Group-IV nanostructured materials, such as hydrogenated nanocrystalline silicon (nc-Si:H), hydrogenated nanocrystalline germanium (nc-Ge:H), diamond, silicon carbide, and carbon nanotubes, find numerous applications ranging from solar cells to filters, heat sinks, field emitters, high-power RF semiconductor devices, molecular electronics, and more recently as biological or chemical sensors.  One common technique used to grow these technologically important materials is plasma-enhanced chemical vapor deposition (PECVD), whereby reactive chemical species created in the plasma react on a surface to yield a thin film.  It has been well established that hydrogen plays a critical role in determining the crystallinity of these nanostructured materials.  For example, the ability to deposit diamond thin films from hydrocarbons heavily diluted in hydrogen has been studied extensively.  Another example is the transformation of amorphous silicon films to nanocrystalline ones upon exposure to atomic hydrogen.  Recently, we have shown that crystallization of amorphous silicon is mediated by the insertion into strained Si-Si bonds of H atoms as they diffuse through the film.¹  Concerted rearrangements of the bonds in the vicinity of the insertion reaction lead to a hydrogen-induced amorphous-to-crystalline transformation.^1,2    

Based on the hypothesis that this chemically-induced crystallization mechanism could be operative in other covalently-bonded solids, we have exposed multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (MWCNFs) to atomic hydrogen produced by an inductively coupled radio frequency plasma source.  Indeed, 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.

  ¹S. Sriraman, S. Agarwal, E. S. Aydil, and D. Maroudas, Nature 418, 62 (2002).

²S. Sriraman, M. S. Valipa, E. S. Aydil, and D. Maroudas, J. Appl. Phys. 100, 053514 (2006); M. S. Valipa, S. Sriraman, E. S. Aydil, and D. Maroudas, J. Appl. Phys. 100, 053515 (2006).