Oxidation of Carbon Nanotubes in an Ionizing Environment

Abstract

In this work, we present systematic studies on how an illuminating electron beam which ionizes molecular gas species can influence the mechanism of carbon nanotube oxidation in an environmental transmission electron microscope (ETEM). We found that preferential attack of the nanotube tips is much more prevalent than for oxidation in a molecular gas environment. We establish the cumulative electron doses required to damage carbon nanotubes from 80 keV electron beam irradiation in gas versus in high vacuum. Our results provide guidelines for the electron doses required to study carbon nanotubes within or without a gas environment, to determine or ameliorate the influence of the imaging electron beam. This work has important implications for in situ studies as well as for the oxidation of carbon nanotubes in an ionizing environment such as that occurring during field emission.

Assessing and ameliorating the influence of the electron beam on carbon nanotube oxidation in environmental transmission electron microscopy

“…The recordings reveal that damage had started at 44 sec. The location of breakage – the kink – is one which would cause electric field lines to be concentrated, and is consistent with our earlier hypothesis about image force effects being the cause of damage [22]. It may also be related to the pentagonal carbon rings necessary at these locations to give rise to the curvature.…”

“…This is in contrast to the oxygen beam blanking experiments whereby there is no change in the nanotubes structures. We had attributed this increased reactivity to gas ionization from the electron beam and momentum transfer from the O 2 molecules and O 2 + ion species to the CNTs, with the positive ion species attracted to the nanotubes via image forces with effective field lines concentrated at their tips [22]. …”

“…At lower O 2 pressures (10 −5 mbar to 10 −3 mbar), the presence of electron beam and O 2 gas induces mobility of the amorphous carbon which increases with pressure but the time to remove both the amorphous and graphitic carbon is longer compared to operating at higher O 2 pressures (10 −1 – 10 0 mbar). In the latter, there was rapid removal of both amorphous and CNT structures [22] due to a larger number of gas molecules being present (~ 10 15 at 1 mbar versus ~10 11 at 10 −4 mbar under room temperature and assuming a cube volume with edges equal to objective pole piece gap) and the higher flow rates (number of molecules per unit area per unit time) associated with operating at higher pressure.…”

“…Unlike what had been reported previously based on ex-situ oxidation [19, 20], tube caps are not found to oxidize preferentially [18, 21]. However, when the imaging electron beam was illuminated in the presence of O 2 gas at room temperature, beam induced ionization of gas molecules led to rapid etching and destruction of CNTs at both caps and side walls [22]. This same behavior was also observed when an inert gas (N 2 ) was used in place of O 2 [22].…”

“…The effects of electron dose rate [23–25], total electron dose [22, 25], and beam on / beam off [18, 26] on gas-solid reactions have been reported. We had previously quantified the influence of the imaging electron beam by establishing the cumulative electron dose required to cause onset of visible damage in the CNTs, and showed that there is a two orders of magnitude difference in this parameter to oxidize / damage CNTs in 10 −7 mbar (high vacuum) versus that in approximately ~ 10 0 mbar O 2 [22]. In the present work, we examine the effect of the electron beam on carbon nanotube oxidation behavior using lower O 2 pressures (10 −6 mbar to 10 −1 mbar) and at varying dose rates.…”