Spin Polarization and Transport Properties of Flat Bands in Covalently Functionalized Graphene-Based Junctions

Spin transport properties contributed by the flat bands have become a fascinating issue in device physics. Based on the first-principles method, we investigate the spin polarization and transport properties of covalently functionalized graphene sandwiched between two nonmagnetic electrodes with different contact sites. The analysis of electronic states demonstrates that the two spin-split flat bands originating from covalently functionalized graphene are severely perturbed when the horizontal distance between the Cu tip electrode and the functionalized C atom is less than the length of the C-C bond. The two flat bands degrade into partially polarized midgap states in the case of strong interactions between the Cu tip electrode and functionalized graphene. Spin-dependent transport calculations show that the flat bands may serve efficient transmission in all cases, while a large spin polarization of current of about 90% is achieved when the flat bands are weakly disturbed and a spin-split band gap is preserved. This work reveals the conditions for spin-split flat bands in covalently functionalized graphene in the situation of junctions and their potential prospects in spin-polarized transport in spintronic devices.