Comparative study of the structural and optoelectronic properties of covalently bonded T-graphene-like bilayer nanoflakes: an ab initio investigation.

The exploration of novel two-dimensional (2D) materials with tunable properties is critical for advancing next-generation nano-electronics. In this study, we employ dispersion-corrected B3LYP-D3 ab initio calculations with HSEH1PBE single-point refinement to investigate the structural, electronic, and optical properties of novel tetragonal carbon-based molecular bilayers (MBLs), pristine TG, TBC2N, and heterostructures TG/TBN and TG/TBC2N. Geometry optimizations and frequency analysis confirm that all MBLs are dynamically and energetically stable. FMO analysis shows HOMOs localized on carbon-rich domains and LUMOs on B/N regions, indicating potential for directional charge transfer. Electronic property analyses demonstrate that all systems are semiconductors. The electronic energy gaps are highly tunable, ranging from 0.323 eV to 1.848 eV. MBL-TBC2N exhibits the highest reactivity, as shown by its low global hardness. UV-vis analysis confirms strong absorption in the UV and visible regions, with heteroatom doping and stacking inducing a significant redshift that broadens absorption. A critical finding from QTAIM analysis reveals the strong covalent bonds complemented by π-π* stacking and Reduced Density Gradient (RDG). These results highlight that these covalently-bonded MBLs are robust materials with highly tunable properties, possibly making them promising candidates for applications in sensing, catalysis, and nano-optoelectronics.