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.

Binary borides had been a subject of extensive research. However, the exact compositions and crystal structures of sodium borides remained controversial. Here, using the ab initio variable-composition evolutionary algorithm, a new stable Na2B30 with I212121 symmetry (I212121-Na2B30) is found, which is-7.38 meV/atom lower in energy than the experimental Imma-Na2B30 structure. Interestingly, the Imma-Na2B30 is predicted to be a topological nodal line semimetal, which may result in superior electronic transport. In contrast, I212121-Na2B30 is an ultrahard semiconductor with an unprecedented open-framework structure, whose interstitial helical boron sublattice enhances its hardness and energetic stability.

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We simulate boron on Pb (110) surface by using ab initio evolutionary methodology. Interestingly, the two-dimensional (2D) Dirac P mmn boron can be formed because of good lattice matching. Unexpectedly, by increasing the thickness of 2D boron, a three-bonded graphene-like structure (P 21/c boron) was revealed to possess double anisotropic Dirac cones. It is 20 meV/atom lower in energy than the P mmn structure, indicating the most stable 2D boron with particular Dirac cones. The puckered structure of P 21/c boron results in the peculiar Dirac cones, as well as substantial mechanical anisotropy. The calculated Young's modulus is 320 GPa·nm along zigzag direction, which is comparable with graphene.Graphene is featured by the Dirac cone in the band structure, which leads to the fractional quantum Hall effect, ultrahigh carrier mobility, and some other novel properties [1][2][3][4]. The fancy properties of graphene inspire search for other 2D materials with Dirac cones, and at least seven 2D carbon allotropes with Dirac cones were predicted, i.e., α, β, δ, 6,6,12-, 14,14,14-, 14,14,18-graphyne, and phagraphene [5-8]. Boron has a short covalent radius and the flexibility to adopt sp 2 hybridization as carbon, which favors the formation of low-dimensional forms (clusters, nanotubes, fullerenes and so on) [9][10][11][12][13][14][15][16][17][18][19][20][21]. Owing to the rich variety of bonding configurations, ranging from the common two-center two-electron bonds to the rare eight-center two-electron bonds, an extreme structural diversity is anticipated in 2D boron. Up to now, several boron-related phases were predicted to have distorted Dirac cones, but this prediction remains to be realized [14,17,21]. Most recently, borophenes were successfully grown on Ag (111) surface under ultra-high vacuum conditions [19,20]. These monolayer boron sheets with triangular lattice (without vacancy) or 1/6 vacancies were synthesized and confirmed to be metallic. However, growing multilayer boron sheets on Ag (111) substrate is difficult due to the weak boron-silver interaction. When the boron coverage exceeds one monolayer on Ag (111), 3D boron clusters tend to form easily [19,20]. In contrast, the boroncopper interaction is stronger than boron-silver interaction, thereby 2D multilayer boron sheets were synthe-sized on copper foils by using chemical vapor deposition and turned out to be semiconducting with a bandgap of 2.35 eV [18]. Hunting for 2D boron with Dirac cones are still a big challenge for both experimentalists and theoreticians [22,23]. Whether P mmn boron and P 6/mmm boron can be synthesized and whether other 2D Dirac semimetallic boron allotropes can be formed on specific substrates are therefore unknown. In this work, the facecentered cubic Pb metal was selected as an alternative substrate to grow P mmn boron because the Pb (110) surface matches the lattice of P mmn boron very well. Moreover, Pb is more reactive than Cu and Ag, thus the B-Pb interaction may be strong enough to grow multilayer boron sheets.The evolutionar...