Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

Interfacial interactions at graphene/metal and graphene/dielectric interfaces are likely to profoundly influence the electronic structure of graphene. We present here the first angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy study of single-and bilayered graphene grown by chemical vapor deposition on Cu and Ni substrates. The spectra indicate the presence of new electronic states in the conduction band derived from hybridization of the C-π network with Cu and Ni d-orbitals. In conjunction with Raman data demonstrating charge transfer, the NEXAFS data illustrate that the uniquely accessible interfaces of two-dimensional graphene are significantly perturbed by surface coatings and the underlying substrate. NEXAFS data have also been acquired after transfer of graphene onto SiO 2 /Si substrates and indicate that substantial surface corrugation and misalignment of graphene is induced during the transfer process. The rippling and corrugation of graphene, studied here by NEXAFS spectroscopy, is thought to deleteriously impact electrical transport in graphene.SECTION Surfaces, Interfaces, Catalysis G raphene, a one-atom-thick, two-dimensional (2D) electronic system exhibiting a cornucopia of quantum transport phenomena, is constituted from a single layer of carbon atoms tightly packed within a honeycomb lattice.1-3 Recent advances in the wafer-scale fabrication of graphene by chemical vapor deposition (CVD) methods inspire confidence that it may be possible to harness the remarkable electronic structure of graphene for applications in microelectronics and quantum logic devices. [4][5][6][7] In particular, the massive room-temperature mobilities of charge carriers in graphene 8,9 portends the possible use of this material in ultrahigh frequency transistors with an operational regime extending to the terahertz range.2 The large phase coherence length and room-temperature ballistic conduction observed across micrometer-scale dimensions further tantalizes with possibilities for applications in spin-logic architectures. 10,11 Much of the novel transport phenomena observed for graphene is derived from its unique electronic structure wherein electrons propagating through the honeycomb lattice behave as massless and chiral Dirac fermions, and the valence and conduction bands touch at conical Dirac points with a remarkable linear energy dispersion within (1 eV of the Fermi energy. 3As graphene transitions from being merely an object of academic curiosity to real device applications, there is considerable interest regarding modifications of the characteristic graphene electronic spectrum when graphene is interfaced with other materials including metals and dielectrics.

We have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. Our results are compatible with hydrogen adsorbates inducing intervalley mixing by exerting a short-range scattering potential. The saturation coverages for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach 1.5 × 10 4 cm 2 /V sec in the absence of the hydrogen-adsorbing sites. This affinity to hydrogen is the signature of the most dominant type of native scatterers in graphene-based field-effect transistors on SiO2.Freely suspended graphene sheets display high fieldeffect mobility, reaching 2 × 10 5 cm 2 /V sec. 1,2 High mobilities allows for a wider utilization of graphene sheets in testing relativistic quantum mechanics, exploring twodimensional physics, and creating new electronic, optoelectronic, and spintronic device technologies.3-5 Yet, suspended graphene sheets are fragile and impractical for most experiments and applications. Substrate-bound graphene sheets are easier to handle but possess low carrier mobilities, which can even vary by an order of magnitude from sample to sample. Poor and unpredictable transport properties reduce the utility of substrate-bound graphene sheets for both fundamental and applied sciences. Therefore, understanding the impact of substrates is crucial for graphene science and technology.Charged impurities,ripples, 8and resonant scatterers [9][10][11][12] have been considered for modeling the transport property of graphene field-effect transistors (FETs). Previous experimental studies have explored the impact of charged impurities 13 and resonant scatterers 14-17 by using adsorbed impurities or creating vacancies on graphene sheets. Yet, these studies revealed only the impact of adsorbates or vacancies and did not shed information on the nature of the native scatterers already present in the samples. Furthermore, experiments using different dielectric environments have provided contradictory results on the role and importance of charged impurities.18,19 Thus, there are no conclusive experimental results revealing the nature of the native scatterers that limit the transport properties of graphene on SiO 2 .We have measured the impact of low-energy atomic hydrogen on the transport properties of graphene as a function of coverage and the initial field-effect mobility. Our transport and Raman spectroscopy measurements show that hydrogen exerts a short-range scattering potential which introduces intervalley scattering. Hydrogen transfers a small but finite amount of charge, as indicated by the gate-dependent transport measurements. The resistivity added by hydrogen remains proportional to the number of adsorbed hydrogen and, therefore, adheres to Matthiessen's rule even...

The complex permittivity for Pt, Pd, Ni, and Ti-silicide films as well as heavily doped p-and n-type silicon were determined by ellipsometry over the energy range 0.031 eV to 4.0 eV. Fits to the Drude model gave bulk plasma and relaxation frequencies. Rutherford backscattering spectroscopy, X-ray diffraction, scanning electron microscopy, secondary ion mass spectrometry, and four-point probe measurements complemented the optical characterization. Calculations from measured permittivities of waveguide loss and mode confinement suggest that the considered materials are better suited for long-wavelength surface-plasmon-polariton waveguide applications than metal films.

Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 μm. The bulk plasma frequency ω p and high-frequency limit to the permittivity ε ∞ were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time τ and effective mass m Ã. This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity.