Inspired by recent experiments on the successful fabrication of monolayer Janus transition metal dichalcogenides [Nat. Nanotechnol. 12 (2017) 744] and ferromagnetic VSe 2 [Nat. Nanotechnol. 13 (2018) 289], here we for the first time predict a highly stable room temperature ferromagnetic Janus monolayer (VSSe) by ab initio evolutionary and density functional theory methods. Monolayer VSSe exhibits a large valley polarization due to the broken space and timereversal symmetry. Moreover, the low symmetry C3v point group of VSSe monolayer results in giant in-plane piezoelectric polarization. Most interestingly, a strain-driven 90° lattice rotation is occurred in magnetic VSSe monolayer with an extremely high reversal strain (73%), indicating an intrinsic ferroelasticity. The combination of multiferroic, piezoelectricity, and valley polarization will render magnetic 2D Janus VSSe for potential applications in nanoelectronics, optoelectronics and valleytronics.

Three-dimensional diborides MB2, featured in stacking the M layer above the middle of the honeycomb boron layer, have been extensively studied. However, little information on the two-dimensional counterparts of MB2 is available. Here, by means of evolutionary algorithm and first-principles calculations, we extensively studied the monolayer MB2 crystal with M elements ranging from group IIA to IVA covering 34 candidates. Our computations screened out eight stable monolayers MB2 (M = Be, Mg, Fe, Ti, Hf, V, Nb, Ta), and they exhibit Dirac-like band structures. Dramatically, among them, groups IVB–VB transition-metal diboride MB2 (M = Ti, Hf, V, Nb, Ta) are predicted to be a new class of auxetic materials. They harbor in-plane negative Poisson’s ratio (NPR) arising mainly from the orbital hybridization between M d and Boron p orbitals, which is distinct from previously reported auxetic materials. The unusual NPR and the Dirac transport channel of these materials are applicable to nanoelectronics and nanomechanics.

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In this study, two-dimensional (2D) and three-dimensional (3D) freestanding reduced graphene oxide-supported CuO composites (CuO-rGO) were synthesized via simple and cost-efficient hydrothermal and filtration strategies. The structural characterizations clearly showed that highly porous 3D graphene aerogel-supported CuO microcrystals (3D CuO-GA) have been successfully synthesized, and the CuO microcrystals are uniformly assembled in the 3D GA. Meanwhile, paper-like 2D reduced graphene oxide-supported CuO nanocrystals (2D CuO-rGO-P) have also been prepared by a filtration process. It was found that the products prepared from different precursors and methods exhibited different sensing performances for HO detection. The electrochemical measurements demonstrated that the 3D CuO-GA has high electrocatalytic activity for the HO reduction and excellent sensing performance for the electrochemical detection of HO with a detection limit of 0.37 μM and a linear detection range from 1.0 μM to 1.47 mM. Meanwhile, the 2D CuO-rGO-P structure also showed good electrochemical sensing performance toward HO detection with a much wider linear response over the concentration range from 5.0 μM to 10.56 mM. Compared to the previously reported sensing materials, the as-obtained 2D and 3D CuO-rGO materials exhibited higher electrochemical sensing properties toward the detection of HO with high sensitivity and selectivity. The 2D and 3D CuO-rGO composites also exhibited high sensing performance for the real-time detection of HO in human serum. The present study indicates that 2D and 3D graphene-CuO composites have promising applications in the fabrication of nonenzymatic electrochemical sensing devices.

Two-dimensional (2D) materials are promising for use in lithium (Li) electrodes due to their high surface ratio. By using density functional theory (DFT) calculations, we investigate the adsorption and diffusion of Li on a newly predicted 2D GeP material [Nano Lett., 2016, 17, 1833]. The most favourable adsorption sites for Li are identified, and a semiconducting to metallic transition induced by Li adsorption is found, which indicates excellent electrical conductivity. The GeP monolayer has an estimated capacity of 648 mA h g, which is almost twice that of commercially used graphite (375 mA h g). During full Li intercalation, the GeP layer undergoes only 1.2% lattice parameter reduction. Moreover, GeP possesses the advantages of a small diffusion barrier (∼0.5 eV) and low average open-circuit voltages (∼0.4 V). Our results highlight a new class of promising anode materials, i.e. 2D phosphide, as potential rechargeable lithium batteries with ultrahigh-capacity, superior ionic conductivity, and low average open-circuit voltage.

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The interlayer coupling in 2D van der Waals (vdW) heterostructures (HTS) plays the main role in generating new physics. However, the interlayer coupling is often weak, and little information on the strength of interlayer interaction in HTS is available. On the basis of density functional theory, we demonstrate that an effective electron coupling can be created in 2D CB/CN vdW HTS. The experimentally synthesized monolayers CB and CN are p- and n-type doped large gap semiconductors, respectively. However, the formed vdW HTS exhibits novel Dirac fermion. The strong interlayer electron coupling results in a large interlayer built-in electric field and improved optical properties of the 2D CB/CN vdW HTS. Moreover, a simple tight-binding model of CB/CN HTS with the nonzero interlayer hopping parameters captures the physical picture of interlayer coupling. Our results demonstrate the importance of interlayer electron coupling in the modulation of materials properties of 2D vdW HTS.