Bipolar magnetic semiconductors emerging in graphene nanoribbons with zigzag edges and internal defects

Bipolar magnetic semiconductors (BMSs) enable controlled spin polarization for advanced spintronic applications. However, their applications are greatly hindered by the limited number of available BMS materials. We demonstrate a family of egg-tray graphene nanoribbons (EGNRs) featuring both zigzag edges and internal defects (pentagons and heptagons) constructed by slicing egg-tray graphene. First-principles calculations show that these EGNRs exhibit distinguished BMS behavior, along with other spintronic properties such as half-metallicity and spin-gapless semiconductivity (SGS). Most notably, when the width of EGNRs is small, the internal defect regions significantly influence the spin density distribution of the entire ribbon, resulting in spin polarization not only localized at the edge carbon atoms but also across the defect areas. The synergistic effect between these distinct regions promotes the formation of stable BMSs. The calculations also show a gradual transition from BMS to semiconductor or half-metal, controlled by the edge states while the width of EGNRs increases. Furthermore, including pentagons and heptagons at the edges of EGNRs compresses BMS behavior, transforming the material into an SGS. In this paper, we provide a route to achieve and control BMSs in a family of carbon nanoribbon materials based on the combination of edge cutting and topology control of graphene materials.