Böttcher et al. Nanoscale Research Letters 2011, 6:214 http://www.nanoscalereslett.com/content/6/1/214 hybridization and corresponds to the antibonding orbital between a carbon atom C-top and an interface Ni atom. The second peak in the XAS spectrum at 287.1 eV of photon energy is due to the dipole transition of an elec- tron from the C 1s core level to the interface state above the Fermi level (around the M-point in the hexa- gonal Brillouin zone) which originates from C pz - Ni px,py,3d hybridization and corresponds to a bonding orbital between C-top and C-fcc atoms, involving a Ni interface atom. As was found in the experiment, the observed hybridization leads to the orbital mixing of the valence band states of graphene and Ni and to the appearance of the effective magnetic moment of carbon atoms in the graphene layer. This moment was detected in the recent XMCD measurements of this system [6], which allow estimating the spin-magnetic moment of carbon in the range 0.05-0.1 μB per atom. The XAS spectra of the H2O/graphene/Ni(111) and the NH3/graphene/Ni(111) systems measured at the C K absorption threshold are shown in Figure 3b,c, respec- tively. These results demonstrate the controllable way of the graphene functionalization by water and ammonia. The corresponding adsorbate-induced states in the region of the unoccupied valence band states were detected (Figure 3: the photon energies in the region of 280-290 eV correspond to the C 1s ® π* transitions; the photon energies in the region of 290-320 eV corre- spond to the C 1s ® s* transitions). In this context, it is worth emphasizing that the presented XAS measure- ments were recorded at the C K absorption edge and that they reflect (to some extent) the partial density of states of the carbon atoms in the system [10], and they clearly demonstrate the appearance of the orbital hybri- dization of the graphene-, water-, and ammonia-related states. The absence of the strong angular variations of the water- and ammonia-induced XAS signals might be explained by the statistically uniform distribution of the orientations of H2O and NH3 molecules on graphene/Ni (111). The interpretation of the XAS spectra measured after water or ammonia adsorption can be performed on the basis of the peak-assignment, which has been presented above. For the water adsorbate, the new structure in the XAS spectra appears at the photon energy range corre- sponding to the hybrid state in the electronic structure of graphene/Ni(111) involving both carbon atoms in the unit cell of graphene and interface Ni atom. This leads to the assumption that water molecules are adsorbed either in the center or in the on-bond position on gra- phene/Ni(111) (Figure 1). Ammonia-induced spectral features in the C K XAS spectra are observed in the photon energy range corresponding to the hybrid state which is a result of hybridization of the pz orbital of the C-top atom and the 3dz2 state of the Ni interface atom. Page 4 of 7 On the basis of this analysis, one can conclude that ammonia molecules are placed in the on-top position on graphene/Ni(111) with the lone-pair toward carbon atoms and N-H bonds along C-C bond of the graphene layer. Figure 4 shows a series of ARPES collected with the photon energy hν = 75 eV along the Γ-K direction of the Brillouin zone for the graphene/Ni(111), H2O/graphene/ Ni(111), and NH3/graphene/Ni(111) systems. In all the series, one can clearly discriminate the dispersions of gra- phene π- and s-derived states in the region below 2 eV of the binding energy as well as Ni 3d-derived states near EF. The binding energy difference of ≈2.4 eV for the π states and ≈1 eV for the s states in the center of the Bril- louin zone (in the Γ point) between graphite and gra- phene on Ni(111) is in good agreement with previously reported experimental and theoretical values [4,5,8], and it is explained by the differential strengths of hybridi- zation for π and s states in relation with Ni 3d states. The effect of hybridization between Ni 3d and graphene π states can be clearly demonstrated in the region around the K point of the Brillouin zone: (i) one of the Ni 3d bands at 1.50 eV changes its binding energy by ≈150 meV to larger binding energies when approaching the K point; (ii) a hybridization shoulder is visible in photoe- mission spectra which disperses from approximately 1.6 eV to the binding energy of the graphene π states at the K point. The full analysis of the electronic band structure and magnetic properties of the graphene/Ni(111) system were performed in Ref. [9]. The adsorption of 1/2 of ML of water and ammonia molecules on graphene/Ni(111) leads to the appearance of the additional photoemission signal in the spectra at 6.5 and 7.3 eV, respectively (Figure 4). In these spectra, these emissions are associated with the H2O-3a1 and NH3-1e states, respectively. As can be clearly seen from the photoemission spectra, the adsorption of H2O or NH3 on graphene/Ni(111) leaves the electronic structure of graphene π- and Ni 3d-states almost intact. This observation can be taken as an indication of the inert- ness of the graphene layer on Ni(111) as was earlier demonstrated in Ref. [4]. There are only small changes of the electronic structure of graphene/Ni(111) upon adsorption of water or ammonia. The small shift of about 150 meV of the graphene π band to the small binding energies is detected at the Γ point of the Bril- louin zone in both cases. At the K point, there is a shift of this band to the higher binding energies of about 50 and 70 meV for the water and ammonia adsorptions, respectively. Thus, ARPES and XAS data allow us to refer the inter- action between considered molecules and the graphene/ Ni(111) system as physisorption. From a theoretical point of view, physisorption can be considered as weak