Paper PS1-WeM3
Diamonds and New Carbon Allotropes from Carbon Nanotubes at Room Temperature

Wednesday, October 22, 2008, 8:40 am, Room 304

Interactions of reactive plasmas with nanostructured materials enable the synthesis of materials that would not be expected to form at low temperature and in vacuum. As a remarkable example, in this presentation, we report the plasma synthesis of diamonds and other new carbon allotropes from multiwalled carbon nanotubes (MWCNTs) at room temperature. We exposed MWCNTs to hydrogen atoms created by plasma dissociation of dihydrogen gas in a downstream inductively coupled plasma and observed the transformation of the nanotubes to various crystalline carbon structures, even at room temperature. Examination of the H-exposed MWCNTs with transmission electron microscopy (TEM) revealed webs of long strings of crystallites, ~2-20 nm in diameter, in locations occupied initially by nanotubes. High-resolution TEM, selected-area electron diffraction, and convergent-beam electron diffraction techniques were used to identify the lattice structures of these carbon nanocrystals as cubic diamond, cubic n-diamond, lonsdaleite (hexagonal diamond), and a new carbon allotrope with face-centered cubic symmetry and lattice parameter a = 0.426 nm. This H-induced transformation was observed over the temperature range from 300 K to 1073 K, and investigated as a function of atomic hydrogen dose. Combining synergistically our experimental findings with molecular-dynamics simulations and first-principles density functional theory calculations, we show how H atoms produced in the dihydrogen plasma can induce sp2-to-sp3 C-C bonding transitions in MWCNTs and help nucleate various crystalline carbon allotropes such as cubic diamond and lonsdaleite.