C-BN single-walled nanotubes from hybrid connection of BN/C nanoribbons : prediction by ab initio density functional calculations

We demonstrated for the first time by ab initio density functional calculation and molecular dynamics simulation that C0.5(BN)0.5 armchair single-walled nanotubes (NT) are gapless semiconductors and can be spontaneously formed via the hybrid connection of graphene/BN Nanoribbons (GNR/BNNR) at room temperature. The direct synthesis of armchair C0.5(BN)0.5 via the hybrid connection of GNR/BNNR is predicted to be both thermodynamically and dynamically stable. Such novel armchair C0.5(BN)0.5 NTs possess enhanced conductance as that observed in GNRs. Additionally, the zigzag C0.5(BN)0.5 SWNTs are narrow band gap semiconductors, which may have potential application for light emission. In light of recent experimental progress and the enhanced degree of control in the synthesis of GNRs and BNNR, our results highlight an interesting avenue for synthesizing a novel specific type of C0.5(BN)0.5 nanotube (gapless or narrow direct gap semiconductor), with potentially important applications in BNC-based nanodevices.

Reactions of Nb2 and Nb3 with CO, D2, N2, and O2 : reconciling experimental kinetics with density functional theory-calculated reaction profiles

 Calculated energy profiles for the reactions of neutral Nb₂ and Nb₃ metal clusters with CO, D₂, N₂, and O₂ are presented. In each reaction path, both a physisorption energy minimum, where the reactant remains intact, and a chemisorption energy minimum, where the reactant has dissociated, are calculated and linked by saddle points. We calculate branching ratios for the forward (dissociative) and reverse reactions which we compare with the experimental kinetic data. It is found that a combination of average thermal energies and barrier heights leads to wide variation in branching ratios which compares favourably to previously determined experimental reaction rates.

Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel

We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (MnγAl anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site.

Magnetism of C adatoms on BN nanostructures : implications for functional nanodevices

Spin-polarized density functional calculations reveal that magnetism can be induced by carbon adatoms on boron nitride nanotubes (BNNTs) and BN hexagonal sheets. As a result of the localization of impurity states, these hybrid sp-electron systems are spin-polarized, with a local magnetic moment of 2.0 μB per C adatom regardless of the tube diameter and the bonding between the C atom and the BNNTs/BN sheets. An analysis of orbital hybridization indicates that two valence electrons participate in the bonding and the remaining two electrons of the C adatom are confined at the adsorption site and contribute to the magnetism accordingly. The effective interaction distance between the C-induced magnetic moments is evaluated. In terms of the diffusion barrier and the adsorption energy of C adatoms on the BN nanotubes/ sheets, a fabrication method for BN-C-based functional nanodevices is proposed, and a series of virtual building blocks for functional devices are illustrated.