Packing polymorphism definition

A ML can be simply defined as a one-molecule thick 2D film, but the molecular surface density has to be defined for each molecule-substrate system because it depends on the shape, size and relative orientation of the molecules. To clarify this point let us consider the examples of PTCDA and Ceo on the Ag(l 11) surface. The surface density of the substrate is 1.4 x 10 atoms cm , which is usually defined as 1 ML as a reference limit. The surface density of the (102) plane of PTCDA, the cleavage plane, is 8.4 x 10 and 8.3 x 10 cm (molecules cm ) for the monocliiuc a and polymorphs, respectively. Therefore, full coverage corresponds to 0.02 ML according to this definition. On the other hand, the surface density of a full hexagonal layer of closed-packed Ceo molecules corresponding to the (111) plane in the fcc-Ceo crystal is 1.2 x 10 " cm . Thus, Ceo would fully cover the Ag(l 11) surface at a coverage of 0.09 ML. However, other authors define 1 ML as... 

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