Böttcher et al. Nanoscale Research Letters 2011, 6:214 http://www.nanoscalereslett.com/content/6/1/214 Page 6 of 7 The results obtained for the (√3 × √3)R30° overstruc- tures of adsorbed molecules on graphene/Ni(111) are presented in Table 2. Owing to the symmetry breaking by the Ni(111) support, two inequivalent carbon atoms in on-top positions have to be considered in these cases. The difference between adsorption behaviors of water on graphene and graphene/Ni(111) indicates the effect of the substrate underneath of the graphene layer, and can be explained by the fact, that in the latter case, the electron charge density is shifted to the interface between the graphene layer and the Ni(111) support. At the same time, similar to the case when free-standing graphene is used as a substrate, for the H2O/graphene/ Ni(111) system, DOWN orientation is the energetically most favorable one and the preferable adsorption site is the center of the carbon ring. This theoretical observa- tion confirms our prediction based on the interpretation of XAS spectra. It can be noted that during these calcu- lations, structural optimization of the system was not preformed, and only the distance between graphene and adsorbate is relaxed. Full optimization of H2O geometry in the case of C_DOWN configuration leads to d0 = 2.51 Å, that is a deviation of 2% with respect to the non-relaxed value. The corresponding interaction energy is lower by 4%, than Eint given in Table 2. For the most stable arrangement of H2O on top of graphene/Ni(111), the band structure calculations were performed. One finds the H2O-related states at the fol- lowing binding energies: 3.97, 5.96, and 9.85 eV, which satisfactorily match the APRPES data. One can see, when looking at data listed in Table 2, that in the case of ammonia, its interaction energy with the substrate is higher compared to the values obtained for the H2O/graphene/Ni(111) system, which is also in good agreement with the experimental results, where the modification of the XAS C K spectra was observed. In this context, the UP orientation is preferable for any Table 2 The interaction energies (Eint) and the equilibrium distances (d0) between H2O (NH3) and the graphene/Ni(111) substrate as obtained for the eight selected geometries at PBE-D2 level of theory System H2O/graphene/Ni(111) NH3/graphene/Ni(111) d0 (Å) 2.55 3.03 2.64 3.11 2.63 3.14 2.62 3.13 Eint (meV) 123 64 111 58 110 56 111 58 d0 (Å) 3.19 2.93 3.21 2.95 3.12 2.89 3.12 2.91 Eint (meV) 127 143 124 141 123 148 125 146 Geometry C_DOWN C_UP B_DOWN B_UP T(C1)_DOWN T(C1)_UP T(C2)_DOWN T(C2)_UP adsorption position. Although on-top (T_C1) adsorption yields the highest interaction energy, one has to be aware that the present calculations cannot give exact answer regarding the energetically most favorable adsorption position since the obtained interaction ener- gies are very close to each other (within 3%). Geometry optimization can make this difference more pronounced, especially when taking into account stronger interaction between ammonia and the considered substrate. Overall, from a theoretical side, one can see good agreement between the experimental data and the ones obtained by means of DFT calculations. However, further investigations are required before making the final conclusion regarding the position and orientation of the adsorbate with respect to the substrate under study. First, all possible arrangements of H2O and NH3 on top of graphene/Ni(111) have to be considered. Opti- mization of molecular geometry as well as relaxation of interlayer distances within the substrate has to be per- formed. Furthermore, parameter-free way of accounting for dispersion corrections is preferable. The latter is possible via van der Waals density functional, developed by Dion et al. [17]. In conclusion, the authors have studied the modifica- tion of the electronic structure of the graphene/Ni(111) system upon adsorption of water and ammonia mole- cules at low temperature. Adsorption of both types of adsorbates leads to the modifications of the XAS C K- edge spectra indicating the orbital mixing of the valence band states of graphene and adsorbates. For the occu- pied states, the small shifts of the graphene π states were detected in both cases with overall shift of the gra- phene π states to the lower binding energies reflecting the effect of p-doping (with respect to the initial state) after adsorption of water and ammonia on graphene/Ni (111). Analysis of experimental results leads us to the idea of the site-selective adsorption: water is adsorbed either in the center of carbon ring or on the bond between two carbon atoms; ammonia molecules are adsorbed on the carbon atom, which is located above the Ni interface atom. This assumption is supported by the results obtained via DFT calculations. Abbreviations ARPES: angle-resolved photoelectron spectroscopy; DFT: density-functional theory; PBE: Perdew-Burke-Ernzerhof; VASP: Vienna Ab Initio Simulation Package; XAS: X-ray absorption spectroscopy; XMCD: X-ray magnetic circular dischroism. Acknowledgements The authors would like to thank A. Preobrajenski (Max-lab) for his technical assistance during experiment. S.B., M.W., Y.D. acknowledge the financial support by MAX-laboratory (Lund). Y.D. acknowledges the financial support by the German Research Foundation (DFG) under project DE 1679/2-1. E.V. appreciates the support from the German Research Foundation (DFG) through the Collaborative Research Center (SFB) 765 “Multivalency as