In recent years, various highly regular supramolecular architectures of self-organized molecules on crystals have been designed and used in the bottom-up device technology. In the past metal surfaces have frequently been used as substrate. However, metal substrates induce screening and quenching effects and thus hamper the detailed spectroscopy of the adsorbed overlayer. In contrast, molecular adsorption on ionic crystals like  sodium or potassium chloride opens the possibility to study electronically decoupled molecules.

Here we present density-functional theory calculations on the adsorption of PTCDA on KCl and NaCl (100) surfaces. Thereby, the adsorption on flat surfaces as well as stepped substrates steps is investigated. The adsorption of PTCDA at special defective step sites is strongly favored over flat terraces and initiates film growth.   Experimentally, both flat-lying (P-type) [1,5] as well as standing molecules (S-type) [1,2] are observed depending on the preparation conditions. In order to understand and rationalize the adsorption mechanisms leading to the formation of these structures, a large variety of interface geometries was studied and analyzed with respect to the contribution of ionic, covalent and van-der-Waals interactions between adsorbates and substrate. The influence of the substrate and bonding mechanism on the molecular electronic structure is investigated in detail and compared with the experimental data available [3,4].  

[1] M. Möbus et al. Thin Solid Films 175, 89 (1989).

[2] M. Möbus et al. Journal of Crystal Growth 116, 495 (1992).  

[3] T. Dienel et al. Advanced Materials, 20, 959 (2008).

[4] H Karacuban et al. Nanotechnology, 22, 295305 (2011).

[5] Bruke et al PRL 100,186104 (2008)

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