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Carbon nitride two-dimensional (2D) materials are among the most attractive class of nanomaterials, with wide range of application prospects. As a continuous progress, most recently, two novel carbon nitride 2D lattices of C 3 N 5 and C 3 N 4 have been successfully experimentally realized. Motivated by these latest accomplishments and also by taking into account the well-known C 3 N 4 triazine-based graphitic carbon nitride structures, we predicted two novel C 3 N 6 and C 3 N 4 counterparts. We then conducted extensive density functional theory simulations to explore the thermal stability, mechanical, electronic and optical properties of these novel nanoporous carbon-nitride nanosheets. According to our results all studied nanosheets are found to exhibit desirable thermal stability and mechanical properties. Non-equilibrium molecular dynamics simulations on the basis of machine learning interatomic potentials predict ultralow thermal conductivities for these novel nanosheets. Electronic structure analyses confirm direct band gap semiconducting electronic character and optical calculations reveal the ability of these novel 2D systems to adsorb visible range of light. Extensive first-principles based results by this study provide a comprehensive vision on the stability, mechanical, electronic and optical responses of C 3 N 4 , C 3 N 5 and C 3 N 6 as novel 2D semiconductors and suggest them as promising candidates for the design of advanced nanoelectronics and energy storage/conversion systems. Image 1 [ABSTRACT FROM AUTHOR]

• The stability of H-, F- and Cl-diamanes 2D lattices is examined. • Mechanical and electronic properties of H-, F- and Cl-diamanes are studied. • Effects of number of carbon atomic layers on the intrinsic properties are explored. Most recently, fluorinated diamond monolayer so called "F-diamane" has been successfully experimentally realized via fluorination of bilayer-graphene (Nat. Nanotechnol. 15 (2020), 59–66). Motivated by this experimental advance, we conduct density functional theory simulations to explore the stability, mechanical, electronic and optical properties of diamane nanosheets. In this work we consider diamane nanosheets functionalized with H, F and Cl atoms. We particularly examine thickness effect on the properties of functionalized diamane nanomembranes. According to phonon dispersions and ab-initio molecular dynamics results, thermal and dynamical stability of studied systems are confirmed. It is found that by increasing the number of carbon layers in diamane the elastic modulus and tensile strength increase. Analysis of electronic band-structures reveal that while H- and F-diamane are wide band gap semiconductors and insulators, respectively, Cl-diamanes are direct band gap semiconductors, highly promising for nanoelectronics applications. Optical calculations show that Cl-diamane can absorb the visible light whereas H and F counterparts absorb ultraviolet range of light. First-principle results by this study provide a comprehensive vision on the thickness dependent mechanical, electronic and optical responses of functionalized diamane nanosheets and can serve as a useful guide for future studies. [ABSTRACT FROM AUTHOR]