Ab-initio Simulations of Diamondoid Nanostructures

Significant Achievements, Anticipated Outcomes and Future Work

This project undertook theoretical investigations of the relative phase stability of quasi-one dimensional carbon nanostructures, using a heat of formation model previously used successfully to compare the phase stability of diamond nanocrystals and fullerenes. The results of this study indicate that carbon nanotubes represent the most energetically preferred form for fine one dimensional carbon nanostructures, and that the sp3 analog (diamond nanowires) occupy a 'window' of stability. This window ranges from approximately 2.7 nm to 3.7–9 nm in (lateral) diameter, beyond which graphite is energetically preferred. The limits of this range are sensitive both to the nanowire morphology, and the method used to scale the graphite structures (required to ensure dimensional consistency). It has also been found that in all cases the nanowires with the principal axis in the [110] direction are structurally unstable, whereas nanowires with the principal axis in the [100] direction are stable, irrespective of the stability of the corresponding repeatable quasi-zero dimensional nanodiamond unit. This suggests that diamond nanowires having a principal axis in the [110] direction do not represent the optimal choice, unless hybrid nanowire–tube structures (such as bucky-wires) are desired. Similarly, dodecahedral diamond nanowires (with axes in the [100] direction) were found to be an energetically preferred morphology among the stable nanowires, via calculations using the same heat of formation model used to compare diamond nanowires and carbon nanotubes mentioned above. These results are considered to be useful in estimating the size range for which diamond nanowires may be expected during synthesis, and as a guide to the relative stability of some sp2 and sp3 carbon in one dimensional nanostructures. Further work is required to investigate the relative stability of other nanocarbon structures such as nanopeapods, bucky-wires and amorphous carbon nanostructures in one dimension.

Computational Techniques Used

The first principles calculations performed in this project have been carried out using Density Functional Theory (DFT) within the Generalized-Gradient Approximation (GGA), with the exchange-correlation functional of Perdew and Wang (PW91). This has been implemented via the Vienna Ab initio Simulation Package (VASP), which spans reciprocal space with a plane-wave basis. In this case a basis was used up to a kinetic energy cutoff of 290 eV. We have used the Linear Tetrahedron Method (LTM) with a 4x4x4 Monkhorst-Pack k-point mesh. Although this choice of k-mesh results in some superfluous k-points in the non-periodic directions, it was found that the inclusion of these k-points is more consistent with the LTM. The electronic relaxation technique used here is an efficient matrix diagonalization routine based on a sequential band-by-band residual minimization method of single-electron energies, with direct inversion of the iterative subspace, whereas the ionic relaxation involves minimization of the Hellmann-Feynman forces. The relaxations and the final static single point energy calculations used to characterize the final structures employed gradient-corrected Vanderbilt-type pseudopotentials (US-PP).

Publications, Awards and External Funding

External Funding and Awards

None.

Publications

A. S. Barnard, S. P. Russo, I. K. Snook, From nanodiamond to diamond nanowires: structural properties affected by dimension, Philosophical Magazine, 84, (2004), 899–907
A. S. Barnard, I. K. Snook, Phase stability of nanocarbon in one dimension: Nanotubes versus diamond nanowires, Journal of Chemical Physics, 120, (2004), 3817-3821