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Fundamental hydrogen interactions with beryllium : a magnetic fusion perspective

Kolasinski, Robert K.; Bartelt, Norman C.; Whaley, Josh A.; Felter, Thomas E.; Wampler, William R.

Increasingly, basic models such as density functional theory and molecular dynamics are being used to simulate different aspects of hydrogen recycling from plasma facing materials. These models provide valuable insight into hydrogen diffusion, trapping, and recombination from surfaces, but their validation relies on knowledge of the detailed behavior of hydrogen at an atomic scale. Despite being the first wall material for ITER, basic single crystal beryllium surfaces have been studied only sparsely from an experimental standpoint. In prior cases researchers used electron spectroscopy to examine surface reconstruction or adsorption kinetics during exposure to a hydrogen atmosphere. While valuable, these approaches lack the ability to directly detect the positioning of hydrogen on the surface. Ion beam techniques, such as low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS), are two of the only experimental approaches capable of providing this information. In this study, we applied both LEIS and DRS to examine how hydrogen binds to the Be(0001) surface. Our measurements were performed using an angle-resolved ion energy spectrometer (ARIES) to probe the surface with low energy ions (500 eV - 3 keV He{sup +} and Ne{sup +}). We were able to obtain a 'scattering maps' of the crystal surface, providing insight on how low energy ions are focused along open surface channels. Once we completed a characterization of the clean surface, we dosed the sample with atomic hydrogen using a heated tungsten capillary. A distinct signal associated with adsorbed hydrogen emerged that was consistent with hydrogen residing between atom rows. To aid in the interpretation of the experimental results, we developed a computational model to simulate ion scattering at grazing incidence. For this purpose, we incorporated a simplified surface model into the Kalypso molecular dynamics code. This approach allowed us to understand how the incident ions interacted with the surface hydrogen, providing confirmation of the preferred binding site.

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TYPE SAND Report YEAR 2012
OSTIDOI

Graphene islands on Cu foils: The interplay between shape, orientation, and defects

Nano Letters

Wofford, Joseph M.; Nie, Shu N.; McCarty, Kevin F.; Bartelt, Norman C.; Dubon, Oscar D.

We have observed the growth of monolayer graphene on Cu foils using low-energy electron microscopy. On the (100)-textured surface of the foils, four-lobed, 4-fold-symmetric islands nucleate and grow. The graphene in each of the four lobes has a different crystallographic alignment with respect to the underlying Cu substrate. These "polycrystalline" islands arise from complex heterogeneous nucleation events at surface imperfections. The shape evolution of the lobes is well explained by an angularly dependent growth velocity. Well-ordered graphene forms only above ∼790 °C. Sublimation-induced motion of Cu steps during growth at this temperature creates a rough surface, where large Cu mounds form under the graphene islands. Strategies for improving the quality of monolayer graphene grown on Cu foils must address these fundamental defect-generating processes. © 2010 American Chemical Society.

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TYPE Journal Article YEAR 2010
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Results 51–75 of 108
Results 51–75 of 108