2D hafnium halides and hetero halides: Bridging topological properties and quantum capacitance

Two-dimensional (2D) topological insulators (TIs) have become an intriguing family of materials due to their potential applications in spintronics, quantum computing, and nanoelectronics, as well as their resilient edge states shielded by time-reversal symmetry. The structural, electronic, optical and topological characteristics of hafnium halides and their hetero halides (${\text{Hf}}_2{X}_2$, where X = one or two halogen from Cl, Br and I) are examined in this work. We verify their non-trivial topological character by analyzing their stability, band structures, and topological invariants using first-principles density functional theory (DFT) computations. Our findings show that halide composition and spin-orbit coupling (SOC) have a major impact on band inversion. The topological insulator nature is confirmed by the presence of gapless edge states and the computed z2 invariant. Optical properties of these materials, such as their absorption spectra and dielectric function, demonstrate their potential for use in optoelectronics. Additionally, calculations of quantum capacitance show that it has a large capacity for storing charge. This thorough investigation of hafnium halides and hafnium hetero halides identifies them as viable options for 2D TIs with a variety of applications in optoelectronics and energy storage.