Gary Lee Kellogg

Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide

Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moores law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.

Direct observation of surface diffusion and atomic interactions on metal surfaces

Abstract The unique attributes of the field ion microscope permit the direct observation of surface diffusion of single atoms and simple atomic clusters on perfect crystal planes. A brief review of the principles and techniques of such studies is presented along with a collection of much of the existing experimental data. Emphasis is placed on more recent work such as the diffusion of adatoms under the influence of a driving force, the kinetics of cluster formation and dissociation, and how the interatomic force between two atoms on a surface is related to the diffusion parameters and the pair distribution function.

Oxide formation and reduction on rhodium surfaces

Abstract The growth of thin oxide layers on Rh and their reduction by CO has been investigated by imaging atom-probe mass spectroscopy and field-ion microscopy. Surface oxides were produced by heating Rh field-emitter tips in oxygen at pressures between 0.01 and 1.0 Torr and temperatures between 400 and 650 K. The oxidized samples were transferred under ultrahigh vacuum to an imaging atom-probe/field-ion microscope for compositional and structural analysis. Oxygen uptake was found to follow a logarithmic law with an initial activation energy of 3.1 kcal/mol. Imaging atom-probe analysis indicated that the oxide formed in 1 Torr O 2 was stoichiometric Rh 2 O 3 for temperatures of 500 K and above. The onset pressure for oxidation at 500 K was found to be ≈0.01 Torr, with only a weak pressure dependence in the range from 0.01 to 1 Torr. Field-ion microscope images of the oxide showed ring structures suggestive of epitaxial growth above the (111) plane, and images of the substrate after removal of the oxide indicated that the oxide was thicker above the stepped regions of the surface than above the low-index planes. The oxide was quickly reduced in 1 Torr CO at temperatures above 420 K, and partially-reduced oxides were found to be substoichiometric throughout the oxide region. CO reduction exhibited a much stronger temperature dependence than surface oxide formation indicating a different rate-controlling step for the two processes. The time dependence for CO reduction at 418 K suggested that the COO surface reaction was rate-determining in the reduction process.

Temperature dependence of surface self-diffusion on Pt(001)

Abstract The surface diffusion of individual Pt atoms on the Pt(001) plane has been investigated by field ion microscopy. The map of binding sites visited by an atom as it diffuses across the surface in the temperature range from 170 to 205 K indicates that individual displacements occur exclusively in the [100] and [010] directions. Migration in these directions is consistent with diffusion by an atomic exchange mechanism. No evidence for diffusion by ordinary hopping was found in this temperature range. The activation energy of surface diffusion and diffusivity (Arrhenius prefactor) were found to be 0.47 ± 0.01 eV and 1.3 × 10 −3 cm 2 /s, respectiv Diffusion off the plane edge was not observed at temperatures below 215 K.

Measurement of the charge state distribution of field evaporated ions: Evidence for post-ionization☆

Abstract Charge state distributions of field evaporated Si, Ni, Mo, Rh, W, Re, Ir and Pt ions have been measured as a function of electric field strength using the pulsed laser atom-probe. The results are compared to previously published theoretical calculations based on the post-ionization model of field evaporation. The agreement between theory and experiment is sufficient to establish the general validity of the post-ionization model. Measurements of the charge state distributions as a function of evaporation rate at constant temperature (increasing field) and constant field (increasing temperature) are also presented for W, Mo and Si. The observation that the fractional abundances of different charge states for the same material do not change with changing temperature indicates that the activation energies of desorption are the same for the different charge states and provides further support for the post-ionization model. The anomalous field evaporation behavior observed at high temperatures (e.g., desorption from localized areas on the surface and the occurrence of ionic clusters) is also discussed.

Surface diffusion of Pt adatoms on Ni surfaces

Abstract The diffusion of individual Pt adatoms on the (100), (110), (311), and (111) surfaces of Ni has been investigated by field ion microscopy. On the (100) and (110) surfaces Pt adatoms replace Ni substrate atoms at temperature below the onset of hopping displacements. The minimum temperature required for replacement is 250 and 105 K for the (100) and (110) surfaces, respectively. On the (311) and (111) surfaces, Pt adatoms migrate by conventional site-to-site hopping. The activation barriers for displacements on the (100), (110), and (311) planes are estimated from the migration temperatures and compared to activation barriers measured previously for self-diffusion on Pt and Ni.

Surface diffusion and clustering of nickel atoms on the (110) plane of tungsten

The surface diffusion and clustering of Ni atoms on the W(110) plane has been studied by field ion microscopy. Individual Ni atoms, deposited at 77 K, were found to become mobile at temperatures above 178 K and diffused with an activation energy of 0.49±0.02 eV. Atoms deposited at temperatures above 200 K formed into clusters. At 200 K the clusters were linear chains following along the direction of the close-packed channels of the substrate. Two-dimensional clusters eere formed at 230 K and above. These clusters were found to migrate as a unit and coalesce into larger clusters at temperatures in the range of 300–375 K. Heating the surface above 375 K caused the clusters to disappear indicating dissociation of the cluster followed by migration of the atoms off the plane. No three-dimensional island growth was observed at any temperature, even on larger planes formed by thermal annealing. Tungsten defects (small clusters of W atoms left by field evaporation) were found to serve as nucleation sites for cluster growth. Ni atoms would stick to the top of these clusters, but only on top of the W “defect” atoms.

Field ion microscopy and imaging atom‐probe mass spectroscopy of superconducting YBa2Cu3O7−x

The structure and composition of the superconducting oxide YBa2Cu3O7−x have been examined in atomic detail by field ion microscopy and imaging atom‐probe mass spectroscopy. The field ion samples were prepared from hot‐pressed disks of the oxide powders. Atomic resolution images were obtained with either argon or hydrogen as the imaging gas. Individual layers of atoms were observed which could be field evaporated in a uniform, layer‐by‐layer manner. Imaging atom‐probe analysis of the field ion tips indicated a metal composition which varied noticeably from sample to sample and an oxygen concentration which was consistently much too low.

LEEM Investigation of the Faceting of the Pt Covered W (111) Surface

A low energy electron microscope (LEEM) has been used to investigate the faceting of W(111) as induced by Pt. The atomically rough W(111) surface, when fully covered with a monolayer film of Pt and annealed to temperatures higher than {approximately} 750 K, experiences a significant morphological restructuring: the initially planar surface undergoes a faceting transition and forms three-sided pyramids with {211} faces. The experiments demonstrate the capability of LEEM for imaging both the fully and partially faceted surface. In addition, we have observed the formation of the facets in real time, when Pt is dosed onto the heated surface. We find that the transition from planar surface, to partially faceted surface, and to fully faceted surface proceeds through the nucleation and growth of spatially separated faceted regions.

Surface Morphology Changes During Pb Deposition on Cu(100): Evidence for Surface Alloyed Cu(100)-c(2x2) Pb

Using Low Energy Electron Microscopy (LEEM), the authors have followed Cu(100) surface morphology changes during Pb deposition at different temperatures. Surface steps advance and 2-D islands nucleate and grow as deposited Pb first alloys, and then dealloys, on a 125 C Cu(100)surface. From LEEM images, they determine how much Cu is being displaced at each stage and find that the amount of material added to the top layer for a complete Pb/Cu(100) c(4x4) reconstruction (a surface alloy) is consistent with the expected c(4x4) Cu content of 0.5 monolayer. However, as the surface changes to the Pb/Cu(100) c(2x2) overlayer, they find that the displaced material from surface dealloying, 0.22ML, is more than a factor of two lower than expected based on a pure Pb c(2x2) overlayer. Further, they find that in the 70 to 130 C range the amount of Cu remaining in c(2x2) increases with increasing substrate temperature during the deposition, showing that surface Cu is alloyed with Pb in the c(2x2) structure at these temperatures. When holding the sample at 125 C, the transformation from the c(2x2) structure to the higher coverage c(5{radical}2 x{radical}2)R45{degree} overlayer structure displaces more Cu, confirming the c(2x2) surface alloy model. They also find the c(2x2) surface has characteristically square 2-D islands with step edges parallel to the (100) type crystallographic directions, whereas the c(5{radical}2 x{radical}2)R45{degree} structure has line-like features which run parallel to the dislocation double rows of this surfaces atomic structure and which expand into 2-D islands upon coarsening.