K. F. McCarty

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (157)552.49 Total impact

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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. · 3.84 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.
    06/2014;
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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. · 31.20 Impact Factor
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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 01/2014; 104(10):101606-101606-4. · 3.79 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. · 2.22 Impact Factor
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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; · 31.20 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.
    10/2013;
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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. · 3.12 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.
    09/2013;
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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.
    07/2013;
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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; · 10.68 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; · 3.83 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). · 3.77 Impact Factor
  • David Siegel, Kevin McCarty, Farid El Gabaly
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    ABSTRACT: Ion transport through materials driven by electric potential is essential to many processes, including electrical energy storage. Here we study in situ the behavior of oxide surfaces in the presence of applied electric fields with low-energy electron microscopy (LEEM), angle-resolved photoemission spectroscopy (ARPES), and related structural and spectroscopic measurement techniques. We measure with high spatial resolution the electric potential on the surface of yttria-stabilized zirconia (YSZ), a prototypical oxygen ion conductor, as a function of distance from a metallic electrode. The dependence of the potential distribution on temperature and oxygen gas pressure is determined. Finally we explore which types of surface sites facilitate the gas-surface reactions that create and annihilate the oxygen ions.
    03/2013;
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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).
    03/2013;
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    ABSTRACT: The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ~ 40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50 % state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely de-lithiated or lithiated. Specifically, only ~ 2 % of individual particles were a mixture of the two LFP phases. From this small fraction of particles that were actively undergoing phase transformation, we conclude that the time needed to charge a particle is ~ 1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of de-lithiation and the particle size, contrary to the common expectation that smaller particles phase transform before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggests that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but the rate for phase-transformation initiation.
    Nano Letters 01/2013; · 13.03 Impact Factor
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    ABSTRACT: Twisted bilayer graphene (tBLG) provides us with a large rotational freedom to explore new physics and novel device applications, but many of its basic properties remain unresolved. Here we report the synthesis and systematic Raman study of tBLG. Chemical vapor deposition was used to synthesize hexagon- shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Superlattice structures are revealed by the observation of two distinctive Raman features: folded optical phonons and enhanced intensity of the 2D-band. Both signatures are strongly correlated with G-line resonance, rotation angle and laser excitation energy. The frequency of folded phonons decreases with the increase of the rotation angle due to increasing size of the reduced Brillouin zone (rBZ) and the zone folding of transverse optic (TO) phonons to the rBZ of superlattices. The anomalous enhancement of 2D-band intensity is ascribed to the constructive quantum interference between two Raman paths enabled by a near-degenerate Dirac cone. The fabrication and Raman identification of superlattices pave the way for further basic study and new applications of tBLG.
    Applied Physics Letters 01/2013; 103:123101. · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Twisted bilayer graphene (tBLG) provides us with a large rotational freedom to explore new physics and novel device applications, but many of its basic properties remain unresolved. Here we report the synthesis and systematic Raman study of tBLG. Chemical vapor deposition was used to synthesize hexagon- shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Superlattice structures are revealed by the observation of two distinctive Raman features: folded optical phonons and enhanced intensity of the 2D-band. Both signatures are strongly correlated with G-line resonance, rotation angle and laser excitation energy. The frequency of folded phonons decreases with the increase of the rotation angle due to increasing size of the reduced Brillouin zone (rBZ) and the zone folding of transverse optic (TO) phonons to the rBZ of superlattices. The anomalous enhancement of 2D-band intensity is ascribed to the constructive quantum interference between two Raman paths enabled by a near-degenerate Dirac cone. The fabrication and Raman identification of superlattices pave the way for further basic study and new applications of tBLG.
    01/2013;
  • N.C. Bartelt, K.F. McCarty
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    ABSTRACT: The exceptional properties of graphene originate from its two-dimensional polymeric structure of sp 2-bonded carbon. This feature also causes graphene to grow on metal substrates through mechanisms that are strikingly different from those of conventional heteroepitaxy. We provide here a brief review of graphene growth on metals, a subject with a rich history even before the recent explosion of interest in graphene. The current activities related to graphene growth on metals have been motivated by the need to develop low-cost, scalable processes for graphene synthesis and to understand how graphene–metal interfaces behave in devices. In this article, we examine the current state of the art, emphasizing the basic processes that distinguish graphene growth from normal crystal growth.
    MRS Bulletin. 12/2012; 37(12).
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    ABSTRACT: We use spatially resolved photoelectron spectroscopy performed in operando to identify the reaction intermediates of the hydrogen electro-oxidation reaction on yttria-stabilized zirconia (YSZ) electrolytes with Pt electrodes. We find that hydroxyl on the zirconia electrolyte is a reaction intermediate in the hydrogen oxidation reaction and that it participates in the rate-determining step. In contrast to the general wisdom, the limiting step does not involve the transfer of charge. These results allow us to propose the detailed reaction pathway, which provides direct insight into how to accelerate the kinetics.
    Chemical Communications 07/2012; 48(67):8338-40. · 6.38 Impact Factor

Publication Stats

1k Citations
552.49 Total Impact Points

Institutions

  • 1988–2014
    • 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
  • 2009
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 2005–2009
    • Universidad Autónoma de Madrid
      • Department of Condensed Matter Physics
      Madrid, Madrid, Spain
  • 2008
    • Spanish National Research Council
      • Institute of Physical Chemistry "Rocasolano"
      Madrid, Madrid, Spain
    • University of Hamburg
      Hamburg, Hamburg, Germany
  • 1992–1996
    • CSU Mentor
      Long Beach, California, United States