Impurity Decoration for Crystal Shape Control: C$_{60}$ on Ag(111)

The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C$_{60}$, observed via STM at 300K, dramatically changes their shape and fluctuations. We tune coverage so that a single C$_{60}$ chain fully decorates each Ag island boundary.\footnote{C.G. Tao et al., PRB 73, 125436 (2006); Nano Letters 7, 1495 (2007).} The C$_{60}$-induced rounding appears due to competing energetic and entropic effects.\footnote{T.J. Stasevich \& TLE, (SIAM) Multiscale Model. Simul. 6, 90 (2007)} We estimate the Ag - C$_{60}$ and C$_{60}$ - C$_{60}$ attractions as $\sim$0.13 eV and $\sim$0.04 eV, respectively.\footnote{T.J. Stasevich et al., submitted.} The edge fluctuations are remarkable: 1) C$_{60}$ decoration does not notably impede the step-edge diffusion (SED) and 2) while the bare-island fluctuations are driven by SED, the decorated island has the signature of non-conserved dynamics, even though the C$_{60}$s remain at the island edge. We suggest that rapidly diffusing Ag atoms randomly attracting the nearby C$_{60}$s. Generalizations of our model show that both spherical and rectangular decorating molecules will similarly lower the energy of highly-kinked boundaries, leading to similar island shape changes.