Graphene is a nonmagnetic semimetal and cannot be directly used as electronic and spintronic devices. Here, we demonstrate that zigzag graphene nanoflakes (GNFs), also known as graphene quantum dots, 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 calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GNFs (\({\mathrm{C}}_{6n^2}\)H_6n, n = 1–25) become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron–electron interactions along the edges of GNFs, ultimately resulting in a magnetic configuration transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm (C₄₈₀H₆₀). Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GNFs at the nanoscale (around 10 nm) can be stabilized at room temperature and is much stronger than that exists between two parallel zigzag edges in graphene nanoribbons, which cannot be stabilized at ultra-low temperature (3 K). Furthermore, such strong and localized edge states also induce GNFs 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 GNFs at the nanoscale.

石墨烯是一种非磁性的半金属，不能直接用作电子和自旋电子器件。在这里，我们证明了之字形石墨烯纳米薄片（GNF），也称为石墨烯量子点，由于存在局部边缘态，因此可以表现出强大的边缘磁性和可调节的能隙。通过大规模的第一原理密度泛函理论计算和基于模型哈密顿量的详细分析，我们可以证明GNF（\（{\ mathrm {C}} _​​ {6n ^ 2} \） H _{6 n}，n = 1–25）随着系统规模的增加而变得更加强大和更加本地化。当直径大于4.5 nm（C ₄₈₀ H _60）时，增强的边缘态会沿着GNF的边缘引起强烈的电子-电子相互作用，最终导致磁性结构从非磁性转变为边缘内铁磁性和边缘间反铁磁性）。我们的分析表明，纳米级（约10 nm）的GNF中两个最相邻的之字形边缘之间的反铁磁态的边缘间超交换相互作用可以在室温下稳定，并且比石墨烯中两个平行的之字形边缘之间存在的强得多。纳米带，在超低温（3 K）下无法稳定。此外，这种强而局限的边缘状态还诱导了具有可调能隙的GNF半导体，主要是通过调节系统尺寸来控制的。我们的结果表明，量子限制效应，边缘间的超交换（反铁磁）和边缘内的直接交换（铁磁）相互作用对于锯齿形GNF在纳米级的电子和磁性至关重要。