Current-Driven Restructuring of Ion-Induced Cu-Incorporated Carbon Nanofibers: In Situ TEM Observation and Electro-Thermal Analysis

In this study, we observed the structural change of a Cu-incorporated carbon nanofiber (Cu-CNF) under current flow by in situ transmission electron microscopy (TEM), and estimated the power needed for the change using an electro-thermal model. The Cu-CNFs were grown on the edge of a Cu foil by Ar+ ion irradiation at room temperature. The selected-area electron diffraction (SAED) rings of the as-grown fiber gave d-spacings of 0.2087, 0.1808, 0.1282 and 0.1095 nm, indexed to fcc Cu with no separate graphite reflection. Under a constant bias, no structural change was observed for the first 2.0 s. Between 2.0 and 2.8 s, the fine Cu crystallites agglomerated into a continuous core of about 1.5 µm in length and 60–120 nm in width, and the surrounding carbon transformed into graphitic layers with an interlayer spacing of about 0.38 nm. At 3.2 s, the fiber broke at a Cu-depleted segment about 0.5 µm above the probe. A one-dimensional steady-state heat conduction model showed that 12–390 µW is required to heat the fiber to 600–800 °C, whereas the nominal circuit (200 mV, 200 kΩ, up to 1 µA) supplies 0.2 µW or less. The model also suggests that the heating concentrates in the Cu-free segment once a Cu core forms, which can explain the breaking point. The results indicate that the power actually delivered to the fiber was higher than the nominal value, or that a non-thermal process, such as electron-beam irradiation, assisted the change. These two cases could not be separated, because the voltage across the fiber was not recorded. The finding can be significant for designing in situ electrical processing of metal–carbon nanofibers.