First principles study of flat silicon rich 2D alloys Si_xBe_y

Silicon-based semiconductors underpin modern electronics due to their exceptional electrical-optical performance, and two-dimensional (2D) systems offer promising alternatives for miniaturization in pursuit of next-generation materials. Here, we present a computational study on five planar silicon-rich 2D alloys consisting of silicon and beryllium (Si_xBe_y; x > y), structurally derived from silicon analogs of benzene, naphthalene, anthracene, pyrene, and coronene, interconnected via beryllium atoms. We investigate their structural flatness, dynamical and thermal stability, electronic and mechanical properties. Four of the five alloys exhibit planar geometries as local minima without imaginary frequency at the Γ point, a key factor for air stability. Notably, we achieve direct band gaps in these materials, contrasting with the indirect gap of bulk silicon and the zero-gap nature of silicene. Phonon dispersion and ab initio molecular dynamics simulations confirm the dynamical and thermal stability of the direct-gap semiconductor. Additionally, their reduced in-plane Young's modulus and comparable minimum thermal conductivity compared to graphene suggest high mechanical flexibility and efficient heat dissipation. These findings highlight the potential of flat, air-stable, free-standing silicon-based monolayers with direct band gaps, paving the way for miniaturized silicon-based electronics beyond the Moore era.