Optical absorption spectra of nanocrystal gold molecules

The optical absorption spectra of a series of nanocrystal gold molecules-larger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)s-have been measured across the electronic range (1.1-4.0 eV) in dilute solution at ordinary temperature. Each of the similar to 20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (similar to 70 to similar 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 (similar to 1.7 eV) of the interbandgap (5d --> 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 i 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 evolution-and thereby yield a consistent core-sizing-only 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 (similar to 200 atoms), as proposed earlier.