We critically discuss the stability of edge states and edge magnetism in zigzag edge graphene nanoribbons (ZGNRs). We point out that magnetic edge states might not exist in real systems, and show that there are at least three very natural mechanisms - edge reconstruction, edge passivation, and edge closure - which dramatically reduce the effect of edge states in ZGNRs or even totally eliminate them. Even if systems with magnetic edge states could be made, the intrinsic magnetism would not be stable at room temperature. Charge doping and the presence of edge defects further destabilize the intrinsic magnetism of such systems.Comment: 9 pages, 6 figures, 2 table

Solid α-B12 rhombohedral and γ-B28 orthorhombic boron as well as boron nanostructures in the form of spheres, sheets, and multirings beside a ring consisting of icosahedral B12 units were investigated using ab initio quantum chemical and density functional methods. The structure of the B100 fullerene exhibits unusual stability among all noninteracting free-standing clusters, and is more stable than the B120 cluster fragment of the γ-B28 solid, recently predicted and observed by Oganov et al. (Nature 2009, 457, 863). In addition, we compared the stability of the multirings and reported the structural transition from double-ring to triple-ring systems. This structural transition occurs between B52 and B54 clusters. We confirm that the noninteracting free-standing triangular buckled-sheet is more stable than the γ-sheet, assembled in this work, and than the α-sheet, proposed by Tang and Ismail-Beigi (Phys. Rev. Lett. 2007, 99, 115501). In contrast, however, when these sheets are considered as infinite periodic systems, then the α-sheet remains the most stable one.

A molecular dynamics simulation based on the embedded-atom method was conducted at different sizes of single-crystal Ag nanoparticles (NPs) with diameters of 4 to 20 nm to find complete melting and surface premelting points. Unlike the previous theoretical models, our model can predict both complete melting and surface premelting points for a wider size range of NPs. Programmed heating at an equal rate was applied to all sizes of NPs. Melting kinetics showed three different trends that are, respectively, associated with NPs in the size ranges of 4 to 7 nm, 8 to 10 nm, and 12 to 20 nm. NPs in the first range melted at a single temperature without passing through a surface premelting stage. Melting of the second range started by forming a quasi-liquid layer that expanded to the core, followed by the formation of a liquid layer of 1.8 nm thickness that also subsequently expanded to the core with increasing temperature and completed the melting process. For particles in the third range, the 1.8 nm liquid layer was formed once the thickness of the quasi-liquid layer reached 5 nm. The liquid layer expanded to the core and formed thicker stable liquid layers as the temperature increased toward the complete melting point. The ratio of the quasi-liquid layer thickness to the NP radius showed a linear relationship with temperature.

ABSTRACT:The electronic and geometric structures, total and binding energies, first and second energy differences, harmonic frequencies, point symmetries, and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of small and neutral B n (n ϭ 2-12) clusters have been investigated using density functional theory (DFT), B3LYP with 6-311ϩϩG(d,p) basis set. Linear, planar, convex, quasi-planar, three-dimensional (3D) cage, and open-cage structures have been found. None of the lowest energy structures and their isomers has an inner atom; i.e., all the atoms are positioned at the surface. Within this size range, the planar and quasi-planar (convex) structures have the lowest energies. The first and the second energy differences are used to obtain the most stable sizes. A simple growth path is also discussed with the studied sizes and isomers. The results have been compared with previously available theoretical and experimental works.

10 × 10) SWNT consisting of 400 atoms with 20 layers is simulated under tensile loading using our developed O(N ) parallel TBMD algorithms. It is observed that the simulated carbon nanotube is able to carry the strain up to 122% of relaxed tube length in elongation and up to 93% for compression. The Young's modulus, tensile strength and Poisson ratio are calculated and the values found are 0.311 TPa, 4.92 GPa and 0.287, respectively. Stress-strain curve is obtained. The elastic limit is observed at the strain rate of 0.09 while the breaking point is at 0.23. The frequency of vibration for the pristine (10 × 10) carbon nanotube in radial direction is 4.71 × 10 3 GHz and it is sensitive to the strain rate.