First-Principles Study of Topological Nodal Line Semimetal I229-Ge via Cluster Assembly.

Group IV element-based topological semimetals (TSMs) are pivotal for next-generation quantum devices due to their ultra-high carrier mobility and low-energy consumption. However, germanium (Ge)-based TSMs remain underexplored despite their compatibility with existing semiconductor technologies. Here, we propose a novel I229-Ge allotrope constructed via bottom-up cluster assembly that exhibits a unique porous spherical Fermi surface and strain-tunable topological robustness. First-principles calculations reveal that I229-Ge is a topological nodal line semimetal with exceptional mechanical anisotropy (Young's modulus ratio: 2.27) and ductility (/ = 2.21, = 0.30). Remarkably, the topological property persists under spin-orbit coupling (SOC) and tensile strain, while compressive strain induces a semiconductor transition (bandgap: 0.29 eV). Furthermore, I229-Ge demonstrates strong visible-light absorption (10 cm) and a strong strain-modulated infrared response, surpassing conventional Ge allotropes. These findings establish I229-Ge as a multifunctional platform for strain-engineered nanoelectronics and optoelectronic devices.