Photofragmentation of the - American Chemical Society Publications

7284
Photofragmentation of the closo-Carboranes Part II: VUV Assisted Dehydrogenation in the
closo-Carboranes and Semiconducting B10C2Hx Films
Introduction
Eckart Rühl, † 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 UniVersität 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.
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 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 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: pdowben@unl.edu.
† Freie Universität 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
10.1021/jp103805r © 2010 American Chemical Society
Published on Web 06/23/2010
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 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 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 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

Photofragmentation of the closo-Carboranes J. Phys. Chem. A, Vol. 114, No. 27, 2010 7285
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) meta-
1-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 decaborane 20 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 a3m
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 ∼10 15 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 eV 26 and
10.1 ( 0.2, 27 although significantly smaller than the photoelectron
value of 10.62 eV. 28 Similarly, our value for the vertical

7286 J. Phys. Chem. A, Vol. 114, No. 27, 2010 Rühletal.
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 ( 0.2 27 ) and photoelectron
spectroscopy (10.1. eV, 28 10.19 eV, 29 and 10.78 ( 0.05 28 ). 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 eV 27 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 (DFT-
B3LYP), 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 10 B (19.8%) and 11 B (80.2%), will
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 10 B 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 ( 10 B and 11 B) 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, 1,2-
PCB10H11 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 icosahedra 34 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 (DFT-
B3LYP) using standard 6-31 G* basis set and the Perdew-Wang

Photofragmentation of the closo-Carboranes J. Phys. Chem. A, Vol. 114, No. 27, 2010 7287
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 Hartree-
Fock (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 B10C2H10 + H2 + e -
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
(1)
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, 1,7-
C2B10H12, 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-

7288 J. Phys. Chem. A, Vol. 114, No. 27, 2010 Rühletal.
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 0Dforpara-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 eV 14,35-37 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.

Photofragmentation of the closo-Carboranes J. Phys. Chem. A, Vol. 114, No. 27, 2010 7289
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-1,2-
PCB10H11 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
NH 3 + hV f NH 2 *(Ã 2 A 1 ) + H (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 -1 cm -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 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 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

7290 J. Phys. Chem. A, Vol. 114, No. 27, 2010 Rühletal.
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 eV 43 ). 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
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, 1,2-
PCB10H11, and 1,7-PCB10H11.

Photofragmentation of the closo-Carboranes J. Phys. Chem. A, Vol. 114, No. 27, 2010 7291
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 BESSY-
II 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.
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