Two-Dimensional Graphullerene C24: Strain-Induced Tunable Electronic and Optical Properties for Applications in Wearable UV-Protective Devices

The synthesis of C60-based two-dimensional (2D) fullerene networks has attracted wide interest in graphullerene. In this study, based on the C24 fullerenes, we theoretically construct quasi-tetragonal 2D graphullerene and its three-dimensional (3D) structures, denoted as GrF-C24 and 3D-C24, respectively. First-principles calculations reveal that both structures release energy during polymerization, together with double [2 + 2] cycloaddition bonding between C24s, promoting better stability. The 2D GrF-C24 possesses a moderate indirect band gap and is highly sensitive to external stress: under compressive stress, it transforms into a direct band gap semiconductor, with the band gap decreasing linearly as strain increases; under tensile strain, it maintains an indirect band gap, which initially increases and then sharply decreases with stress. In terms of optical properties, GrF-C24 exhibits nearly 100% transmittance for infrared light but relatively significant absorption in the ultraviolet (UV) range. Under stress, the optical absorption peak undergoes a blue shift, since the band gap becomes narrow. Combined with a small Young's modulus of 235 GPa, GrF-C24 can serve as a wearable, flexible UV-protective device. Meanwhile, 3D-C24 represents a lightweight, superhard material with a large insulating band gap. Our research further enriches the graphullerene family and provides a foundation for the application of graphullerene C24.