Graphene revisited: From orbital mapping to its impact as a substrate*

Graphene, the material of the 21st century, is without doubt one of the best characterized solids. Despite the enormous amount of investigations and related publications, it still it offers a variety of exciting aspects to explore, in particular in view of its excitations. Combining density-functional theory with many-body perturbation theory, as implemented in the all-electron full-potential package \textbf{exciting }[1], provides a powerful framework for this purpose. (i) The first example concerns the question, whether we can ``see'' orbitals in an electron microscope. Indeed, transmission electron microscopy can be used for mapping atomic orbitals, as demonstrated recently by a first-principles approach [2]. For defected graphene, exhibiting either an isolated vacancy or a substitutional nitrogen atom different kinds of images are to be expected, depending on the orbital character. (ii) Graphene/BN heterostructures absorb light over a broad frequency range, from the near-infrared to the ultraviolet region, exhibiting novel features induced by the stacking [3]. Peculiar features of their excitations are inter-layer excitons that can be modulated upon layer patterning. By choosing the stacking arrangement, the electronic coupling between the individual components can be tuned to enhance light-matter interaction. (iii) As demonstrated for azobenzene monolayers, graphene as a substrate strongly impacts the photo-switching behavior of molecules [4]. Despite the weak hybridization, the photo-absorption of the molecules is remarkably modulated. While substrate polarization reduces the band-gap of the adsorbate, enhanced dielectric screening weakens the attractive interaction between electrons and holes. Furthermore, excitations corresponding to intermolecular electron-hole pairs, which are dark in the isolated monolayers, are activated by the presence of the substrate. (iv) Finally, we ask how first- and second-order Raman spectra of graphene [5] are affected by strain that may be induced by an underlying substrate. References: [1] A. Gulans, et al., J. Phys.: Condens. Matter 26, 363202 (2014). [2] L. Pardini, S. L\"{o}ffler, G. Biddau, R. Hambach, U. Kaiser, C. Draxl, and P. Schattschneider, Phys. Rev. Lett. 117, 036801 (2016). [3] W. Aggoune, C. Cocchi, D. Nabok, K.Rezouali, M. Belkhir, and C. Draxl, preprint. [4] Q. Fu, C. Cocchi, D. Nabok, A. Gulans, and C. Draxl, preprint. [5] A. Hertrich, C. Cocchi, P. Pavone, and C. Draxl, in preparation.