The Electronic Structure of 1,7-PCB 10 H 11 Molecular Films
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The Electronic Structure of 1,7-PCB10H11 Molecular Films Snjezana Balaz1, Neil M Boag2, Neil P Platt2, Dimtcho I Dimov2, Jennifer I Brand3, and Peter A Dowben4 1 Engineering, University of Nebraska-Lincoln, 116 Brace Lab, P.O. Box 880111, Lincoln, NE, 68588-0111 2 University of Salford, Cockcroft Building, Salford, M5 4WT, United Kingdom 3 Engineering, University of Nebraska-Lincoln, 245N WSEC, Lincoln, NE, 68588-511 4 Physics and Astronomy, University of Nebraska-Lincoln, 116 Brace Lab, P.O. Box 880111, Lincoln, NE, 68588-0111
ABSTRACT The electronic structure and highest occupied to lowest unoccupied molecular orbital gap of undoped films of the molecular icosahedra of closo-1-phospha-7-carbadodecaborane (1,7PCB10H11, meta-phosphacarborane) are reported. For 1,7-PCB10H11 adsorbed on Au and Ag, the Fermi level is placed closer to the lowest unoccupied molecular orbital than has been observed with closo-1-phospha-2-carbadodecaborane (1,2-PCB10H11, ortho-phosphacarborane) adsorbed on Au and Ag. INTRODUCTION Partly hydrogenated icosahedral boron carbide can be a very useful semiconductor material. The fabrication of solid state semiconductor devices from boron carbide has only recently been realized, although a semiconducting boron carbide has been sought since 1959 [1]. Boron carbide is a chemically stable material, mechanically strong and easily deposited at relatively low temperatures. Boron carbide devices may be appropriate for use in harsh conditions since they should be resistant to corrosive, high temperature, and mechanically abrasive surroundings. Prototype devices now include real-time solid-state neutron detectors [26], homojunction [7] and a variety of heterojunction diodes [2,3,8], including all boron-carbide heterojunction [9] and heteroisomeric diodes [5,6]. A starting point to understanding these semiconductors, and how these materials are grown is to undertake a detailed study of the molecular precursors. Specifically, ortho-phosphacarborane (closo-1-phospha-2carbadodecaborane or 1,2-PCB10H11) and meta-phosphacarborane (closo-1-phospha-7carbadodecaborane or 1,7-PCB10H11) are both potential n-type semiconductors [9]. Here, we compare experimentally and theoretically thin layers of these two closo-phosphacarboranes adsorbed molecules on Au and Ag substrates.
THEORY AND EXPERIMENT The ground state energies were calculated using a PM3-NNDO model calculation with the HyperChem package. These semi-empirical calculations of occupied and unoccupied molecular orbitals were undertaken following geometry optimization and the calculation of the lowest restricted Hartree-Fock (RHF) energy states. The molecular thin film sample preparation and subsequent spectroscopies were carried out in two UHV chambers with base pressure of 5×10-11 Torr. Prior to film growth, the metal substrates (Ag(100) or Au(111)) were cleaned using Ar+ ion bombardment and annealed. Preceding the deposition, the substrate Fermi level was determined. The substrates were then cooled to 150 - 200 K, and the molecular films grown
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