Near-zero Poisson's ratio monolayer bismuthene

First-principles calculations identify monolayer P2/m bismuthene (P2/m-Bi) as a rare 2D material that combines pronounced mechanical anisotropy with a highly tunable electronic structure. This system exhibits strong mechanical anisotropy: uniaxial strain along the armchair (X) direction yields a conventional out-of-plane Poisson's ratio of 0.737, in marked contrast to the anomalous, nearly zero value of 0.056 for strain along the zigzag (Y) direction. In addition to its unconventional mechanical response, the electronic band gap can be continuously tuned from 0.12 to 0.39 eV under uniaxial strain between -5% and 5% applied along either the X or Y direction. Most notably, the material undergoes a reversible, strain-induced transition from an indirect- to a direct-band-gap semiconductor. The interplay between its anisotropic mechanical response and highly tunable electronic properties highlights its potential for flexible optoelectronic applications. Overall, these results provide essential theoretical insights to guide the future experimental realization of P2/m-Bi.