Carbon nanotubes (CNTs) can be used as field emission electron sources in X-ray tubes for medical applications [1, 2]. In a laboratory setting, field emission measurements of CNTs are usually carried out in an ultra-high vacuum system with base pressure of about 1E-7 mbar or better. Under less stringent vacuum conditions, CNTs are found to exhibit lower emission currents and reduced lifetimes [3, 4].
Here, we report the direct study on the structural changes in CNTs as we heated and oxidized them in situ using an aberration-corrected environmental TEM [5]. We established a protocol whereby heating and oxidation were performed without an imaging beam and changes on identifiable nanotubes were documented after purging the gas from the chamber, to ensure that they were due to the effect of gaseous oxygen molecules on the nanotubes, rather than the ionized gas species [5]. Contrary to earlier reports that CNT oxidation initiates at the end of the tube and proceeds along its length, our findings show that only the outside graphene layer is being removed and, on occasion, the interior inner wall is oxidized, presumably due to oxygen infiltrating into the hollow nanotube through an open end or breaks in the tube [5]. The CNT caps are not observed to oxidize preferentially [5, 6].
In the environment of an ETEM, interaction between fast electrons and gas leads to ionization of gas molecules and increased reactivity. It is very important to evaluate the results to determine or ameliorate the influence of the imaging electron beam. We found that there is a two orders of magnitude difference in the cumulative electron doses required to damage carbon nanotubes from 80 keV electron beam irradiation in gas versus in high vacuum [7]. We anticipate that experimental conditions that delineate the influence of the imaging electron beam can be established, which will enable us to study the CNT field emission process in situ in an ETEM.
[1]. G. Cao et al., Med. Phys. 37 (2010), pp. 5306–5312.
[2] X. Qian et al., Med. Phys. 39 (2012), pp. 2090–2099.
[3] K. A. Dean and B. R. Chalamala, Appl. Phys. Lett. 75 (1999), pp. 3017–3019.
[4] J.-M. Bonard, et al., Ultramicroscopy 73 (1998), pp. 7–15.
[5] A. L. Koh et al., ACS Nano 7(3) (2013), pp. 2566–2572.
[6] R. Sinclair et al., Advanced Engineering Materials 16(5) (2014), pp. 476-481.
[7] A. L. Koh et al., Nano Lett. 16(2) (2016), pp. 856-863.
[8] The authors acknowledge funding from the National Cancer Institute grants CCNE U54CA-119343 (O.Z.), R01CA134598 (O.Z.), CCNE-T U54CA151459-02 (R.S.) and CCNE-TD #11U54CA199075. (R.S.) Part of this work was performed at the Stanford Nano Shared Facilities.
Figures:

Figure 1. Aberration-corrected TEM images showing the structural changes in a double-walled carbon nanotube acquired at (a) 300°C before oxidation, (b) 300°C after 15 min exposure to 1.5 mbar oxygen, and (c) 400°C after 15 min exposure to 1.5 mbar oxygen. (d)- (f) are higher magnification TEM images of insets (a)-(c) indicated by the red boxes. Scale bars in (a)-(c) and (d)-(f) are 5 and 2 nm respectively. [5]

Figure 2. Aberration-corrected TEM images of the same nanotube at (a) 400°C and (b) after oxidation at 1.5 mbar at 400°C, showing that the nanotube cap does not oxidize preferentially. [6]

Figure 3. Cumulative electron dose to damage MWNTs by continuous 80 keV electron beam irradiation in high vacuum and in gas environments in an ETEM at room temperature. [7]
To cite this abstract:
Ai Leen Koh, Emily Gidcumb, Otto Zhou, Robert Sinclair; Oxidation of Carbon Nanotubes Using Environmental TEM and the Influence of the Imaging Electron Beam. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/oxidation-of-carbon-nanotubes-using-environmental-tem-and-the-influence-of-the-imaging-electron-beam/. Accessed: September 21, 2023« Back to The 16th European Microscopy Congress 2016
EMC Abstracts - https://emc-proceedings.com/abstract/oxidation-of-carbon-nanotubes-using-environmental-tem-and-the-influence-of-the-imaging-electron-beam/