Correlation of Dirac ordering with optical activity in AlB2-type Dirac MBenes

Using first-principles calculations, this study systematically investigates the electronic properties and optical activity of AlB₂-type Dirac MBenes alongwith their correlations. Insights from phonon-spectral calculations andab-initiomolecular dynamics simulations substantiates the thermally and dynamically stable character of Dirac MBenes. Electronic dispersions reveals that all Dirac MBenes exhibits finitely gapped Dirac cones (DCs) at the fermi level, while FeB₂MBene behaves as a zero-band-gap semimetal akin to graphene. Such gap in DCs is desirable and crucial for optoelectronic applications. The interplay of out-of-planed_xzandd_yzorbitals of metal atom and hybrids in-plane dxyand dx2-y2orbitals from metal atom with p orbitals from boron atoms can be attributed to the emergence of DCs in MBenes. The calculations clearly reveal that the static dielectric constant and the energy gap within the DCs are critical factors influencing the electron-hole screening effect, consequently effecting the exciton binding energy. Further, it has been demonstrated that the exciton binding energies are consistent with predictions made by the two-dimensional (2D) Mott-Wannier model, confirming the Mott-Wannier characteristics of excitons in AlB₂-type Dirac MBenes, with the exception of partial Frenkel character in FeB₂MBene. Furthermore, it is demonstrated that Dirac MBenes exhibit a significant light absorption capacity in the near-infrared (NIR) and visible regions, with electron-hole interactions slightly modifying the optical spectral profile, making them promising for optoelectronic and photovoltaic applications. Subsequently, covariance analysis indicates that moderate energy gaps and high static dielectric constants are conducive to hosting Mott-Wannier excitons. Additionally, careful control of d-state valence electrons and hole doping can regulate the Mott-Wannier and Frenkel character of excitons in 2D Dirac materials. Thus, this comprehensive and systematic analysis of the electronic properties and optical-excitonic behavior of AlB₂-type Dirac MBenes, alongwith their correlations, enhances our understanding of this emerging family of 2D materials.