Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001)

Abstract

We have performed low temperature scanning tunnelling spectroscopy measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interaction strength, varies spatially from one position to other of the graphene supercell. This inhomogeneity in the electronic properties of graphene on Ru(0001) results from local variations of the carbon-ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices. The experimental realization of graphene 1 has boosted the research on this material, finding unique electronic properties.2,3 The prospective use of graphene in optics, electronic devices, 4 and chemical sensors 5 has also motivated a large number of studies devoted to develop different fabrication methods and characterization techniques. Among a variety of growth methods, epitaxial growth of graphene on transition metal substrates has been intensively studied because of its high efficiency and sample quality. [6][7][8] Nevertheless, the role of the substrate in the electronic properties of graphene devices can be very important 9 and it has to be studied in detail. For example, the extraction of photogenerated carriers in graphene photodetectors relies on spatial variations of the potential at the graphene/metal contacts. Often epitaxially grown graphene presents a variety of moir e patterns due to the lattice mismatch with the different transition metal substrates and a spatial modulation of the electronic structure, e.g., the local density of states of the graphene layer has been observed. 10 The electron-phonon coupling (EPC) in graphene, which is responsible of a variety of properties from ballistic transport to excited-state dynamics, depends on the electronic density or doping level through the deformation potential.11 A possible spatial modulation of the electron-phonon interaction, however, has not been studied in these systems.In this letter, we present low-temperature scanning tunnelling microscopy (STM) and inelastic electron tunnelling spectroscopy (IETS) measurements in monolayer graphene epitaxially grown on a Ru(0001) surface, which show a strong inelastic feature that can be attributed to the interaction between the tunnelling electrons and a breathing vibrational mode of the graphene layer. The intensity of this inelastic feature, which is proportional to the electronphonon coupling strength and phonon density of states, varies spatially following the periodic Moir e pattern originated by the graphene/Ru(0001) lattice mismatch.The samples were prepared in an ultra-high vacuum (UHV) chamber with a base pressure in the range of 10 À11 Torr. The substrate is a single crystal of Ru exposing the (0001) surface, which was cleaned in UHV by ion sputtering and annealing to 1400 K. The graphene samples are grown under UHV c...

Local Probes of Graphene Lattice Dynamics

“…[ 42 ] The observed d I /d V gap around zero bias voltage was traced to a K 6 acoustic phonon (ZA phonon mode at K ; Figure 1) with energy ≈67 meV. In a later report, [ 43 ] signatures in IET spectra appeared at ±360 mV, which were attributed to the second overtone of a graphene optical phonon. Importantly, the signal strength was demonstrated to depend on the lateral position atop the moiré pattern induced by the graphene and Ru(0001) lattice mismatch.…”

“…[ 40,41 ] In addition, graphene phonons throughout the BZ [ 34 ] or at the M ‐points of the BZ [ 37,38 ] were mapped in d I /d V spectroscopy. Moreover, alkali metal‐intercalated graphene [ 38 ] as well as graphene on Ru(0001), [ 42,43 ] which supposedly exhibit a stronger hybridization with the substrate, likewise exhibit clear phonon signatures in IETS. These results call for an extension or even alteration of the emerging picture and insistently demonstrate that IETS of graphene phonons is a vividly debated topic.…”

Strong Asymmetric Charge Carrier Dependence in Inelastic Electron Tunneling Spectroscopy of Graphene Phonons

“…Previously, IETS has had limited success in measuring phonons in graphene because of weak signals and spectral overlap with other miscellaneous excitations. To date only a few reports of phonons in graphene have been published [13][14][15][16]. Reference [13] characterized an excitation at 63 meV and assigned it to a K-point phonon, while reference [14] revealed an additional feature at 150 meV and also assigned it to a K-point phonon.…”

“…Reference [13] characterized an excitation at 63 meV and assigned it to a K-point phonon, while reference [14] revealed an additional feature at 150 meV and also assigned it to a K-point phonon. Reference [15] detected an excitation at 360 meV, which was tentatively assigned to a breathing mode. followed by e-beam lithography to deposit Au contacts [19].…”

“…Many techniques have been applied to measure graphene and graphite phonons, including neutron scattering [5], x-ray scattering [6], Raman spectroscopy [7][8][9], electron energy-loss spectroscopy [10], and inelastic electron tunneling spectroscopy (IETS) [11][12][13][14][15][16].…”