Two-Dimensional Kagome Monolayer Hf₃P₂X₆ (X = Cl, Br, I) with Robust Direct Band Gap and Ultrahigh Electron Mobility

As the cornerstone of the next generation of microelectronics and optoelectronics technology, two-dimensional (2D) semiconductors have shown great potential. In this study, we successfully predicted three highly stable 2D hafnium-based kagome monolayers, Hf3P2X6 (X = Cl, Br, I), through a synergistic approach combining first-principles calculations, surface passivation techniques, and charge balance strategies. Electronic structure analysis reveals that Hf3P2X6 exhibits direct band gap characteristics with values ranging from 0.74 to 1.10 eV (HSE06 + SOC level). Notably, these direct band gap features remain robust under strain engineering. Although Hf3P2X6 adopts a hafnium-based kagome lattice, its kagome-derived electronic bands are relatively weak due to an unusually large bond length ratio. Furthermore, deformation potential theory predicts ultrahigh electron mobility in Hf3P2X6, reaching up to 5.83 × 105 cm2 V–1 s–1, while hole mobility remains moderate (∼103 cm2 V–1 s–1). The excellent visible light absorption capability has also been confirmed by theoretical calculations. This study not only expands the exploration of 2D kagome materials but also highlights the potential of Hf3P2X6 for high-performance, low-dimensional optoelectronic devices.