Zigzag double-chain C–Be nanoribbon featuring planar pentacoordinate carbons and ribbon aromaticity 

   Low-dimensional materials (LDMs) involving planar hypercoordinate carbon bonding were predicted to have applications in electronic devices, energy materials, and optical materials, etc. The majority of carbon atoms in such LDMs adopt a tetracoordinate structure, while examples with a higher coordination number are extremely rare and the bonding geometries of those carbons are not perfectly planar. In this work, we designed ribbon-like clusters C_nBe_3n+2H_2n+2²⁺ with planar pentacoordinate carbons (ppCs) and extended the corresponding structural model under 1D periodic boundary conditions (PBCs), leading to a zigzag double-chain C–Be nanoribbon. The beryllium atoms in such a nanoribbon arrange in a cosine shape around the perfect ppCs, which are unprecedented in LDMs. Detailed analyses revealed that the perfect ppC structure in the nanoribbon was geometrically achieved by opening a Be–Be edge of small Be 5 rings, thereby making the intra-ring space adjustable to fit the size of the carbons. Electronically, the structure is stabilized by a favourable sandwich type charge distribution and satisfaction of the octet rule for ppCs. Note that all the valence electrons in the nanoribbon are locally delocalized within each ppC moiety, representing a new type of ribbon aromaticity, which should be useful in nanoelectronics. The nanoribbon and its cluster precursor C₂Be₈H₆²⁺ are thermodynamically stable, and are promising targets for experimental realization. The nanoribbon was predicted to be an indirect band gap semiconductor; thus it has potential applications in designing light-weight electronic devices.