  We predict the existence of new two dimensional silicon carbide nanostructure
employing ab initio density-functional theory calculations. These structures
are composed of tetragonal and hexagonal rings with C-C and Si-C bonds arranged
in a buckling plane. They are proven to be thermodynamically and mechanically
stable with relatively low formation energy, implying potential fabrication in
lab. They exhibit strong ductility and anisotropicity from their strain-stress
relation and directional dependence of mechanical moduli. The materials
maintain phonon stability upon the application of mechanical strain up to 27%
with fantastic ductile property. The SiC2 structure possesses a tiny direct
band gap of 0.02 eV predicted using HSE06 functional and the band gap can be
opened up through multiple approaches such as hydrogenation and strain
application. The gap values can be strategically tuned in the range of 0.02 ~
1.72 eV and the direct/indirect gap nature can be further manipulated. In
contrast, a closely related structure of SiC shows an indirect HSE band gap of
1.80 eV and strain engineering its value between 0.0 ~ 1.95 eV. The unique
properties in these newly proposed structures might have potential applications
in future nanomechanics and electronics.
