Two-Dimensional Tetrahex-GeC₂: A Material with Tunable Electronic and Optical Properties Combined with Ultrahigh Carrier Mobility

Based on first-principles calculations, we propose a novel two-dimensional (2D) germanium carbide, tetrahex-GeC₂, and determine its electronic and optical properties. Each Ge atom binds to four C atoms, in contrast to the known 2D hexagonal germanium carbides. Monolayer tetrahex-GeC₂ possesses a narrow direct band gap of 0.89 eV, which can be effectively tuned by applying strain and increasing the thickness. Its electron mobility is extraordinarily high (9.5 × 10⁴ cm²/(V s)), about 80 times that of monolayer black phosphorus. The optical absorption coefficient is ∼10⁶ cm^-1 in a wide spectral range from near-infrared to near-ultraviolet, comparable to perovskite solar cell materials. We obtain high cohesive energy (5.50 eV/atom), excellent stability, and small electron/hole effective mass (0.19/0.10 m₀). Tetrahex-GeC₂ turns out to be a very promising semiconductor for nanoelectronic, optoelectronic, and photovoltaic applications.