National Center for Electronic Microscopy

Berkeley, CA, United States

National Center for Electronic Microscopy

Berkeley, CA, United States
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Marquardt K.,German Research Center for Geosciences | Marquardt K.,National Center for Electronic Microscopy | Ramasse Q.M.,National Center for Electronic Microscopy | Ramasse Q.M.,Daresbury Laboratory | And 2 more authors.
American Mineralogist | Year: 2011

Atomic diffusion along grain boundaries in solids is a key process in many geological environ-ments and in ceramics research. It is closely related to the grain boundary width, which is an important parameter in numerous equations describing diffusional or rheological processes, including plastic deformation of polycrystals, intergranular failure, and recrystallization. Here, we studied diffusion along a single well-characterized, near S5 grain boundary in yttrium aluminum garnet (YAG) using different transmission electron microscopy methods at atomic resolution. For the diffusion experiment, YAG thin-films containing Yb as diffusant were deposited perpendicular to the grain boundary on the bicrystal. We investigated the grain boundary using a focal series in combination with multislice calculations that yield the electron exit wave. This, coupled with chemically sensitive Z-contrast images, as well as the Yb distribution over the grain boundary measured using electron energy loss spectroscopy, show the zone of enhanced Yb diffusion parallel to the grain boundary. This zone of enhanced diffusion is often considered as the effective grain boundary width. Profiles from the boundary into the crystal volume suggest a highly permeable zone of about 18 nm, which we assume to be the effective grain boundary width for diffusional processes. The effective width strongly differs from the structural grain boundary width of about 2 nm in the present study. Furthermore, it is much shorter compared to the calculated volume diffusion profile. We conclude that the combination of small samples and transmission electron microscopy at atomic resolution are excel-lent tools to study varying processes, such as diffusion, deformation, or reactions at the atomic scale.

Turchanin A.,Bielefeld University | Weber D.,Physikalisch - Technische Bundesanstalt | Buenfeld M.,Bielefeld University | Kisielowski C.,National Center for Electronic Microscopy | And 6 more authors.
ACS Nano | Year: 2011

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tunable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfill this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well-defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility, and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects toward the engineering of free-standing 2D carbon materials with tunable properties on various solid substrates and on holey substrates as suspended membranes. © 2011 American Chemical Society.

Zhang X.,Tsinghua University | Zhang X.,National Center for Electronic Microscopy | Wu L.,Tsinghua University | Wu L.,National Center for Electronic Microscopy | And 6 more authors.
Journal of Nanoscience and Nanotechnology | Year: 2011

The decay of spin polarization poses serious problems for spintronic devices. It will be greatly helped by the availability of spintronic materials with a long spin diffusion length. Carbon has small spin-orbital interaction and longer coherent length. This makes carbon suitable material for exploitation in the spintronic materials and devices. A great deal of magnetoresistance (MR) research has been carried out in carbon nanotubes, grapheme and small carbon molecules. However, the MRs of these materials are normally observed at low temperature, making these carbon materials difficult used in information industry. In this paper, we introduce a novel class of carbon based hybrid materials Fe x-C 1-x/Si structure which show larger MR at room temperature. These materials have also some other novel physical properties, such as electromagnetoresistance, switch effect, pressure sensitivity, gas sensitivity and photoconductivity. This kind of carbon based materials has shown early sign of being excellent candidates for spintronic materials operating at room temperature. © 2011 American Scientific Publishers.

Erickson K.,Lawrence Berkeley National Laboratory | Erni R.,National Center for Electronic Microscopy | Lee Z.,Lawrence Berkeley National Laboratory | Alem N.,Lawrence Berkeley National Laboratory | And 2 more authors.
Advanced Materials | Year: 2010

The local atomic structure of graphene oxide (GO) and reduced annealed graphene oxide (raGO) is determined via ultra-high-resolution transmission electron microscopy. We find that the proposed and desired return to graphene from GO is not possible through the synthetic route employed. The detailed structure of GO, previously unknown, is revealed as mottled, with few square nanometer graphitic regions separated by highly oxidized regions. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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