Two‐Dimensional Aluminum Boride AlB₄ Nanosheets as a Platform to Architecture Very Rare Semiconducting Phase and Superconductor

ABSTRACT Semiconducting phase is extremely rare and difficult to be realized in two‐dimensional (2D) aluminum borides. Here, we for the first time report the discovery of a rarely semiconducting allotrope (labeled as AlB 4 ‐1) in 2D AlB 4 nanosheets. This semiconductor is the global minimum structure in 2D space with two layers stacked together connected by strong Al‐B bonds. Systematic studies demonstrate the high thermodynamic, lattice dynamic, thermal, and mechanical stabilities of AlB 4 ‐1. More importantly, this semiconducting AlB 4 ‐1 shows fascinating properties and promising applications, such as, the optimal band gap (1.156 eV at HSE06 level), high carrier mobility (up to 3.14 × 10 3 cm 2 V −1 s −1 ), substantially high solar energy conversion efficiency (21.9 %) and large optical response (10 6 cm −1 ) in the visible region. Extensive studies indiate that h‐BN can serve as an effective substrate to support and encapsulate AlB 4 ‐1 with minimal impact on the electronic properties of AlB 4 ‐1, laying the foundation for the real application of AlB 4 ‐1 in electronic devices. Besides this semiconducting phase, other low‐lying allotropes (AlB 4 ‐2 to ‐11) also display high stabilities, exotic properties and diverse applications. For example, the metallic AlB 4 ‐4 shows Dirac cone near Fermi level and superconductivity with T C as high as 23.4K, which can be substantially enhanced to 34.1K at tensile strain of 11%. These allotropes with different shapes show diverse hypercoordinate motifs with unusual bonding patterns. Comprehensive studies demonstrate that 2D AlB 4 nanosheets is a class of highly stable, multifunctional nanomaterials for diverse applications in electronics, optics, optoelectronics, nanodevices, solar energy conversion, superconductivity, nanomechanics, and so on. The present study will provide useful guidance in fabricating these interesting nanostructures and stimulate both experimental and computational efforts in this direction.