Fe-filled CNTs have been proposed as perfect candidates for a large and varied number of applications, ranging from the biological to optoelectronics or memory storage devices ¹. For any of these applications, it seems crucial to understand the interactions, possible phase changes or reactions that can take place as a consequence of exposure to their real working conditions.

Conventional bright-field transmission electron microscopy (BF-TEM) has been the tool traditionally employed to observe in-situ the dynamical effects that take place when individual CNTs are exposed to high electrical currents and Joule heating. Despite being a much more powerful technique, scanning TEM-annular dark field (STEM-ADF) imaging has been scarcely for this purpose ². One of the main advantages of STEM-ADF over BF-TEM is that the intensity in the images is highly dependent on the atomic number of the species which are present and therefore the images show compositional in addition to structural information. This gives a much better understanding of current-induced migration effects, formation of intermediate phases or alloying, or phase separation phenomena, something that using BF-TEM alone would miss. In the STEM-ADF configuration it is also possible to perform complementary analytical spectroscopy simultaneously with imaging and with comparable spatial resolution.

Here we show the significant advantages of combining in-situ experiments with STEM-ADF and related analytical techniques to gain new insights into the electrical transport induced transformations in P, N doped Fe-filled carbon nanotubes. It has been possible to monitor in real time a multistage process (Figure 1) in which the Fe filling reacts with nitrogen to form an intermediate alloy phase, which then decomposes into smaller particles. The presence of N₂ gas within the inner channel of the tube is found to be crucial to this process.

1. S. Costa et al.,  Phys. Stat. Sol. B 2007, 244, 4315-4318.
2. Y. Beyer et al., Micron 2012, 43, 428–434.

Figures:

A sequence of STEM-ADF images showing a) the initial Fe-filling in a unchanged during stage 1; followed by the formation of a intermediate contrast particle in (b) and (c); and (d) its conversion back to particles

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EMC Abstracts - https://emc-proceedings.com/abstract/electrical-transport-induced-transformations-in-filled-carbon-nanotubes-imaged-in-situ-by-scanning-transmission-electron-microscopy/