Photofragmentation of the closo-Carboranes Part II: VUV Assisted

Caruso , A. N.; Dowben , P. A.; Balkir , S.; Schemm , N.; Osberg , K.; Fairchild , R. W.; Flores , O. B.; Balaz , S.; Harken , A. D.; Robertson , B. W...
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Photofragmentation of the closo-Carboranes Part II: VUV Assisted Dehydrogenation in the closo-Carboranes and Semiconducting B10C2Hx 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 (B10C2Hx) films as well as the three closo-carborane isomers of dicarbadodecaborane (C2B10H12) and two isomers of the corresponding closo-phosphacarborane (PCB10H11) 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 (C2B10H10+ or PCB10H9+) is, in all cases, the loss of H2. 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 (B10C2Hx) 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 NH3. 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 functionalization chemistry,1 the resurgence of interest in closo-1,2dicarbadodecaborane (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 “C2B10Hx” (where x represents up to 40 at% fraction of hydrogen13), exhibit a range of electronic properties but are all semiconductors. The hydrogen content suggests that decomposition of the closo-carboranes, to form the C2B10Hx 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 * To whom correspondence should be addressed. Phone: 402-472-9838. Fax: (402) 472-2879. E-mail: [email protected]. † 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.

the icosahedral carborane cage during semiconducting boron carbide film growth as otherwise the resulting semiconducting film would exhibit a much higher band gap4,5,14-16 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 semiconducting 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 closocarborane decomposition have been undertaken14,16,18,19 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 dehydrogenation of the free closo-carborane and related phosphacarboranes. 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 similarities with the observed dehydrogenation for the various isomers of closo-dicarbadodecaborane and closo-phosphacarbadodecaborane, schematically shown in Figure 1, investigated by photoionization mass spectrometry in the gas phase. The NH3 reaction with the semiconducting inorganic nonvolatile

10.1021/jp103805r  2010 American Chemical Society Published on Web 06/23/2010

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Figure 1. Schematic representations of the three different isomers of closo-dicarbadodecaborane: (a) ortho-carborane (1,2-C2B10H12), (b) metacarborane (1,7-C2B10H12), and (c) para-carborane (1,12-C2B10H12), and (d) ortho-1-phospha-2-carbadodecaborane (1,2-PCB10H11), and (e) meta1-phospha-7-carbadodecaborane (1,7-PCB10H11).

film allows insight into bond-breaking chemistry of semiconducting boron carbide, as the NH3 reaction at photoinduced radical sites would yield either B(1s) or C(1s) chemical shifts observable by X-ray photoelectron spectroscopy (XPS). Experimental Methods 7-Me3N-7-CB10H12 was prepared from decaborane20 and converted first to 1,2-PCB10H11 and then to 1,7-PCB10H11 by the methodology of Todd and Little.21 The sublimed crystalline 1,7-PCB10H11 was recrystallized from ethanol and resublimed at 48 °C (0.01 mmHg) in 45% yield. All the isomers of C2B10H12, that is, ortho-carborane (closo-1,2-dicarbadodecaborane or 1,2-C2B10H12), meta-carborane (closo-1,7-dicarbadodecaborane or 1,7-C2B10H12), and para-carborane (closo-1,12dicarbadodecaborane or 1,12-C2B10H12), were purchased from either Katchem or Aldrich and resublimed prior to use, with purity in all cases confirmed by NMR spectroscopy, as described elsewhere.15 The gas phase photoionization and photodecomposition of the isomers of closo-dicarbadodecaborane and closo-phosphacarbadodecaborane (Figure 1) were investigated by photoionization mass spectrometry using synchrotron vacuum ultraviolet (VUV) radiation. Monochromatic light was provided by a 3 m normal incidence monochromator (3 m NIM-1), at BESSY II, which was equipped by a 600 L/mm grating. For higher fluences, undispersed VUV synchrotron radiation was used. Inserting a LiF cutoff filter allows us to measure mass spectra without higher order radiation or stray light. A quadrupole mass filter was used for photoion detection.22 Experimental studies of dehydrogenation and concomitant reactions with ammonia of the inorganic semiconducting boron carbide films were carried out in a two-chamber system consisting of an analysis chamber (base pressure 5 × 10-10 Torr) with hemispherical analyzer and dual anode X-ray source, and an introduction/photochemistry chamber (base pressure 1 × 10-7 Torr). This system has been described previously in detail.23 The selective chemistry experiments were performed with irradiation from a sealed Xe lamp (hυ ) 8.4 eV), in the presence

of 10-4 Torr NH3. A MgF2 window sealed the atmosphere within the Xe line source lamp (Resonance Ltd.) from the rest of the chamber. A photon flux of ∼1015 cm-2 s-1 on the sample, with a photon energy of 8.4 eV, is estimated. The XPS spectra were acquired with the analyzer in constant pass energy mode (50 eV) using Mg KR radiation at 300 W/15 keV. Standard Gaussian-Lorentzian line-shapes, corresponding to Voigt profiles, were used to simulate spectra for comparison with experiment. Boron carbide films ∼1 µm thick were formed by plasmaenhanced chemical vapor deposition (PECVD) on thermally oxidized Si substrates using ortho-carborane as the source gas, as procedure described previously,13 and subsequently characterized by ellipsometry and FTIR. Samples 1 cm ×1 cm were scribed from the substrate and mounted on a Ta sample holder. Sample temperature was monitored with a type K thermocouple attached to the substrate. Following insertion into UHV, the boron carbide samples were lightly Ar+ sputtered with 1000 eV Ar+ to remove surface contamination. Free Molecular closo-Carborane Ionization Energies. As preliminary characterization, we measured and compared the ionization energies of the three different isomers of closodicarbadodecaborane (ortho-carborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), para-carborane (1,12-C2B10H12)), and two related icosahedral cage molecules, 1-phospha-2-carbadodecaborane (1,2-PCB10H11) and 1-phospha-7-carbadodecaborane (1,7-PCB10H11). We determined the adiabatic ionization energy from the first onset of appearance of the parent ion mass peak, similar to previous work.24,25 As illustrated in Figure 2, the parent ion signal for 1,7-PCB10H11 provides for the adiabatic ionization energy 10.10 ( 0.02 eV. In addition, the vertical ionization energy is derived from a linear extrapolation of the ion signal, as indicated in Figure 2, yielding 10.22 ( 0.03 eV. The vertical ionization energy of closo-1,2-C2B10H12 derived in this manner, yielding IE ) 10.17 ( 0.05 eV. This is in good agreement with prior photoionization values of 10.13 eV26 and 10.1 ( 0.2,27 although significantly smaller than the photoelectron value of 10.62 eV.28 Similarly, our value for the vertical

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Figure 2. The parent ion current for the closo-phosphacarborane 1,7 PCB10H11, as a function of photon energy. Higher order light (Ephoton > 11.9 eV) has been suppressed with a LiF window. The inset is the doubly differential parent ion current as a function of photon energy for the same molecule to illustrate the vertical parent molecule ionization energy. The vertical ionization energy lies at about 10.22 eV.

ionization energy of closo-1,7-C2B10H12, IE ) 10.23 ( 0.05 eV, is in good agreement with prior values obtained from photoionization (10.14 eV,26 10.1 ( 0.227) and photoelectron spectroscopy (10.1. eV,28 10.19 eV,29 and 10.78 ( 0.0528). Not too surprisingly, the highly symmetric closo-1,12-C2B10H12 has a slightly higher vertical ionization energy of IE ) 10.42 ( 0.05 eV in our studies, again in general agreement with the prior photoionization value of 10.3 ( 0.2 eV27 and the prior photoelectron spectroscopy result of 10.6 eV.30 These experimental photoionization yield values for the ionization energy of the three closo-carboranes (C2B10H12) isomers are only slightly higher than those energetic values calculated by density function theory 9.95 to 10.12 eV,18 so overall the agreement with expectations is excellent. As such, this is a good test of the effectiveness of hybrid density function theory (DFTB3LYP), using standard 6-31 G* basis set and the Perdew-Wang 91 exchange correlation potential, used below (vide infra) and elsewhere to model the favored fragmentation energetics of the closo-carboranes.15,18,31 We can estimate the error in the calculated energetics to be about 3% for these systems, and these errors are largely systematic. The first vertical ionization energies (IE) have now been determined for the first time for two closo-phosphacarboranes: ortho-1,2-PCB10H11, IE ) 10.17 ( 0.05 eV (adiabatic ionization energy: 10.00 ( 0.05 eV), and meta-1,7-PCB10H11, IE ) 10.23 ( 0.05 eV (adiabatic ionization energy: 10.10 ( 0.02 eV). It is above these ionization threshold that cation fragmentation occurs, and although multiple fragmentation routes are possible for the closo-carboranes,18,19 of interest here are the hydrogen loss from the parent cation as occurs at energies near the ionization energies.18 closo-Carborane Dehydrogenation in the near VUV. Because of the limited count rates, total flux was important for obtaining photoionization mass spectra at sufficient ion intensity. Photoionization mass spectra were taken in the range of 126-146 amu with zero-order light from the normal incidence monochromator. The geometry of the normal incidence monochromator cuts off the zero order synchrotron light at above about 35 eV. On the other hand, photofragmentation of the closo-carborane cation should occur at photon energies above the photoionization thresholds,18 just discussed. It is at these low energies of 10-35 eV that dehydrogenation is believed to dominate the fragmentation process.18 Because of the multiplicity of boron atoms (10), even just the relative abundance of 10B (19.8%) and 11B (80.2%), will

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Figure 3. The part ion mass spectrum of closo-meta 1,7-dicarbadodecaborane in the region of 126 to 146 m/q. Photoionization was done with zero order light cut off at 35 eV by the geometry of the beamline (see text). Multiple mass peaks are seen as the result of hydrogen loss and the statistical distribution of the natural abundance of the 10B and 11 B isotopes of boron.

lead to a multiplicity of parent ion masses, as seen in Figure 3. Any photoionization mass spectroscopy geared toward identifying dehydrogenation must take into account the natural isotopic abundance of boron (10B and 11B) and carbon. Analysis of the parent ion mass spectrum was done using the procedures applied to other polyatomic clusters with a natural multiple isotope distribution.32 Even with careful consideration of the isotopic abundances of the main group elements, the distribution of masses in the parent ion region of the photoionization mass spectrum tend toward smaller masses than expected for the three different isomers of closo-dicarbadodecaborane, if no loss of hydrogen is present. This is illustrated in Figure 3. Figure 4 for 1,2-C2B10H12, 1,7-C2B10H12, and 1,12-C2B10H12. Similarly, for the two related icosahedral cage molecules, 1,2PCB10H11 and 1,7-PCB10H11, the distribution of parent molecule cation masses, from VUV photoionization, tends toward smaller masses than expected for ionization without loss of hydrogen, as seen in Figure 5. These observed smaller masses for the parent cations resulting from VUV photoionization are the result of hydrogen loss. The experimental photoionization mass distribution requires corrections for both hydrogen loss, and the boron and carbon isotopic abundance, as shown in Figures 4 and 5. We find that the photofragmentation of the parent cation, in the VUV, results in the loss of an even number of hydrogen atoms. This likely corresponds to the formation of H2, which requires about 4.56 eV lower energy for formation compared to 2 H formation.33 Note that the neutrals cannot be directly measured in the present experimental setup, so we cannot provide much information about neutrals formed, from the data available to us. The hydrogen loss from the parent cation is shown in Figure 6a for 1,2-C2B10H12, 1,7-C2B10H12, and 1,12-C2B10H12, and in Figure 6b, for the two related icosahedral cage molecules, 1,2-PCB10H11 and 1,7-PCB10H11. This hydrogen loss is apparently largely H2, and almost completely even multiples of molecular hydrogen (Figure 6). We conclude that, for energetic reasons, dehydrogenation of the parent closo-carborane cations is dominated by H2 loss. Regrettably, photoionization mass spectroscopy does not identify which hydrogens are preferentially lost, and we must resort to theory and the solid state for guidance. Nonetheless, the favored loss of H2 should tend to lead toward the formation of edge bonded icosahedra34 in the solid state, and this surmise is certainly consistent with recent local structural information.3 Modeling the Dehydrogenation. The ground state and dehydrogenation energies for a variety of carborane clusters were calculated using the hybrid density function theory (DFTB3LYP) using standard 6-31 G* basis set and the Perdew-Wang

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Figure 4. The predicted parent cation mass distribution, with boron and carbon isotopic abundance considered, compared to the photoionization experiment for three different isomers of closo-dicarbadodecaborane: (a) 1,2-C2B10H12, (b) 1,7-C2B10H12, and (c) 1,12-C2B10H12. There is good agreement with the experimental data if there are corrections made to the parent ion mass distribution for both hydrogen loss and the boron and carbon isotopic abundance, as shown.

91 exchange correlation potential, which has proved to be a successful approach for modeling the energetics closo-carborane decomposition.15,18,31 The initial state ab initio calculations were geometry optimized to obtain the lowest unrestricted HartreeFock (UHF) energy states. We have calculated all the symmetrically unique pairwise combinations for dihydrogen (H2) loss from the parent cation for the three different isomers of closo-dicarbadodecaborane: (a) 1,2-C2B10H12, (b) 1,7-C2B10H12, and (c) 1,12-C2B10H12; and (d) ortho-1,2-PCB10H11 and (e) meta-1,7-PCB10H11. This is summarized in Figure 7, using the number notation schemes of Figure 1 for the various closo-carboranes. In all cases, the reaction: B10C2H12 + hV f B10C2H+ 10 + H2 + e

(1)

favors loss of adjacent hydrogen atoms farthest away from the carbon atoms or farthest away from the carbon and phosphorus (Figure 7). The range of bond dissociation energies possible in reaction 1 is greatest with hydrogen atom loss from 1,2-C2B10H12 (roughly 11.5-14.5 eV, or a range of energies covering 3 eV) and least with ortho-1,2-PCB10H11 and meta-1,7-PCB10H11 (11.8-13.5 eV or 10.8-12.8 eV respectively, or a range of energies 2 eV or less), as summarized in Figure 7. Except for ortho-1,2-PCB10H11, the fraction of dihydrogen (H2) loss from

the parent cation follows the trend of the calculated energetics. In the case of ortho-1,2-PCB10H11, the energy range of symmetrically different pair combination of H2, varies only over a small range of less than 2 eV (11.8-13.5 eV), but the fraction of H2 loss relative to other even and odd numbers of hydrogen loss (40%) is larger than seen for meta-1,7-PCB10H11, 1,7C2B10H12, and 1,12-C2B10H12, as illustrated in Figure 6. As summarized in the plots of the energetics of pair wise loss of hydrogen (Figure 7), the loss of hydrogen from boron atoms is always favored over the loss of a hydrogen from a carbon atom. Indirectly, this boron dehydrogenation is substantiated by the propensity for most boron-rich solids whose majority carriers are holes, that is, there is an introduction of acceptor states into the growing solid semiconductor. Although direct confirmation of the favored site for hydrogen loss is not available from the data we have presently in hand, some experimental support for our contention is available from studies the heavily hydrogenated boron carbide semiconducting films “C2B10Hx”, as is discussed below. The hydrogen loss occurs on the boron atoms with the smallest excess electron populations (0.01e to 0.15e) and farthest away from the carbons with an excess electron charge of roughly 0.31e, determined using Mulliken method, as summarized in Figure 8. It is not, however, related to the overall dipole that ranges from 4.42 D for ortho-carborane, 3.55 D for ortho-

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Figure 5. The predicted parent cation mass distribution, with boron, carbon and phosphorus isotopic abundance considered, compared to the photoionization experiment for (a) ortho-1,2-PCB10H11 and (b) meta-1,7-PCB10H11. There is again good agreement with the experimental data if there are corrections made to the parent ion mass distribution for both hydrogen loss and the boron and carbon isotopic abundance, as shown.

Figure 6. The loss of hydrogen from the parent cation for (a) the three different isomers of closo-dicarbadodecaborane (1,2-C2B10H12, 1,7-C2B10H12, and 1,12-C2B10H12) and (b) two isomers of the closo-phosphacarborane (ortho-1,2-PCB10H11 and (b) meta-1,7-PCB10H11), as determined from the photoionization mass spectra of Figures 4 and 5.

phosphacarborane, 2.79 D for meta-carborane, 2.24 D for metaphosphacarborane, and 0 D for para-carborane.5 Selective Bond Breaking in Semiconducting C2B10Hx Boron Carbide Films. There is a significant variation in the hydrogen content of semiconducting C2B10Hx boron carbide films fabricated by PECVD using ortho-carborane as a percursor.13,17 Loss of hydrogen leads to film densification, but overall, such semiconducting C2B10Hx boron carbide films have a much smaller band gap of 0.7-1.5 eV14,35-37 than the free or adsorbed molecular highest occupied molecular

orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of 9-11.3 eV for the closo-dicarbadodecaboranes and 7.8-9.5 eV for the closo-phospha-carbadodecaboranes.5,6,14,15 Consequently, far less than 10 eV photon energy, the lower limit of the photoionzation threshold in the gas phase, is needed for photoionization of the icosahedra in the semiconducting C2B10Hx boron carbide film. Indeed, we find that the Xe line, with a photon energy of 8.4 eV, is adequate for photoemission/photoionization of the inorganic solid state films.

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Figure 8. The charge densities of the frontier orbitals for ortho-1,2 closo-dicarbadodecaborane, based on the Mulliken charge populations, calculated using the hybrid density function theory (DFT-B3LYP). The larger blue is carbon, red is boron.

Figure 7. The energy cost for the loss of dihydrogen from the parent cation for the three different isomers of closo-dicarbadodecaborane: (a) 1,2-C2B10H12, (b) 1,7-C2B10H12, (c) 1,12-C2B10H12, (d) ortho-1,2PCB10H11 and (e) meta-1,7-PCB10H11, calculated using the hybrid density function theory (B3LYP). These pairwise hydrogen atom loss energies are in eV and indexed according to the icosahedral site numbering schemes shown in Figure 1. Not all pair combinations are plotted: some hydrogen pair combinations identical by symmetry considerations are not shown.

To identify the site location of dehydrogenation, we used ammonia as a “probe” molecule, as has been used previously.38 The VUV excitation energy used here (8.4 eV) is below the ionization threshold of gaseous NH3, and the most significant excitation reaction is:38,39

˜ 2A1) + H NH3 + hV f NH*(A 2

(2)

Given a source-sample distance of ∼2 cm, an NH3 pressure of 10-4 Torr, and an NH3 absorption coefficient (K) at this wavelength of ∼90 atm-1cm-1,38 the relative fraction of photons absorbed by NH3 molecules, according to the Beer-Lambert law, is only 2 × 10-5. Therefore, absorption of photons by the gas can be neglected. Similarly, an estimation of the “effective pressure” due to NH2* is ∼10-9 Torr, indicating that such effects can be neglected under the conditions of these experiments. In the absence of VUV irradiation of the semiconducting C2B10Hx boron carbide films, there is no uptake, absorption, or adsorption of nitrogen-containing species at ambient temperatures upon exposure to NH3. It is only in the presence of both

Figure 9. Evolution of core level spectra photoelectron spectra (XPS) of heavily hydrogenated semiconducting C2B10Hx boron carbide films as a function of exposure to 8.4 eV photon flux in the presence of 10-4 Torr NH3. The core level XPS spectra are shown for the (a) B(1s), (b) C(1s), and (c) N(1s) core levels for the surface after sputter cleaning procedures and after (i) 0 min, (ii) 30 min, (iii) 90 min, and (iv) 180 min exposure to ammonia at 10-4 Torr.

ammonia and VUV radiation (8.4 eV) that nitrogen adsorption is observed, as seen in Figure 9. Referencing the energy of the main C(1s) feature (Figure 9b,i) to the binding energy of aliphatic carbon, 285 eV, and a C(1s) shoulder feature that is observed at a binding energy of 283 eV, yields a B(1s) maximum at 187.9 eV, which is in excellent agreement with results in the literature.40-42 A small N(1s) feature (Figure 9d,i) is also observable for the pristine

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sample, even in the absence of a sample (empty sample holder), and is due to signal contamination from the Ta sample holder (the signal was observed with only the sample holder in the chamber and no semiconducting boron carbide sample present). The lower electronegativity of B relative to C indicates that B atoms bonded to C (B-C) should have lower charge densities and therefore higher core level binding energies than boron atoms bound to other boron atoms (B-B), neglecting possible final state effects. Accordingly, the B(1s) core level spectrum (Figure 9a,i) could be accurately decomposed into B(1s) components at binding energies of 187.8 and 189.3 eV for the B-B and B-C environments, respectively.42 The C(1s) spectrum (Figure 9b,i) can be similarly decomposed into features at 286.7, 285, and 282.7 eV attributable to C-O, C-C, and C-B (carbide) environments, respectively.40 From the core level intensities, corrected from core level cross sections and transmission function of the analyzer, the boron to carbon atomic ratio is estimated to be 4.3:1, consistent with the carbon atoms of the upmost icosahedra of the semiconducting C2B10Hx boron carbide films facing the vacuum interface. The relative intensities of B-B/B-C components, however, is 2.9 and reflects the ratio of 2.9 B-B bonds for every C-B bond, although we expect deviations from calculated 2.9 bond ratio due to the expected edge bonding of the icosahedra in the solid state.3 The evolution of the B(1s), C(1s), and N(1s) core level spectra as a function of exposure to 8.4 eV photons in the presence of 10-4 Torr NH3 is displayed in Figure 9a-c, ii-iv. Notably, exposure results in a decrease in B(1s) intensity near the low binding energy portion of the spectrum, and the growth of a new feature near a binding energy of 192 eV (Figure 9a, ii-iv). This B(1s) binding energy feature is similar in nature to the core level feature identified as being due to B-N bonding environments in boron carbide nanoparticles milled in N2,40 and close to the B(1s) binding energy for boron in boron nitrides.43 During VUV exposure in the presence of NH3, the B-C feature (Figure 9a) near 189.3 eV remains unchanged in relative intensity, indicating that B-C sites are not affected by the B-N bond formation process. The growth of the N(1s) feature near a binding energy of 398.3 eV has been previously identified as due to N bound to B,40 but is somewhat less that the N(1s) binding energy for the nitride (399.1 eV43). This indicates that the nitrogen atoms contributing to this feature are not in a similar environments as would occur in c-BNscovalently bound to other boron atoms, and that a boron nitride phase is not being formed. The B(1s) and N(1s) data are instead consistent with formation of a -NH2 species. There is no evidence for C-N bond formation with ammonia exposure even in the presence of VUV light at 8.4 eV photon energy, in spite of the “carbon” rich surface of the semiconducting C2B10Hx boron carbide films. In the C(1s) core level spectra we would be expected to lead to additional intensity at binding energies >285 eV,40,44 but the high binding energy C(1s) features in the region near 286.7 eV (Figure 9b) actually decreases with increasing VUV + NH3 exposure. The data in Figure 9, therefore, suggests N to B bond formation at B-B sites, without any evidence of nitrogen bonding to carbon sites. The absence of any evidence of C-N bond formation or nitrogen reaction at boron atoms bound to carbon atom sites (Figure 9) indicates that N bond formation occurs primarily

Ru¨hl et al. at reactive sites formed by B-H bond scission at B atoms bound only to boron, rather than to C nearest neighbors:

Although the exact mechanism for hydrogen loss and the sitespecific amine bond formation mechanism are not known from our experimental data of the heavily hydrogenated semiconducting boron carbide, the results are consistent with studies of gas phase carboranes just discussed above. In the gas phase dehydrogenation cation reaction, the lowest energy pathway for photodissociation near the ionization threshold is pairwise loss of atomic hydrogen, corresponding to the formation of H2. In the solid state, the semiconducting boron carbides are excitonic insulators, so “local” cation formation is possible with a long lifetime charge restricted to the icosahedral cage “building bock” of the semiconductor material. The site-specific nitrogentation of solid boron carbide (Figure 8, eqs 3a,b) is therefore consistent with the gas phase experimental and theoretical data (Figures 4-6), indicating that formation of a cation near the ionization threshold is followed by correlated B-H bond scission at B sites opposite carbon atoms and loss of H2. There is an important and obvious caveat: H2 production from the cation parent molecule likely occurs from adjacent cage B-H sites, as discussed above; but in the solid state, the amidization of eq 3a itself may produce H2 from a single B-H site. The energy gain for the reaction of eq 3 is calculated using DFT (B3LYP) to be slight for the closo-carboranes (about -0.12 eV per bond). So a photoactivated reaction is thus not a complete surprise. Because of the uncertainties of concerning the structure of semiconducting boron carbides, local density approximation electronic structure calculations only roughly corresponds with experiments.35 In general, such LDA/DFT calculations are often flawed in accurately assessing such semiconductors,45 although there has been some success in applying density functional theory to clusters known to correspond to the semiconducting boron carbide structure in the vicinity of a transition metal dopant atom in semiconducting boron carbide.46,47 Summary The irradiation of PECVD-deposited hydrogen-rich semiconducting C2B10Hx boron carbide films by 8.4 eV photons in the presence of 10-4 Torr NH3 results in B-N bond formation, specifically at boron sites with boron nearest neighbors. No reaction is observed at carbon sites or at the site of boron atoms bound to carbon, and the amine reaction does not occur in the absence of VUV light. These results are in excellent agreement with DFT calculations, which indicate that lowest energy H2/ cationic B10C2H10 formation involves B-H bond breaking at sites opposite the carbon atoms for pairwise H-B and H-C dissociation/H2 formation from a variety of closo-carborane molecules. What is clear is dehydrogenation of the parent closo-carborane cations is dominated by H2 loss. The largely H2 loss by the parent cation occurs at energies just above the photoionization threshold for 1,2-C2B10H12, 1,7-C2B10H12), 1,12-C2B10H12, 1,2PCB10H11, and 1,7-PCB10H11.

Photofragmentation of the closo-Carboranes Acknowledgment. This work was supported by the Defense Threat Reduction Agency (Grant No.HDTRA1-09-1-0060), the Office of Naval Research (N00014-10-1-0419), the Deutsche Forschungsgemeinschaft through grant RU 420/8-1, and the Fonds der Chemischen Industrie. We thank the staff of BESSYII at Helmholtz Center Berlin for their assistance and also thank David Graves for the loan of a Xe lamp. The authors note, with regret, the passing of our colleague Dr. Hans-Werner Jochims on 8 May, 2010 just prior to publication. References and Notes (1) (a) Heying, T. L.; Ager, J. W.; Clark, S. L.; Mangold, D. J.; Goldstein, H. L.; Hillman, M.; Polak, R. J.; Szymanski, J. W. Inorg. Chem. 1963, 2, 1089. (b) Schroeder, H.; Heying, T. L.; Reiner, J. R. Inorg. Chem. 1963, 2, 1092. (c) Heying, T. L.; Ager, J. W.; Clark, S. L.; Alexander, R. P.; Papetti, S.; Reid, J. A.; Trotz, S. I. Inorg. Chem. 1963, 2, 1097. (d) Papetti, S.; Heying, T. L. Inorg. Chem. 1963, 2, 1105. (e) Fein, M. M.; Bobinski, J.; Mayes, N.; Schwartz, N.; Cohen, M. S. Inorg. Chem. 1963, 2, 1111. (f) Fein, M. M.; Grafstein, D.; Paustian, J. E.; Bobinski, J.; Lichstein, B. M.; Mayes, N.; Schwartz, N. N.; Cohen, M. S. Inorg. Chem. 1963, 2, 1115. (g) Grafstein, D.; Bobinski, J.; Dvorak, J.; Smith, H.; Schwartz, N.; Cohen, M. S.; Fein, M. M. Inorg. Chem. 1963, 2, 1120. (h) Grafstein, D.; Bobinski, J.; Dvorak, J.; Paustian, J. E.; Smith, H. F.; Karlan, S.; Vogel, C.; Fein, M. M. Inorg. Chem. 1963, 2, 1125. (I) Grafstein, D.; Dvorak., J. Inorg. Chem. 1963, 2, 1128. (2) (a) Byun, D.; Spady, B. R.; Ianno, N. J.; Dowben, P. A. Nano Struct. Mat. 1995, 5, 465. (b) Hwang, S.-D.; Byun, D.; Ianno, N. J.; Dowben, P. A.; Kim, H. R. Appl. Phys. Lett. 1996, 68, 1495. (c) Hwang, S.-D.; Remmes, N. B.; Dowben, P. A.; McIlroy, D. N. J. Vac. Sci. Technol. B 1996, 14, 2957. (d) Hwang, S.-D.; Remmes, N.; Dowben, P. A.; McIlroy, D. N. J. Vac. Sci. Technol. A 1997, 15, 854. (3) (a) Carlson, L.; LaGraffe, D.; Balaz, S.; Ignatov, A.; Losovyj, Ya. B.; Choi, J.; Dowben, P. A.; Brand, J. I. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 195. (b) Dowben, P. A.; Kizilkaya, O.; Liu, J.; Montag, B.; Nelson, K.; Sabirianov, I.; Brand, J. I. Mater. Lett. 2009, 63, 72. Liu, J.; Luo, G.; Mei, W.-N.; Kizilkaya, O.; Shepherd, E. D.; Brand, J. I.; Dowben, P. A. J. Phys. D: Applied Physics 2010, 43, 085403. (4) (a) Hwang, S.-D.; Yang, K.; Dowben, P. A.; Ahmad, A. A.; Ianno, N. J.; Li, J. Z.; Lin, J. Y.; Jiang, H. X.; McIlroy, D. N. Appl. Phys. Lett. 1997, 70, 1028. (b) McIlroy, D. N.; Hwang, S.-D.; Yang, K.; Remmes, N.; Dowben, P. A.; Ahmad, A. A.; Ianno, N. J.; Li, J. Z.; Lin, J. Y.; Jiang, H. X. Appl. Phys. A: Mater. Sci. Process. 1998, 67, 335. (5) Balaz, S.; Dimov, D. I.; Boag, N. M.; Nelson, Kyle; Montag, B.; Brand, J. I.; Dowben, P. A. Appl. Phys. A: Mater. Sci. Process. 2006, 84, 49. (6) Caruso, A. N.; Billa, R. B.; Balaz, S.; Brand, J. I.; Dowben, P. A. J. Phys. Cond. Mat. 2004, 16, L139. (7) Robertson, B. W.; Adenwalla, S.; Harken, A.; Welsch, P.; Brand, J. I.; Dowben, P. A.; Claassen, J. P. Appl. Phys. Lett. 2002, 80, 3644. (8) (a) Robertson, B. W.; Adenwalla, S.; Harken, A.; Welsch, P.; Brand, J. I.; Claassen, J. P.; Boag, N. M.; Dowben, P. A. AdV. Neutron Scatt. Instrum.; Anderson, I. S.; Gue´rard, B., Eds.; Proc. SPIE: 2002; Vol. 4785, pp 226; (b) Adenwalla, S.; Billa, R.; Brand, J. I.; Day, E.; Diaz, M. J.; Harken, A.; McMullen-Gunn, A. S.; Padmanabhan, R.; Robertson, B. W. Penetrating Radiation Systems and Applications V, Proc. SPIE 2003, 5199, 70. (9) (a) Osberg, K.; Schemm, N.; Balkir, S.; Brand, J. I.; Hallbeck, S.; Dowben, P. A.; Hoffman, M. W. IEEE Sensors J. 2006, 6, 1531. (b) Osberg, K.; Schemm, N.; Balkir, S.; Brand, J. I.; Hallbeck, S.; Dowben, P. 2006 IEEE International Symposium on Circuits and Systems (ISCAS 2006) Proceedings 2006, 1179. (10) Caruso, A. N.; Dowben, P. A.; Balkir, S.; Schemm, N.; Osberg, K.; Fairchild, R. W.; Flores, O. B.; Balaz, S.; Harken, A. D.; Robertson, B. W.; Brand, J. I. Mat. Sci. Engin. B 2006, 135, 129. (11) (a) Day, E.; Diaz, M. J.; Adenwalla, S. J. Phys. D: Appl. Phys. 2006, 39, 2920. (b) Hong, N.; Langell, M. A.; Liu, J.; Kizilkaya, O.; Adenwalla, S. J. Phys. D: Appl. Phys. 2010, 107, 024513. (12) Emin, D.; Aselage, T. J. Appl. Phys. 2005, 97, 013529. (13) Schulz, D. L.; Lutfurakhmanov, A.; Maya, B.; Sandstrom, J.; Bunzow, D.; Qadri, S. B.; Bao, R.; Chrisey, D. B.; Caruso, A. N. J. NonCryst. Sol. 2008, 354, 2369. (14) Byun, D.; Hwang, S.-D.; Zhang, J.; Zeng, H.; Perkins, F. K.; Vidali, G.; Dowben, P. A. Jap. Journ. Appl. Phys. Lett. 1995, 34, L941. (15) Balaz, S.; Caruso, A. N.; Platt, N. P.; Dimov, D. I.; Boag, N. M.; Brand, J. I.; Losovyj, Ya. B.; Dowben, P. A. J. Physical Chemistry B 2007, 111, 7009.

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