Adsorption, clustering and reactions of H atoms on graphene

Recent years have witnessed an ever increasing interest in studying the interaction of hydrogen atoms with graphenic surfaces. Such interest comes from its importance in fusion reactors, possible relevance in hydrogen storage, necessity in understanding hydrogen formation in interstellar medium, and from the opportunities it offers in the rapidly exploding field of graphene-related device fabrication. In this contribution we summarize the basic features of this interaction and the dynamical behaviour it gives rise to, as they result from a combined theoretical study using electronic structure (TB, DFT, MRPT) and quantum wave packet calculations. We start considering adsorption of a single H atom on graphene, which has been long known[1] to be an activated process requiring substantial lattice reconstruction. We then show how, differently from metals, formation of the adsorbate-substrate bond strongly modifies the electron properties of the substrate at the Fermi level, through the formation of so-called midgap states (itenerant electrons). The latter, in turn, introduce a bias in the adsorption properties[2] which is responsible for the observed, markedly non-random adsorption pattern, i.e. clustering of H atoms on the surface[3]. We show how binding (barrier) energies increase (decrease) linearly as a function of the site-integrated magnetization, which is a measure of the site-occupation for the itinerant electron. Midgap states, and the related bias to adsorption, are common to all graphenic substrates, and we show that they lead to analogous results in Polycylic Aromatic Hydrocarbons (PAH). The latter further highlight the importance of edge sites which, having a reduced hindrance, show better adsorption properties[4]. Next, we consider dynamics of hydrogen atoms on graphene, focusing in particular on the Eley-Rideal reaction out of a single, adsorbed H atom. We consider the rigid, flat surface model[5] and provide exact, quantum results for this model in both high and low collision energy regimes, showing evidence for unusual quantum effects at high energies[6] and quantum reflection at low energies[7]. We conclude by discussing how controlled adsorption of hydrogen atoms can be used to engineer graphene electronic structure, e.g. opening that band-gap which is of paramount importance for graphene-based logic-devices. In particular, we show how a symmetry-preserving band-gap opening can be achieved by arranging H atoms to form certain honeycomb superlattices[8]. [1] Jeloaica L. and Sidis V., Chem. Phys. Lett., 1999, 300, 157; Sha X. and Jackson B, Surf. Sci., 2002, 496, 318 [2] Casolo S., LØvvik O. M., Martinazzo R. and Tantardini G.F., J. Chem. Phys., 2009, 130, 054704 [3] Hornekaer L., Rauls E., Xu W., Sljivancanin Z. Otero R., Stensgaard I., Laegsgaard E., Hammer B., Besenbacher F. , Phys. Rev. Lett., 2006, 97, 186102 [4] Bonfanti M. Casolo S., Tantardini G.F. and Martinazzo R., in preparation [5] Persson M and Jackson B, J .Chem. Phys., 1995, 102, 1708; Lemoine D. and Jackson B., Comp. Phys. Comm., 2001, 137, 415 [6] Martinazzo R. and Tantardini G.F., J. Phys. Chem. A, 2005, 109, 9379; J .Chem. Phys., 2006, 124, 124702; J .Chem. Phys., 2006, 124, 124703. [7] Casolo S., Bonfanti M., Martinazzo R. and Tantardini G.F., J. Phys. Chem. A, 2009, 113, 14545. [8] Martinazzo R., Casolo S. and Tantardini G.F., Phys. Rev. B, 2010, 81, 245420.