Adsorption geometry of glycine on Cu(001) determined with low-energy electron diffraction and scanning tunnelling microscopy

Using low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) it has been found that glycine molecules adsorbed on Cu(001) can form but only the (2x4) and c(2x4) superstructures. On the basis of the missing LEED spots of the surface, it has been concluded that: each (2x4) unit cell consists of two molecules, one being the mirror image of the other; the C-C axis of both molecules lies in the mirror plane of the Cu substrate without a significant shift and twist from the plane; and the two O atoms of the carboxylate group of both molecules locate at the same height level without significant buckling. According to these conclusions, a structural model has been proposed for the (2x4) superstructure (a model for the c(2x4) superstructure already exists). We argue that the (2x4) and c(2x4) superstructures must have similar specific surface free energy, that their hydrogen bonds must be of N-H-O-H type, and that their local adsorption geometry must be similar or even the same. The advantage of combining STM with LEED to determine surface structures is clearly demonstrated.