Preparation and structural studies of gold nanocrystals and their arrays

The properties of nanocrystalline substances have emerged as a subject of great fundamental and technological importance. Mesoscopic materials hold a tremendous potential due to the wide range of unusual size-dependent phenomena that occur during the evolution from the purely atomic/molecular toward bulk properties. By bringing together the methods of cluster physics (mass-spectrometric assessment of material purity) and chemical approaches to preparation and size-purification, thiol-passivated Au nanocrystals have been isolated in the size range spanning from ∼20 to ∼1000 atoms (0.9–3.3 nm). Through extensive investigation of the reaction parameters, conditions optimal for the production of highly stable small clusters have been found. Mass spectrometric characterization demonstrated that nanocrystal formation is dominated by the punctuated abundance patterns corresponding to thermochemically stable structures. Structural characteristics of Au nanocrystal cores have been revealed by large-angle x-ray diffraction, and a structural transition between bulk-fcc and non-crystallographic motifs has been deduced in the vicinity of ∼200 atoms. These nanocrystal assemblies have also been investigated by a combination of various experimental techniques, including optical, NMR, and x-ray photoelectron spectroscopy. X-ray diffraction and transmission electron microscopy were used to determine structures of spontaneously formed highly ordered 2D and 3D nanocrystal arrays. An unprecedented degree of ordering and a wide variety of crystal lattices have been found. One particular nanocrystal compound, with Au core mass near 29 kDa has been investigated in greater detail due to its particular stability and ease of high-yield preparation with various thio-adsorbate groups.