Femto-magnetism and electron transport at core-excited organic molecule/graphene interfaces

We investigate by ab initio theoretical methods phenomena occurring on the femtosecond time scale as a result of core level excitations in organic molecules adsorbed on graphene. Molecular chemisorption induces a magnetic ground state in graphene, that relaxes towards a non-spin polarized configuration upon the excitation of a molecular core state due to the coupling of the adsorbate energy levels with the graphene mid-gap (defect) ones. Conversely, physisorbed molecules shift from a non-magnetic to a magnetic state [1]. Femtosecond electron transfer times at interfaces can be measured by resonant core-level spectroscopies, where backward transfer (substrate-to-molecule) was also observed following the excitation at the molecule. We describe this phenomenon within a theoretical framework based on density-functional theory (DFT) and a molecular break junction setup [2,3]. The ultrafast transfer (τ=4fs) induced by N 1s excitation for bipyridine molecules on epitaxial graphene/Ni(111) [4] is significantly slowed down by the addition of a second layer of graphene [5]. This is rationalized by the transition from the strong hybridization between C and metal states in epitaxial graphene, to a decoupled interface for bilayer graphene where the C layer in contact with the molecule is less hybridized with Ni underneath. The absence of transfer in principle expected by the current approach for molecules on free-standing graphene is a stimulus for further developments. References 1. A. Ravikumar, A. Baby, H. Lin, G. P. Brivio, and G. Fratesi, Scientific Reports 6, 24603 (2016), doi:10.1038/srep24603. 2. G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118, 8775 (2014), doi:10.1021/jp500520k. 3. D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18, 22140 (2016), doi:10.1039/c6cp04099c. 4. O. Adak, G. Kladnik, G. Bavdek, A. Cossaro, A. Morgante, D. Cvetko, and L. Venkataraman, Nano Lett. 15, 8316, (2015), doi:10.1021/acs.nanolett.5b03962. 5. A. Ravikumar, G. Kladnik, M. Müller, A. Cossaro, G. Bavdek, L. Patera, D. Sánchez Portal, L. Venkataraman, A. Morgante, G.P. Brivio, D. Cvetko, and G. Fratesi, submitted.