Bundling up Carbon Nanotubes through Wigner Defects

Abstract

We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right besides a vacancy, which are present in irradiated graphite can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizeable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.Single-walled carbon nanotubes (SWNT) have attracted an enormous amount of attention since its discovery[1] about ten years ago. No doubt this is related to all their extraordinaire materials properties and potential for applications [2]. From a mechanical point of view, SWNT have exceedingly large elastic modulus [3] around 1 TPa, which would make them perfect for lowdensity high-modulus fibers. When in bundles, however, a much worse performance is usually obtained, mostly related to the weak interaction between tubes, which makes it relatively easy for them to slide against each other [4]. A breakthrough has been recently obtained [5] using electron-beam irradiation to generate crosslinks between the tubes. This increased the shear modulus by a factor of approximately 30 thus indicating a new way to fabricate stronger fibers.Irradiation by electrons or ions is being used by many groups [7] as an important tool to modify the structure and properties of nanotubes. As examples, we can mention the welding of tubes [6], and the dramatic increase of shear modulus for a bundle of nanotubes [5]. In order to have a better control of the nanotube engineering, it is fundamental to have a detailed microscopic understanding of the defects that are created upon irradiation. Properties like formation energy, geometry, stability and electronic structure are critical in determining what type of defects will be produced and how they will affect the mechanical behavior of the carbon fiber. Some theoretical works have investigated the effects of electron or ion irradiation on SWNT and SWNT bundles [8,9]. These studies consisted of Molecular Dynamics simulations with the interaction between the atoms being described by empirical potentials. Even though these works are relevant and indicative of the processes that may occur, they may miss important aspects since empirical potentials cannot describe appropriately features such as re-hybridization, rebonding or charge redistribution, all of them quite important when chemical bonds are being broken and made, specially in stressed configurations that may result from the irradiation procedure. Moreover, recombination barriers and formation energies are usually poorly described by these potentials. Yet another difficulty with these potentials is the important fact that carbon atoms have many possible hybridi...

Early stages of radiation damage in graphite and carbon nanostructures: A first-principles molecular dynamics study

“…14 This proves the crucial role of the iIV defects in the reinforcement of carbon nanotube bundles 3,35 produced by 80 keV electron irradiation. Our results suggest the optimal conditions for the modification of mechanic and electronic properties of carbon-based layered nanostructures by means of the formation of iIV defects.…”

“…10,29,35 However, MD simulations on a longer time scale indicate that the asymmetric iIV 1 defect in graphite is not stable against recombination at 350 K if the shear of neighboring graphite layers is allowed. By contrast, the symmetric iIV 2 defect is stable throughout our MD simulations.…”

Ion and electron irradiation-induced effects in nanostructured materials

“…11͑b͒. The bonds can be due to vacancies or the socalled Wigner defects, 332 a metastable atom configuration formed by a vacancy and a nearby interstitial. Such structures were experimentally found in TEM images of irradiated double-walled nanotubes ͑DWNTs͒, 39 Fig.…”

Curvature and chirality dependence of the properties of point defects in nanotubes

“…A connection between theory and experiments is therefore necessary for an unambiguous classification [6,7]. Individual point defects, such as adatoms [8], topological defects like the Stone-Wales defect (SW) [9], single vacancies [10,11], double vacancies [12] and vacancy-interstitial complexes [13], have been studied by tight-binding (TB) and density-functional (DFT) calculations in order to complement the experimental information. However, a complete theory for point defects in nanotubes is still not available.…”