Structure prediction of organic - metal interfaces: Going beyond monolayer coverage
Alexander Egger
Institute of Solid State Physics, TU Graz
11:15 - 12:15 Wednesday 20 June 2018 PH01150 In presence of a surface organic molecules often crystallize in special phases with properties superior to those of bulk phases. However, resolving those surface-induced thin film phases remains a huge challenge, in particular for first-principle calculations. While efficient structure determination strategies exist for the adsorption of isolated molecules none of these are efficiently applicable at full monolayer coverage.
We demonstrate a strategy to bridge this gap using the SAMPLE[1] approach: By combining single molecule adsortion geometries we first create an exhaustive set of monolayer structures. Then we choose a small subset of these structures using experimental design (D-optimality criterion) and calculate their adsortion energies using dispersion-corrected density functional theory (DFT). Based on these calculations we apply Bayesian linear regression to predict adsorption energies of all discretized monolayer structures. This enables us not only to find energetically optimized structures at low coverage but also to employ ab initio thermodynamics in order to determine phases that emerge above monolayer coverage. To compare these phases with selected multilayer structures we then consider this first layer as new surface on which a second layer adsorbes.
Here we use this approach to predict phases of TCNE (tetracyanoethene) adsorbed on Cu(111) at various coverages. For this system, we found that structures consisting of face-on geometries are energetically favorable at low coverage. We then investigate whether at increasing coverage a phase transition to densely packed, upright standing molecules occurs, or whether a TCNE bilayer forms. Finally we compare those predictions with experimental results.[2]
[1] Scherbela, Hörmann, Jeindl, Obersteiner, and Hofmann, Phys. Rev. Materials 2, 043803 (2018)
[2] W. Erley and H. Ibach J. Phys. Chem., 1987, 91 pp 2947-2950
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Institute of Solid State Physics
