Fluorination Effects in Hexagonal XY (X, Y = C, Ge, Si, Sn; X ≠ Y) Monolayers and vdW Heterostructures: Insights from Ab Initio Calculations.

By combining hybrid ab initio electronic-structure calculations that include spin-orbit coupling with phonon-spectrum calculations and molecular-dynamics simulations, we systematically investigate a family of hexagonal XY and fluorinated XYF2 monolayers (X, Y = C, Si, Ge, Sn; X ≠ Y). All compounds are confirmed to be dynamically and thermally stable, with fluorination being exothermic and further enhancing lattice robustness. Fluorination universally drives a transition from planar sp2 to buckled sp3 structures, which strongly affects the electronic structure. For instance, CSi, CGe, and CSn already display sizable gaps (3.28, 2.85, and 1.96 eV, respectively) that shift slightly upon fluorination to 3.21, 2.97, and 2.27 eV. In contrast, narrow-gap systems such as SiGe (0.19 eV) and SiSn (0.41 eV) exhibit pronounced bandgap openings to 1.43 and 1.02 eV, respectively, corresponding to relative increases of ∼653 and ∼149%. GeSn, however, remains a narrow-gap semiconductor (0.33 → 0.28 eV), showing little sensitivity to functionalization. We further investigate the corresponding van der Waals bilayers and trilayers and determine their absolute band-edge positions relative to the vacuum level to evaluate their potential for photocatalytic water splitting. For CSi+CSiF2, SiSn+SiSnF2, and GeSn+SiSnF2 heterobilayers, we identify both type-II and type-III alignments. In CSi-based trilayers, fluorination drives a systematic transformation from a graphitic sp2 lattice to a fully diamondized sp3 lattice, driving substantial hybridization changes and a pronounced reshaping of the band structure.