(358e) First-Principles Theoretical Analysis of Pure and Hydrogenated Crystalline Carbon Phases and Nanostructures

Carbon exists in a variety of different allotropes, such as cubic diamond, hexagonal diamond (lonsdaleite), graphite, carbynes, fullerenes, and carbon nanotubes. In addition to these well known forms, various other forms such as i-carbon, a cubic phase with lattice parameter a = 4.25 Å, a body-centered cubic (BCC) carbon phase with a = 3.1 Å, and n-diamond, a face-centered cubic (FCC) phase with a = 3.57 Å have been reported in experiments. These new carbon phases have been synthesized by various techniques, including plasma-assisted deposition from dilute hydrocarbon gases and hydrogen plasma exposure of bulk diamond and other carbon surfaces. However, the structure and stability of some of these new phases of carbon (i-carbon, n-diamond, and the new BCC phase) remain elusive.

In this presentation, we discuss a detailed analysis of various crystalline phases of carbon, which are candidate structures to explain the aforementioned experimentally observed carbon phases, as well as the potential role of hydrogen in determining the structures of the resulting carbon nanocrystals. Toward this end, we have employed first-principles density functional theory (DFT) calculations in order to compute the optimal lattice parameters and formation energies of pure, as well as hydrogenated crystalline phases of carbon. Our DFT calculations were carried out within the generalized gradient approximation and employed plane-wave basis sets, ultra-soft pseudopotentials, and supercell models.

We examined various crystalline cubic phases of pure carbon, including simple cubic, diamond-cubic, FCC, and BCC phases, as well as more complex crystalline phases of pure carbon such as supercubane, BC-8 carbon (the g-Si equivalent of carbon), and another BCC phase with space group I23 (#197), an optimal lattice parameter a = 3.31 Å, and a cohesive energy of 5.81 eV. We found that none of the pure carbon allotropes examined could explain satisfactorily the experimentally observed structures of i-carbon, n-diamond, or the new observed BCC carbon phase with a = 3.1 Å. Hence, we considered the incorporation of H in high-symmetry interstitial sites of diamond-cubic, BCC, and FCC carbon lattices, including bond-center, octahedral, and tetrahedral sites, and investigated the effect on the equilibrium lattice parameter of varying the composition, C_xH_y (x,y ≤ 3), of the carbon phases. H incorporation into the lattices of the various cubic carbon phases causes increased lattice expansion with increasing H concentration and enhances lattice stability compared to that of the corresponding pure carbon phases. As a result, by assuming interstitial H incorporation at varying concentrations in the cubic carbon lattices, we could provide possible interpretations for the structure of i-carbon, n-diamond, and the new BCC carbon phase.