We present experimental data on the topography and dissipation of the organic molecules 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) and N-N´-dimethylperylene-3,4,9,10 -dicarboximide (DiMe-PTCDI) on a Ag(111) substrate by non-contact atomic force microscopy (NC-AFM). Both molecules have been subject to a variety of studies using different surface science methods including STM. They are known to form stable islands at sub monolayer coverage [1]. We achieved molecular resolution in the topography and the damping signal alike. Great care has been taken to exclude scanning artefacts. Additionally the damping signal of the PTCDA molecules shows a sub molecular contrast which can be assigned to the location of the functional groups. For a better understanding we investigated another molecule namely DiMe-PTCDI. Similar to the previous investigation we could achieve sub molecular resolution that can be attributed to the functional methyl groups of the molecule.
While the dissipation signal for PTCDA is still under debate we propose a model for the dissipation mechanism in the DiMePTCDI molecule. The model is based on calculations using AMBER force fields. This method has been successfully applied to the growth of PTCDA on KBr(001) [2, 3].
[1] K. Glöckler et al., Surf. Sci. 405, 1 (1998)
[2] T. Kunstmann et al., Phys. Rev. B 71, 121403(R) (2005)
[3] M.Fendrich et al. Nanotechnology 18 084004 (2007)
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