Ai Leen Koh1, Emily Gidcumb2, Otto Zhou2,3 and Robert Sinclair4
1 Stanford Nano Shared Facilities, Stanford University, Stanford, CA 94305, USA
2 Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
3 Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
4 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
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 10^-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.
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[2] X. Qian et al., Med. Phys. 39 (2012), pp. 2090–2099.
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[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.
