Raman Spectroscopy of Lattice-Matched Graphene on Strongly Interacting Metal Surfaces.

Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm-1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials.

Nature of the binding of a c(2×2)-CO overlayer on Ag(001) and surface mediated intermolecular coupling.

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.

Ab initio calculations of the dispersion of surface phonons of a c(2 × 2) CO overlayer on Ag(001).

We examine the phonon dispersion of c(2 × 2)-CO on Ag(001) by applying density functional perturbation theory with the generalized-gradient approximation. Our calculations indicate that the c(2 × 2)-CO overlayer on Ag(001) is dynamically stable. We find that the bond length of CO is expanded and its stretch mode (ν(1)) softened by ∼ 9 meV upon adsorption on Ag(001), in excellent agreement with experiments. We show that ν(1) at [Formula in text] alone cannot gauge the metal-CO interaction since it is not entirely determined by the C-O intramolecular force constant. Further softening of ν(1) on Ag(001) is obtained outside [Formula in text], indicative of CO-CO interactions even at a distance of ∼ 4 Å. The frequency of the Ag-CO stretch mode (ν(2)) is ∼ 30 meV and it is nearly dispersionless, implying that the perturbation corresponding to this mode is short-ranged. The frustrated rotation mode of CO (ν(3)) overlaps with the bulk band and mixes with substrate modes inside the SBZ, suggesting this as one of the key features for the enhanced diffusivity of CO on Ag surfaces over that on Cu surfaces. The frustrated translation mode of CO (ν(4)) is everywhere below ∼ 2.8 meV and therefore mixes with substrate modes in the region of the SBZ around [Formula in text]. Depending on the q vector, vertical and in-plane surface modes may soften or stiffen with respect to their counterpart on clean Ag(001). Although the response of most Ag(001) modes to CO adsorption is similar to that of corresponding Cu(001) modes, there are some contrasting features between the dynamics of the two surfaces concerning the changes in the surface force constants and mixing of S(6) with ν(3) on Ag(001).

Momentum dependence of the electron-phonon coupling and self-energy effects in superconducting YBa2Cu3O7 within the local density approximation.

Using the local density approximation and a realistic phonon spectrum we determine the momentum and frequency dependence of alpha(2)F(k,omega) in YBa(2)Cu(3)O(7) for the bonding, antibonding, and chain band. The resulting self-energy Sigma is rather small near the Fermi surface. For instance, for the antibonding band the maximum of ReSigma as a function of frequency is about 7 meV at the nodal point in the normal state and the ratio of bare and renormalized Fermi velocities is 1.18. These values are a factor of 3-5 too small compared to the experiment showing that only a small part of Sigma can be attributed to phonons. Furthermore, the frequency dependence of the renormalization factor Z(k,omega) is smooth and has no anomalies at the observed kink frequencies which means that phonons cannot produce well-pronounced kinks in stoichiometric YBa(2)Cu()3)O(7), at least, within the local density approximation.