Novel 3D Pentagraphene Allotropes: Stability, Electronic, Mechanical, and Optical Properties

Abstract: Carbon-based materials have attracted great attention due to their exceptional structural diversity and wide-ranging applications. Recently, a new two-dimensional carbon allotrope, named pentagraphene (PG), was proposed. In this study, we proposed three novel three-dimensional (3D) PG allotropes, named 3D-PG-$\alpha$, -$\beta$, and -$\gamma$, engineered through biaxial strain and controlled compression of 2D PG layers. Comprehensive stability analyses, including phonon dispersion and ab initio molecular dynamics simulations (AIMD), confirm their thermodynamic stability under room and high-temperature conditions. 3D-PG-$\alpha$ is the most stable, exhibiting a cohesive energy 0.5 eV/atom lower than the least stable structure, 3D-PG-$\gamma$. Electronic property characterization reveals semiconducting behavior for all structures, with indirect electronic band gaps ranging from 0.91 to 2.67 eV. The analyses of the mechanical properties showed significant anisotropy, with higher stiffness along the in-plane ($xy$-plane) direction. Optical properties highlight strong absorption along a wide range and a pronounced anisotropic response. Additionally, the absorption spectra exhibit activity in the visible region, and the refractive index and reflectivity indicate potential use in ultraviolet-blocking devices.