Abstract:

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 ~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 (crystalline 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 that 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 -- 6sp), and (iii) the appearance in the smallest crystallites of a 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 (~200 atoms), as proposed earlier.

Download complete document in Adobe Acrobat format.