First-principles study of graphene nanoflakes with large spin property
Authors: Zhang Shu-Ting  , Sun Zhi  , Zhao Lei  ,   Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Heilongjiang Provincial Key Laboratory of Dielectric Engineering, School of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150080, China
Published: December 2020

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 -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 -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.