Very recently, two-dimensional (2D) boron sheets (borophene) with rectangular structures were grown successfully on single crystal Ag(111) substrates (Mannix et al 2015 Science 350 1513). The ...fabricated boroprene is predicted to have unusual mechanical properties. We performed first-principle calculations to investigate the mechanical properties of the monolayer borophene, including ideal tensile strength and critical strain. It was found that monolayer borophene can withstand stress up to 20.26 N m−1 and 12.98 N m−1 in a and b directions, respectively. However, its critical strain was found to be small. In the a direction, the critical value is only 8%, which, to the best of our knowledge, is the lowest among all studied 2D materials. Our numerical results show that the tensile strain applied in the b direction enhances the bucking height of borophene resulting in an out-of-plane negative Poisson's ratio, which makes the boron sheet show superior mechanical flexibility along the b direction. The failure mechanism and phonon instability of monolayer borophene were also explored.
At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets ...will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.
Despite many decades of research of diodes, which are fundamental components of electronic and photoelectronic devices with p–n or Schottky junctions using bulk or 2D materials, stereotyped ...architectures and complex technological processing (doping and multiple material operations) have limited future development. Here, a novel rectification device, an orientation‐induced diode, assembled using only few‐layered black phosphorus (BP) is investigated. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge‐transport conditions abruptly along the different crystal orientations. Rectification ratios of 6.8, 22, and 115 can be achieved in cruciform BP, cross‐stacked BP junctions, and BP junctions stacked with vertical orientations, respectively. The underlying physical processes and mechanisms can be explained using “orientation barrier” band theory. The theoretical results are experimentally confirmed using localized scanning photocurrent imaging. These orientation‐induced optoelectronic devices open possibilities for 2D anisotropic materials with a new degree of freedom to improve modulation in diodes.
Orientation‐induced diodes assembled using only few‐layered black phosphorus (BP) materials are developed as novel rectification devices. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge‐transport conditions abruptly along the different crystal orientations. These devices can open possibilities for two‐dimensional anisotropic materials to improve modulation in diodes.
Abstract
Strengths of nanograined (ng) and nanotwinned (nt) metals increase with decreasing grain size and twin thickness, respectively, until reaching a critical value, below which strength ...decreases. This behavior is known as the reverse Hall–Petch effect (RHPE), which has also been observed in nanograined cubic boron nitride (cBN) and diamond. Surprisingly, however, hardness of nt-cBN and nt-diamond increases continuously with decreasing twin thickness down to several nanometers, suggesting the absence of RHPE in these covalent materials. The mechanism responsible for such a behavior remains controversial. Here we investigate the strengthening mechanisms in ng- and nt-diamond using molecular dynamics and first-principles calculations. For ng-diamond, the competition between shuffle-set dislocation (SSD) and grain boundary atom motions gives rise to RHPE. For nt-diamond, SSDs remain dominant but their slips along twin boundaries energetically show no advantage over those along other slip planes. Twin domains are locked and mechanically stable, resisting SSD propagation and inhibiting RHPE. These findings provide new insights into the hardening mechanism of nanotwinned covalent materials.
The nudged elastic band (NEB) method, as a popular technique for studying reaction paths due to its efficiency, has not been extensively used in solid state physics because of the need to deal with ...the variation of the unit cell during solid–solid transformations. Here we present an extended technique—a variable-cell NEB (VC-NEB) technique combined with the ab initio method, implemented in the USPEX code. Our technique is applied to reconstructive solid–solid phase transitions of GaN: from wurtzite to rocksalt (B4→B1) and from zincblende to rocksalt (B3→B1). As a more challenging application, we study the mechanism of the recently predicted insulator–metal phase transition of BH. The results reveal that the VC-NEB technique is an efficient and general method and should have wide applications for studying the paths and mechanisms of reconstructive phase transitions. Results of the VC-NEB method can be considered as a starting point for a more sophisticated treatment using the transition path sampling method, the main prerequisite of which is to have an initial transformation trajectory.
Using systematic evolutionary structure searching we propose a new carbon allotrope, phagraphene fæ’græfi:n, standing for penta-hexa-hepta-graphene, because the structure is composed of 5-6-7 carbon ...rings. This two-dimensional (2D) carbon structure is lower in energy than most of the predicted 2D carbon allotropes due to its sp2-binding features and density of atomic packing comparable to graphene. More interestingly, the electronic structure of phagraphene has distorted Dirac cones. The direction-dependent cones are further proved to be robust against external strain with tunable Fermi velocities.
Abstract
With the motivation of discovering high-temperature superconductors, evolutionary algorithm USPEX is employed to search for all stable compounds in the Sn-H system. In addition to the ...traditional SnH
4
, new hydrides SnH
8
, SnH
12
and SnH
14
are found to be thermodynamically stable at high pressure. Dynamical stability and superconductivity of tin hydrides are systematically investigated. I"Equation missing"
m2-SnH
8
, C2/m-SnH
12
and C2/m-SnH
14
exhibit higher superconducting transition temperatures of 81, 93 and 97 K compared to the traditional compound SnH4 with T
c
of 52 K at 200 GPa. An interesting bent H
3
–group in I"Equation missing"
m2-SnH
8
and novel linear H"Equation missing"
in C2/m-SnH
12
are observed. All the new tin hydrides remain metallic over their predicted range of stability. The intermediate-frequency wagging and bending vibrations have more contribution to electron-phonon coupling parameter than high-frequency stretching vibrations of H2 and H3.
It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two-dimensional (2D) phases and likely precursors for boron nanostructures. ...Here we predict, based on an ab initio evolutionary structure search, a novel 2D boron structure with nonzero thickness, which is considerably, by 50 mev / atom, lower in energy than the recently proposed alpha -sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless Dirac fermions. The buckling and coupling between the two sublattices not only enhance the energetic stability, but also are the key factors for the emergence of the distorted Dirac cone.
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