Due to their extraordinary mechanical, electrical, optical, thermal … properties, single wall carbon nanotubes (SWCNTs) have attracted tremendous studies in the last 20 years. However, most of these properties are closely related with the defects present in the carbon layer and with the chirality of the SWNCTs. Two specific studies have been performed in order to better understand the high temperature behavior of SWCNTs, using the high temperature device attached with the environmental scanning electron microscope (HT-ESEM mode).

SWCNTs imaging (Fig. 1): Isolated SWCNTs cannot be observed directly in the HT-ESEM due to the relatively poor resolution of this technique. The SWCNTs are deposited on an insulating substrate and low voltage (1-3kV) charge contrast imaging is used to directly observe the CNTs. While the ESEM is not supposed to operate in such high voltage conditions at high temperature, aligned CNTs images have been recorded under various gas compositions and pressures using the gaseous secondary electron detector. These images have demonstrated the possibility to observe the SWCNTs at high temperature in the ESEM and have opened the path for further in situ experiments in the microscope chamber.

We studied the chemical reactivity of individual single-walled carbon nanotubes on oxide substrates toward oxygen etching using HT-ESEM (Fig. 2) coupled with AFM observations. Our in situ observations show that the reactivity of carbon nanotubes on substrates is different from that of free-standing ones. In particular, semiconducting nanotubes appear as or slightly more reactive than metallic ones, showing that the nanotube type has a secondary influence compared with that of the substrate. In addition, carbon nanotubes are not progressively etched from their ends as frequently assumed but disappear segment by segment. Atomic force microscopy before and after oxidation reveals that nanotube oxidation proceeds first by a local cutting leading to two separate nanotube segments, which is then followed by a rapid etching of the segment that has been electrically disconnected from others. In addition, our study shows that exposure to electron and laser beams can strongly increase the chemical reactivity of single-walled carbon nanotubes on substrates. These results are rationalized by considering i) the effect of substrate-trapped charges on the nanotube density of states close to the Fermi level, and ii) the effect of electron and laser beams on the density of these surface charges.

[1] H. A. Mehedi, J. Ravaux, Y. Khadija, T. Michel, S. Taïr, M. Odorico, R. Podor, V. Jourdain Oxidation Mechanism of Individual Single-Walled Carbon Nanotubes on Substrate Monitored by in situ Scanning Electron Microscopy. Nano Research (2016) 9 519-527

Figures:

Figure 1: HT-ESEM micrographs of a SWNCTs performed at room temperature (E0=3kV, high vacuum) and at T=1000°C (E0=2.4kV, 200Pa CH3CH2OH).

Figure 2: Isothermal oxidation of SWCNTs in pure oxygen observed by HT-ESEM at T=480°C (E0=1.8kV, 11Pa O2).

Figure 3: Comparison of HT-ESEM and AFM images recorded before and after 17 min isothermal oxidation of SWCNTs in 10Pa O2 observed by HT-ESEM at T=540°C. The colored points indicate the positions of the SWCNT.

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-high-temperature-esem-study-of-carbon-nanotubes-reactivity-under-oxidative-conditions/