STM study of gold-overlayer formation onC60monolayers

Scanning tunneling microscopy was used to study Au-overlayer formation on the first and second monolayers of ${\mathrm{C}}_{60}$ on a Au(111) substrate. At 300 K, Au overlayers were clustering on top of the first monolayer of ${\mathrm{C}}_{60}$, and the stronger Au-Au bonding did not break the underlying long-range close-packed order of ${\mathrm{C}}_{60}$ molecules. However, Au diffused under the second ${\mathrm{C}}_{60}$ layer and formed two-dimensional clusters. These clusters provided multiple nucleation sites and the second ${\mathrm{C}}_{60}$ monolayer was rearranged around them. As a result, the second monolayer granulated. Bonding between the second monolayer and underlying Au clusters, characterized by charge transfer, was evidence that density of states near the Fermi level became higher. Our real-space images clarified some details presented in a recent photoelectron spectroscopy study.