Theoretical Insights into Two-Dimensional Semiconducting Transition Metal Borides: MB₉ (M = Ag and Tm)

In recent years, borophene has garnered significant attention due to its diverse physical and chemical properties. However, most known allotropes of borophene exhibit metallic or semimetallic characteristics. This study employs first-principles calculations and an evolutionary algorithm to predict a novel 2D B₉’ borophene structure, which displays impressive thermal, dynamic, and mechanical stability, suggesting its viability at room temperature. Subsequently, we introduced transition metal (M) atoms into the predicted borophene structures, resulting in a variety of MB₉ configurations. Through high-throughput screening, we identified two stable semiconducting transition metal borophenes: AgB₉ and TmB₉. In these structures, the Ag and Tm atoms occupy the centers of the hexagonal or nonagonal boron vacancies. Electronic structure calculations reveal that AgB₉ has an indirect band gap of 0.58 eV, while TmB₉ features a direct band gap of 0.72 eV. Notably, both AgB₉ and TmB₉ exhibit high carrier mobility, with TmB₉ achieving a remarkable carrier mobility of up to 11,391 cm² V^-1 s^-1. Compared to 2D materials such as MoS₂, h-BN, BAs, and BSb, AgB₉ and TmB₉ stand out due to their narrow band gaps and high carrier mobility, indicating promising applications in electronic devices.