Tarek Ragab Electronic Packaging Laboratory, Department of Civil, Structural and Environmental Engineering, State University of New York at Buffalo, Buffalo, New York 14260; Faculty of Engineering, Alexandria University, Egypt 21526; Nanotechnology Research Laboratory, University of Tabuk, Saudi Arabia 71491 Cemal Basaran 1 Electronic Packaging Laboratory, Department of Civil, Structural and Environmental Engineering, State University of New York at Buffalo, Buffalo, New York 14260 e-mail: cjb@buffalo.edu The Unravelling of Open-Ended Single Walled Carbon Nanotubes Using Molecular Dynamics Simulations The unravelling of (10, 10) and (18, 0) single-walled carbon nanotubes (SWCNTs) is simulated using molecular dynamics simulations at different temperatures. Two different schemes are proposed to simulate the unravelling; completely restraining the last atom in the chain and only restraining it in the axial direction. The forces on the terminal atom in the unravelled chain in the axial and radial directions are reported till the separation of the atomic chain from the carbon nanotube structure. The force-displacement relation for a chain structure at different temperatures is calculated and is compared to the unravel- ling forces. The axial stresses in the body of the carbon nanotube are calculated and are compared to the failure stresses of that specific nanotube. Results show that the scheme used to unravel the nanotube and the temperature can only effect the duration needed before the separation of some or all of the atomic chain from the nanotube, but does not affect the unravelling forces. The separation of the atomic chain from the nanotube is mainly due to the impulsive excessive stresses in the chain due to the addition of a new atom and rarely due to the steady stresses in the chain. From the simulations, it is clear that the separation of the chain will eventually happen due to the closing structure occur- ring at the end of the nanotube that would not be possible in multiwalled nanotubes. [DOI: 10.1115/1.4003866] Keywords: carbon nanotubes, molecular dynamics simulations, unravelling, atomistic simulations, field emission 1 Introduction Since 1991 [1] when carbon nanotubes were re-identified experi- mentally, they have attracted considerable curiosity to investigate their electrical [2–5] and thermomechanical behavior [6–9] as well as the electromechanical behavior [10,11]. Experiments show that carbon nanotubes have extraordinary electrical and mechanical properties. Mechanically, carbon nanotubes (CNTs) have a tensile strength that is 20 times that of high strength steel [12] and a Young’s modulus in the order of a terapascal [13]. These extraordi- nary mechanical properties can easily be explained by the strong hybrid sp 2 carbon–carbon bond, which is considered to be the strongest bond in nature [14]. Geometrically, CNTs can be classi- fied into single-walled carbon nanotubes (SWCNTs) formed from folding a single sheet of graphite or multiwalled carbon nanotubes (MWCNTs) that is formed of SWCNTs that are concentrically aligned inside each other. The direction of folding the graphite sheet is defined by the chiral vector C h ðn; mÞ [15], where CNTs can be classified into armchair nanotubes ðn; nÞ and zigzag nanotubes ðn; 0Þ or chiral nanotubes ðn; mÞ. Both the mechanical and the elec- trical properties of CNTs depend on the chirality of the nanotube. Mechanically, it was found that armchair nanotubes can sustain higher stresses than zigzag nanotubes [12]. After the first proposals of the usage of MWCNT as field emit- ters in 1995 [16,17] a lot of research was directed to study their applicability [18–20], and showed a lot of success. In Ref. [16], the enhanced field emission of a single MWCNT was attributed to the unravelled atomic chain from the open-ended nanotube. It was pro- posed that the electric field generated the forces that caused the unravelling process. In 1997 [21], ab initio density functional for- malism was used to simulate the unravelling process in double- walled CNTs as proposed in Ref. [16]. In this paper, molecular dynamics (MD) Simulation is used to investigate the mechanical unravelling of (10, 10) armchair and (18, 0) zigzag SWCNT till failure, using different mechanical schemes at different temperatures. MD simulations can serve as a powerful tool for studying CNTs in different applications [22–25] that allows for the investigation of the applied atomic forces and stresses as well as the atomic trajectories during the course of the simulation. 2 Molecular Dynamics Simulation The second generation, reactive empirical bond order (REBO) potential [26,27] based on the Abell–Tersoff potential [28,29] is used to represent the covalent bonding between the carbon atoms (taking into account different possible hybridizations). Mylvaga- nam and Zhang [12] have shown that for CNTs Tersoff–Brenner potential is more accurate and cost effective. The potential energy of a set of covalently bonded carbon atoms is given by U ¼ X N i¼1 X j>i ½U R ðr ij Þ b ij U A ðr ij Þ (1) where r ij is the distance between atom i and atom j, U R and U A are the pair-additive interactions that represent the interatomic repulsions and attractions, respectively, and b ij is the bond order between atom i and atom j , which takes into account the effect of the many-body interactions between the carbon atoms. When the distance between two carbon atoms exceeds the maximum bond 1 Corresponding author. Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received April 13, 2010; final manuscript received February 10, 2011; published online June 7, 2011. Assoc. Editor: Jianmin Qu. Journal of Electronic Packaging JUNE 2011, Vol. 133 / 020903-1 Copyright V C 2011 by ASME Downloaded 06 Jul 2011 to 128.205.19.117. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm