Böttcher et al. Nanoscale Research Letters 2011, 6:214 http://www.nanoscalereslett.com/content/6/1/214 Page 3 of 7 Figure 2 (Central panel) Photoemission intensity map shows the modification of the valence band of the graphene/Ni(111) system at the Γ point upon adsorption of water molecules (partial water-pressure p = 5 × 10-8 mbar; t is the deposition time). (Upper panel) Photoemission intensity profiles are shown for several time-points demonstrating the main photoemission features: Ni 3d states, graphene π states, and water-induced states (I and II). (Right panel) Photoemission intensity profiles as a function of water deposition time (t) taken at particular binding energies: red solid line, blue solid circles, and green open squares show intensity profiles at 7, 8.3, and 10 eV of the binding energies, respectively. (t) taken at particular binding energies (red solid line, blue solid circles, and green open squares show intensity profiles at 7, 8.3, and 10 eV of the binding energies, cor- respondingly) are shown in the upper and right panels, respectively. The behavior of the water-related photoe- mission features, I and II, allows us to conclude that island-type growth of water on graphene/Ni(111) takes place: (i) These features start to grow simultaneously at t = 130 s, but slopes of the intensities growth are differ- ent; (ii) After t = 170 s, the intensity of feature I decreases via the exponential law, and there is a small plateau for the feature II (first ML is complete); (iii) At t = 230 s, when the thickness of deposited water is more than 2ML, probably, the structural phase transi- tion takes place–formation of ice. Since ice is an insula- tor, the rapid decrease of the photoemission intensities of the Ni-related features and the shift of some states to higher binding energies can be explained by the forma- tion of an insulating thin film of ice on top of the gra- phene/Ni(111) system. The delay in starting of the growth of the water-related photoemission features is somewhat puzzling (130 s until the first water-related signal appears in the spectra), but this delay could be because some clustering centers on the graphene/Ni (111) surface are necessary to allow water growth pro- cess to start. As soon as sufficient numbers of such cen- ters are formed, the process of growth is accelerated. The general trend in the observation of the ammonia- related photoemission features in the similar experi- ments is the same. In subsequent XAS and ARPES experiments, the thicknesses of water and ammonia layers were chosen to be 1/3-1/2 of the ML (as dis- cussed above with regard to the structure). The effects of the possible orbital mixing of the valence band states of the graphene layer on Ni(111) and orbitals of water and ammonia molecules were stu- died by XAS (Figure 3). This figure shows the angular dependence of the C K-edge XAS spectra of (a) gra- phene/Ni(111) and this system after adsorption of 1/2 of the ML of (b) H2O and (c) NH3, respectively. The XAS spectra of the clean graphene/Ni(111) sys- tem (Figure 3a) were analyzed in detail in Refs. [6,9]. According to the theoretical calculations for this system, the first sharp feature in the XAS spectrum at 285.5 eV of photon energy is due to the transition of the electron from the C 1s core level to the interface state above the Fermi level (around the K point in the hexagonal Bril- louin zone), which originates from the C pz - Ni 3d Figure 3 XAS studies of water and ammonia adsorption on grapheme. Angular dependence of the C K-edge XAS spectra of (a) graphene/ Ni(111) and this system after adsorption of one-half of the ML of (b) H2O and (c) NH3, respectively.