# First-Principles Study of Topological Nodal Line Semimetal I229-Ge48 via Cluster Assembly

## TL;DR

A new germanium-based topological semimetal is proposed with unique properties for quantum devices and optoelectronics.

## Contribution

A novel I229-Ge48 allotrope is introduced with strain-tunable topological robustness and multifunctional properties.

## Key findings

- I229-Ge48 is a topological nodal line semimetal with a porous spherical Fermi surface.
- It shows strain-induced semiconductor transition and strong visible-light absorption.
- The material exhibits a strong strain-modulated infrared response and mechanical anisotropy.

## Abstract

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-Ge48 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-Ge48 is a topological nodal line semimetal with exceptional mechanical anisotropy (Young’s modulus ratio: 2.27) and ductility (B/G = 2.21, ν = 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-Ge48 demonstrates strong visible-light absorption (105 cm−1) and a strong strain-modulated infrared response, surpassing conventional Ge allotropes. These findings establish I229-Ge48 as a multifunctional platform for strain-engineered nanoelectronics and optoelectronic devices.