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 firstprinciple 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.Boron is a fascinating element because of its chemical and structural complexity. Although it is carbon's neighbor in the periodic table with similar valence orbitals, the electron deficiency prevents it from forming graphene-like planar structures. In spite of numerous theoretical proposals [1-6], borophene had not been synthesized successfully until very recently on single crystal Ag(111) substrates under ultrahigh-vacuum conditions [7]. The monolayer borophene with rectangular structure has shown some extraordinary properties [2, 7-9], including the anisotropic metallic character and unique mechanical properties. For example, it exhibits an extremely large Young's modulus of 398 GPa nm along the a direction [7], which exceeds the value of graphene. The borophene shows great potential for applications in nano-scale electronic devices and microelectro-mechanic systems (MEMS) due to these novel properties. An adventitious strain is almost unavoidable experimentally, therefore, it is highly desirable to explore the mechanical properties of borophene.For 2D materials, the ideal tensile strength [10,11], is a crucial mechanical parameter which fundamentally characterizes the nature of the chemical bonding and the elastic limit of the single-or few-layer thin films. So far, the elastic limit of many 2D materials, such as graphene [12][13][14], h-BN [15][16][17][18][19], MoS 2 [20-24], black phosphorene (BP) [25][26][27][28], and silicone [29][30][31][32][33], have been characterized by the ideal tensile stress and critical strain. Compared to these materials, monolayer borophene is a stiffer material because of a higher Young's Modulus [7]. In this work, we presented systematic analysis on the strain-induced mechanical properties of monolayer borophene, including the ultimate stress and critical strain, the change of bucking height, and the failure mechanism when approaching the limit strain, and compared them with other representative 2D mat...

Black phosphorus (P) has been considered as a promising candidate for anodes due to its ability to absorb a large amount of Li atoms. Unfortunately, lithiation of bulk black P induces huge structural deformation, which limits its application. Here, on the basis of the density functional theory calculation, we predict that the newly found two-dimensional (2D) black and blue P are good electrodes for high-capacity lithium-ion batteries. Our theoretical calculations indicate that, in contrast to bulk black P, the monolayer and double-layer black and blue P can maintain their layered structures during lithiation and delithiation cycles. Moreover, it is found that Li diffusion on the surfaces of black and blue P has relatively low energy barriers (<0.4 eV), and the single-layer blue P and doublelayer black and blue P possess high charge capacities.

Very recently, a new single-element two-dimensional (2D) material borophene was successfully grown on a silver surface under pristine ultrahigh vacuum conditions which attracts tremendous interest. In this paper, the lattice thermal conductivity, phonon lifetimes, thermal expansion and temperature dependent elastic moduli of borophene are systematically studied by using first-principles. Our simulations show that borophene possesses unique thermal properties. Strong phonon-phonon scattering is found in borophene, which results in its unexpectedly low lattice thermal conductivity. Thermal expansion coefficients along both the armchair and zigzag directions of borophene show impressive negative values. More strikingly, the elastic moduli are sizably strengthened as temperature increases, and the negative in-plane Poisson's ratios are found along both the armchair and zigzag directions at around 120 K. The mechanisms of these unique thermal properties are also discussed in this paper.

The heterostructured electrodes assembled with various two-dimensional materials break the limitation of the restricted properties of individual building blocks and combine the advantages of single material systems. Here, we design a novel blue phosphorus (BP)/borophene heterostructure by combing BP and borophene monolayers together. We investigate the adsorption and diffusion of Li along the outside surfaces and the interlayer of BP/borophene to assess its suitability as the Li-ion battery anode material. It is revealed that the BP/borophene heterostructure possesses excellent structural stability and high mechanical stiffness. In contrast to the semiconductor character of pristine BP monolayer, BP/borophene is metallic. The adsorption energy of Li on the BP side of the BP/borophene heterostructure is higher than that on the BP monolayer, and Li adsorption on the borophene side of the BP/borophene system is also stronger than that on the borophene monolayer. In addition, the BP/borophene heterostructure possesses a specific capacity of 1019 mA h g–1, which is larger than those of pristine BP monolayer and other BP-based heterostructures. Moreover, it is found that the energy barriers are relatively low as Li diffuses in the BP/borophene. Given these advantages, that is, large adsorption energies, low diffusive energy barriers, high capacity, and electrical conductivity, we conclude that the BP/borophene heterostructure can be an excellent candidate as anode material for Li-ion batteries.