Adsorption and migration of carbon adatoms on zigzag carbon nanotubes

doi:10.1016/j.carbon.2003.12.025

Carbon

Volume 42, Issues 5–6, 2004, Pages 1021-1025

https://doi.org/10.1016/j.carbon.2003.12.025 Get rights and content

Abstract

Using density-functional tight-binding and ab initio methods we study the adsorption and migration of carbon adatoms on surfaces of single-walled zigzag carbon nanotubes. We demonstrate that the adatoms form strong covalent bonds with the nanotubes and that the migration is strongly anisotropic. The adatom adsorption energy and migration barrier depend on the nanotube diameter and chirality. The migration barriers, being in the range of 0.6–1 eV, are in a good agreement with the experimental values (about 0.8 eV) reported in the literature.

Introduction

The outstanding mechanical and electronic properties of single-walled carbon nanotubes (SWNTs) have stimulated much research on their mass production techniques. However, despite a considerable effort, further progress is hampered by the lack of the control over SWNT chiralities and diameters at the growth stage. This is in part due to the insufficient understanding of SWNT growth mechanisms.

A number of microscopic SWNT growth models have been developed [1], [2], [3], [4], [5], [6] with growth taking place either at the nanotube edge [1], [2], [3] (capped or open) or its root [4], [5], [6]. Depending on the temperature range and catalyst used, any of the mechanisms may work. However, whatever the mechanism is, the quantitative understanding of the synthesis process is not possible without knowing how the “building blocks”––carbon atoms and clusters––are supplied to the place where the SWNT growth occurs.

Carbon atoms coming from the feedstock (plasma, gas, etc.) can be captured directly at the end of the SWNT [3], especially if dangling covalent bonds are present. However, it seems to be more plausible that the atoms are first absorbed on the SWNT surface and then they migrate to the SWNT growing end [1], [2]. The adsorbed atoms (adatoms) can also aggregate and form clusters (amorphous carbon) and detach from nanotube surface. Thus, knowing the adatom migration mechanism and such key quantities like adsorption and migration energies is indispensable for the comprehensive theory of SWNT synthesis.

At the same time, there exists very little knowledge about how carbon adatoms migrate over the SWNT surface. There have been studies on the migration of carbon adatoms on a graphite (flat) surface [1], [7] but effects of SWNT surface curvature on the carbon adatom diffusion have not yet been studied by the proper methods.¹ Moreover, the reported values of the adatom migration barrier (about 0.1 eV) [1], [7] seem to be much lower than the value (∼0.8 eV) obtained in experiments on the annealing of the irradiation-induced damage in SWNTs [11]. Annealing, similar to the SWNT growth, is governed by the migration of adatoms which play the role of interstitials in SWNTs [12].

In this paper we study the adsorption and diffusion of carbon adatoms on SWNTs. By using two different computational techniques we evaluate the adatom adsorption energy E_a and migration barrier E_m for zigzag SWNTs with various chiralities.

Section snippets

Calculation details

In our simulations, we employed density-functional theory (DFT) implemented in the non-orthogonal real-space tight-binding (DFTB) code [13] and the plane wave (PW) basis set VASP [14] code. Although the PW DFT method has very well been established as the leading edge in electronic structure calculations, we were unable to carry out all calculations using this method because of computational limitations. However, as shown below, the DFTB approach in which the parameters of the Hamiltonian are

Carbon adatom on a graphene sheet

To check the applicability of DFTB to the problem of adatom migration, we started by calculating E_a and E_m for the adatom on small graphene flakes consisting of 60–216 atoms. We found that the equilibrium position of the adatom was a bridge-like structure (the adatom is above the middle of the carbon-carbon bond) with the perpendicular distance to the graphite surface being 1.9 Å. Our recent DFT PW calculations gave exactly the same configuration with a distance of 1.87 Å [10]. E_a (with account

Carbon adatoms on zigzag nanotubes

In this work we dwell upon zigzag (n,0) SWNTs which, depending on the SWNT chiral index n, can be metals or narrow-band semiconductors. This made it possible to study effects of not only nanotube curvature, but also electronic structure on the diffusion and adsorption.

In the DFTB calculations, finite SWNTs (having a length of 12.7 Å and composed of up to 200 atoms) with periodical boundary conditions were considered. The same systems were used for the DFT PW simulations. To check how the

Conclusion

To conclude, using density-functional tight-binding and ab initio methods we studied the adsorption and migration of carbon adatoms on zigzag nanotubes. We found that the adatoms form strong covalent bonds with the nanotubes and that the migration is strongly anisotropic. The adatom adsorption energy and migration barrier depend on the nanotube diameter and chirality, which should be taken into account in models of nanotube growth and radiation damage annealing. The migration barriers, being in

Acknowledgments

We would like to thank F. Banhart for useful discussions and Th. Frauen-heim for the permission to use the DFTB code. The research was supported by the Academy of Finland under project nos. 48751 and 50578. Grants of computer time from the Center for Scientific Computing in Espoo, Finland, are gratefully acknowledged.

References (21)

Y.H. Lee et al.
Catalytic growth of single-wall carbon nanotubes: An ab initio study
Phys. Rev. Lett.
(1997)
O.A. Louchev et al.
Morphological stabilization, destabilization, and open-end closure during carbon nanotube growth mediated by surface diffusion
Phys. Rev. E
(2002)
S. Iijima et al.
Growth model for carbon nanotubes
Phys. Rev. Lett.
(1992)
J. Gavillet et al.
Root-growth mechanism for single-wall carbon nanotubes
Phys. Rev. Lett.
(2001)
X. Fan et al.
Nucleation of single-walled carbon nanotubes
Phys. Rev. Lett.
(2003)
A. Maiti et al.
Kinetics of metal-catalyzed growth of single-walled carbon nanotubes
Phys. Rev. B
(1997)
M. Heggie et al.
LDF calculations of point defects in graphites and fullerenes
Electrochem. Soc. Proc.
(1998)
D.J. Shu et al.
Curvature effect on surface diffusion: the nanotube
J. Chem. Phys.
(2001)
K. Nordlund et al.
Formation of ion irradiation induced small-scale defects on graphite surfaces
Phys. Rev. Lett.
(1996)
P.O. Lehtinen et al.
Magnetic properties and diffusion of adatoms on a graphene sheet
Phys. Rev. Lett.
(2003)

There are more references available in the full text version of this article.

Cited by (56)

Low-temperature radiation damage studies in single-walled carbon nanotube bundles by hopping conductance
2023, Diamond and Related Materials
The paper shows the results of studying radiation defects in single-walled carbon nanotube bundles at low temperatures. The data were obtained in a unique experiment that inseparably combines 1 MeV electron irradiation at a temperature of ∼10 K and electrical conductivity measurements over a wide temperature range of 5–300 K. The experiment allowed us to reveal and separate the influence of stable and unstable radiation defects on the electrical properties of SWCNTs. It is shown that the 2D Mott variable range hopping conduction regime exhibits resistance to electron irradiation. The hopping conductivity is the dominant charge transfer mechanism in the temperature range of 5–50 K in both the pristine and irradiated SWCNT samples. It was found that the parameters of hopping transport demonstrate high sensitivity to the introduction and annealing of radiation defects. The low-temperature electron irradiation revealed the possibility of the emergence of an additional conduction channel in the irradiated SWCNTs. This channel demonstrates thermally activated behavior with a low activation energy of ∼0.5 meV at temperatures of 5–50 K. The obtained data indicate that unstable radiation defects create conditions for the possibility of impurity band conduction in SWCNT bundles.
Low-temperature annealing of radiation-induced defects in carbon nanotube bundles
2017, Diamond and Related Materials
Citation Excerpt :
At the same time, the low-temperature behavior of defects in nanotubes has been investigated mainly theoretically and is still uncertain. Calculations show that the migration of vacancies and adatoms should prevail above room temperature, since the migration energy of vacancies in CNT is ~ 1 eV [14], and adatoms ~ 0.6–1 eV [11,15]. On the other hand, the migration energy of interstitial carbon atoms within single-walled carbon nanotube is in range 0.1–0.4 eV [15].
The annealing of radiation-induced defects in carbon nanotube bundles below room temperature has been investigated experimentally. Significant feature of this study is that electron irradiation (with an energy 1 MeV and fluence up to 10¹⁶ el/cm²) was carried out at liquid helium temperature. A detailed analysis of the resistance change over a wide temperature range (7–300 K) enables us to prove partial annealing of irradiation-induced defects at moderate temperatures. It is shown that such an annealing follows a first-order reaction in whole investigated temperature range. While at temperatures below 40 K, annealing is nonactivation, tunneling process; at higher temperatures (100 − 300 K) it becomes activated, with activation energy of ~ 0.05 eV. The value of discovered activation energy is close to the migration energy of interstitial carbon atom in graphite. Probably, observed annealing of radiation-induced defects in carbon nanotube bundles is also caused by the migration of interstitial carbon atoms between single nanotubes below room temperature.
Migration of a carbon adatom on a charged single-walled carbon nanotube
2017, Carbon
We find that negative charges on an armchair single-walled carbon nanotube (SWCNT) can significantly enhance the migration of a carbon adatom on the external surfaces of SWCNTs, along the direction of the tube axis. Nanotube charging results in stronger binding of adatoms to SWCNTs and consequent longer lifetimes of adatoms before desorption, which in turn increases their migration distance several orders of magnitude. These results support the hypothesis of diffusion enhanced SWCNT growth in the volume of arc plasma. This process could enhance effective carbon flux to the metal catalyst.
Adsorption of hydrogen on single-walled carbon nanotubes with defects
2015, Diamond and Related Materials
We present molecular dynamics (MD) simulations and density functional theory (DFT) calculations of hydrogen adsorption on single-walled carbon nanotubes (SWCNT) with various kinds of defects. The nature of defects, which is characterized here by the number of carbon atoms present in a ring on the surface of nanotube, plays a significant role in determining the hydrogen adsorption capacity of the SWCNT. Nanotubes containing the Stone–Wales defect with 5 and 8-member rings were found to have the largest hydrogen adsorption ability that increases further with the number of rings with such defects. Whereas, the presence of defects with 5, 3-5-8-member rings and the Stone–Wales defect with 5 and 7-member rings decreases the adsorption ability of the defective SWCNT significantly with respect to defect-free nanotubes. Our results indicate that the huge discrepancies in hydrogen storage capacities of SWCNT reported in the literature could be attributed to the nature of defects present in nanotubes. DFT calculations also reveal that the adsorption energy depends not only on the nature and number of defects present on the surface of nanotube but also on the equilibrium structure of rings.
Vacancy mediated clipping of multi-layered graphene: A precursor for 1, 2 and 3D carbon structures
2015, Carbon
Using first principles calculations, we show that the overlapping defects in bi-layer graphene (both AA- and AB-stacked) interact forming inter-layer covalent bonds, giving rise to two-dimensional (2D) clipped structures, without explicit use of functional groups. These clipped structures can be transformed into one-dimensional (1D) double wall nanotubes (DWCNT) or multi-layered three dimensional (3D) bulk structures. These clipped structures show good mechanical strength due to covalent bonding between multi-layers. Clipping also provides a unique way to simultaneously harness the conductivity of both walls of a double wall nanotube through covalently bonded scattering junctions. With additional conducting channels and improved mechanical stability, these clipped structures can lead to a myriad of applications in novel devices.
Oxygen migration on the graphene surface. 2. Thermochemistry of basal-plane diffusion (hopping)
2011, Carbon
Citation Excerpt :
Since graphene’s discovery [1,2], the study of adatoms on its surface [3] has been of interest for their profound effects on graphene’s physical properties [4–10].
Thermodynamic affinities, activation energies and diffusion coefficients for oxygen mobility on the graphene surface are calculated using density functional theory (DFT). We report and discuss the effects of geometry, charge distribution and heteroatom substitution on the migration of epoxy oxygen on the basal plane: both the driving force and the ease of surface hopping are very sensitive to their variations. A significant decrease in the hopping energy barrier is observed when graphene contains free edge sites and oxygen functionalities, as well as upon an increase in electron density; conversely, the barrier increases as a consequence of electron removal, and the propensity for graphene ‘unzipping’ also increases. There is a correlation between the hopping barrier and the C–O bond strength of the leaving epoxide group. Under the most favorable conditions investigated, oxygen mobility is quite high, of the same order as that of gas-phase O₂ in micropores (ca. 10⁻⁹ m²/s). This is consistent with the increasingly acknowledged role of basal-plane oxygen as a protagonist (e.g., reaction intermediate), instead of a spectator, in the wide variety of adsorption and reaction processes involving sp²-hybridized carbon materials.

View all citing articles on Scopus

View full text

Adsorption and migration of carbon adatoms on zigzag carbon nanotubes

Abstract

Introduction

Section snippets

Calculation details

Carbon adatom on a graphene sheet

Carbon adatoms on zigzag nanotubes

Conclusion

Acknowledgments

Catalytic growth of single-wall carbon nanotubes: An ab initio study

Phys. Rev. Lett.

Morphological stabilization, destabilization, and open-end closure during carbon nanotube growth mediated by surface diffusion

Phys. Rev. E

Growth model for carbon nanotubes

Phys. Rev. Lett.

Root-growth mechanism for single-wall carbon nanotubes

Phys. Rev. Lett.

Nucleation of single-walled carbon nanotubes

Phys. Rev. Lett.

Kinetics of metal-catalyzed growth of single-walled carbon nanotubes

Phys. Rev. B

LDF calculations of point defects in graphites and fullerenes

Electrochem. Soc. Proc.

Curvature effect on surface diffusion: the nanotube

J. Chem. Phys.

Formation of ion irradiation induced small-scale defects on graphite surfaces

Phys. Rev. Lett.

Magnetic properties and diffusion of adatoms on a graphene sheet

Phys. Rev. Lett.