Session Z12: Graphene: Electronic Structure and Interactions - Spectroscopy

Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center ($q $= 0) wave-vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene (1LG) in the frequency range from 2350 to 2750 cm$^{-1}$, which shows the G* and the G'-band features originating from a double-resonant Raman process with \textit{q $\ne $} 0. The observed phonon renormalization effects are different from what is observed for the zone-center $q $= 0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with non-zero wave-vectors (\textit{q $\ne $ } 0) in 1LG in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center $q $= 0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G* Raman feature at 2450 cm$^{-1}$ to include the iTO+LA combination modes with \textit{q $\ne $} 0 and the 2iTO overtone modes with $q $= 0, showing both to be associated with wave-vectors near the high symmetry point \textbf{K }in the Brillouin zone.   

Impact of Graphene-Metal Interfaces on the Raman and Transport Properties of Graphene Devices

Graphene is an amazing nano-material with many exciting properties and applications. However, due to its low dimensionality, the performance of this material is mainly limited by interfaces and surface properties. One of these interfaces, important for graphene field effect transistors and catalysts supported on graphene membranes, is that between the graphene and a metal layer. In this study, we experimentally examine the impact of various metals on graphene through Raman and Transmission Electron Microscopy. We find that strong graphene-metal interactions have significant impacts on the phonon structure in graphene. Furthermore, we observe changes in our Raman spectra relating to the crystallographic orientation between a metal and graphene.   

Raman spectroscopy of sputtered metal-graphene and metal-oxide-graphene interfaces

In this talk, we report our recent development in sputtering deposition of magnetic and non-magnetic metal and metal-oxide thin films on graphene for applications in spintronics and nanoeleoctronics. TEM and SEM images demonstrate homogeneous coverage, uniform thickness, and good crystallinity of the sputtered films. Raman spectroscopy shows that the structure of the underlying graphene is well preserved, and the spectral weight of the defect D mode is comparable to that of the e-beam evaporated samples. Most significantly, we report the first observation of graphene-enhanced surface excitations of crystalline materials. Specifically, we discover two pronounced dispersive Raman modes at the interface of graphene and the nickel-oxide and cobalt-oxide films which we attribute to the strong light absorption and high-order resonant scattering process in the graphene layer. We will present the frequency-dependent, polarization-dependent Raman data of these two modes and discuss their microscopic origin.   

Effects of sliding on Raman spectrum of bilayer graphene

Electronic properties of graphene are changed by various external effects such as strain, doping, and defects, etc. Such effects can be measured by Raman spectroscopy. For example, doping and defects increase the intensities of D-peak of single layer graphene, while the external strains generate splitting of G- and 2D-peaks respectively. In bilayer graphene, sliding motions between two layers also can change its electric and optical properties. Based on the first principles calculation methods, the sliding between two layers of bilayer graphene is shown to change the electronic and phonon dispersions, thereby altering its Raman spectrum significantly.   

Electronic Structure of Suspended Bilayer Graphene

Bilayer graphene is an important medium for achieving band-gap tuning for graphene applications in digital electronics, such as in graphene nanoribbon FETs. ARPES is an important tool for measuring the electronic structure of crystals in the vicinity of the Dirac point. While accurate measurements have been made on the bilayer graphene band-structure on SiC, similar effort on free-standing graphene has not yet been reported. In this paper, we describe measurements of the band-structure of a $\sim $ 650 $\mu $m$^{2}$ sample of exfoliated bilayer graphene suspended over 5-$\mu $m-diameter wells formed in SiO$_{2}$ on a Si substrate. The measurements, performed at the Spectromicroscopy Beamline at ELETTRA with photon energy of 27eV, show that this material is essentially undoped and we compare its band structure with that expected from tight-binding calculations.   

Electrical transport between single-crystal domains in graphene: bigger is not always better

Single-layer graphene can now be produced on the centimeter or even meter scale using chemical vapor deposition. These large-scale graphene films are polycrystalline, consisting of many separate single-crystal domains, as was recently identified using transmission electron microscopy. Understanding the electrical transport across these domains is relevant not only for device applications, but has also been the focus of many fundamental studies. Here, we first examine the structure of graphenes produced under different growth conditions using dark-field transmission electron microscopy (DF-TEM). We find three classes of grain boundaries--continuous, amorphous, and overlapped. Next, we study the electrical properties of graphene devices consisting of individual grain boundaries that have been first imaged by DF-TEM. We find that the grain boundaries exhibit an additional gate-dependent resistance that is keenly sensitive to growth conditions. Surprisingly, this resistance is an order of magnitude greater for growths with larger grain size due to more poorly-connected domains. Our results show that domain size is not the single most important parameter determining electrical performance of large-scale graphene films--the quality of inter-domain connections is just as crucial.   

Trapping Image State Electrons on Graphene Layers and Islands

The understanding of graphene-metal interfaces is of utmost importance in graphene transport phenomena. To probe this interface we use time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and the smooth, epitaxial graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited image-state electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.   

Image potential states of Graphene/Ru(0001) interface

Graphene, the parent of all graphitic forms, has become one of the most exciting topics in condensed matter physics. It is now well understood that the low-energy electronic properties of graphene are described by two-dimensional Dirac equation for massless particles, but the unoccupied bands are hardly unexplored in experimental research. In this talk we will present new results on the unoccupied electronic structure, namely the image potential states (IPS), measured by two-photon photoemission spectroscopy (2PPS). The single layer graphene is prepared by thermal decomposition of ethylene on a Ru(0001) surface. Through low energy electron diffraction (LEED) and 2PPS, we verify formation of high-quality, single layer thick graphene samples.Based on the graphene/Ru(0001) system, we first measured the surface structure and IPS on Ru(0001) by angle-resolved 2PP. From the angle-resolved spectra, we obtain the effective mass $m_{eff} = 1.1m_{e}$, which is close to the expected value for a free electron state of $m_{eff} = 1m_{e}$.With the same method, we measured the IPS on the graphene/Ru(0001) surface. However, we observe complex structure consisting of two nearly degenerate states that reflect the mutual interactions between graphene and Ru(0001) substrate.   

Energy loss spectroscopy of epitaxial and free-standing multilayer graphene

We present a formalism and numerical results for the energy loss of a charged particle scattered at an arbitrary angle from epitaxially grown multilayer graphene (MLG). It is compared with that of free-standing graphene layers. Specifically, we investigated the effect of the substrate induced energy gap on one of the layers. The gap yields collective plasma oscillations whose characteristics are qualitatively and quantitatively different from those produced by Dirac fermions in gapless graphene. The range of wave numbers for undamped self-sustaining plasmons is increased as the gap is increased, thereby substantially increasing and red-shifting the MLG stopping power for some range of charged particle velocity. We also applied our formalism to interpret several distinct features of experimentally obtained electron energy loss spectroscopy (EELS) data.   

Interface Science in Graphene Materials: An Electronic Structure View of Soft X-Ray Spectroscopy

The ability to control the morphology of nanostructured carbon-based materials is of crucial importance for the applications of photovoltaic, energy storage such as Li-ion batteries, etc. The properties of matter at nanoscale dimensions are dramatically different from the bulk. The differences arise through quantum confinement, altered thermodynamics or changed chemical reactivity. In general, electronic structure ultimately determines the properties of matter, thus understanding of electronic structure is crucial for tailoring the properties of nanoscale systems. The graphene/Cu and SiO$_{2}$ composites have been studied using XAS, XES and RIXS. New electronic states in the conduction band are observed, which are ascribed to the monovacancy defect state and interfacial interaction. The polarized XAS spectra demonstrate that the graphene/Cu exhibits high alignment and weak corrugation. Significant intensity modulation of resonant XES spectral shape upon different excitation energies near the C K-edge, indicates that graphene preserves an intrinsic symmetry and the interaction between graphene and Cu has unique influence on the electronic structure of graphene. The broad RIXS features and subtle shifts are observed in the RIXS spectra of graphene/Cu, which can be attributed to the strong electron-phonon scattering, charge transfer from the Cu sites   

Electron-Phonon Coupling and Large Intrinsic Bandgap in Highly-Screened Graphene

The electron-electron and electron-phonon interactions are two of the most fundamental interactions in many-body physics, leading to Mott insulating behavior and superconductivity. In graphene, the pointlike Fermi surface and linear band dispersions give these interactions distinctive characteristics with respect to normal metals. Here we present data on the electron-phonon interaction in highly-screened graphene, which we have analyzed by angle-resolved photoemission spectroscopy (ARPES). In contrast with every previous ARPES study, the magnitude of our extracted electron-phonon coupling constants generally agree with theoretical predictions. We also demonstrate the presence of a 400 meV bandgap at the Dirac point in epitaxial graphene on a copper substrate. These results shed light on the nature of the electron-phonon interaction and the tunability of the Dirac quasiparticles in graphene.   

Photoemission on highly-ordered arrays of structured epitaxial graphene

We use the recently-demonstrated method of growing pre-patterned epitaxial graphene directly from structured silicon carbide (SiC) to produce dense, highly-ordered arrays of graphene nanoribbons suitable for macroscopic characterization methods. Specifically, we use angle-resolved photoemission spectroscopy (ARPES) to observe the ensemble-averaged band structure of nanoribbons with nominal widths of 10nm and 30nm. A Dirac-like cone is visible down to 10nm and, although the graphene is a single layer, it is undoped, in contrast to single layer graphene nanoribbons on either the Si- or C-terminated faces of SiC.   

Spectroscopic Ellipsometry, Auger and STM Characterization of Epitaxial Graphene grown on 6H-SiC (0001)

Graphene grown by the thermal decomposition of SiC has become of interest to the semiconductor industry due to its unique, high-mobility electronic structure. The growth is of a more scalable nature when compared to exfoliated flakes produced from the ``scotch tape'' method. The resulting film rests on a ``buffer layer'' separating the graphene from the underlying substrate, which is thought to consist of a mixture of sp$^{2}$ and non-sp$^{2}$ bonding due to the sp$^{3}$ bonding of the SiC substrate. The mobilities of the graphene layer have previously been shown to differ from that of the interface layer. We investigate the difference in the optical response of the two layers using Spectroscopic Ellipsometry and find a red-shift of the $\sim $4.5 eV absorbance found in graphene due to the exciton-domianted transition at the M point of the Brilloun Zone. The structural characterization of the films are performed through Auger and STM on substrates which were cleaned by CMP and chemical etching methods prior to the epitaxial growth in UHV.   

Vibrational spectroscopy of molecules adsorbed on graphene

Graphene, being a monolayer membrane, is extremely sensitive to the environment. Understanding how it interacts with adsorbed molecules and polymers is fundamental for improving graphene electronic devices. Previous studies show that electrons in graphene can couple efficiently to phonon vibrations of the substrate, which can become a limiting factor for graphene mobility. Here we investigate the interactions between vibrations of adsorbed molecules with graphene using vibrational spectrsocopy. We performed the vibrational spectroscopy using a broadband tunable infrared laser source and a high precision spectrometer. We will discuss how the vibration frequencies of the adsorbed molecules get modified through their interactions with graphene.   

Magneto-phonon resonance of shear modes of bilayer graphene

We describe the frequency renormalization of the recently observed shear phonon mode of bilayer graphene due to electron-phonon coupling. In presence of a relatively strong magnetic field resonances with electronic inter-Landau level transitions are possible. When the resonance condition is staisfied a fine structure of the Raman line corresponding to the shear mode arises which is linear in electron-phonon coupling constant. This effect can be used to measure the strength of the electron-phonon coupling for the shear mode.