Benzene-like N6 rings in a Be2N6 monolayer: a stable 2D semiconductor with high carrier mobility

Abstract

Designing new two-dimensional (2D) semiconductors with high carrier mobilities is highly desirable for material innovation, especially when the configuration has novel topological properties. Here, we proposed a first-principles-based design of a 2D crystal, namely a Be₂N₆ monolayer. In which, each N atom is shared by two neighboring N atoms and one Be atom, forming a novel moiety of benzene-like N₆ rings. Rather than the instability of hexazine, the Be₂N₆ monolayer has a moderate cohesive energy, good kinetic and thermodynamic stability, due to the stabilization effect of the Be element by forming twelve classical two-centre–two-electron (2c–2e) σ-bonds and five multicenter 6c–2e π-bonds. There are ten π electrons in a unit cell, which satisfies the Hückel rule [4n + 2] (n = 2), indicating the Be₂N₆ monolayer aromaticity. As a result, the Be₂N₆ monolayer has an ultra-high mechanical strength of up to 200 J m⁻². Particle-swarm optimization (PSO) computations reveal that a cyclo-N₆-containing Be₂N₆ monolayer is the lowest-energy configuration in 2D forms with a stoichiometry of 1 : 3, and therefore could be synthesized experimentally. Furthermore, the Be₂N₆ monolayer is an indirect semiconductor with a band gap of 1.71 eV at the hybrid functional level, close to that of the bulk amorphous silicon ∼1.6 eV widely used in solar cells. At this point, the high electron mobility of up to ∼10⁴ cm² V⁻¹ s⁻¹ and visible-light absorption of ∼10⁵ cm⁻¹ are observed for the Be₂N₆ monolayer. If realized, it will not only enrich the knowledge of the bonding nature of nitrogen but could also have potential applications in electronics and optoelectronics.