石墨烯纳米片大自旋特性第一性原理研究

通过基于密度泛函理论的全电子数值轨道第一性原理电子结构计算,研究了各种形状有限石墨烯片段(石墨烯纳米片, GNF)的磁特性,证明GNF的自旋磁有序来源于由其形状决定的π键拓扑挫折(topological frustration)作用.锯齿形边缘的三角形GNF的净自旋不为零,如同一个人造铁磁性原子团,总自旋随尺度线性增加.根据拓扑挫折原理,可以在GNF中引入较大的净自旋和独特的自旋分布,如由三角形GNF单元构成的复杂分形结构,总自旋随分形级数呈指数上升.通过刻蚀技术制作具有一定拓扑结构的GNF可以实现可控自旋电子纳米材料和器件应用.

First-principles study of graphene nanoflakes with large spin property

Based on density functional theory, the extraordinary magnetic properties of finite graphene fragments (graphene nanoflake, GNF) with different shapes are studied by first-principles electronic structure calculations with all electron numerical-orbital basis set scheme as implemented in DMol3 code of Materials Studio 8.0 software package. According to the graph theory, the spin characteristics of several typical hydrogen-terminated GNF shaped into 3-fold and 6-fold highly rotational symmetries as well as two specific geometrical structures related to graphene nanoribbon are analyzed and verified by first-principles calculations. In some characteristic GNFs shaped into a singular graph, the electron energy matrix becomes singular and multiple states of zero eigenvalue appear which is called nonbonding state (NBS). In these singular graph structures, all the π-bonds cannot be satisfied simultaneously and spin-aligned singly occupied molecular orbitals are generated from degeneracy at Fermi-level, which means that the topological frustration occurs. It is proved that the electronic spin magnetic order of GNF originates from topological frustration of conjugate π-bonds determined by its shape. The net spin of triangular GNF with zigzag edges is not zero, like an artificial ferromagnetic atom, increasing proportionally with its linear dimension. According to the principle of topological frustration, the large net spins and specific spin distributions can be reasonably introduced into graphene nanocrystals, such as by triangulation. The NBSs of zigzag-edged triangular GNFs with nanoscale dimension create 0.4-0.7 eV energy gaps at Fermi-level to achieve stable spin-alignment at ambient temperature. Even though the linear size of zigzag-edged triangular GNF increases beyond nanoscale, the maximum energy gap is still ~0.68 eV and thus the magnetic moment measurement is feasible at room ambient temperature. The total spin of the complex fractal structure constructed by zigzag-edged triangular GNF unit increases exponentially with the fractal level, due to the increased possibility of topological frustration from aggrandizing boundary. In addition, a large net spin can also be acquired by hollowed-out zigzag triangle in graphene with a net spin value of at least 1.00 and a spin-polarization split band gap of ~0.40 eV. The basic design principle for obtaining large spin and controlling spin state distribution by etching GNF of various patterns in graphene atomic layer is presented. In contrast to traditional chemical synthesis of obtaining large spin limited by complicated reaction pathways, the GNF with large spin easily exceeding the reported highest spin in conjugated polymers can be practically produced by carving lithography. It is suggested that the GNF with designed topological structures fabricated by pattern carving technique can be efficiently used to realize the controllable spintronic nanomaterials and devices.