly is driven by favorable (van der Waals) interactions among the long, ordered alkane chains in concert with a weak chemisorption of the sulfur head group to the metal X Abstract published in AdVance ACS Abstracts, April 15, 1997. 3706 J. Phys. Chem. B 1997, 101, 3706-3712 S1089-5647(96)02922-7 CCC: $14.00 © 1997 American Chemical Society