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.

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The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance.

Grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution transmission electron microscopy (HRTEM) were used to investigate phase separation in hafnium silicate films after rapid thermal annealing between 700 and 1000 °C. 4-nm-thick Hf–silicate films with 80 and 40 mol % HfO2, respectively, were prepared by metalorganic vapor deposition. Films of the two compositions showed distinctly different phase-separated microstructures, consistent with two limiting cases of microstructural evolution: nucleation/growth and spinodal decomposition. Films with 40 mol % HfO2 phase separated in the amorphous by spinodal decomposition and exhibited a characteristic wavelength in the plane of the film. Decomposition with a wavelength of ∼3 nm could be detected at 800 °C. At 1000 °C the films rapidly demixed with a wavelength of 5 nm. In contrast, films with 80 mol % HfO2 phase separated by nucleation and growth of crystallites, and showed a more random microstructure. The factors determining specific film morphologies and phase separation kinetics are discussed.

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.