Detailed combined scanning tunneling microscopy and low-energy electron diffraction measurements reveal that the structure of a ${\mathrm{C}}_{60}$ monolayer on Ag(100) is not the previously accepted commensurate $c(6\mathrm{}\times 4)$ phase but rather an incommensurate (111) close-packed phase. The film exhibits a characteristic molecular contrast pattern with merely short-range order, and room-temperature fluctuations of the contrast show that a thermal equilibrium state is reached. The nature of this controversial bright-dim ${\mathrm{C}}_{60}$ contrast is clarified as a topographic feature due to ${\mathrm{C}}_{60}$-induced reconstruction underneath the dim ${\mathrm{C}}_{60}$ molecules. Due to interactions between the incommensurate ${\mathrm{C}}_{60}$ adlayer and the reconstructed substrate, the (111) phase is distorted laterally, forming a novel “tetramer” configuration of specific contrast order. The spatial distribution of these tetramers is aperiodic; this has crucial implications for the peculiar short-range contrast order observed experimentally. A lattice gas model with anisotropic nearest-neighbor interactions and a configuration energy of the tetramer is developed. Quantitative agreements between observation and modeling are achieved with reasonable phenomenological parameters derived within experimental constraints.

• Received 30 June 2003

DOI:https://doi.org/10.1103/PhysRevB.68.245414