Chapter 5 Coupling Surface geometry and electronic properties of
5.6 Comparison of different graphene/TM
Many of our conclusions reached for Rh(111) hold true for other TM substrates. Nevertheless, the observed trend in the local adsorption strength (ring-hollow < ring-bridge) is at odds, because the strongest local adsorption registry for graphene/TM is reported experimentally and/or theoretically in the literature to be ring-hcp for the close-packed surfaces of Ru10,15-16, Pd40, Pt41, and Ir11,14,42. In addition, the surfaces of Co(0001) 43-44 and Ni(111)45-48, both 3d metals with a lattice mismatch of about 1%, have been experimentally shown to adsorb graphene in a (1 × 1) epitaxy with the ring-hollow geometry. As previously mentioned, ring-bridge sites have escaped the attention of most studies to date. Swart et al. reported that this registry is significantly less stable on
fcc-Co(111)25. It is worth mentioning that the ring-bridge structure is favoured over ring-hcp on Ni(111) by the theoretical study of Fuentes-Cabrera et al., 49 but this
conclusion is in conflict with the recent experimental findings 46-48. We now present a qualitative explanation to rationalise the differences in the bonding of graphene to the various group VIIIB TMs. With reference to studies of graphene adsorbed on 3d, 4d and 5d TM surfaces, the overall agreement between experimental and theoretical data based on DFT is rather good. For most of these studies, the d band model is invoked in first instance. This model was developed to explain trends in binding strength of adsorbed molecules on TMs38, predicting a binding strength that (1) gradually decreases with the occupation of the d band, and (2) increases on going from the 5d to the 3d metals. However, the d band model can not give a satisfactory explanation on the trends in adsorption energy and graphene-TM bond distances. We compare for instance the binding energies obtained by DFT calculations: the averaged values over extended supercells are 40 meV per carbon atom over graphene/Ru(0001)15, 7.8 meV/C over graphene/Rh(111) (from our DFT data), and 2 meV/C over graphene/Ir(111)14. The d band model, however, predicts that the values for graphene/Rh(111) should be slightly lower than for graphene/Ru(0001) and significantly larger than for Ir(111). Moreover, we can deduced from the d band model that the overall physical characteristics of the graphene/Rh(111) interface should be comparable to the graphene/Ru(0001) system, with only slightly larger C-TM bond distances for the former. The C-TM distances for the bonding configurations (ring-bridge and ring-hollow) reported in the literature are about
2.0–2.2 Å for the close-packed surfaces of both 3d and 4d TM substrates (Ni45, Co43, Ru36-37, Pd50), and 3.3-3.8 Å for 5d metals (Pt41, Ir14). The case of Rh(111) is hence very intriguing in the sense that the C-Rh bond follows a “d band trend” with respect to the ring-bridge configuration, whereas the ring-hollow configurations differs from what is expected from the “dband trend”.
To appropriately understand trends in binding energy of graphene on TM surfaces, we need to separate two contributing factors: electronic states (energy and symmetry) and lattice structure (type and mismatch). To a first approximation, the electronic factor can be described by the d band model.38 However, this model necessarily fails under the two following conditions: (1) the large lattice mismatch exists at the graphene/TM interface, and (2) electronic effects (exchange and correlation energies) arising from an appropriate many-electron description can not be neglected. This also explains why DFT-based calculations can yield a reliable description of the graphene/TM interface and the results are in good agreement with experimental observations, when extended supercell is adopted to account for the lattice mismatch.
In summary, we have investigated the electronic surface properties and chemical bonding of graphene/Rh(111) by means of high-resolution STM, site-specific RTS and extended DFT calculations. The observed Φ variation of about 250 meV is significant, especially considering the spatial extent over which it occurs (~ 3 nm, the size of a supercell). We have confirmed that epitaxial graphene is geometrically buckled when adsorbed on Rh(111). We furthermore identified the local C-Rh registries of maximum and minimum chemical bonding to the rhodium substrate and showed that Rh(111) is a very different substrate for graphene as compared to Ru(0001) and Ir(111).
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