Surface Science 605 (2011) L61–L66
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Surface Science
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Surface Science Letters
Initial oxidation stages of hydrogen- and styrene-terminated Si(100) surfaces: A
molecular dynamics study
Bhavin N. Jariwala a, Cristian V. Ciobanu b,⁎, Sumit Agarwal a,⁎
a
b
Department of Chemical Engineering, Colorado School of Mines, Golden, CO 80401, United States
Division of Engineering, Colorado School of Mines, Golden, CO 80401, United States
a r t i c l e
i n f o
Article history:
Received 26 March 2011
Accepted 29 June 2011
Available online 7 July 2011
Keywords:
Oxidation
Surface chemical reaction
Silicon
Molecular dynamics
a b s t r a c t
We have studied the initial oxidation of H- and styrene-terminated Si(100)-2 × 1 films in O2 atmosphere at
500 K using molecular dynamics (MD) simulations based on a reactive force field. Our simulations show that
for both surface terminations the primary reactions observed are the dissociation of the oxygen molecules and
the simultaneous insertion of atomic oxygen in the Si\Si back-bonds. On the H:Si(100)-2 × 1 surface, another
reaction is the formation of isolated Si\OH bonds via the insertion of an oxygen atom in a Si\H bond.
Detailed analysis of MD configurations shows that different vibrational modes of the surface Si\H and the
tilting of Si dimers at 500 K facilitate the breaking of the O2 molecule and the oxygen attack at backbonds. The
combination of these reactions leads to increased amorphization of the surface as the oxidation proceeds. In
the case of styrene-terminated Si(100)-2 × 1, the rate of O2 attack was much lower than on H-terminated
surface and O-atom insertions were not observed in back-bonds of Si\C bonds. In addition to lesser number
of Si\H sites on styrene-Si(100)-2 × 1, another significant reason for the lower rate of O2 attack was the
repulsion of oxygen molecules resulting from the movement of phenyl rings in styrene at 500 K.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Oxidation of H-terminated crystalline silicon (c-Si) surfaces has been a
topic of interest for a broad range of experimental and theoretical
investigations over the last few decades. It is well known that exposure of
H-terminated c-Si wafers to air results in the formation of a native oxide
on the surface, even at room temperature [1–3]. In contrast to the
thermally grown oxide used in various Si-based devices, the native oxide
contains large number of defects and growth of such oxide therefore
needs to be suppressed. Moreover, it is also necessary to prevent
atmospheric oxidation of various H-terminated c-Si structures, such as
porous Si and Si quantum dots (QDs), since oxidation significantly affects
their opto-electronic properties [4,5]. While oxidation of Si surfaces is well
studied, there are a few discrepancies in the literature regarding oxidation
via O2 and H2O attack. Morita and coworkers concluded that presence of
both oxygen and moisture is required for the growth of native oxide on
Si(111) and Si(100) surfaces at room temperature [1]. Zhou et al. also
proposed that oxidation of H-terminated Si(111) requires presence of
both an oxidant (O2) and a nucleophile (H2O) [6]. On the other hand,
numerous studies have demonstrated oxidation of H-terminated c-Si
surfaces in a controlled dry O2 environment [4,5,7,8]. In spite of the
differences, experimental studies have conclusively shown that three
main reactions occur during oxidation of H-terminated Si surfaces:
⁎ Corresponding authors.
E-mail addresses: cciobanu@mines.edu (C.V. Ciobanu), sagarwal@mines.edu
(S. Agarwal).
0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2011.06.028
(i) O-atom insertion in Si\Si back bonds resulting in OySiHx sites, (ii)
formation of Si\O\Si bonds, and (iii) formation of Si\OH surface sites
[5,8,9]. Most theoretical reports to date have dealt with O2 chemisorption
on bare Si surfaces [10–12], or with oxide structures on various Si surfaces
[13–15]. Therefore, there is a need to understand the oxidation
mechanism of H-terminated c-Si surfaces by oxygen and water.
In an attempt to prevent oxidation, various passivation/functionalization schemes have been employed for Si surfaces. Amongst
them, highly stable organic monolayers attached to the Si surface
through covalent Si\C bonds have proven to be an attractive
passivation layer [16,17]. An added interest in organic termination
also emanated from the prospect of incorporating various chemical
functionalities into hybrid organic/Si microelectronic devices [18].
Wolkow and coworkers demonstrated self-directed growth of styrene
nanostructures on H-terminated Si(111) [19] and Si(100) [18,20]. On
the H:Si(100)-2 × 1 surface, the formation of 1-D lines of styrene along
the dimer rows has been observed using scanning tunneling
microscopy [18]. Passivation of Si(111) and Si(100) has also been
demonstrated via alkyl-termination [16,17]. Further, significant
resistance to atmospheric oxidation has been shown via attachment
of alkyl ligands to the surface of H-terminated Si QDs [21]. Although
alkyl ligands provide better passivation than hydrogen, oxidation at
relatively low rates has been reported for alkyl-terminated Si surfaces
[22]. Oxidation of alkyl-terminated Si could be attributed to the
limited coverage of alkyl ligands on the surface. It has been reported
that steric hindrance on the surface results in a maximum alkyl
coverage of ~ 50% on Si single crystal surfaces using long-chain
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hydrocarbons [23,24] which leaves the rest of the unsubstituted
surface Si\H sites susceptible to oxidation. However, a threshold
surface-alkyl coverage of ~ 42% is sufficient to provide long-term
passivation on Si(111) surfaces [25]. The reason for this was
attributed to the formation of an o monolayer with intermolecular
channels narrower than the diameter of the water molecule, which
then cannot penetrate into the monolayer and attack the unsubstituted Si\H bonds [25]. However, this explanation does not take into
account O2-based oxidation. Few computational studies have been
performed to understand oxidation of alkyl-terminated Si surfaces as
compared to H-terminated surfaces. These studies have been limited
to dependence of structure and stability of organic layers on the
different possible suboxide structures [26]. The mechanism of O2
attack and the reason for reduced oxidation rates on alkyl-terminated
surfaces has not yet been investigated at the atomic level.
In this letter, we have employed large-area molecular dynamics
(MD) simulations in order to understand the atomic-scale mechanisms
of the oxidation of H and styrene-terminated Si(100)-2 × 1 films. The
reason for choosing styrene-Si(100)-2 × 1 as a model surface is that the
styrene termination is well understood on Si(100)-2 × 1 [18]. Using MD,
we have identified the different reactions that occur on H- and styreneSi(100)-2 × 1 surfaces during the initial period of oxidation. Further,
through detailed analysis of the atomic trajectories of O2 we present
mechanisms of O2 attack on Si\Si back-bonds and surface Si\H bonds.
The reasons for reduced oxidation rates on styrene-terminated surfaces
are also presented.
2. Computational details
The interaction of O2 with H- and styrene-Si(100)-2 × 1 surfaces is
studied using classical MD as employed in the Large-scale Atomic/
Molecular Massively Parallel Simulator (LAMMPS) code [27]. The
atomic interactions for Si, C, H and O atoms are described by a manybody reactive force field (ReaxFF) [28]. This potential has been shown to
describe correctly the relative stability of the bulk phases of Si, certain
atomic clusters, as well as the vacancy formation energy in diamond
silicon; more relevant for our current studies, ReaxFF reproduces closely
the reactions of water molecules (pathways, barriers, adsorption
energies) with the Si(001) surface [28]. In ReaxFF, the total potential
energy contains bond-order terms that are updated at each time step,
allowing for breaking and formation of bonds during the MD simulation.
Further, the ReaxFF also contains long-range interaction terms and
dynamically computes atomic charges thus accounting for polarization
effects. Unit cells consisting of 2112 atoms (8 Si layers; 1536 Si and 576
H atoms) and 3392 atoms (8 Si layers: 1536 Si, 1216 H and 640 C atoms)
with dimensions of 61.4 × 46.1 × 11.6 Å and 61.4 × 46.1× 18.1 Å were
used to simulate H- and styrene-Si(100)-2× 1 slabs, respectively. The
slabs were periodic in the in-plane x and y directions. Based on previous
experimental results [18], in the case of styrene-Si(100)-2× 1, the
styrene molecules were attached to Si atoms on one side of the dimers
along the dimer rows. When kept close to 50%, the precise value of the
styrene coverage is not expected to influence our conclusions about the
reaction mechanisms but does affect directly the frequency with which
O2 oxidation attacks occur on the substrate. For this reason, we use in
our starting structures an experimentally relevant styrene coverage [29]
that is somewhat smaller than the saturation level of 50%. Alternate
dimer rows contained 12 and 8 styrene molecules per unit cell, resulting
in an effective coverage of ~42% of styrene while the remainder of the
surface Si atoms were H-terminated [Fig. 1(c)].
Full atomic relaxations were performed using the conjugate gradient
algorithm within the LAMMPS code. Fig. 1(a) and (c) shows the side
view of the relaxed H- and styrene-Si(100)-2 × 1 slabs, respectively.
Fig. 1(c) also shows a schematic view of the top of the styrene-Si(001)
which realizes an effective coverage of ~42%. The slabs were thermalized
in the canonical ensemble at 500 K (Nose–Hoover thermostat) using a
time step of 0.1 fs for 10 ps. To simulate oxygen exposure of the surface,
O2 molecules were repeatedly impinged at random locations. The initial
positions of the molecules were at 5 Å above the topmost atom of the
surface and initial velocities corresponded to a temperature of 500 K
directed normal to the surface. In order to ensure sufficient relaxation of
the surface, the entire systems were equilibrated for 20 ps between any
two consecutive O2 impingement events. The atomic trajectories were
visualized by taking snapshots of the atomic configurations at 1 fs
intervals. To investigate the energetics of the reactions observed, we
have studied the temporal evolution of the total energy of the system
during the MD runs. Further, MD configurations before and after the
reactions were also relaxed using the conjugate-gradient algorithm, and
exothermicities were calculated based on the difference in these
energies were in good agreement with the values obtained from the
energetics during the MD runs.
3. Results and discussion
We have observed that a large fraction of the oxygen molecules that
were impinged onto the H:Si(100)-2×1 slab resulted in the dissociation
of O2. The dissociation was also accompanied by O-atom insertions in
back-bonds (Si\Si) of the surface Si\H bonds and in subsurface Si\Si
bonds. In the case of styrene-Si(100)-2×1 surface, the majority of O2 were
reflected back into the gas phase. However, the oxygen molecules that
reacted with the styrene-terminated surface resulted in O-atom insertions in Si\Si back-bonds and subsurface bonds, similar to the case of
H-Si(100)-2×1. Further, O-atom insertions in the back-bonds of Si\C
bonds were not observed during the MD runs. Fig. 1(b) and (d) show the
different configurations for O-atom insertions in Si\Si bonds on H- and
styrene-Si(100)-2×1 surfaces, respectively. The configurations shown in
Fig. 1(b) and (d) represent the different sites occupied by the O atoms
formed via O2 dissociation followed by O-atom insertions, and were
obtained via relaxation of the final atomic configurations attained after
the MD runs. The relaxations were performed simply to arrive at nearest
local minima, and no reactive processes occurred as a consequence of
these relaxations. The initial O-atom insertions resulted in Si\H(O) bonds
(O-atom insertion in a single back-bond) as well as Si\O\Si bonds.
Further O2 impingement led to the formation of Si\H(O2) bonds (not
shown in the figure), which resulted from O-atom insertions in one of the
remaining back-bonds of the Si\H(O) sites. All the O-atom insertion
events observed were highly exothermic. Various MD configurations
before and after the reactions were relaxed, and the exothermicity of the
reactions was calculated based on the difference in the energies of these
relaxed configurations. Interestingly, the exothermicity for the O2
dissociation followed by O-atom insertions on the H- as well as styreneSi(100)-2×1 surfaces (Fig. 1(b) and (d)) was about 10.0±0.2 eV. The
comparable exothermicities can be explained based on binding energies
of the surface Si\H bonds. The binding energies of an H atom to a surface
Si with one dangling bond on the H- and styrene-terminated surfaces is
calculated as the difference in the (relaxed) potential energies between
the situation with H from away from the surface and that with H
passivating the dangling bond. The values obtained were 3.50 eV and
3.52 eV, respectively. Since the Si\H bonds on the two surfaces have
similar binding energies, the back-bonds that are initially attacked by O2
also have similar bond strengths, which led to comparable exothermic
energies for the O-atom insertions. Furthermore, since a large fraction
(~80%) of O2 resulted in O-atom insertions on H-Si(100)-2×1 surface at a
relatively low temperature of 500 K, the activation energy barrier for such
insertion events is very low.
The events observed during MD simulations are consistent with
experiments that employed controlled O2 exposure of Si surfaces. In
previous studies, IR measurements of H-terminated Si surfaces as
well as porous Si showed that the oxidation of these surfaces leads to
the formation of SiHx(Oy) back-bonds and Si\O\Si bonds on the
surface [5,7,8]. It was demonstrated that initial oxidation of these
surfaces resulted in the formation of SiHx(Oy) back-bonds and also a
shift in the SiHx (x = 1,2) vibrational modes to higher frequencies
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(a)
L63
(b)
(c)
[110]
H H H H H H H H H H H H
s s H s s H s s H s s H
H H H H H H H H H H H H
s s s s s s s s s s s s
[001]
side view, along [110]
[110]
schematic top view, along [001]
(d)
Fig. 1. (a) Side view of relaxed H-Si(100)-2 × 1 slab. (b) Configurations showing the different atomic sites occupied by O atoms after the O2 dissociation followed by O-atom
insertions in Si\Si back-bonds on H-Si(100)-2 × 1 slab. (c) Side view of relaxed styrene-Si(100)-2 × 1 slab (left) and schematic top view (right) showing the repeated surface cell
with two dimer rows. As shown in the schematic view, each surface atom is passivated either with hydrogen (H) or with styrene (s) for an effective styrene coverage of 41.66%.
(d) Configurations showing the different atomic sites occupied by O atoms after O2 dissociation followed by O-atom insertions in Si\Si back-bonds on styrene-Si(100)-2 × 1 slab. The
configurations in (b) and (d) were obtained after relaxation of the final atomic configurations attained after the respective MD runs. For clarity only the first three Si layers and atoms
in the vicinity of the reaction events are shown.
[5,8]. The shift was attributed to an increase in the σ-bond character
of the Si\H bonds due to electron withdrawal by the O atoms, since
the electronegativity of O is greater than that of Si [5]. This is indeed
what was observed through the MD simulations where the Si\H
bond lengths before and after the O-atom insertion were 1.49 Å and
1.46 Å, respectively, indicating a stronger Si\H bond after the
O-atom insertion in the back-bond. Although O-atom insertion has
(a)
been observed experimentally, the mechanism for the insertion into
a Si\Si back-bond is not well understood.
To understand the mechanism of O2 attack, we analyzed the atomic
configurations at various time intervals during MD runs. Fig. 2 shows the
temporal evolution of such atomic configurations illustrating an O2
molecule attack on H:Si(100)-2× 1 surface at 500 K. When the O2
molecule is away from surface (Fig. 2(a)), the Si\H bond length is 1.49 Å
(c)
(b)
(d)
(e)
Si1-Si 2= 2.32 Å
Si-Si 1= 2.32 Å
118.3
116
119
122
117
121
Si-Si1 = 2.42 Å
Si1-Si 2= 2.5 Å
Si-H = 1.46 Å
Si-H = 1.56 Å
Si-H = 1.49 Å
t = 0.00 ps
t = 3.00 ps
t = 3.06 ps
t = 3.76 ps
t = 10.00 ps
Fig. 2. Temporal evolution of the atomic configurations showing O2 dissociation followed by O-atom insertions on H-Si(100)-2 × 1 during an MD run. For clarity only three topmost Si
layers and atoms in the vicinity of the reaction events are shown.
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a
b
Total Energy (eV)
0.00
c
-2.00
~8 eV
-4.00
2nd O-atom
insertion
-6.00
-8.00
-10.00
1st O-atom
insertion
0.00
2.00
d
~2 eV
e
4.00
6.00
8.00
10.00
time (ps)
Fig. 3. Plot of the total energy of the supercell as a function of time during the MD run
described in Fig. 2. The points highlighted on the curve correspond to the atomic
configurations shown in Fig. 2.
and the bond angles between the three Si\Si back-bonds are 109.8°,
111.2°, and 113.4°, representing a tetrahedral configuration. Fig. 2(b)
shows O2 approaching the surface and also seen in the sideview is the
alternate tilting of Si\Si dimers along the dimer rows [30]. It was
observed that at 500 K the Si atoms in the dimers oscillate in a
coordinated fashion resulting in alternate tilting along the dimer rows.
The tilting affects the bond angles and leads to an approximate coplanarity of the surface Si atom and its first-order neighbors. Moreover,
the dimer tilting in combination with the Si\H vibrations leads to the
weakening of the back-bonds and the Si\H bond, as is indicated by an
increase in their lengths (refer to Fig. 2(c)). This disruption of bond
angles and bond lengths facilitates the O2 attack, and since the Si\H
bond is stronger than the Si\Si bond, the O-atom insertion occurs in the
back-bond as seen in Fig. 2(d). Once the O-atom is inserted in one of the
back-bonds, the neighboring bonds are weakened and the second O
atom can insert either in a neighboring back-bond (Fig. 1(b)) or in a
Si\Si bond that lies beneath the surface bilayer as shown in Fig. 2(e).
Thus, the changes in the bond angles and bond lengths resulting from
the vibrations of the surface Si\H and rocking of Si\Si dimers facilitate
O2 attack on an H-Si(100)-2 × 1 surface, and results in O-atom insertions
in Si\Si back-bonds.
Fig. 3 shows the energetics of the reaction in Fig. 2. Initially, as O2
approaches the surface, the total energy of the system is nearly constant.
However, as soon as O2 reaches the surface, it dissociates and attacks a
Si\Si back-bond, which results in rapid and substantial decrease of the
total energy indicating that O2 dissociation along with the initial O-atom
insertion is a highly exothermic event (~8 eV). The insertion of the
second O atom in a subsurface Si\Si bond is also exothermic resulting in
further decrease in the total energy by ~2 eV. Thus, the overall
exothermicity of the reaction is ~10 eV. As mentioned above and as
can also be seen in Fig. 3, the activation energy barriers for O2
dissociation followed by O-atom insertions are extremely low. It is
worth noting that the attack of the Si-Si bonds can occur even at lower
temperatures than 500 K. During our MD simulations, we have not
observed the O2 molecule to first attack the surface dimers, even though
these dimers are more exposed than the Si\Si backbonds. A possible
(a) t = 0.50 ps (b) t = 0.70 ps (c) t = 0.90 ps (d) t = 1.00 ps (e) t = 10.00 ps
(f)
Total Energy (eV)
0.00
(a)
-2.00
-4.00
-6.00
(e)
-8.00
0.00
2.00
4.00
6.00
8.00
10.00
time (ps)
Fig. 4. (a)–(e) Atomic configurations during the O2 attack at a surface dimer in the vicinity of an already reacted site. This attack shows the formation of a surface \OH and the onset
of amorphization of the substrate. For clarity, only atoms in the vicinity of the reaction events are shown. (f) Total energy of the supercell as a function of time during the MD run.
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t = 3.50 ps
t = 6.00 ps
t = 6.50 ps
t = 7.00 ps
L65
t = 7.50 ps
Fig. 5. Temporal evolution of the atomic configurations during O2 impingement on styrene-terminated Si(100)-2 × 1 surface, showing the repulsion of O2 due to movement of phenyl
rings on the surface.
reason could be that the dissociation of O2 requires the close proximity
of the oxygen molecule to three or more silicon atoms so that these Si
atoms chemically interact with each of the two oxygens.
Another reaction that was observed after the initial oxidation was
the formation of a \OH group, as shown in Fig. 4. This figure shows
the oxygen molecule reacting with the surface dimer after the nearby
site has already been attacked at its backbonds. The formation of \OH
groups during H2O-based oxidation of Si(100) and Si(111) surfaces is
well known [2,9,31]. In contrast, the formation of a \OH group has
not been reported so far for O2-based oxidation of c-Si surfaces.
However, systematic IR measurements performed during O2-based
oxidation of porous silicon demonstrated the formation of isolated
Si\OH bonds via O-atom insertion in Si\H bonds [5]. Similar
reactions were observed on the H:Si(100)-2 × 1 surface during the
MD simulations, where O2 impingement resulted in isolated \OH
bonds on the surface. The reason for O2 attacking a Si\H bond can be
attributed to disruption of the surface periodicity from the previous Oatom insertions in the back-bonds resulting in consecutive weakening
of the neighboring bonds via change in bond angles and bond lengths
[Fig. 4(a)–(e)]. The \OH formation occurred after the insertion of an
O atom in a backbond [at t = 0.9 ps, Fig. 4(c)] which was accompanied
by the simultaneous collapse of the attacked surface dimer, and was
followed by the migration of H atoms on the surface. Fig. 4(f) shows
the energetics of the entire reaction. In this case, the O2 dissociation
followed by O-atom insertion and \OH formation resulted in an
exothermicity of ~8 eV and no discernible barrier was observed for
the reaction. Thus, subsequent oxidation due to O-atom insertions in
various Si\Si and Si\H bonds results in increased amorphization of
the surface and the subsurface layers.
The O insertions in Si\Si backbonds observed on H:Si(100)-2× 1
surface were also observed on the styrene-Si(100)-2 × 1 surface as
discussed above (Fig. 1(d)). However the rate of O2 attack was
considerably low on the styrene-terminated surface, which is consistent
with experimental observations on alkyl-terminated Si surfaces
showing reduced oxidation rates in air [17,32–34]. The primary reason
for the relatively low rate of O2 attack is explained in Fig. 5. The figure
shows the temporal evolution of the atomic configurations illustrating
O2 attack on styrene-Si(100)-2 × 1 surface at 500 K. Further, as seen in
Fig. 5, the phenyl rings in styrene can rotate around the C\C bonds [35]
and therefore result in repulsion of an O2 that comes close to a styrene
molecule. Thus, steric crowding due to the rotation and movement of
phenyl rings limits the number of surface atoms that are accessible to
the incoming O2 molecules. However, O2 can still intercalate through the
styrene-layer on top and reach the surface Si\H bonds resulting in Oatom insertions as shown in Fig. 1(d). Moreover, O-atom insertions in
back-bonds of Si\C bonds were not observed, most likely because there
the O2 molecules are deflected by the motion of the phenyl ring and
cannot reach the back-bond sites on the styrene side of the dimers. Since
42% of the surface sites are terminated with Si\C bonds, the number of
Si\H bonds available for O2 attack is smaller than that on a Hterminated surface. A combination of the above mentioned reasons
explains the reduced oxidation rates that are experimentally observed
for alkyl-terminated bulk c-Si surfaces in air.
4. Conclusions
In conclusion, we have identified the different reactions that occur
on H- and styrene-terminated Si(100)-2 × 1 surfaces in an O2
environment at 500 K. We observed the dissociation of O2 molecules
along with insertions of oxygen atoms in the Si\Si back-bonds of the
Si\H bonds, as well as in subsurface Si\Si bonds. Molecular dynamics
simulations show that the O-atom insertions in Si\Si bonds can be
attributed to the different Si\H vibrational modes and buckling of
Si\Si dimers at 500 K, which result in changes in the bond angles and
bond lengths and hence facilitate O2 molecule attack at such sites. On
the H-Si(100)-2 × 1 surface, we also observed the formation of Si\OH
bonds via the insertion on an oxygen in an Si\H bond. The formation
of Si\OH also resulted in migration of H on the Si surface. Thus,
combination of these reactions resulted in increased amorphization of
the surface as the oxidation proceeded. Further, as observed
experimentally, in the case of styrene-Si(100)-2 × 1 the rate of O2
attack was much lower than on H-terminated surface and O-atom
insertions were not observed in back-bonds of Si\C bonds. In
addition to smaller number of unsubstituted Si\H sites on styreneSi(100)-2 × 1, a major reason for lower rate of O2 attack was the O2
repulsion resulting from the rotation and movement of phenyl rings
in styrene at 500 K.
Acknowledgments
This research was supported by NSF (through grant nos. CMMI0846858 and CBET-0846923), and the Renewable Energy MRSEC
program at the Colorado School of Mines (NSF grant no. DMR0820518). We also acknowledge the use of supercomputing resources
provided by the Golden Energy Computing Organization at CSM.
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