Two-Dimensional Self-Assembled Gold Silicide Honeycomb Nanonetwork on Si(111)7x7

The growth evolution of Au on Si(111)7X7 at room temperature under ultrahigh vacuum conditions is studied by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Both STM filled-state and empty-state imaging show that on the 7X7 surface two distinct Au layers form one after another, each by connecting patches of adjoining Au clusters, before individual islands start to grow on top. XPS measurements of the same coverages reveal that Au exists as gold silicide in the two interfacial distinct layers and as metallic Au in the islands. The critical thickness of the gold silicide interface region is found to be two monolayers, which marks the transition from layer-by-layer to island growth. These results provide direct observation and chemical-state characterization of Au growth on Si(111)7x7 in the Stranski-Krastanov mode. Of special interest is the formation of the gold silicide honeycomb nanonetwork at 0.76 monolayer coverage, which is made up of six triangular Au clusters (around each corner hole) interconnected to one another at the dimer rows of the Si(111)7x7 substrate. With the corner holes of the 7X7 surface exposed, this new gold silicide nanonetwork, in effect, forms a two-dimensional template of nanopores (similar to 1 nm in pore size) for molecular trapping application. The gold silicide honeycomb nanonetwork also offers a new conducting phase of fundamental interest to semiconductor device fabrication and to potential applications in biofunctionalization.