Isolation of Smaller Nanocrystal Au Molecules:  Robust Quantum Effects in Optical Spectra

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.