Geometric and electronic structures of sulfur-edge-terminated zigzag edge graphene nanoribbons

The stable geometric and electronic structures of the fully and half sulfur-edge-functionalized ZGNRs at their widths of four zigzag carbon chains (S-4-ZGNRs) have been studied by using the ab initio density-functional method. It is found from our calculations that (1) under the periodic boundary condition, the two-dimensional plane structures are the most stable ground states in all the possible isomers of the S-4-ZGNRs at both 100% and 50% terminations, which are all metallic. (2) A much delocalized characteristic S-px lone-pair electronʹs band crossing its Fermi level appears in the case of fully S-edge-termination, which is more extended in a large energy range of over 8.0 eV, in contrast to the corresponding oxygen-px (O-px) band of the O-4-ZGNR, covering only a small energy range of 3.2 eV. In the case of half S-edge-termination, however, the S-px band is found to be much more localized, which forms two almost flat bands at about+3.0 eV above its Fermi level. (3) More interestingly, at 50% S-edge-termination, a flat portion of the π-electron edge states is found to lie a little bit below its Fermi level, making its unpolarized ground state unstable. And thus the spin-polarized antiferromagnetic (AFM) state is found to be the real ground state of the half S-4-ZGNR, which is a semiconductor with an indirect energy gap of about 0.16 eV. In the AFM ground state, there exists magnetic moments of about 0.2μB on each edge carbon atom, which is FM coupling along the same edge, but AFM coupling between its two edges.