Strain driven negative Poisson's ratio and extraordinary light-harvesting response of Penta-BCN monolayer

We report here, structural, dynamic, and mechanical stability in pentagonal boron carbon nitride (p-BCN) monolayer, a new member of direct bandgap two-dimensional (2D) semiconductor. The identified visible range bandgap with excellent mechanical strength allows it to be a promising candidate material in optoelectronics and nanomechanics. By employing density functional theory (DFT), we reveal a unique geometrical reconstruction with rigidity in B$ - $N and C$ - $N bond lengths with applied strain. These quasi-sp$ ^3$ hybridized short and strong covalent bonding and unique geometry support the monolayer to possess extraordinary mechanical response. Remarkably, the very rare, negative Poisson's ratio (NPR), with softening and hardening, mechanical anisotropy to isotropy is achieved with the application of a small value of strain. Similarly, the desired bandgap is manipulated by loading the biaxial strain. Most importantly, our predictions on p-BCN show excellent optical response such as good static dielectric constant and refractive index, strong optical absorption (up to 1.08$\times$10$ ^5 $ $ {cm}^{-1}$ in VR and 7.01$\times$10$ ^5 $ $ {cm}^{-1}$ in UV) with small energy loss and reflectance both appearing in visible and ultraviolet regions (UV). The desired optical response along with the blue and red shift is demonstrated by tailoring with tensile and compressive strain. Additionally, the predicted strong optical anisotropy provides it's application in polarized photodetection.