K. F. McCarty

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (173)630.85 Total impact

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    ABSTRACT: Graphene films grown by vapor deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, i.e., "ripening"; domain-boundary motion within islands; and continuous lattice-rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moir\'e corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials.
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    ABSTRACT: Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750 - 900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.
    Nano Letters 11/2014; 15(1). DOI:10.1021/nl503340h · 12.94 Impact Factor
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    ABSTRACT: We have used low-energy electron microscopy and diffraction to examine the significance of lattice orientation in graphene growth on Cu(001). Individual graphene domains undergo anisotropic growth on the Cu surface, and develop into lens shapes with their long axes roughly aligned with the Cu<100> in-plane directions. The long axis of a lens-shaped domain is only rarely oriented along a C<11> direction, suggesting that carbon attachment at "zigzag" graphene island edges is unfavorable. A kink-mediated adatom attachment process is consistent with the behavior observed here and reported in the literature. The details of the ridged moire pattern formed by the superposition of the graphene lattice on the (001) Cu surface also evolve with the graphene lattice orientation, and are predicted well by a simple geometric model. Managing the kink-mediated growth mode of graphene on Cu(001) will be necessary for the continued improvement of this graphene synthesis technique.
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    ABSTRACT: Using selected-area low-energy electron diffraction analysis, we showed strict orientational alignment of monolayer hexagonal boron nitride (h-BN) crystallites with Cu(100) surface lattices of Cu foil substrates during atmospheric pressure chemical vapor deposition. In sharp contrast, the graphene-Cu(100) system is well-known to assume a wide range of rotations despite graphene's crystallographic similarity to h-BN. Our density functional theory calculations uncovered the origin of this surprising difference: The crystallite orientation is determined during nucleation by interactions between the cluster's edges and the substrate. Unlike the weaker B- and N-Cu interactions, strong C-Cu interactions rearrange surface Cu atoms, resulting in the aligned geometry not being a distinct minimum in total energy. The discovery made in this specific case runs counter to the conventional wisdom that strong epilayer-substrate interactions enhance orientational alignment in epitaxy and sheds light on the factors that determine orientational relation in van der Waals epitaxy of 2D materials.
    Proceedings of the National Academy of Sciences 11/2014; DOI:10.1073/pnas.1405613111 · 9.81 Impact Factor
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    ABSTRACT: We study where and how hematite (alpha-Fe2O3) nucleates and grows during the oxidation of magnetite(100) single crystals. Hematite inclusions grow along < 110 > directions of the magnetite (Fe3O4), leading to a biaxial array of hematite slabs in an electrically conducting matrix of magnetite. The slab arrays form in both bulk single crystals and thin films of magnetite. Atomic force microscopy reveals that the surface growth of magnetite that accompanies hematite formation is faster adjacent to the hematite slabs. In situ X-ray photoelectron and X-ray absorption spectroscopies at 600 degrees C in an oxygen environment reveal that the conversion of the Fe2+ in magnetite to Fe3+ in hematite occurs without the formation of the metastable phase maghemite (gamma-Fe2O3). We offer an explanation of why Fe3O4(100) oxidizes faster than Fe3O4(111).
    The Journal of Physical Chemistry C 08/2014; 118(34):19768-19777. DOI:10.1021/jp5037603 · 4.84 Impact Factor
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    ABSTRACT: We report the observation of a strong 2D band Raman in twisted bilayer graphene (tBLG) with large rotation angles under 638 nm and 532 nm visible laser excitations. The 2D band Raman intensity increased four-fold as opposed to the two-fold increase observed in single-layer graphene. The same tBLG samples also exhibited rotation-dependent G-line resonances and folded phonons under 364 nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive interference between two double-resonance Raman pathways, which were enabled by a nearly degenerate Dirac band in the tBLG Moiré superlattices.
    Nanotechnology 07/2014; 25(33):335201. DOI:10.1088/0957-4484/25/33/335201 · 3.67 Impact Factor
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    ABSTRACT: The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this `triple phase boundary' is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttria-stabilized zirconia (YSZ) undergoes a metal-insulator transition near Pt negative electrodes. YSZ's electron conducting region functions as an extended triple phase boundary that can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.
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    ABSTRACT: Using in situ low-energy electron microscopy and density functional theory calculations, we follow the growth of monolayer graphene on Pd(111) via surface segregation of bulk-dissolved carbon. Upon lowering the substrate temperature, nucleation of graphene begins on graphene-free Pd surface and continues to occur during graphene growth. Measurements of graphene growth rates and Pd surface work functions establish that this continued nucleation is due to increasing C adatom concentration on the Pd surface with time. We attribute this anomalous phenomenon to a large barrier for attachment of C adatoms to graphene coupled with a strong binding of the non-graphitic C to the Pd surface.
    Applied Physics Letters 03/2014; 104(10):101606-101606-4. DOI:10.1063/1.4868386 · 3.52 Impact Factor
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    ABSTRACT: By adapting the concept of epitaxy to two-dimensional space, we show the growth of a single-atomic-layer, in-plane heterostructure of a prototypical material system--graphene and hexagonal boron nitride (h-BN). Monolayer crystalline h-BN grew from fresh edges of monolayer graphene with atomic lattice coherence, forming an abrupt one-dimensional interface, or boundary. More important, the h-BN lattice orientation is solely determined by the graphene, forgoing configurations favored by the supporting copper substrate.
    Science 01/2014; 343(6167):163-7. DOI:10.1126/science.1246137 · 31.48 Impact Factor
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    ABSTRACT: We study how FeO wüstite films on Ru(0001) grow by oxygen-assisted molecular beam epitaxy at elevated temperatures (800-900 K). The nucleation and growth of FeO islands are observed in real time by low-energy electron microscopy (LEEM). When the growth is performed in an oxygen pressure of 10(-6) Torr, the islands are of bilayer thickness (Fe-O-Fe-O). In contrast, under a pressure of 10(-8) Torr, the islands are a single FeO layer thick. We propose that the film thickness is controlled by the concentration of oxygen adsorbed on the Ru. More specifically, when monolayer growth increases the adsorbed oxygen concentration above a limiting value, its growth is suppressed. Increasing the temperature at a fixed oxygen pressure decreases the density of FeO islands. However, the nucleation density is not a monotonic function of oxygen pressure.
    Journal of Physics Condensed Matter 11/2013; 25(48):484001. DOI:10.1088/0953-8984/25/48/484001 · 2.22 Impact Factor
  • 224th ECS Meeting; 10/2013
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    ABSTRACT: The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface oxygen enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment-limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to mechanically exfoliated graphene, in spite of being grown in the presence of oxygen.
    Science 10/2013; DOI:10.1126/science.1243879 · 31.48 Impact Factor
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    ABSTRACT: Using low-energy electron diffraction, we show that the room-temperature $(\sqrt{2}\times\sqrt{2})R45^\circ$ reconstruction of Fe$_3$O$_4$(100) reversibly disorders at $\sim$450 $^\circ$C. Short-range order persists above the transition, suggesting that the transition is second order and Ising-like. We interpret the transition in terms of a model in which sub-surface Fe$^{3+}$ is replaced by Fe$^{2+}$ as the temperature is raised. This model reproduces the structure of antiphase boundaries previously observed with STM as well as the continuous nature of the transition. To account for the observed transition temperature, the energy cost of each charge rearrangement is 82 meV.
    Physical Review B 10/2013; 88(23). DOI:10.1103/PhysRevB.88.235436 · 3.66 Impact Factor
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    ABSTRACT: We report the observation of anomalously strong 2D band in twisted bilayer graphene (tBLG) with large rotation angles under 638-nm and 532-nm visible laser excitation. The 2D band of tBLG can reach four times as opposed to two times as strong as that of single layer graphene. The same tBLG samples also exhibit rotation dependent G-line resonances and folded phonons under 364-nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive quantum interference between two double-resonance Raman pathways which are enabled by nearly degenerate Dirac band in tBLG Moir\'e superlattices.
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    ABSTRACT: We determine the atomic structure of the (111) surface of an epitaxial ceria film using low-energy electron diffraction (LEED). The 3-fold-symmetric LEED patterns are consistent with a bulk-like termination of the (111) surface. By comparing the experimental dependence of diffraction intensity on electron energy (LEED-I(V) data) with simulations of dynamic scattering from different surface structures, we find that the CeO2(111) surface is terminated by a plane of oxygen atoms. We also find that the bond lengths in the top few surface layers of CeO2(111) are mostly undistorted from their bulk values, in general agreement with theoretical predictions. However, the topmost oxygen layer is further from the underlying cerium layer than the true bulk termination, an expansion that differs from theoretical predictions.
    The Journal of Chemical Physics 09/2013; 139(11):114703. DOI:10.1063/1.4820826 · 3.12 Impact Factor
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    ABSTRACT: We examine the magnetic easy-axis directions of stoichiometric magnetite films grown on SrTiO3:Nb by infrared pulsed-laser deposition. Spin-polarized low-energy electron microscopy reveals that the individual magnetic domains are magnetized along the in-plane <100> film directions. Magneto-optical Kerr effect measurements show that the maxima of the remanence and coercivity are also along in-plane <100> film directions. This easy-axis orientation differs from bulk magnetite and films prepared by other techniques, establishing that the magnetic anisotropy can be tuned by film growth.
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    ABSTRACT: We study how the (100) surface of magnetite undergoes oxidation by monitoring its morphology during exposure to oxygen at ~650 ˚C. Low-energy electron microscopy (LEEM) reveals that magnetite's surface steps advance continuously. This growth of Fe3O4 crystal occurs by the formation of bulk Fe vacancies. Using Raman spectroscopy, we identify the sinks for these vacancies, inclusions of α-Fe2O3 (hematite). Since the surface remains magnetite during oxidation, it continues to dissociate oxygen readily. At steady state, over one quarter of impinging oxygen molecules undergo dissociative adsorption and eventual incorporation into magnetite. From the independence of growth rate on local step density, we deduce that the first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetite's terraces, not preferentially at its surface steps. Since we directly observe new magnetite forming when it incorporates oxygen, we suggest that catalytic redox cycles on magnetite involve growing and etching crystal.
    Journal of the American Chemical Society 06/2013; 135(27). DOI:10.1021/ja402599t · 11.44 Impact Factor
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    ABSTRACT: Nickel is the most commonly used anode for solid-oxide fuel cells (SOFC) due to its fast kinetics and low price. A leading cause of degradation in Ni electrodes is oxidation. Here we use operando ambient-pressure X-ray photoelectron spectroscopy (XPS) to chemically characterize the Ni electrode of a fuel cell anode during oxidation in a H2/H2O atmosphere. We find three different stages of Ni oxidation in the model SOFC. In the first two stages, the Ni exposed to the gas remains metallic but the Ni at the interface with the zirconia electrolyte is oxidized. In the third oxidation stage, we find that Ni transforms to NiOOH, a phase not previously considered in the SOFC literature. We show that the transformation between Ni and NiOOH is reversible and is initiated at the Ni/gas interface. In addition we find that NiOOH stores charge, as evidenced by the stable discharge plateau (voltage) measured as this oxyhydroxide phase reduces to metallic Ni.
    Physical Chemistry Chemical Physics 04/2013; DOI:10.1039/c3cp50366f · 4.20 Impact Factor
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    ABSTRACT: The reflectivity of low energy electrons from graphene on copper substrates is studied both experimentally and theoretically. Well-known oscillations in the reflectivity of electrons with energies 0 - 8 eV above the vacuum level are observed in the experiment. These oscillations are reproduced in theory, based on a first-principles density functional description of interlayer states forming for various thicknesses of multilayer graphene. It is demonstrated that n layers of graphene produce a regular series of n-1 minima in the reflectance spectra, together with a possible additional minimum associated with an interlayer state forming between the graphene and the substrate. Both (111) and (001) orientations of the copper substrates are studied. Similarities in their reflectivity spectra arise from the interlayer states, whereas differences are found because of the different Cu band structures along those orientations. Results for graphene on other substrates, including Pt(111) and Ir(111), are also discussed.
    Physical review. B, Condensed matter 03/2013; 87(24). DOI:10.1103/PhysRevB.87.245414 · 3.66 Impact Factor
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    ABSTRACT: Graphene growth of aligned domains on Ir(111) and Ru(0001) is controlled by the attachment of clusters of carbon adatoms. Here we study the growth of rotational variants on Ir(111) and show that the growth is dependent on both cluster attachment and kink kinetics. We simultaneously measure the growth velocity of individual facets and the local concentration of carbon adatoms. The faceted domains tend to lie along the equilibrium zigzag or armchair direction. As the carbon adatom concentration increases, the facets deviate from their equilibrium orientation. This increases the kink density, allowing faster growth. The kink density is a function of the carbon adatom supersaturation. We will discuss how these findings account for the different growth velocities between aligned and rotated domains. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy Contract No. De-Ac04-94AL85000 (SNL). ODD acknowledges support from the NSF (Grant No. DMR-1105541). PCR acknowledges support from a DoD NDSEG fellowship (32 CFR 168a).

Publication Stats

4k Citations
630.85 Total Impact Points

Institutions

  • 1988–2015
    • Sandia National Laboratories
      • • Surface and Interface Sciences Department
      • • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
  • 2011–2013
    • Complutense University of Madrid
      • Department of Material physics
      Madrid, Madrid, Spain
  • 2012
    • Texas State University
      San Marcos, Texas, United States
  • 2010
    • University of California, Berkeley
      • Department of Materials Science and Engineering
      Berkeley, MO, United States
    • University of California, Los Angeles
      • Department of Materials Science and Engineering
      Los Angeles, CA, United States
  • 2006–2009
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, California, United States
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 2008
    • University of Hamburg
      Hamburg, Hamburg, Germany
  • 2005
    • Universidad Autónoma de Madrid
      • Department of Condensed Matter Physics
      Madrid, Madrid, Spain
  • 1992–1997
    • University of California, Davis
      • • Department of Chemical Engineering and Materials Science
      • • Department of Physics
      Davis, California, United States
  • 1994
    • Arizona State University
      • Department of Physics
      Phoenix, Arizona, United States