Nanotube coalescence-inducing mode: A novel vibrational mode in carbon systems

The appearance of a resonant Raman mode which is related to vibrations of linear carbon chains and is observed as a precursor to the merging of highly purified double-walled carbon nanotubes (DWNT) is discussed. The mode, termed as coalescence-inducing mode (CIM), which initiates the coalescence process is induced by thermal annealing and its effect is enhanced by boron doping. The onset of the CIM vibration occurs at a much lower heat-treatment temperature upon boron addition, showing boron to act as a catalyst or welding agent for the coalescence process. The CIM mode arises from the generation of short 1D carbon chains established covalently between adjacent tubes. Molecular dynamics calculation shows that linear carbon chains are first established between the tubes and these chains trigger the nanotube coalescence via a zipper model. Raman spectroscopy monitors the initiation of the nanotube coalescence process in DWNTs and is capable of identifying the 1D carbon chains that participates in the early stages of DWNT coalescence.