4 research outputs found
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Chromium nanostructures formed by dewetting of heteroepitaxial films on W(100)
In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer onW(100).We show that these square islands are composed of
inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands
thicker than the centers. The islands’ shapes do not represent traditional equilibrium crystal shapes as expected
for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved
spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between
the islands remains covered by a thin pseudomorphic wetting layer of ∼1 ML. Low-energy electron diffraction
and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a
predominance of tilted Cr(100) facets ±5◦ off of the substrate normal bound by (210) planes at ∼26◦. The
epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal.We suggest
that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morpholog
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Magnetic iron oxide nanowires formed by reactive dewetting
The growth and reactive dewetting of ultra-thin films of iron oxides supported on Re(0001) surfaces have been imaged in-situ in real time. Initial growth forms a non-magnetic stable FeO (wüstite like) layer in a commensurate network upon which high aspect ratio nanowires of several microns length but less than 40nm width can be fabricated. The nanowires are closely aligned with the substrate crystallography and imaging by X-ray magnetic circular dichroism shows that each contain a single magnetic domain. The driving force for dewetting appears to be the minimization of strain energy of the Fe3O4 crystallites and follows the Tersoff and Tromp model in which strain is minimized at constant height by extending in one epitaxially matched direction. Such wires are promising in spintronic applications and we predict that the growth will also occur on other hexagonal substrates
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Epitaxial growth of ultrathin palladium films on Re{0001}
Ultrathin bimetallic layers create unusual magnetic
and surface chemical effects through the modification of electronic structure brought on by low dimensionality, polymorphism, reduced screening, and epitaxial strain. Previous studies have related valence and core-level shifts to surface reactivity through the d-band model of Hammer and Nørskov, and in heteroepitaxial films this band position is determined by competing effects of coordination, strain, and hybridization of substrate and overlayer states. In this study we employ the epitaxially matched Pd on Re{0001} system to grow films with no lateral strain. We use a recent advancement in low-energy electron diffraction to expand the data range sufficiently for a reliable determination of the growth sequence and out-of-plane surface relaxation as a function of film thickness. The results are supported by scanning tunneling
microscopy and X-ray photoemission spectroscopy, which show that the growth is layer-by-layer with significant core-level shifts due to changes in film structure, morphology, and bonding
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Pyrene-Modified quartz crystal microbalance for the detection of polynitroaromatic compounds
The synthesis of a dithiol-functionalized pyrene
derivative is reported, together with studies of interactions between this receptor (and other related pyrenes) and nitroaromatic compounds (NACs), in both solution and in the solid state. Spectroscopic analysis in solution and X-ray crystallographic analysis of cocrystals of pyrene and NACs in the solid state indicate that supramolecular interactions lead to the formation of defined pi-pi stacked complexes. The dithiolfunctionalized
pyrene derivative can be used to modify the
surface of a gold quartz crystal microbalance (QCM) to create a unique π-electron rich surface, which is able to interact with electron poor aromatic compounds. For example, exposure of the modified QCM surface to the nitroaromatic compound 2,4-dinitrotoluene (DNT) in solution results in a reduction in the resonant frequency of the QCM as a result of supramolecular interactions between the electron-rich pyrenyl surface layer and the electron-poor DNT molecules. These results suggest the potential use of such modified QCM surfaces for the detection of explosive NACs