Structural properties and strain engineering of a BeB2 monolayer from first-principles

Boron-based two-dimensional materials have extremely rich structures and excellent physical properties. Using a particle-swarm optimization (PSO) method and first-principles calculations, we performed a comprehensive search for the structure of a two-dimensional BeB2 monolayer. We found new configurations with lower energy compared with the previously reported alpha phase, namely the beta, gamma, and delta structures. Among those structures, the gamma phase is found to have the lowest energy and we examined its dynamic as well as its thermodynamic stabilities. Then through strain engineering, we found a metal-semimetal transition in the alpha phase (under about 5% biaxial compressive strain) and in the delta phase (under about 3.2% and 7% biaxial tensile strain). As the compressive strain increases to 7%, the BeB2 sheets of the beta phase and gamma phase strongly twist, becoming more stable than the delta system. More interestingly, we found that Be atoms could penetrate the B atomic layer in the gamma system under 2.5% tensile strain. All the predicted effects demonstrate the rich physical properties of the two-dimensional BeB2 monolayer.