Free Energy of a 1D Metal-Molecule Interface: C$_{60}$-Decorated Ag Islands*

We study the structural and dynamical properties of one- dimensional metal-molecule interfaces by investigating Ag monolayer islands on Ag(111) decorated by C$_{60}$. At 300K bare Ag(111) islands have hexagonal equilibrium shapes. When C$_{60}$ is deposited on the surface, it preferentially nucleates along island step edges, near the island corners, making them round.\footnote{C. Tao et al., Phys. Rev. B \textbf{73}, 125436 (2006).} We tune coverage so that a single chain of C$_{60}$ fully decorates the island, forming a closed ring, circular in shape. From a simple model for the C$_{60}$ step decoration, we derive the decorated step free energy as a function of step angle, yielding the equilibrium shape of the decorated islands via the Wulff construction. By comparing the model to experiment, we estimate the Ag-C$_{60}$ attraction. Using fast STM scanning, we also study the fluctuations of the C$_{60}$ decorated islands. By fitting the time correlation function of the fluctuation component Fourier modes, we show the decorated step dynamics are consistent with attachment-detachment (AD) kinetics, in contrast to the step-edge diffusion of the bare island. Finally, from our analysis, we extract the decorated step free energy and estimate the C$_{60}$-C$_{60}$ attraction.