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A two-dimensional carbon allotrope,H-net,is proposed using first principle calculations.H-net incorporates C4 distorted squares,C6 hexagons,and C8 octagons.Unlike previously reported planar graphene and other theoretical carbon sheets,H-net is a two-atom thick polymorph with identical C6+C4+C6 components cross-facing and covalently buckled to feature a handshake-like model.The feasibility of H-net is evident from its dynamic stability as confirmed by phonon-mode analysis and its lower total energy.H-net is energetically more favorable than synthesized graphdiyne and theoretical graphyne,BPC,S-graphene,polycyclic net,a-squarographite,and lithographite.We explored a possible route for the synthesis of H-net from graphene nanoribbons.Electronic band structure calculations indicated that H-net is a semiconductor with an indirect band gap of 2.11 eV,whereas graphene and many other two-dimensional carbon sheets are metallic.We also explored the electronic structure of one-dimensional nanoribbons derived from H-net.The narrowest H-net nanoribbon showed metallic behavior,whereas the other nanoribbons are semiconductors with band gaps that increase as the nanoribbons widen.H-net and its tailored nanoribbons are expected to possess more electronic properties than graphene because of their exceptional crystal structure and different energy band gaps.