High-resolution transmission electron microscopy (HR-TEM) first directly imaged pentagon-heptagon pair defects migrating and accumulating in a single-walled carbon nanotube (SWNT) heated to 2,273 K, suggesting that dislocation-like motion of topological defects governs SWNT plasticity, but left the mechanism, dynamics and kinetics unverified. The open problem is to establish, for SWNTs, (i) whether observed defect migration and accumulation is an intrinsic thermally driven process or is assisted by electron-beam effects (heating, ionization, knock-on displacement) during imaging, (ii) how to observe individual bond-switching and defect-glide events in real time at technologically relevant elevated temperatures, where theory predicts nanosecond-scale motion, (iii) whether plastic deformation of SWNTs can be reproduced under purely mechanical or thermal driving without massive carbon-atom loss, thereby isolating dislocation motion as the principal deformation mechanism, and (iv) the quantitative activation energies, rates and strain dependence of defect nucleation, glide and annihilation under controlled temperature and stress. Theory (density-functional and molecular-dynamics studies) has progressed substantially, and real-time dislocation dynamics has been separated from beam effects in the two-dimensional analog graphene, but no atomically resolved, beam-controlled, quantitatively kinetic account of topological-defect motion in nanotubes exists.

Background

A single-walled carbon nanotube (SWNT) is a graphene sheet — a single atomic layer of carbon atoms bonded in hexagons — rolled into a cylinder. Its diameter is fixed by the rolling direction, so any plastic (permanent) change of the tube's diameter or shape cannot occur by ordinary diffusion as in bulk solids; it requires topological defects, i.e. non-hexagonal carbon rings, to be created and to migrate along the tube wall. The elementary way to create such defects is the Stone-Wales transformation (named after the 1986 theoretical study of C60), in which one carbon-carbon bond rotates by 90 degrees, turning four hexagons into a pentagon-heptagon pair (5-7, often paired as 5-7-7-5). Buongiorno Nardelli, Yakobson and Bernholc (1998) predicted that these pairs behave as dislocation cores — line defects whose motion (glide) produces plastic strain in crystals — so that a 5-7 defect gliding along the tube wall changes the tube's chirality (the winding angle of the hexagon lattice) and its diameter. Experiments around the same time saw only mesoscopic consequences: Huang et al. (Nature 2006) observed superplastic elongation of nanotubes by roughly 280 percent at about 2,000 K, but the tubes thinned dramatically because carbon atoms were continuously lost by evaporation and electron-beam sputtering during imaging, so the atomistic mechanism remained hidden; Huang et al. (Phys. Rev. Lett. 2006) observed the motion of kinks (visible bends) at high temperature and interpreted it as dislocation-like, again without atomic-scale identification of the moving defects. Suenaga et al. (Nature Nanotechnology 2, 358-360, 2007) then used aberration-corrected HR-TEM with single-atom sensitivity to image, for the first time, individual pentagon-heptagon pair defects in an SWNT that had been heated to 2,273 K, and their in-situ imaging recorded defects accumulating near the kink of a deforming tube — evidence, but not proof, that active topological defects carry the plastic deformation. The authors themselves flagged what remained unresolved: the electron beam could be assisting or even causing the observed defect motion (thermal, ionization, or knock-on mechanisms, where knock-on means an incident electron transfers enough momentum to displace a carbon atom from its site); the temporal resolution of the microscope (video-rate, i.e. seconds per frame) cannot resolve individual bond-switching events, since theory predicts defect motion at high temperature on nanosecond timescales; the observed deformation was accompanied by substantial carbon-atom loss, so dislocation motion could not be cleanly isolated as the mechanism; and no quantitative activation energies, rates, or strain dependence for defect nucleation, glide and annihilation were available. These four gaps jointly define what must be measured to confirm and quantify the dislocation theory of SWNT plasticity.

Problem Statement

Establish the atomistic mechanism, real-time dynamics and quantitative kinetics of topological-defect-mediated plasticity in single-walled carbon nanotubes, as left open by the first direct HR-TEM imaging of active pentagon-heptagon defects (Suenaga et al., Nature Nanotechnology 2, 358-360, 2007). Concretely, for single-walled carbon nanotubes: (1) determine whether the migration and accumulation of topological defects observed at elevated temperature are intrinsic, thermally driven processes, or are assisted or altered by electron-beam effects (beam heating, ionization, or knock-on displacement) during imaging; (2) observe, in real time and at atomic resolution, individual bond-switching (Stone-Wales) events and the full glide dynamics of pentagon-heptagon defects at technologically relevant elevated temperatures, where theory predicts nanosecond-scale motion, far beyond the seconds-per-frame imaging used in 2007; (3) demonstrate plastic deformation of single-walled nanotubes under purely mechanical or purely thermal driving, without concomitant massive carbon-atom loss from vaporization or beam-induced sputtering, so that dislocation motion of topological defects can be verified as the principal mechanism of plastic deformation; and (4) measure quantitatively the activation energies, rates and strain dependence of defect nucleation, glide and annihilation under controlled temperature and stress, and confront them with the existing theoretical predictions. These four deliverables jointly support one objective: converting the 2007 qualitative suggestion that active topological defects carry SWNT plasticity into a verified, time-resolved and quantitative atomistic account.

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