Predicting experimentally stable allotropes: Instability of penta-graphene

International audienceIn recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp2-carbon–containing pentagons and heptagons, and “penta-graphene,” consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the “Cairo pattern,” penta-graphene is elegant and aesthetically pleasing. However, we dispute the author’s claims of its potential stability and experimental relevanc

Low energy graphene edge termination via small diameter nanotube formation

We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp$^3$-carbon atoms, and introduce Van Hove singularities into the graphene density of states.Comment: To appear in Phys. Rev. Let

Stable hydrogenated graphene edge types: Normal and reconstructed Klein edges

Hydrogenated graphene edges are assumed to be either armchair, zigzag or a combination of the two. We show that the zigzag is not the most stable fully hydrogenated structure along the direction. Instead hydrogenated Klein and reconstructed Klein based edges are found to be energetically more favourable, with stabilities approaching that of armchair edges. These new structures "unify" graphene edge topology, the most stable flat hydrogenated graphene edges always consisting of pairwise bonded C2H4 edge groups, irrespective the edge orientation. When edge rippling is included, CH3 edge groups are most stable. These new fundamental hydrogen terminated edges have important implications for graphene edge imaging and spectroscopy, as well as mechanisms for graphene growth, nanotube cutting, and nanoribbon formation and behaviour.Fundação para a Ciência e a Tecnologia (FCT

Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition

Abstract Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS 2 ) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown

Structure Determination and Compositional Modification of Body-Centered Tetragonal PX-Phase Lead Titanate

The fibrous PX-phase of lead titanate (PT), which can be synthesized by a hydrothermal method, has been studied to clarify its structural aspects on the atomic level. By combining synchrotron X-ray diffraction and electron microscope analysis with a first-principles calculation, a unique open-channel structure with a 5.529 angstrom diameter bore through the whole wire is determined. First-principle modeling reveals that the PX-phase structure has an indirect wide band gap and a higher formation enthalpy than the tetragonal perovskite phase. Both the frequency and symmetry of Raman-active lattice vibrational modes have been identified with polarized Raman spectra on individual monocrystalline PX-phase PT wires, showing a good match with the first-principles calculation. Furthermore, the doping of Zr in the PX-phase PT and the influence on the structure were investigated, showing a limit of 17% incorporation of Zr in the PX-phase PT