Insights on the Excitation Spectrum of Graphene Contacted with a Pt Skin.

The excitation spectrum in the region of the intraband (Dirac plasmon) and interband ( π plasmon) plasmons in graphene/Pt-skin terminated Pt 3 Ni(111) is reproduced by using an ab-initio method and an empirical model. The results of both methods are compared with experimental data. We discover that metallic screening by the Pt layer converts the square-root dispersion of the Dirac plasmon into a linear acoustic-like plasmon dispersion. In the long-wavelength limit, the Pt d electron excitations completely quench the π plasmon in graphene at about 4.1 eV, that is replaced by a broad peak at about 6 eV. Owing to a rather large graphene/Pt-skin separation (≈3.3 Å), the graphene/Pt-skin hybridization becomes weak at larger wave vectors, so that the π plasmon is recovered with a dispersion as in a free-standing graphene.

. Panels (a) and (b) report the real-time evolution of the C 1s signal during the heating of the Pt3Ni(111) in an ethylene environment. Panels (c), (d), and (e) represent the C 1s core level measured with the sample kept at 350, 650, and 800 K, respectively. The black regions represent the Shirley background, which has been subtracted before fitting the resulting spectrum with Voigt line-shapes. The inset of panel (d) reports the corresponding vibrational spectrum. The inset of panel (e) shows the behaviour of the photoemission intensity at a BE of 284.1 eV, corresponding to the graphene phase, as a function of temperature.
From the temperature evolution of the C 1s core level for ethylene-dosed Pt3Ni(111) in the range between 438 and 715 K, we can assert that CCH3 evolves into -CH groups, as evidenced by the relative increase of the component at BE 284.2 eV. The spectral weight of the peak at 283.6 eV changes with temperature and it disappears at 741 K.
The C 1s signal for temperature in the range 777 -823 K has a single narrow component at 284.1 eV. Both the sharpness of the line-shape and its BE suggest that a graphene phase is formed in this temperature range.
However, for temperature higher than 823 K, the intensity of the C 1s signal abruptly decreases (Error! Reference source not found., panels a and b) and disappears at 921 K. This finding indicates that carbon atoms dissolve into the bulk of the Pt3Ni crystal.
Remarkably, we are led to conclude that the presence of Ni atoms in the alloy does not induce any change in the thermodynamics of the nucleation of graphene islands, as instead reported for the Ni/Au alloy [4]. The presence of Ni atoms underneath the Pt skin does not change the temperature of formation of Gr, while it coincides with that of graphene on Pt(111) [5]. On the contrary, in the case of Ni/Au [4], graphene nucleates on the residual Ni patches at the surface and this explains why the growth temperature corresponds to that reported for graphene/Ni(111) [6] and graphene/Ni(110) [7]. Therefore, we can assert that the mere presence of Ni atoms in a crystalline lattice of an alloy does not play any role in lowering the temperature for graphene formation. By contrast, we suggest that the presence of residual Ni nanoplatelets over the surface could catalyze graphene nucleation at a temperature close to that found in Ni single crystals.
It is noteworthy that no temperature-induced segregation of Ni has been observed during timeresolved XPS experiments in the region of Pt 4f and Ni 3p core levels. In fact, the ratio between Pt 4f and Ni 3p peaks remains unchanged during the heating.

S2. Orientation of the graphene lattice with respect to the Pt skin
The low-energy electron diffraction (LEED) pattern observed for graphene grown on Pt3Ni(111) at a fixed temperature of 780 K (top part of Figure S2) exhibits spots from the hexagonal lattice of Pt3Ni(111) (inner spots) superimposed with semi-arcs arising from diffraction from the graphene overstructure (outer spots). The presence of semi-arcs indicates that differently rotated domains exist in the graphene superlattice. Such a LEED pattern is similar but not identical to that of graphene on Pt(111). In graphene on Pt(111), three domains are predominant: one aligned with the substrate, other ones rotated by ±19° and another one rotated by 30° [5]. The quantitative analysis of the intensity of diffraction spots as a function of the angle I(θ) reveals that, compared to graphene/Pt(111) ( Figure S2  Therefore, as for graphene/Pt(111) [11,12] , the graphene overlayer grown on the Pt skin of Pt3Ni(111) is characterized by a moiré superlattice, originated from the minimization of the absolute value of the strain between the graphene and the Pt skin for the different orientations between both atomic lattices.

S3. Electronic properties of graphene supported by Pt-skin-terminated Pt3Ni(111)
Both the C 1s BE (284.1 eV, Figure S1) and the LEED pattern ( Figure S2) indicate that graphene formed on the Pt3Ni(111) surface is supported by the Pt-skin. In details, the BE of the C 1s resembles that of quasi-freestanding graphene on Pt(111) [13], i.e. 284.1 eV, while for graphene/Ni(111) the C 1s has two components at 284.4 and 284.8 eV [6,14,15], due to the coexistence of top-fcc and bridge-fcc structures [16].
Near-edge X-ray absorption fine structure (NEXAFS) experiments for graphene/Pt skin are reported in Figure S3 for two geometries, with photon incidence grazing (red curve) and nearlynormal (black curve) with respect to the sample in order to probe the 1s → π* (~285 eV) and 1s → σ * (291-292 eV) transitions, respectively [17]. The highly dichroic C K-edge signal, due to the inherent nature of graphene band structure, with in-plane σ* and out-of-plane π* states, confirms the excellent quality of the graphene layer on Pt3Ni(111). The pre-edge shoulder observed at ~284 eV has been revealed in graphene/Pt(111) [18], with a weak feature at 288.1 eV [19]. Features in the region between 1s → π * and 1s → σ * transitions are due to interface states, as observed for graphene/Ni(111) [20]. These excitations correspond to electronic transitions toward the interface state above the Fermi level (around the M-point in the Brillouin zone) originating from the hybridization of C pz orbitals with Ni d bands. The intensity of interface states is reduced in the graphene/Pt3Ni(111) system, because of the presence of Ni atoms only in the second layer of the Pt3Ni surface [21]. Moreover, their energy is different with respect to graphene/Ni(111), for which interface states are observed at 286.3 eV [20]. The occurrence of interface states in graphene/Pt3Ni(111) indicates orbital mixing, which is instead absent in graphene/Pt(111) [18].
The 1s → σ * transition is composed of two distinct features at 291.4 eV and 292.4 eV. The sharp peak at 291.4 eV corresponds to the creation of an exciton [22]. Its intensity reflects strong correlation effects of electron-hole pairs within the graphene sheet [23]. The broad peak at 292.6 eV is related to the transition from the C 1s level to the relatively nondispersing σ* states at the Γ point of the Brillouin Gr/Pt(111) Gr/Pt 3 Ni (111) zone [23,24]. NEXAFS features related to 1s → σ * transitions in graphite and graphene/Pt(111) are quite similar to those of graphene/Pt3Ni(111), while the two features in strongly interacting graphene/metal interfaces, as graphene/Ni(111), overlap resulting into a broad peak [20]. Figure S3. X-ray absorption spectra at the C K-edge measured for graphene/Pt3Ni(111) at grazing (20°, red curve) and normal incidence (90°, black curve). Figure S4 shows the angle-resolved photoemission spectroscopy (ARPES) characterization of the grown graphene layer, taken after focusing on a single micrometric graphene domain.
The analysis of ARPES experiments indicates that Gr-derived states around the Fermi level hybridize with 5d states of Pt, whose centroid is localized in the nearness of the Fermi level of the Pt3Ni alloy [25]. On the contrary, the 3d band of Ni, located at a BE of ~2 eV [25], is not hybridized with the Dirac cone of the graphene sheet.
Remarkably, we note that the graphene doping results to be 0.0 ± 0.1 eV, i.e. the Fermi level crosses just the Dirac point. Due to the coincidence of the work function of graphene [26] with that of Pt3Ni(111) [27,28], graphene/Pt3Ni(111) forms a charge-neutral graphene/metal contact. By contrast, the graphene sheet on Pt(111) is p-type doped by 0.4 eV [29,30].