Reinforcing multiwall carbon nanotubes by electron beam irradiation
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Structural changes in MWNTs subject to electron beam irradiation. High-resolution transmission electron micrographs of MWNTs exposed to a current density of during (a) 24 s, (b) 49 s, and (c) 3 min 44 s. Dark lines in the enlarged segments of the micrographs on the right indicate the position of walls. The effect of electron irradiation within the wall region, indicated in (d), is shown schematically in (e)–(g). (e) Presence of large scale defects in multiwall nanotubes reduces significantly the Young’s and the shear modulus, resulting in large displacements in response to shear stress. (f) The main effect of electron beam irradiation is to create vacancy defects in adjacent walls that spontaneously cross-link adjacent walls, thus increasing the bending modulus and reducing the shear motion in response to shear stress. (g) Defect percolation in nanotubes of either initial poor quality or tubes subject to excessive irradiation continues to lower the bending modulus upon additional irradiation. Displacement in response to shear stress increases in this case.
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Elastic properties of MWNTs subject to heat treatment or electron beam irradiation. (a) AFM setup to measure the deflection of a suspended nanotube as a function of the tip position . The nanotube is suspended over a trench and anchored at both ends. (b) Illustration of the fact that in response to the force exerted by the AFM tip the deformation of ideally clamped nanotubes with many walls involves both stretch and shear. (c) AFM measurement of the deflection profile of a 20 nm wide carbon nanotube, suspended across a 350 nm wide trench. The deflection of the nanotube in response to a loading force exerted by the AFM tip, labeled “as-grown” and depicted by the dark data points (◼), is rather large and exhibits plateaus due to stick-slip motion, as illustrated in the inset. Following irradiation with the optimal dose , these plateaus are suppressed and the deflection is reduced when the tube is subject to an even larger loading force , as shown by the light data points (●). (d) Observed dependence of the bending modulus of various carbon nanotubes on tube treatment. In absence of electron irradiation, annealing in the temperature range between , labeled on the upper abscissa, does not change the bending modulus of nanotubes with within the error bars of the observation in Ref. 17; this is depicted by the dotted line as a guide to the eye connecting the data points (⧓). Unlike annealing only, electron beam irradiation does cause significant changes in the bending modulus of multi-wall nanotubes. For the sake of consistency, we performed AFM measurements on the same nanotube exposed to a varying degree of electron irradiation. As-grown nanotubes with display an initial enhancement upon moderate irradiation, which reverts to a gradual decrease in beyond an optimum irradiation dose. No initial enhancement is observed in nanotubes with low initial bending modulus , as shown by the data points (▶). (e) Dependence of on the diameter of as-grown and irradiated nanotubes. (f) Comparison between in as-grown and irradiated nanotubes. Results for irradiated nanotubes in (e) and (f) refer to the irradiation dose that maximizes structural reinforcement.
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Results of ab initio MD simulations of cross-link formation and structural reinforcement at exposed edges of an extended vacancy defect, modeled by a narrow bilayer graphene strip. The cross-linking dynamics, shown by a sequence of snap shots depicting spontaneous formation of bonds connecting the edges of a graphene bilayer with (a) ZZ and (b) AC edges at . (c) Morphology of a ZZ graphene bilayer strip with AA and AB layer stacking in top view. (d) Morphology of an AC graphene bilayer strip with AA and AB layer stacking in top view. The bottom layer is shown by a lighter color than the top layer, and the size the unit cell is further enhanced by the darker color in (c) and (d). Energy cost to displace the topmost layer by in the direction of the edge per 1 nm length of one edge in cross-linked (e) ZZ and (f) AC bilayer strips. We find in all cases when the layers are not cross-linked.
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