(127a) Plasma-Induced Structural Transitions In Materials

Atomic hydrogen plays a key role in plasma assisted deposition of group IV films. Indeed, it is well known that plasma deposition with silane, methane and germane gas heavily diluted in hydrogen results in microcrystalline silicon, diamond and germanium, respectively. In fact, thin films of hydrogenated amorphous silicon undergo chemically-induced disorder-to-order structural transformations and crystallize upon exposure to H atoms created through plasma. Remarkably, this transition occurs at temperatures (~25-300 ^oC) much lower than those required for thermal crystallization. While the critical role of atomic H created through plasma dissociation of H₂ has been established experimentally, the mechanism of how H enables the deposition of microcrystalline group IV films had remained a mystery. Recently, we have shown that hydrogen induced crystallization of amorphous silicon is mediated by the insertion into strained Si-Si bonds of H atoms as they diffuse through the film and by the concerted rearrangements of the bonds in the vicinity of the insertion reaction. Taking advantage of these reactions we have even deposited microcrystalline silicon at room temperature.

Based on the hypothesis that this H-induced crystallization mechanism is general, and may be operative in other covalently-bonded group IV solids, we have exposed multiwall carbon nanotubes (MWCNTs) to H atoms produced by electron impact dissociation of H₂ molecules in an inductively coupled radio-frequency (rf) plasma. We hypothesized that reactions with H atoms like those observed in silicon may help bind carbon atoms in neighbouring concentric CNTs and transform them into diamond nanocrystals. Indeed, we observed hydrogen-induced transformation of MWCNTs to various crystalline carbon structures, even at room temperature. Specifically, exposure of multiwall carbon nanotubes (MWCNTs) or multiwall carbon nanofibres to atomic hydrogen transformed them into other carbon allotropes including cubic diamond and hexagonal diamond (lonsdaleite). Nanometer-size crystals of diamond appear gradually in webs of long strings of crystallites, ~ 5-50 nm in diameter, where nanotubes once laid. This H-induced transformation of MWCNTs to diamond is observed even at room temperature. Moreover, high-resolution transmission electron microscopy of the H-exposed material reveals the presence of carbon nanocrystals whose electron diffraction patterns and lattice spacings could not be accounted for by known crystalline phases of carbon, such as diamond and lonsdaleite, or by contaminants. In addition to cubic and hexagonal diamond, interactions of H atoms with the concentric graphene layers of the MWCNTs produce new crystalline carbon phases that have not been observed previously. Specifically, we show, unambiguously, the existence of a face-centered cubic carbon phase with lattice constant a=0.426 nm.