Coverage-dependent adsorption superstructure transition ofC60/Cu(001)

Abstract

We have investigated the growth and structure of a ${\text{C}}_{60}$ monolayer film on Cu(001) with scanning tunneling microscopy at room temperature and 100 K. We discovered that the equilibrium adsorption structure of annealed ${\text{C}}_{60}$ films depends sensitively on the initial deposition coverage; for a coverage of 0.5 monolayer ${\text{C}}_{60}$ orders in an one-bright-and-one-dim (1B1D) row sequence along the [110] direction whereas for a coverage close to one monolayer ${\text{C}}_{60}$ orders in a two-bright-and-one-dim (2B1D) sequence. At the transition region of the bright and dim row segments, ${\text{C}}_{60}$ often appears ``frizzy'' at room temperature. This indicates that a ${\text{C}}_{60}$ rotates and adopts molecular orientations with inequivalent symmetry. Upon annealing, the ${\text{C}}_{60}$ film exhibits a high thermal stability before ${\text{C}}_{60}$ fragmentation and desorption occur at $\ensuremath{\sim}880--960\text{ }\text{K}$, depending on its adsorption superstructure. The duality of equilibrium superstructure in ${\text{C}}_{60}/\text{Cu}(001)$ is unique among studied ${\text{C}}_{60}$ monolayers on metals. We argue that different boundary energy of the 1B1D and 2B1D phases offers a plausible explanation on the observed tunability of superstructure versus coverage.