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

Abstract

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.

High-temperature multigap superconductivity in two-dimensional metal-borides

“…Calculated lattice constant (a), electronic DOS at the Fermi level (N F ), average Fermi velocity (v F ), e-ph coupling (λ), logarithmic average of phonon frequencies (ω ln ), and Allen-Dynes superconducting transition temperature (T c ) for all the considered MBenes. lattice constants listed in the Supplementary Material are in good agreement both with available experimental values for bulk structures[18,26,34,56] and with calculated values for the same single-layer crystals in the literature[29,32,57]. The phonon dispersions presented in the Supplementary Material clearly point out the dynamical stability of all the considered crystals except BeB 2 , TiB 4 , ZrB 4 , HfB 4 , and ReB 4 .…”

“…Subsequently, we investigated the electronic properties of each material and we found nearly all the structures to be metallic. However, TiB 2 has a graphene-like band structure possessing a Dirac cone at the Fermi level as previously reported [57]. Also, BeB in terms of electronic properties, as one can see in Supplementary Material.…”

Giant negative Poisson's ratio in two-dimensional V-shaped materials

“…17 After the first realization of 2D auxetic effects in graphene with thermally-induced ripples, 18 a number of 2D materials have been reported to have NPR. [19][20][21][22][23][24][25][26][27][28][29][30] For example, monolayer black phosphorene with a buckled structure was predicted to have NPR of about -0.027 which has been confirmed in experiments. 11,19 Generally, the auxetic behaviors of 2D materials are correlated to the geometric structures, the deformation mechanisms of the materials dominated by the inter-atomic interaction.…”

“…[20][21][22][23][24][25][26][27][28] While in other cases, the auxetic behaviors are relevant to the electronic structures of 2D materials which determine the atomic interactions, rather than the geometry. [29][30][31][32] Strong auxetic effect will greatly improve the mechanical performance of materials, such as dentation resistance or hardness. However, the auxetic effects revealed in 2D materials remain very weak due to the low NPR values.…”

Efficient mechanical modulation of the phonon thermal conductivity of Mo6S6 nanowires

“…5b and d, respectively, both inner Mo and outer S atoms showed displacement vectors toward the outside of the nanowire when it was axially stretched, resulting in a negative Poisson's ratio. To date, negative Poisson's ratios have been reported for 2D materials [80][81][82][83][84][85][86][87] as well as artificially designed nanostructures; [87][88][89][90][91] however, 1D materials with negative Poisson's ratios have rarely been observed.…”