Si9C15 monolayer: A silicon carbide allotrope with remarkable physical properties

Zero band gap in graphene and silicene is an important obstacle to their application in nanodevices. Combining carbon and silicon atoms to form a silicon carbide (SixCy) sheet is an applicable proposal to solve the band gap issue. Very recently, a large-scale monolayer Si9C15 was successfully synthesized and exhibited suitable band gap and environment stability [Gao et al., Adv. Mater. 34, 2204779 (2022)]. Motivated by this recent experiment, we explore the electronic, mechanical, optical, and thermoelectric properties of a Si9C15 monolayer using first-principles calculations. Our results reveal that the monolayer is an auxetic material with a negative Poisson ratio of 0.175. The monolayer is a semiconductor with a direct band gap of 2.54 eV calculated by the Heyd-Scuseria-Ernzerhof functional (HSE06) functional. The optical properties of the monolayer are investigated by solving the Bethe-Salpeter equation. It is revealed that the optical band gap of the monolayer is 2.73 eV, and the exciton binding energy is equal to 1.14 eV, indicating a strongly bound bright exciton. Thermoelectric properties of the monolayer are calculated by combining density functional theory and Green's function formalism in the linear response regime. The lattice thermal conductance of the monolayer is 0.85 nW/K at room temperature, and the maximum figure of merit is 1.2 at 800 K, higher than that of other silicon carbide allotropes. High flexibility, a suitable band gap, high mobility, high optical absorption in the UV-visible region, and high thermoelectric efficiency make the Si9C15 monolayer a promising candidate for applications in mechanics, optoelectronics, and energy converters.