Topology of two-dimensional C60 domains.

Two-dimensional systems possess a unique topological ordering that is not found in either three- or one-dimensional systems1. Using high-resolution scanning tunnelling microscopy, we show here that a 60-carbon-atom (C60) array on a self-assembled monolayer of an alkylthiol forms an ideal two-dimensional system which has another novel topological order originating from the orientational degrees of freedom. At a temperature of 5 K, the two-dimensional C60 forms a domain structure in which the correlation function of the molecular orientation within a domain is constant anywhere (so every C60 has the same orientation) but changes abruptly at domain boundaries. Remarkably, the positional order and the bond-orientational order are both fully preserved across domain boundaries.