Photofragmentation of the closo-Carboranes Part II: VUV Assisted Dehydrogenation in the closo-Carboranes and Semiconducting B 10 C 2 H x Films Eckart Ru ¨ hl, † Norman F. Riehs, † Swayambhu Behera, ‡,§ Justin Wilks, ‡ Jing Liu, | H.-W. Jochims, † Anthony N. Caruso, ⊥ Neil M. Boag, # Jeffry A. Kelber, ‡ and Peter A. Dowben* ,| Physikalische und Theoretische Chemie, Freie UniVersita ¨t Berlin, Takustr. 3, D-14195 Berlin, Germany, Departments of Chemistry and Physics and Center for Electronic Materials Processing and Integration, UniVersity of North Texas, Denton, Texas 76203, U.S.A., Department of Physics and Astronomy, Nebraska Center for Nanostructures and Materials, UniVersity of Nebraska-Lincoln, Lincoln, Nebraska 68588-0111, U.S.A., Department of Physics, UniVersity of Missouri-Kansas City, Kansas City, Missouri 64110, U.S.A., and Functional Materials, Institute for Materials Research, Cockcroft Building, UniVersity of Salford, Salford M5 4WT, United Kingdom ReceiVed: April 27, 2010; ReVised Manuscript ReceiVed: June 3, 2010 The dehydrogenation of semiconducting boron carbide (B 10 C 2 H x ) films as well as the three closo-carborane isomers of dicarbadodecaborane (C 2 B 10 H 12 ) and two isomers of the corresponding closo-phosphacarborane (PCB 10 H 11 ) all appear to be very similar. Photoionization mass spectrometry studies at near-threshold gas phase photoionization indicate that the preferred pathway for dissociation of the parent cation species (C 2 B 10 H 10 + or PCB 10 H 9 + ) is, in all cases, the loss of H 2 . Ab initio density functional theory (DFT) calculations indicate that energetically preferred sites for exopolyhedral hydrogen (B-H) bond dissociation are in all cases at B atoms opposite the C atoms in the parent cage molecule. The site of photodissociation of hydrogen from semiconducting boron carbide (B 10 C 2 H x ) films, fabricated by plasma-enhanced chemical vapor deposition, is a cage boron atom that can bond to nitrogen upon exposure to VUV light in the presence of NH 3 . Shifts in core level binding energies due to nitrogen bond formation indicate that B-N bond formation occurs only at B atoms bound to other boron atoms (B-B sites) and not at B-C sites or at C sites, in agreement with gas phase results. Introduction There has been a resurgence of interest in carborane chemistry. Although the discovery of closo-carboranes in 1963 was followed quickly accompanied by a flurry of functional- ization chemistry, 1 the resurgence of interest in closo-1,2- dicarbadodecaborane (ortho-carborane) has been to some extent driven by its extensive use as a precursor source molecule for the fabrication of a semiconducting boron carbide, 2-11 a material suitable for the fabrication of solid state neutron detectors. 5-10,12 The method of choice, at present, for making semiconducting boron carbide thin films is to use closo-1,2-dicarbadodecaborane (ortho-carborane), and its isomers, as a source gas(es) for radical-induced polymerization via plasma-enhanced chemical vapor deposition (PECVD). The resulting boron carbides, of approximate stoichiometry “C 2 B 10 H x ” (where x represents up to 40 at% fraction of hydrogen 13 ), exhibit a range of electronic properties but are all semiconductors. The hydrogen content suggests that decomposition of the closo-carboranes, to form the C 2 B 10 H x boron carbide semiconductor, does not result in complete fragmentation of the icosahedral cage. Despite the determined 40 at% H concentration in carborane saturated plasma grown films, 13 some dehydrogenation must occur off the icosahedral carborane cage during semiconducting boron carbide film growth as otherwise the resulting semiconducting film would exhibit a much higher band gap 4,5,14-16 and would not form stable semiconductor devices to high temperatures, as is observed, 3 although densification is observed with some boron carbides, 17 presumably due to hydrogen loss. Knowledge of the molecular decomposition and fragmentation of boron-containing cage molecules are of central importance to our understanding of the relationships between the PECVD process and resulting materials properties. The structures of the different semicon- ducting boron carbides, and the relationship between the different polytypes, are not well understood at present, in spite of much successful device fabrication. Some studies of closo- carborane decomposition have been undertaken 14,16,18,19 but have not investigated the favored dehydrogenation routes in both the free closo-carboranes and the semiconducting boron carbide films grown from the closo-carboranes. In an effort to understand the radical-induced polymerization of the carboranes (i.e., semiconducting film growth), based on their partial dehydrogenation during plasma-enhanced chemical vapor deposition, we have investigated the cation dehydroge- nation of the free closo-carborane and related phosphacarbo- ranes. We present here the bond-specific photochemistry induced in a solid boron carbide film, as characterized by core level X-ray photoelectron spectroscopy (XPS) and find strong simi- larities with the observed dehydrogenation for the various isomers of closo-dicarbadodecaborane and closo-phospha- carbadodecaborane, schematically shown in Figure 1, investi- gated by photoionization mass spectrometry in the gas phase. The NH 3 reaction with the semiconducting inorganic nonvolatile * To whom correspondence should be addressed. Phone: 402-472-9838. Fax: (402) 472-2879. E-mail: pdowben@unl.edu. † Freie Universita ¨t Berlin. ‡ Department of Chemistry, University of North Texas. § Department of Physics, University of North Texas. | University of Nebraska-Lincoln. ⊥ University of Missouri-Kansas City. # University of Salford. J. Phys. Chem. A 2010, 114, 7284–7291 7284 10.1021/jp103805r  2010 American Chemical Society Published on Web 06/23/2010