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Possible occupancy sites for carbon atoms on Pd(111): T (top, C atom atop a Pd atom), B (bridge, C atom directly above the midpoint of a surface Pd-Pd bond), H (hollow, C atom above the hollow site), and G (gap, C atom at the center of the smallest TBB triangles).
The areal density of the individual types of sites occupied by the C atoms of the graphene sheet as a function of orientation.
Graphene on Pd(111) oriented at , 10.9°, 19.1°, and 30.0°, with the carbon atoms colored according to the site they occupy (defined in Fig. 1 ). The moiré surface unit cells are outlined in white. Despite the nearly random distribution of top, bridge, hole, and gap sites, we note the periodic repetition of distinctive clusters of sites such as TB3 [three bridge site carbons (yellow) surrounding a top site (red)], TH3 (three green, central red), BG3 (three cyan, central yellow), HT3 (three red, central green), HH3 (three green, central green), HG3 (three cyan, central green), GB3 (three yellow, central cyan), and GG3 (three cyan, central cyan).
(a) Density of different types of 4-site clusters for the orientations of , 10.9°, 19.1°, and 30.0°. (b) The binding energy Eb of graphene on Pd(111) increases as a function of orientation θ. The only cluster whose areal density increases monotonously with θ is TB3, which we show to be responsible for the bonding of graphene to the substrate. (c) The mean distance between the C atoms and the top Pd layer; the vertical bars around each point show the range of distances spanned between C atoms and the top Pd layer at each orientation.
Site and angular momentum projected density of states (PDOS) for the top and the bridge site carbons in a TB3 cluster and for the corresponding Pd atom at the 30.0° orientation. The presence of common peaks (vertical gray lines) for the C–pz , Pd–dxz , Pd–dyz , and projected density of states is consistent with the formation of hybridized orbitals at the interface.
Electron density transfer for (a) 19.1° and (b) 30.0° orientations, showing regions of charge accumulation (red-orange spectrum) and depletion (blue spectrum). Three types of bonds can be identified at these orientations, marked in (a) as ovoid, oblong, and TB3. Only the TB3 type exists at (b), and no bonds are formed at orientations of 5.7° or 10.9°. Panel (c) shows side-views of the charge transfer regions corresponding to different types of bonds identified in (a).
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