Aperiodic incommensurate phase of a C60 monolayer on Ag(100) -: art. no. 245414

Detailed combined scanning tunneling microscopy and low-energy electron diffraction measurements reveal that the structure of a C-60 monolayer on Ag(100) is not the previously accepted commensurate c(6x4) 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 C-60 contrast is clarified as a topographic feature due to C-60-induced reconstruction underneath the dim C-60 molecules. Due to interactions between the incommensurate 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.