Chemisorption of Pentacene on the Pt(111) with little molecular distortion

Aromatic molecules are promising building blocks for active and carrier-injection layers in hybrid metal-organic opto-electronic devices[1]. A complete characterization of the interface is of paramount importance in defining the properties and the efficiency of such systems, therefore they have to be carefully modelled. For example, the van der Waals (vdW) interactions play a crucial role in determining the correct adsorption geometry and energy[2]. In our work we investigated theoretically and experimentally the adsorption of a member of the acene family, namely Pentacene (Pc) (C 22 H 14 ), on the (111) surface of Platinum (Pt). We determined the adsorption energetics and geometry for several configurations by the means of density functional theory, also accounting for vdW dispersion. We performed our simulations using Quantum ESPRESSO and VASP softwares, employing the Grimme D2[3] and the modified Tkatchenko-Scheffler[4,5] TS surf vdW correction schemes. We investigated several adsorption sites. The two most favorable geometries are: the first one displaying a flat molecular profile and the second a more distorted one. The former also has the long molecular axis parallel to the Pt surface crystal directions. We verified our findings by comparison with scanning tunnelling microscopy (STM) data. In all the configurations that we tested, we observed a strong re-hybridization of the molecular orbitals with the substrate states. This result, along with the high adsorption energy and the short C-Pt distances, suggests that the Pc is undergoing chemisoption with the substrate. To get a deeper understanding of this effect we performed an X-ray absorption analysis, by the means of the photoemission (XPS) and near-edge fine structure (NEXAFS) spectra, both experimental and theoretical. In XPS we accounted for the different C-atom contributions to the spectra, due to the varying chemical environment. In NEXAFS we observed a broadening of the features, supporting our hypothesis of strong interaction. References: [1] Fratesi G. et al., Phys. Chem. Chem. Phys. 16, 14834 (2014) [2] Yildirim H. et al., J. Phys. Chem. C 117, 20572−20583 (2013) [3] Grimme S., J. Comput. Chem. 27, 1787–1799 (2006) [4] Tkatchenko A. et al., PRL 102, 073005 (2009) [5] Ruiz V.G. et al., PRL 108, 146103 (2012)