Optical Absorption Spectra of Nanocrystal Gold Molecules

Abstract

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.

Microstructural evolution of Au/TiO2 nanocomposite films: The influence of Au concentration and thermal annealing

“…Another important aspect of noble NPs, when dispersed in a dielectric matrix, is that their optical response strongly depends on their clustering tendency, namely the clusters size, shape and distribution [3,35,36], the interaction between them [7,37], but also on the host matrix dielectric function itself [11,[38][39][40][41].…”

Surface plasmon resonance of capped Au nanoparticles

“…9 The proposed model can also explain the controversy about the metallic or insulating character of thiol-capped Au NPs. For NPs below 2 nm in size, several authors observe localized charge behavior 9,20,21 while recent studies showed the presence of free electrons. 14,15 According to our model ͑see Fig.…”

Morphology of gold nanoparticles determined by full-curve fitting of the light absorption spectrum. Comparison with X-ray scattering and electron microscopy data

“…Having determined R S independently from the SAXS data, we are now in a position to evaluate A (see Figure 4). Its average value is about A = 0.35, to be compared with the literature values of 0.6 − 2 for gold spheres [24][25][26] . …”