A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson's ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.carbon allotrope | carbon pentagon | stability | negative Poisson's ratio | electronic structure C arbon is one of the most versatile elements in the periodic table and forms a large number of allotropes ranging from the well-known graphite, diamond, C 60 fullerene (1), nanotube (2), and graphene (3) to the newly discovered carbon nanocone (4), nanochain (5), graphdiyne (6), as well as 3D metallic structures (7,8). The successful synthesis of graphene (3) has triggered considerable interest in exploring novel carbon-based nanomaterials. A wealth of 2D carbon allotropes beyond graphene has since been studied (see SI Appendix, Table S1 for details). Although some of these polymorphs such as graphdiyne (6) are metastable compared with graphene, they have been successfully synthesized. Moreover, some 2D carbon allotropes are predicted to exhibit remarkable properties that even outperform graphene, such as anisotropic Dirac cones (9), inherent ferromagnetism (10), high catalytic activity (6), and potential superconductivity related to the high density of states at the Fermi level (11). These results demonstrate that many of the novel properties of carbon allotropes are intimately related to the topological arrangement of carbon atoms and highlight the importance of structure-property relationships (12).Pentagons and hexagons are two basic building blocks of carbon nanostructures. From zero-dimensional nanoflakes or nanorings (13) to 1D nanotube, 2D graphene, and 3D graphite and metallic carbon phases (7,8), hexagon is the only building block. Extended carbon networks composed of only pentagons are rarely seen. Carbon pentagons are usually considered as topological defects or geometrical frustrations (14) as stated in the well-known "isolated pentagon rule" (IPR) (15) for fullerenes, where pentagons must be separated from each other by surrounding hexagons to reduce the steric stress. For instance, C 60 ...

Graphene, a two dimensional (2D) carbon sheet, acquires many of its amazing properties from the Dirac point nature of its electronic structures with negligible spin-orbit coupling. Extending to 3D space, graphene networks with negative curvature, called Mackay-Terrones crystals (MTC), have been proposed and experimentally explored, yet their topological properties remain to be discovered. Based on the first-principle calculations, we report an all-carbon MTC with topologically non-trivial electronic states by exhibiting node-lines in bulk. When the node-lines are projected on to surfaces to form circles, "drumhead"-like flat surface bands nestled inside of the circles are formed. The bulk node-line can evolve into 3D Dirac point in the absence of inversion symmetry, which has shown its plausible existence in recent experiments.

In addition to spintronics another motivation for exploring ferromagnetic two-dimensional materials is for biomedical applications such as magnetic labeling and hyperthermia treatment of tumors. Unfortunately, the widely studied Mn-containing monolayer is not biocompatible, although it is ferromagnetic. Here using first principles calculations combined with Monte Carlo simulations based on the Ising model, we systematically study a class of 2D ferromagnetic monolayers CrX3 (X = Cl, Br, I). The feasibility of exfoliation from their layered bulk phase is confirmed by the small cleavage energy and high in-plane stiffness. Spin-polarized calculations, combined with self-consistently determined Hubbard U that accounts for strong correlation energy, demonstrate that CrX3 (X = Cl, Br, I) monolayers are ferromagnetic and that Cr is trivalent and carries a magnetic moment of 3 μ(B); the resulting Cr(3+) ions are biocompatible. The corresponding Curie temperatures for CrCl3, CrBr3 and CrI3 are found to be 66, 86, and 107 K, respectively, which can be increased to 323, 314, and 293 K by hole doping. The biocompatibility and ferromagnetism render these Cr-containing trihalide monolayers unique for applications.

Two-dimensional (2D) topological insulator (TI) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial bandstructure topology identified by the Z 2 number as protected by time-reversal symmetry (TRS).2D TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. In the edge states, the electrons with opposite spins propagate in opposite directions and the backscattering is fully prohibited when the TRS is conserved. This leads to quantized dissipationless "two-lane highway" for charge and spin transportation and promises potential applications. Up to now, only very few 2D systems have been discovered to possess this property. The lack of suitable material obstructs the further study and application. Here, by using first-principles calculations, we propose that the functionalized MXene with oxygen, M 2 CO 2 (M=W, Mo and Cr), are 2D TIs with the largest gap of 0.194 eV in W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recent developed selective chemical etching of transition-metal carbides (MAX phase). This will pave the way to tremendous applications of 2D TIs, such as "ideal" conducting wire, multifunctional spintronic device, and the realization of topological superconductivity and Majorana modes for quantum computing.2

We proposed a facile, low cost, and green approach to produce stable aqueous graphene dispersions from graphite by sonication in aqueous bovine serum albumin (BSA) solution for biomedical applications. The production of high-quality graphene was confirmed using microscopy images, Raman spectroscopy, UV-vis spectroscopy, and XPS. In addition, ab initio calculations revealed molecular interactions between graphene and BSA. The processability of aqueous graphene dispersions was demonstrated by fabricating conductive and mechanically robust hydrogel-graphene materials.

Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene.Comment: 10 pages, 12 figure

Exploring earth-abundant electrocatalysts with Pt-like performance toward alkaline hydrogen evolution reaction (HER) is extremely desirable for the hydrogen economy but remains challenging. Herein, density functional theory (DFT) predictions reveal that the electronic structure and localized charge density at the heterointerface of NiP2–FeP2 can be significantly modulated upon coupling with metallic Cu, resulting in optimized proton adsorption energy and reduced barrier for water dissociation, synergistically boosting alkaline HER. Motivated by theoretical predictions, we developed a facile strategy to fabricate interface-rich NiP2–FeP2 coupled with Cu nanowires (CuNW) grown on Cu foam (NiP2–FeP2/CuNW/Cuf). Benefiting from the superior intrinsic activity, conductivity, and copious active sites, the obtained catalyst exhibited exceptional alkaline HER activity requiring a low overpotential of 23.6 mV at −10 mA/cm2, surpassing the state-of-the-art Pt. Additionally, a full electrolyzer required a cell voltage of 1.42/1.4 V at 10 mA/cm2 in alkaline water/seawater with promising stability. This work highlights a design principle for advanced HER catalysts and beyond.

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