Probing substrate effects in the carbon-projected band structure of graphene on Pt(111) through resonant inelastic x-ray scattering

The unoccupied and occupied σ and π bands of graphene on Pt(111) were measured using near-edge x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the carbon $K$ edge. Elemental specificity and crystal-momentum conservation allows for detection of the dispersive band structure. In addition to features arising from the band structure of free-standing graphene, signatures of symmetry breaking were identified in XAS and in RIXS for excitation around the Fermi level. These additional features are proposed to derive from weak graphene-substrate interactions which manifest as substrate hybridization and surface umklapp processes due to Moiré superlattice vectors.

Figure 1

(a) C 1$s$ XPS spectra ($h{\upsilon }_{\mathrm{in}}$ $=$ 400 eV) fitted with one $s{p}^2$ component. (b) LEED pattern of graphene/Pt(111) taken at $E$ $=$ 200 eV. Pt(111) (blue) and graphene (black) Brillouin zone was indicated in the LEED picture. ${\mathbf{G}}_{\mathrm{Pt}}$, ${\mathbf{G}}_{\mathrm{g}}$, and ${\mathbf{G}}_{\mathrm{gM}}$ represent crystal unit vectors of Pt(111) graphene, and the Moiré superstructure, respectively. (c) XPS spectrum of Pt 4$f_{7/2}$ ($h{\upsilon }_{\mathrm{in}}$ $=$ 165 eV) for pure Pt(111) (red (dashed line)) and graphene on Pt(111) (black) indicates an almost free-standing graphene layer. From low to high binding energy, deconvoluted components (for graphene on Pt(111) spectra) indicate the surface, the bulk state of Pt(111), and the Pt-graphene originated state.

Figure 2

Out of plane (black line) and in-plane polarized [red (gray) line] XAS spectra of graphene. The arrows indicate the excitation energies where RIXS spectra were recorded. The inset is a zoomed version of XAS normalized to π${}^{*}$ resonance peak for π and σ geometry. The preedge feature could be observed in both geometries.

Figure 3

(a) Calculated band structure of graphene (from Ref. 24). (b) RIXS spectra of graphene in grazing (black lines) and normal [red (gray) lines) photon emission geometry for various excitation energies. Indicated with bars are the energy positions for RIXS from high-symmetry critical points: the critical points indicated above the energy axis are related to π geometry (black) and σ geometry [red (gray)]. Fermi level is indicated with a solid black line, while dashed black lines at 9 and 2.5 eV indicate the emission region due to substrate interaction and surface umklapp process for $h{\upsilon }_{\mathrm{in}}$ $=$ 284.5 eV.

Figure 4

RIXS spectra for graphene and graphite (from Ref. 10) for incident photon energies $h{\upsilon }_{\mathrm{in}}$ $=$ 284.5 eV and $h{\upsilon }_{\mathrm{in}}$ $=$ 320 eV. The dotted rectangle indicates the emission region due to surface umklapp process. The sum of a suitable combination of σ and π emission RIXS of graphene (from Fig. 3.) was used to obtain RIXS spectra at an emission angle of 35° since the reported data for graphite were measured at this emission angle.

Figure 5

RIXS spectra of graphene in grazing photon emission geometry for excitation energies in the range $h{\upsilon }_{\mathrm{in}}$ $=$ 284.5–285.75 eV. All spectra were normalized to the emission area excluding the elastic peak. The Fermi level is indicated with a dotted line.