22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-assisted etching of “incipient” carbon nanotubes: a hybrid molecular
dynamics / Monte Carlo study
U. Khalilov, A. Bogaerts and E.C. Neyts
Research Group PLASMANT, department of Chemistry, University of Antwerp, 2610 Antwerpen-Wilrijk, Belgium
Abstract: Hydrogen either enhances or slows down carbon nanotube growth, depending
on its concentration in the plasma. While the negative effect of H species due to etching
has been broadly investigated, its etching behavior is not fully understood at the atomic
level. We thus comparatively studied etching stages of ideal and defective nanotubes in the
hydrogenation process using combined MD/MC simulations. Three distinguished etching
stages are analyzed. Also the effect of H 2 plasma on the cap nucleation is investigated in
detail.
Keywords: carbon nanotubes, hydrogenation, molecular dynamics, Monte-Carlo, ReaxFF
1. Introduction
Thanks to the extraordinary properties and applications
of carbon nanotubes (CNTs), understanding how to
control their growth is of prime importance. The role of
H atoms in plasmas used for CNT growth can be
significant in that respect. It is known that hydrogen
helps in the activation of the catalyst particles and thus
enhances the yield of CNT growth. In contrast, it may
also hinder the formation of graphite-like carbon, if its
concentration is too high [1]. Furthermore, plasma
including H atoms causes the breaking of the sp2 C-C
bonds and the formation of sp3 C-H bonds, resulting in
CNT etching [2]. In spite of a large number of studies
[1-3], however, the precise influence of H on the early
stages of CNT growth and its etching behaviour is still
not fully understood. Therefore, this work is devoted to
study the H etching mechanisms for ideal and defective
nanotubes as well as to understand the H effect on the cap
nucleation stage of the CNT growth.
2. Simulation details
The hydrogenation and etching processes are
investigated by combined (reactive) molecular dynamics
(MD) and force-bias Monte Carlo (tfMC) [4, 5]
simulations at 1600 K. In the simulations, all atomic
interactions are described by the ReaxFF force field,
using parameters developed by Mueller et al. [6]. Prior to
hydrogenation, defect-free (5,5) and defective CNTs are
positioned on a Ni 55 nanoparticle as a surface-bound
cluster, which itself is physisorbed on a virtual Al surface
[7], thereby mimicking catalyzed tip growth experiments.
During the MD simulations, some H atoms are added in
the vacuum surrounding the catalyst and tube, and their
concentration is kept constant at any moment in time.
When a H atom adsorbs on the nanotube or the cluster,
the resulting structure is allowed to relax by application of
tfMC. During this relaxation stage, no new atoms are
allowed to impinge on the tube. We remove gas phase
etching products every 106 MD steps to limit their
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influence.
3. Results and discussions
In Fig. 1, successive etching stages of defective (a) and
ideal (b) nanotubes during the hydrogenation are shown.
There are four specific types of structures that can be
distinguished for both cases, i.e., 1) structures prior to
hydrogenation; 2) structures in the beginning of
hydrogenation; 3) structures with initially broken C-C
bonds (onset of a hole creation or etch pit formation); and
4) damaged or H-etched carbon nanotubes. In the first
stage of the hydrogenation process, H atoms adsorb on the
surface of both a nanotube and a catalyst cluster and they
try to coalesce, leading to the occupation of neighbouring
sites. On the other hand, H atoms may also desorb either
by the Langmuir–Hinshelwood (LH) recombinative
desorption or the Eley–Rideal (ER) abstraction
mechanisms during the hydrogenation [8]. Due to H
adsorption and clustering on both tube surfaces, some
regions contain sp3 C atoms or adsorbed H atoms and
become locally amorphous. While such amorphous sites
are mostly found on the cap region in the ideal structures,
they can be found in any part of the defective tubes.
Consequently, C-C bonds at those sites elongate and can
eventually break, due to pyramidalization and
misalignment of the π-orbitals of the C atoms [9]. The
second stage is characterized by the appearance of the
first defect due to breaking of the now more elongated
C-C bonds. In the third stage, additional H-atoms rapidly
occupy nearby (neighbouring) C atoms, leading to the
formation of a hole or so-called etch pit. Their borders
are indicated by pink coloured C atoms as shown in
Fig. 1. The carbon-carbon bonds connecting these sites
are consecutively broken, which closely resembles the
experimental evidence on graphite and MWNT etching
[3] showing the formation of holes and etch pits upon
H-atom exposure. Moreover, as can be seen in the figure,
the resulting hole in our simulations is surrounded by
H-atoms, as was previously also demonstrated
1
experimentally by EELS measurements [3].
This
indicates that the carbon atoms in that region remain
purely sp3-bonded. After the first etch pit formation, the
ideal short tube is gradually destroyed starting from its
cap toward to the end of the tube. Its defective
counterpart, on the other hand, is disorderly destroyed due
to its interaction with the H-atoms from the plasma, as
shown in Fig. 1.
Fig. 2. (a) Initial and final structures of three different
carbonated Ni clusters (Ni 55 C 89 , Ni 55 C 100 , Ni 55 C 110 ) in
the hydrogenation process; (b) evolution of the number
of carbon and hydrogen atoms in the three structures
during H etching.
Fig. 1. Hydrogenation and etching stages of (a) defective,
and (b) ideal (5,5) nanotubes on a Ni cluster (green
atoms). C atoms of damaged regions are in pink.
Hydrogen-assisted etching of CNT caps can give also
valuable information to understand the H effect in the
early stages of CNT nucleation. In Fig. 2a, three cap-like
carbon structures and the final structures after
hydrogenation are depicted. Due to the hydrogenation,
cap structures can convert to vertical graphene patches
[7]. During the hydrogenation process, cap C atoms are
continuously etched from the cluster surface and therefore
the number of C atoms linearly decreases as shown in
Fig. 2b. On the other hand, the concentration of hydrogen
atoms at the surface initially increases: H atoms easily
adsorb on both the carbon cap and the Ni-catalyst cluster,
binding with C and Ni atoms, respectively. As a result,
the number of adsorbed H quickly saturates. However,
their amount gradually decreases afterward due to either
the cap etching or LH/ER desorption. Fig. 2b shows that
the overall adsorption/desorption and etching trends are
the same for all three cap cases and the rates do not
depend on the amount of C atoms in the initial cap-cluster
structure.
During the hydrogenation/etching process, different
types of etching C x H y species in addition to desorbed H 2
molecules are found in the gas phase (Fig. 3a). As can be
seen in Fig. 3b, CH 4 molecules and CH 3 radicals are the
most abundant species in the gas phase for all cases.
Moreover, the number of formed C 2 H 4 , C 2 H 2 molecules
and C 2 H radicals is also significant for the third structure.
The results show that the occurrence of various gas-phase
C x H y species is due to etching/pyrolysis processes.
4. Conclusion
In this work, the etching of Ni catalyzed carbon
nanotubes by H 2 plasma is investigated. The simulations
indicate three successive hydrogenation/etching stages.
2
Fig. 3.
(a) H-containing cap-cluster structure and
desorbed/etched species in the gas phase during the
hydrogenation; (b) Relative concentration of the etched
hydrocarbon C x H y species.
We found that an ideal short (5,5) nanotube is etched
from its cap to its base which binds with the Ni cluster,
whereas a defective tube, on the other hand, is disorderly
destroyed. The etching rate and adsorption/desorption
rates are found not to depend on the amount of C atoms in
the CNT cap in the early growth stage. Due to the etching
process, CH 4 molecules and CH 3 radicals are abundantly
found in the gas phase, besides H atoms and H 2
molecules.
5. Acknowledgement
U. Khalilov gratefully acknowledges financial support
from the Fund of Scientific Research Flanders (FWO).
This work was carried out in part using the Turing HPC
infrastructure at the CalcUA core facility of the
Universiteit Antwerpen (UA), a division of the Flemish
Supercomputer Center VSC, funded by the Hercules
Foundation, the Flemish Government (department EWI)
and the UA. This research was carried out in the
framework of the network on Physical Chemistry of
Plasma-Surface Interactions - Interuniversity Attraction
Poles, phase VII (http://psi-iap7.ulb.ac.be/), and supported
by the Belgian Science Policy Office (BELSPO).
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6. References
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