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.

In recent years, an increasing number of laboratories have been applying in situ heating (and ultimately, gas reaction) techniques in electron microscopy studies of catalysts and other nanophase materials. With the advent of aberration-corrected electron microscopes that provide sub-Angström image resolution, it is of great interest to study the behavior of materials at elevated temperatures while maintaining the resolution capabilities of the microscope. In collaboration with Protochips Inc., our laboratory is developing an advanced capability for in situ heating experiments that overcomes a number of performance problems with standard heating stage technologies. The new heater device allows, for example, temperature cycling from room temperature to greater than 1000 degrees C in 1 ms (a heating rate of 1 million Centigrade degrees per second) and cooling at nearly the same rate. It also exhibits a return to stable operation (drift controlled by the microscope stage, not the heater) in a few seconds after large temperature excursions. With Protochips technology, we were able to demonstrate single atom imaging and the behavior of nanocrystals at high temperatures, using high-angle annular dark-field imaging in an aberration-corrected (S)TEM. The new capability has direct applicability for remote operation and (ultimately) for gas reaction experiments using a specially designed environmental cell.

Maya blue paint was often used in Mesoamerica. The origin of its color and its resistance to acids and biocorrosion have not been fully understood. High-resolution transmission electron microscopy, electron energy loss spectroscopy, and x-ray microanalysis studies of authentic samples show that palygorskite crystals in the paint form a superlattice that probably occurs as a result of mixing with indigo molecules. An amorphous silicate substrate contains inclusions of metal nanoparticles encapsulated in the substrate and oxide nanoparticles on the surface. The beautiful tone of the color is obtained only when both the particles and the superlattice are present.

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