A DFT Study on the Mechanical, Electronic, Thermodynamic, and Optical Properties of GaN and AlN Counterparts of Biphenylene Network

The biphenylene network (BPN) is a notable achievement in recent fabrication endeavors for conceiving new 2D materials. The stability of its boron nitride counterpart, BN-BPN, has been confirmed through numerical investigations. In this study, we conducted a density functional theory (DFT) analysis to examine the mechanical, electronic, thermodynamic, and optical properties of two other group-III counterparts of BPN: gallium nitride (BPN-GaN) and aluminum nitride (BPN-AlN). Our findings reveal that the band gap values for BPN-GaN and BPN-AlN are 2.3 eV and 3.2 eV, respectively, at the HSE06 level. At the GGA/PBE level, we found band gap values of 1.8 eV and 2.3 eV for BPN-GaN and BPN-AlN, respectively. Phonon calculations and ab initio molecular dynamics (AIMD) simulations suggest that BPN-AlN has good structural and dynamic stabilities. On the other hand, BPN-GaN displayed negative phonon frequencies, suggesting potential instability. Nevertheless, results from AIMD simulations point to its structural integrity with no bond reconstructions at 1000 K. These materials exhibit noteworthy UV activity, presenting promising prospects as UV collectors. The thermodynamic properties reveal that the heat capacity of both BPN-AlN and BPN-GaN increases with temperature, eventually reaching the Dulong-Petit limit at around 800 K. We also performed calculations to determine the elastic stiffness constants, Young's modulus, and Poisson ratio for both BPN-GaN and BPN-AlN, providing valuable insights into their mechanical properties.