Multi-walled carbon nanotube structural instability with/without metal nanoparticles under electron beam irradiation

Abstract

The structural transformation of multi-walled carbon nanotubes (MWCNT) under electron beam (ebeam) irradiation at room temperature is studied, with respect to a novel passivation effect due to gold nanoparticles (Au NPs). MWCNT structural evolution induced by energetic e-beam irradiation leads to faster shrinkage, as revealed via in situ transmission electron microscopy, while MWCNT surface modification with Au NPs (Au-MWCNT) slows down the shrinkage by impeding the structural evolution process for a prolonged time under the same irradiation conditions. The new relationship between MWCNT and Au-MWCNT shrinking radii and irradiation time illustrates that the MWCNT shrinkage rate is faster than either theoretical predictions or the same process in Au-MWCNTs. As compared with the outer surface energy (positive curvature), the inner surface energy (negative curvature) of the MWCNT contributes more to the athermal evaporation of tube wall atoms, leading to structural instability and shrinkage under e-beam irradiation. Conversely, Au NPs possess only outer surface energy (positive curvature) compared with the MWCNT. Their presence on MWCNT surfaces retards the dynamics of MWCNT structural evolution by slowing down the evaporation process of carbon atoms, thus restricting Au-MWCNT shrinkage. Au NP interaction and growth evolves athermally on MWCNT surfaces, exhibits increase in their size, and indicates the association of this mechanism with the coalescence induced by e-beam activated electronic excitations. Despite their growth, Au NPs show extreme structural stability, and remain crystalline under prolonged irradiation. It is proposed that the surface energy of MWCNTs and Au NPs, together with e-beam activated soft modes or lattice instability effects, predominantly govern all the above varieties of structural evolution.

Electron-beam-induced uniform elongation of multi-walled carbon nanotube

“…And the radial Young's modulus sharply increases with the decrease in diameter of MWCNT for constant number of shells. This is consistent with our results for the MWCNT as well as for the SWCNT [16,26]. This is acceptable if we take into account the contributions of the bond length shortening and bond angle changes to the surface energy of MWCNT under the nanoscaled curvature effect [16,25].…”

“…2 illustrates that MWCNT possesses to about ten straight shells, perfect crystalline features, and well-defined cylindrical structure before irradiation. A small defects coarsening by accretion of carbon atoms between the contiguous wall gaps or by immobility of vacancies for extended period at low temperature or by both under prolonged irradiation [16] is inevitable. This can be attributed either to the minor fluctuation in the beam current density or non-uniformity in local atomic structure.…”

“…By thermodynamics, during the uniform elongation/ shrinkage, uniform diffusion of internal carbon atoms along the tube shells in the tube axis direction is driven by the nanoscaled curvature (high surface energy) of MWCNT. By taking into account the thermodynamic driving forces, shrinkage in the fixed length L 1 is precisely tracked by a new model [16] Fig. 11 (between the two yellow dots, left dot at 0 nm and right dot at 45.5 nm)…”

“…For a static SWCNT, without considering the e-beam activation effect, calculations have been approximated on equilibrium, symmetrical, periodical, and linear nature of carbon nanostructures that cannot account the well-defined nanoscaled surface energy effect. Although, in the modified form of Cauchy-Born rule, inter-atomic potential in continuum analysis has been encompassed and additional surface energy of SWCNT has been predicted to be proportional to 1/r 2 [16]; effect of many body potentials among the carbon atoms has not been replicated clearly. Some local density approximations (simulations) [21][22][23] have also been failed to determine the net large scale electronic disorder of SWCNT.…”

“…Nevertheless, therein [1], probable refilling of the necked region by inward/outward flow of atoms under non-uniform distributions of negative (positive) curvature effect and beam current density over the irradiated segment has made it too intricate to evaluate quantitatively the kinetics of shrinkage and elongation. Although an accurate tracking of the kinetics of uniform elongation and uniform shrinkage of MWCNT and quantitative relationship between the MWCNT shrinkage and nanocurvature effect [16] can be achieved through moderate irradiation or by uniform e-beam current density, the chosen MWCNT should remain straight, smooth, and uniform in the entire irradiation mode to circumvent the effect of non-uniform curvature. Furthermore, such a MWCNT should be thin enough at the very initial stage so that a limited shrinkage could be noted, while irradiating under high enough but non-fluctuating beam current density for an adequate irradiation period.…”