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.

Gold nanocrystals passivated by self‐assembled monolayers of straightchain alkylhiolate molecules have been obtained as highly purified molecular materials of high intrinsic stability. Evidence is presented for a predicted discrete sequence of energetically optimal fcc structures of a truncated octahedral morphological motif (see cover). The nanocrystal materials have a propensity to form extended superlattics, such as that in the Figure.

A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.

Five massive gold-cluster molecules have been isolated in high yield and have undergone separate structural characterization, and their electronic structure has been deduced by optical absorption spectroscopy. These new molecules are distinguished by a crystalline (or quasicrystalline) core of densely packed Au atoms, ranging in size from ∼1.1 nm (∼40 atoms) to ∼1.9 nm (∼200 atoms), surrounded by a compact monolayer of various thio (RS) adsorbates. They are obtained as the thermally and environmentally stable products of the reductive decomposition of nonmetallic (−AuS(R)−) polymer in solution, are separated according to size by fractional crystallization or column chromatography, as monitored by high-mass spectrometry, and are characterized structurally by methods including X-ray diffraction (small and large angle), high-resolution electron microscopy, and scanning tunneling microscopy. The optical absorption spectra of dilute solutions of these molecules show size-dependent steplike structure with an onset near the fcc Au interband edge (Δ = 1.7), indicative of transitions to the discrete lowest unoccupied levels of the conduction band. This structure is evident in the smallest clusters even at room temperature, is enhanced at low temperature, and emerges generally as predicted by Kubo's criterion for quantum size effects. It thus requires no assumption of a transition from the bulk metallic bonding character to a nonmetallic (rehybridized or oxidized) state.

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The electronic dynamics of gold nanocrystals, passivated by a monolayer of alkylthiol(ate) groups, were studied by transient spectroscopy after excitation with subpicosecond laser pulses. Three solution-phase gold samples with average particle size of 1.9, 2.6, and 3.2 nm with size distribution less than 10% were used. The photoexcitation in the intraband (surface plasmon region) leads to the heating of the conduction electron gas and its subsequent thermalization through electron-electron and electron-phonon interaction. The results are analyzed in terms of the contribution of the equilibrated "hot" electrons to the surface plasmon resonance of gold. A different spectral response was observed for different sizes of gold nanoparticles. The results were compared to the dynamics of the large (30 nm diameter) gold nanocrystals colloidal solution. The size-dependent spectral changes are attributed to the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the cooling process in gold nanoparticles with changing the laser intensity is attributed to the temperature dependence of the heat capacity of the electron gas.

The broad, intense peak found near 29 kDa in the laser-desorption mass-spectral abundances of various aurothiol (Au:SR) cluster compounds has been used to optimize the preferential formation of the species in that mass range. Recrystallization gives enriched fractions, on the 10 mg scale, that in several cases appear free of species outside that mass range. Elemental analysis and X-ray photoelectron spectra (XPS) confirm the absence of elements other than Au, S, C, and H, while infrared and NMR (1H,13C) spectra are consistent with intact thio groups. The Au 4f7 /2 XPS peak is only slightly shifted (∼0.2 eV) and broadened from that of bulk Au(0) metal, and intense optical absorption extends far into the infrared region (0.5 eV), consistent with a metallic Au core. Recrystallized samples of the R = C4, C6, and C12 materials readily form highly diffractive crystalline films, powders, and single crystals on the scale of 10 μm, consistent with perfect ordering in >100 nm grains, and a negligible amorphous content. Uniformity is quantified through several independent measures:  (1) the mass spectrometrically determined core mass of 29.2 ± 2 kDa is invariant to that of the thiol used, indicating an equivalent Au core diameter of 1.68 ± 0.05 nm, assuming bulk density. (2) The powder X-ray diffraction intensities are sensitively fit to a Au core of 1.64 ± 0.03 nm equivalent diameter. (3) The powder patterns index unambiguously to bcc packing, with nearest-neighbor distances of 2.68 ± 0.02 nm (R = C6) and 3.15 ± 0.02 nm (R = C12). An fcc packing structure with 2.56 ± 0.04 nm distance (11.9 nm3 volume) is found for R = C4. A formulation consistent with this mass spectral, diffraction, and average compositional information is Au144 - 146(SR)50 - 60, with the structure of the inorganic core being influenced by the type of adsorbate used to produce the entire inorganic/organic assembly.