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The adsorption of organic molecules on metal surfaces is a powerful means to aid in the understanding of the spontaneous emergence of self-organized, extended molecular structures. The interaction between organic molecules and a metal substrate is usually small compared to the strengths of a typical covalent bond. Therefore the molecules retain mobility on the surface which facilitates the formation of extended overlayers that are stabilized by intermolecular interactions. The surface itself acts as a template for the molecules in that it provides a well-defined two-dimensional structure.
Chen and Richardson [1] have observed the chain formation of adenine molecules on a Cu(110) surface by STM and LEED. From the experiments it was concluded that adenine molecules dimerize and that these dimers form extended chains enclosing an angle of about 70 degrees with the copper row direction.
We have studied this system in the framework of density-functional theory calculations with a plane-wave basis set and periodic boundary conditions. A number of possible adenine dimers and the resulting adsorption geometries on the Cu(110) surface have been scrutinized [2]. The molecular overlayer is found to be laterally stabilized by hydrogen bridge bonds between the adenine molecules constituting the dimer and between adjacent surface unit cells. From previous work [3] we know that adenine binds directionally to the Cu surface with the amino group. The directionality and the underlying periodicity of the substrate determine the direction of the dimer chains which thus achieve near commensurability with the surface.
[1] Q. Chen and N. V. Richardson, Nature Materials 2, 324 (2003)
[2] M. Preuss et al., in preparation
[3] M. Preuss, W. G. Schmidt, F. Bechstedt, Phys. Rev. Lett. 94, 236102 (2005)
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