Two-dimensional (2D) metal-diboride ZrB2 monolayers was predicted theoretically as a stable new electronic material [A. Lopez-Bezanilla, Phys. Rev. Mater., 2018, 2, 011002 (R)]. Here, we investigate its electronic transport properties along the zigzag (z-ZrB2) and armchair (a-ZrB2) directions, using the density functional theory and non-equilibrium Green's function methods. Under low biases, the 2D ZrB2 shows a similar electrical transport along zigzag and armchair directions as electric current propagates mostly via the metallic Zr-Zr bonds. However, it shows an electrical anistropy under high biases, and its I−V curves along zigzag and armchair directions diverge as the bias voltage is higher than 1.4 V, as more directional B-B transmission channels are opened. Importantly, both z-ZrB2 and a-ZrB2 show a pronounced negative differential conductance (NDC) effect and hence they can be promising for the use in NDC-based nanodevices.

The electronic transport and photoelectric properties of hydrogenated borophene B4H4, which was realized in a recent experiment by Nishino, et al. [J. Am. Chem. Soc. 139, 13761 (2017)], are systematically investigated using the density functional theory and non-equilibrium Green's function methods. We find that B4H4 exhibits a perfect current-limiting effect and has high (along the zigzag direction) and low (along the armchair one) optional levels due to its strong electrical anisotropy. Moreover, B4H4 can generate sizable photocurrents under illumination, with strong photoelectronic response to blue/green light along the zigzag/armchair direction. Our work demonstrates that B4H4 is promising for the applications of current limiter and photodetectors. 

Several layered transition metal borides can now be realized by a simple and general fabrication method [Fokwa et al., Adv. Mater. 2018, 30, 1704181], inspiring our interest to transition metal borides monolayer. Here, we predict a new two-dimensional (2D) transition metal diboride MoB2 monolayer (ML) and study its intrinsic mechanical, thermal, electronic, and transport properties. The MoB2 ML has isotropic mechanic properties along the zigzag and armchair directions with a large Young's stiffness, and has an ultralow room-temperature thermal conductivity. The Mo atoms dominate the metallic nature of MoB2 ML. It shows an obvious electrical anisotropy and a current-limiting behavior. Our findings suggest that MoB2 ML is a promising multifunctional material used in ultrathin high-strength mechanical materials, heat insulating materials, electrical-anisotropy-based materials, and current limiters.It is helpful for the experimentalists to further prepare and utilize the transition metal diboride 2D materials.

We study the electronic transport properties of two-dimensional (2D) fully-hydrogenated borophene (namely, borophane), using density functional theory and non-equilibrium Green's function approaches. Borophane shows a perfect electrical transport anisotropy and is promising for applications. Along the peak- or equivalently the valley-parallel direction, 2D borophane exhibits a metallic characteristic and its current-voltage (I-V) curve shows a linear behavior, corresponding to the ON state in borophane-based nano-switches. In this case, electrons mainly propagate via the B-B bonds along the linear boron chains. In contrast, electron transmission is almost forbidden along the perpendicular buckled direction (i.e., the OFF state), due to its semi-conductor property. Our work demonstrates that 2D borophane could combine metal and semiconductor features and may be a promising candidate for nano-switching materials with a stable structure and high ON/OFF ratio.