Enhanced CNT nucleation through ion bombardment:
reactive MD simulations
E. C. Neyts, A. Bogaerts
Research group PLASMANT, Department of Chemistry, University of Antwerp, Antwerp, Belgium
Abstract: Reactive molecular dynamics simulations demonstrate that carbon nanotube cap
nucleation can be enhanced by ion bombardment in a limited energy window. The observed
enhancement is explained by an ion induced carbon network restructuring mechanism.
These results are of interest for plasma-assisted growth of carbon nanotubes.
Keywords: molecular dynamics, carbon nanotubes, nucleation
1. Introduction
Carbon nanotubes (CNTs) are hollow cylindrical
structures consisting of a hexagonal carbon network.
Their unique properties offer perspective on a plethora of
applications [1]. They can for example be used as
interconnects in silicon IC fabrication because of their
high current carrying capacity (>107 A cm-2) or as heat
sinks to dissipate heat from computer chips due to their
very high thermal conductivity (> 3500 W m-1 K-1).
Especially single walled carbon nanotubes (SWCNTs)
offer perspective, since they can be either metallic or
semiconducting (with a bandgap ranging from 0 eV to
about 2 eV), depending on their chirality. They allow 1dimensional conduction and are envisaged for use in
nanoscale electronics such as single electron transistors,
as electron field emitters, for hydrogen storage, as
actuators, chemical sensors, in super strong polymeric
composite materials, etc. Furthermore, many properties of
SWNTs are tunable to the required application, such as
their photoelectrochemical activity by controlling their
length. This offers opportunities in e.g. fabrication of
efficient opto-electronic devices, nanotube optical
detectors or emitters [2].
Currently, however, the applicability of CNTs is
limited by a lack of control over their fundamental
properties such as the diameter, length and chirality.
Especially for electronic applications control over these
properties is of crucial importance.
Traditionally (begin ’90s), CNTs were produced by
means of arc discharges and laser ablation [3].
Subsequently, formation of CNTs by thermal chemical
vapor deposition (CVD) was developed, allowing for a
lower production cost and possible large scale production.
Since about 10 years, CNTs are also grown by means of
plasma-enhanced chemical vapor deposition (PECVD)
[4]. PECVD allows the formation process to occur at
lower temperatures, which is beneficial for e.g. deposition
on temperature sensitive polymeric substrates. Very
recently, it has also become possible to generate
SWCNTs in a PECVD process. Besides the lower growth
temperature, PECVD for SWCNT growth has additional
advantages over thermal CVD such as alignment of the
SWCNTs during the growth (instead of forming spaghetti
as in thermal CVD). Furthermore, using PECVD,
freestanding SWCNTs can be produced, and most
importantly, it offers perspective on controlling the
chirality of the growing SWCNTs [5]. Because these are
very recent developments, until now there has been very
little basic research on the precise PECVD-based
SWCNT growth process.
To allow the effective application of SWCNTs in the
micro-electronics industry, control over the growth
process is very desirable. Especially precise control over
the length, diameter and finally also the chirality is
required to produce SWCNTs on a large scale and in a
cost-effective manner, with specific properties for specific
applications. This is, however, not possible with the
current knowledge of, and control over, the exact growth
process. By means of PECVD it is already possible (to a
limited extent) to obtain specific tube lengths, as well as
distributions in diameter and chirality. However, real
control over the chirality has not yet been achieved.
Furthermore, the underlying fundamental growth
mechanisms for PECVD-based SWCNT growth are
currently largely unknown. As shown in Figure 1,
PECVD introduces a number of species not involved in
thermal CVD growth, all of which have their specific
influence on the growth process [6].
Fig. 1 Various species in PECVD of SWCNTs affect the
growth process, which are not present in thermal CVD.
Reproduced with permission from [6].
Indeed, various aspects of PECVD are crucial in the
growth process. One of these is the electric field [7]. It is
known that the electric field can align SWCNTs [4, 6, 7].
However, the exact role of the field on the growth
mechanism is unknown. Furthermore, it is possible to
grow SWCNTs in a PECVD-setup at lower temperatures
than in thermal CVD. Most probably, the electric field
plays a crucial role in this phenomenon, which is not yet
elucidated [4, 6].
Another important factor is ion bombardment [8]. In
contrast to thermal CVD, in PECVD plasma ions can
bombard the substrate on which the SWCNTs are
growing. This can lead to sputtering. Unwanted structures
such as amorphous carbon can hence be sputtered.
However, ion bombardment most probably also affects
the growth of the SWCNTs. In this contribution, we
investigate the influence of ion bombardment on the
nucleation process of SWCNTs.
2. Methodology
In a MD simulation, all atoms in the system are followed
through space and time by integration of the equations of
motion. Forces acting on the atoms are calculated as the
negative gradient of the interatomic potential that
describes all interatomic interactions. The interatomic
potential that we used is the so-called ReaxFF potential
[9], which is based on the bond distance / bond order
relation on the one hand, and the bond order / bond
energy relation on the other hand (Abell formalism). In
contrast to nearly all other reactive potentials, ReaxFF
also includes non-bonded interactions, including Van Der
Waals and Coulomb interactions, and hydrogen bonds.
This potential allows the simulation of chemical reactions
with an accuracy that is comparable to or even better than
PM3, while ReaxFF is about 100 times faster. In turn,
PM3 is about 100x faster than quantum chemistry
methods [9]. Note that while DFT calculations are even
more accurate, they currently require prohibitively long
calculation times to simulate SWNT growth within a
reasonable time.
The initial structure of the simulations was a
previously simulated SWCNT cap structure, on the
surface of a Ni40 nanocatalyst particle. The carboncarbon, carbon-nickel and nickel-nickel Reax parameters
are those developed by Mueller et al. [10]. In previous
simulations, we demonstrated that these parameters are
sufficiently accurate to capture all the essential processes
taking place during SWCNT growth, leading to the
formation of SWCNTs with definable chirality [11, 12].
The initial structure is then bombarded by 200 sequential
Ar ions in the energy range 5 – 50 eV. The argon-carbon
and argon-nickel interactions are modeled through the
Molière potential [13]. The temperature of the system was
set to 1000 K, maintained by the Berendsen heat bath.
The input structure is shown in Figure 2.
Fig. 2 Input structure, consisting of a partial SWCNT cap
on a Ni40 nanocluster, to be bombarded by Ar ions in the
energy range 5 – 50 eV.
3. Results and discussion
In Figure 3, we show the global effect of the ion
bombardment of the SWCNT cap for three different
energies: 10 eV, 20 eV and 50 eV. The top part of the
figure shows the input structure in top and side views,
whereas the bottom of the figure shows the structures
obtained after the 200 consecutive impacts.
Fig. 3 Resulting structures (bottom) as emerging from 200
consecutive Ar impacts with the indicated energies,
starting from the structures shown at the top of the figure.
It is clear from Figure 3, that ion bombardment at low
energy, here exemplified for the 10 eV case, at least
visually does not result in much noticeable result, i.e., the
structure is not destructed, nor does it seem to be
enlarged. At 50 eV, on the other hand, it is clear that the
carbon network is entirely destructed, and no SWCNT
cap remains. At this energy, also a number of Ni-atoms
are actually sputtered. At an intermediate energy,
however, as here exemplified for the 20 eV case, the
carbon network is found to be enlarged due to the ion
bombardment. It should be noted that in this simulation,
no carbon atoms were added. Thus, the ion bombardment
effectively enhances the SWCNT cap nucleation. This
result is indeed validated by dedicated experiments [8].
A more quantifiable measure is the evolution of the
number of rings, or equivalently, the change in the
number of rings relative to the original input structure.
This is shown in Figure 4.
It can be seen in the figure that while visually the
structure does not change very much at low ion impact
energies, the number of graphitic rings does increase,
even at only 5 eV. Thus, even at such low ion energies,
ion bombardment seems to be beneficial for the
nucleation of the SWCNT cap. The figure shows that at 5
eV, the increase in the number of rings is smooth up to
about 120 impacts (corresponding to an increase of about
15-20%), after which there is no further change in the
number of rings relative to the input structure.
% change 60
in
num 40
ber
of
20
ring
s
0
0
-20
50
100
150
Impacts
is limited. In this stage, the network is damaged by the ion
impacts, albeit without destroying it. Only after these first
70 impacts, there is a strong decrease in the number of
rings. The destruction of the network at these energies is
directly related to the carbon displacement energy.
Indeed, the carbon displacement energy in graphene and
CNTs is estimated in the range 14-32 eV [14-17]. Given
that the kinetic mass transfer factor for Ar-C collisions is
71%, an impact energy of 40 eV always has the capability
of sputtering a carbon atom from the network. Note that
also lower ion energy can suffice to remove carbon atoms
from the network, as many carbon atoms are locally not
coordinated in a perfect graphene network, and thus have
a reduced displacement energy.
From these observations, we can conclude that both at
low and intermediate ion energies, the nucleation process
is beneficially influenced by the ion bombardment.
As an example of how the network grows (i.e., how
the number of graphitic rings increases) due to the ion
bombardment at intermediate energies, we show in Figure
5 the evolution of the network.
200
-40
-60
Fig. 4 Evolution of the procentual change in the number
of graphitic rings as a function of the number of impacts
for ion impact energies of 5 eV (dotted line), 15 eV (solid
line) and 40 eV (dashed line).
Fig. 5 Evolution of the carbon network due to the ion
bombardment. The total number of rings in the patch is
indicated, as well as the number of graphitic (i.e.,
pentagons, hexagons and heptagons) rings (values
between brackets). The nickel atoms are not shown for
clarity.
Increasing the ion impact energy to 15 eV, however,
clearly has a much more profound effect. Indeed, while at
5 eV, the procentual increase in the number of rings is
about 10-15%, this increase amounts to almost 50% at 15
eV. This results in the enlargement of the carbon network
as shown in Figure 1 at intermediate energies. It can also
be seen that at this energy, the increase in the number of
rings occurs in two steps: a first steep increase during the
first 30 impacts (up to a relative change of 20%), and then
a second steep increase between 60 and 100 impacts (up
to a relative increase of 50%). After about 100 impacts,
the relative increase in the number of rings does not
further change.
Increasing the energy further to 40 eV, it is clear from
Figure 4 that the extent of the carbon network is now
decreasing, with a clear decrease in the number of
graphitic rings. At 40 eV, this decrease amounts to 50%.
Up to about 70 impacts, the change in the number of rings
Figure 5 shows that in a consecutive manner, the carbon
network grows. Recall that in this simulation, no carbon
atoms are added to the structure. Thus, the network
enlargement is entirely due to the ion impacts.
It should be noted that the effect of ion impacts (i.e.,
adding energy to the structure through momentum
transfer) is not similar to simple thermal heating. Indeed,
while in thermal heating energy is delivered to the entire
structure (i.e., nickel cluster and carbon network), the ion
bombardment delivers most of its energy to the directly
targeted atoms (i.e., the carbon network). This energy is
dissipated to all atoms, but the bonds that are most
affected are those that are directly connected to the
targeted atoms. Note, however, that the underlying bond
switching dynamics in Ni and Ni/C clusters are very fast
in any case, certainly at elevated temperatures [18].
Second, since essentially the carbon network is heated
most by the ion bombardment, it is not impossible that the
ion bombardment delivers the energy needed to enlarge
the network and enhance the nucleation, while the nickel
nanoparticle remains at fairly low temperature.
4. Conclusion
Reactive molecular dynamics simulations were used to
investigate the effect of Ar ion bombardment on the
nucleation of SWCNTs. It was found that the ion
bombardment does not have much of an influence at low
impact energies (below 10 eV), but is beneficial for the
nucleation in a medium energy range (10 – 25 eV). In this
energy range, the ion bombardment leads to a carbon
network restructuring, forming new graphitic rings. At
higher energies (above 25 eV), carbon atoms can be
sputtered from the network, and the ion bombardment
becomes detrimental for the nucleation and growth. These
results demonstrate that ion bombardment is not
necessarily detrimental (as was previously suggested),
and possibly open up the road to a more controlled growth
process, e.g., by PE-CVD.
5. Acknowledgments
The authors would like to thank Ken Ostrikov for the
many interesting discussions and Adri van Duin for
providing the ReaxFF model. The authors also gratefully
acknowledge financial support from the Prime Minister’s
Office through IAP VI. This work was carried out in part
using the Turing HPC infrastructure at the CalcUA core
facility of the Universiteit Antwerpen, a division of the
Flemish Supercomputer Center VSC, funded by the
Hercules Foundation, the Flemish government
(department EWI), and the Universiteit Antwerpen.
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