| AVS 54th International Symposium | |
| Thin Film | Friday Sessions |
| Session TF2-FrM |
| Session: | Nanoparticles |
| Presenter: | M.J. Behr, University of Minnesota |
| Authors: | M.J. Behr, University of Minnesota T. Singh, University of Massachusetts, Amherst D. Maroudas, University of Massachusetts, Amherst E.S. Aydil, University of Minnesota |
| Correspondent: | Click to Email |
Plasma-enhanced chemical vapor deposition (PECVD) using hydrocarbons is a common technique used to grow carbon-based nanostructured materials. It has been well established that hydrogen plays a critical role in determining these materials’ crystallinity. For example, PECVD of diamond films is possible only when hydrocarbon precursors are heavily diluted in hydrogen. The specific role of hydrogen in these carbon systems, however, is unknown. To investigate hydrogen’s role, we have exposed multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (MWCNFs) at room temperature to atomic hydrogen produced by an inductively coupled radio frequency plasma source. We observed hydrogen-induced crystallization of MWCNTs and MWCNFs at room temperature. Nanometer-size crystals of carbon appear gradually during 2-10 hour H-atom exposure of the nanotubes. We characterized these nanocrystals using high-resolution transmission electron microscopy (HRTEM), electron diffraction, and forward recoil elastic spectrometry (FRES). HRTEM of the H-exposed material at room temperature revealed the presence of carbon nanocrystals, ~5 nm in diameter, embedded in amorphous carbon nanorods. The 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. However, two distinct crystalline phases are observed among a variety of others in this amorphous matrix. The observed electron diffraction patterns and lattice spacings from these two phases are consistent with a body-centered cubic (BCC) phase with lattice parameter a = 0.30 nm and a face-centered cubic (FCC) phase with lattice parameter a = 0.425 nm. The lattice parameter of the FCC phase is approximately the same as that of i-Carbon, a previously observed but unidentified phase belonging to the cubic crystal system. However, electron diffraction from i-Carbon and the FCC phase we observe do not follow the same selection rules. First-principles density functional theory calculations within the generalized gradient approximation are used in conjunction with electron microscopy to suggest possible crystal structures for these new phases. Specifically, we consider the possibility that these new ordered phases may contain hydrogen atoms in addition to carbon and examine the dependence of the structural parameters of the crystalline phases on their hydrogen content.