Broken edge spin symmetry induces a spin-polarized current in graphene nanoribbon

Abstract

Zigzag graphene nanoribbons (ZGNRs) are known to possess spin moments at the hydrogen-terminated edge carbon atoms; thus, spin-polarized electron transmission is expected, while the current is longitudinally passed through the ZGNRs. However, in pristine ZGNRs, spin-polarized transmission is not observed due to symmetric anti-parallel distributions of the spin densities between the edges. Here, the hypothesis is that any physical or chemical process that breaks such anti-parallel spin symmetry can induce spin-polarized transmission in ZGNRs. In this work, we have established this proof-of-concept by depositing the trimethylenemethane (TMM) radical on 6ZGNRH and investigating the quantum transport properties by employing density functional theory in conjunction with the nonequilibrium Green’s function method. Although TMM has a high magnetic moment ( 2μB ), it does not induce magnetization in 6ZGNRH when TMM is physisorbed. However, during the chemisorption of TMM, it forms the π−π bond with the 6ZGNRH in a particular geometric configuration, where the p z orbitals of carbon atoms of TMM have maximum overlap with the p z orbitals of carbon atoms of 6ZGNRH. The chemisorption of TMM transfers the spin moment to 6ZGNRH, which breaks the edge spin symmetry of pristine 6ZGNRH. The adsorption of the TMM radical results in transmission dips in the transmission spectra due to interference between localized states of TMM and 6ZGNRH states. This induces spin-polarized transmission with 60% spin-filtering efficiency at zero bias, which can further be enhanced up to 92% by applying a bias voltage of 1.0 V.