Mercury and C 2 B 10 Icosahedra Interaction
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Mercury and C2B10 Icosahedra Interaction Carolina C. Ilie1, Petru Lunca-Popa1, Jiandi Zhang2, Bernard Doudin1, Peter A. Dowben1, 1) Department of Physics and Astronomy and the Center for Materials Research and Analysis, Behlen Laboratory of Physics, University of Nebraska – Lincoln, Lincoln, NE 68588-0111 U.S.A. 2) Department of Physics, Florida International University, Miami, FL 33199 U.S.A ABSTRACT We contrast the interaction of mercury with adsorbed orthocarborane films and semiconducting (dehydrogenated) boron carbide. Photoemission spectra reveal small shifts in orthocarborane (C2B10H12) molecular orbital binding energies as well as the shift in mercury 5d5/2 shallow core level binding energies, suggesting only small interaction between mercury and the molecular film. Mercury does, however, interact with decomposed orthocarboranes i.e. semiconducting boron carbide. INTRODUCTION Semiconducting boron carbide represents a new class of semiconducting materials with potential applications in neutron detection and radioactive decay calorimetry [1-4]. Boron has high cross section for neutrons at lower energies while several isotopes of mercury have a high neutron cross section at higher energies [5]. The combination of mercury doped boron carbide could then be a candidate for neutron detectors over a wider range of neutron energies. Thus coadsorption of mercury and boron carbide as well as an icosahedral carborane like closo-1, 2-dicarbadodecaborane, C2B10H12 (commonly known as orthocarborane) may have some practical value. Such studies are not, however, unique. Mercury and alkali metal mixtures with molecules have a long history in the study of non-metal to metal transitions. Alkali metal mixture with carboranes has been studied through the nonmetal to metal transition [6, 7], while iron [8-11], vanadium [9], chromium [9] and nickel [9, 12-14] doping of boron carbides is known. All of these latter transition metal dopants dope boron and boron carbides n-type. Photoemission measurements are used here to assess if changes of the electronic properties of orthocarborane occur when mercury is co-deposited with orthocarborane. Changes in the electrical properties of the semiconducting boron carbide films reveal if mercury doping has occurred, or (in effect) indicate if there is interaction with the dehydrogenated orthocarborane.
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EXPERIMENTAL The photoemission experiments were performed in a UHV system equipped with a hemispherical energy analyzer. The photon-energy source employed was the 1-GeV Alladin ring of Synchrotron Radiation Center at Stoughton, Wisconsin. The light was dispersed by a four meter, normal incidence monochromator having the energy of 35 eV in a largely p-polarized configuration, 65o off normal. The photoemission spectra were obtained at normal emission. The light source and the electron energy analyzer had a combined energy resolution between 40 and 80 meV. All the binding energies are referenced to the Fermi energy of the clean copper substrate. The substrate was cleaned by Ar+ io
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