Strong Edge Magnetism and Tunable Energy Gaps in Zigzag Graphene Quantum Dots with High Stability

Graphene is a nonmagnetic semimetal and cannot be directly used as electronic or spintronic devices. We demonstrate that graphene quantum dots (GQDs) can exhibit strong edge magnetism and tunable energy gaps due to the presence of localized edge states. By using large-scale first principle density functional theory (DFT) calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GQDs become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron-electron interactions along the edges of GQDs, ultimately resulting in a magnetic phase transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm. Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GQDs is much stronger than that exists between two parallel zigzag edges in GQDs and graphene nanoribbons. Furthermore, such strong and localized edge states also induce GQDs semiconducting with tunable energy gaps, mainly controlled by adjusting the system size. Our results show that the quantum confinement effect, inter-edge superexchange (antiferromagnetic), and intra-edge direct exchange (ferromagnetic) interactions are crucial for the electronic and magnetic properties of zigzag GQDs at the nanoscale.