Predicting Novel 2D MB2 (M = Ti, Hf, V, Nb, Ta) Monolayers with Ultrafast Dirac Transport Channel and Electron-Orbital Controlled Negative Poisson's Ratio.

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.