Morphology‐Dependent Magnetism in Nanographene: Beyond Nanoribbons

Abstract

Imprinting self‐sustainable magnetic features into graphene has recently generated much interest owing to its potential application in spintronics. Several strategies for imprinting magnetic features into graphene are proposed theoretically. However, only a few of them are realized experimentally. Here, the first scalable synthesis of magnetic graphene nanoplatelets with diverse morphologies, including nanoribbons and triangular, pentagonal, hexagonal, and other polyhedral shapes, is reported. This material enters the ferromagnetic regime at a temperature of ≈37 K with magnetization approaching ≈0.45 emu g−1 under high external magnetic fields. Theoretical calculations are used to explain this sort of morphology‐driven magnetism of graphene nanoplatelets, which emerges from the synergistic effects of the size, geometry of nanographenes, edge terminations, and angle between adjacent edges. In addition, they suggest a new way for preparing magnetically ordered graphene nanoplatelets with a higher transition temperature. In this respect, triangular motifs with zigzag edges represent the most promising morphology of graphene nanoplatelets, which can remain magnetically ordered up to ≈107 K. Based on these challenging results, further tuning of the size and morphology in spatially confined nanographenes combined with doping and sp3 functionalization will enable the preparation of magnetically ordered half‐metallic carbon sustainable up to room temperature, thus opening new opportunities in spintronics.

Zigzag sp2 Carbon Chains Passing through an sp3 Framework: A Driving Force toward Room-Temperature Ferromagnetic Graphene

“…60 A highly inhomogeneous fluorine coverage was found to produce multilayer islands, folds, wrinkles, and ripples, all containing a lower content of fluorine, consequently forming a conductive superstructure through which the charge transport, mediated by itinerant π-electrons, occurred preferentially. 60 61,62 (see Methods Section below for details) indicated that the ferromagnetically ordered state was stable up to temperatures exceeding 300 K, in good agreement with the experimentally derived TC. The electronic structure displayed as density of states (DOS) of the C18(OH)4F6 model provided insights into the magnetic exchange mechanism in the G(OH)F system (see Figure 4b).…”

“…The spontaneous formation of zigzag motifs during synthesis of G­(OH)F is supported by bond dissociation energies (BDEs) of fluorine atoms favoring defluorination along the motif (see Figure S26 in the Supporting Information). A transition temperature of 440 K estimated for C 18 (OH) 4 F 6 by using the Ising model on the honeycomb lattice , (see Methods section below for details) indicated that the ferromagnetically ordered state was stable up to temperatures exceeding 300 K, in good agreement with the experimentally derived T C . The electronic structure displayed as density of states (DOS) of the C 18 (OH) 4 F 6 model provided insights into the magnetic exchange mechanism in the G­(OH)F system (see Figure b).…”

“…Thus, our approach may provide an upper estimate of T C . However, besides inherent difficulties of DFT in describing the dynamic magnetic disorder in the paramagnetic state, such local disorder goes beyond the Ising model and has been shown to provide excellent agreement with the measured magnetic transition temperature …”

“…By relating the energy difference (Δ E ) between the ferromagnetic state and spin singlet with all electrons paired (typically higher in energy by up to several hundreds of meV depending on both the stoichiometry and structural features of the G­(OH)F system) to the Ising model of the honeycomb lattice, the transition temperature ( T C ) was estimated. , The exact solution for the honeycomb lattice resulted in k B T C /| J | = 0.3797, with J = Δ E /2 z for z pairs of spins on radical sites (with k B being the Boltzmann constant). In the Ising model, one usually relates the magnetic coupling constant ( J ) to the energy difference between the ferromagnetic and antiferromagnetic spin arrangements .…”

“…However, besides inherent difficulties of DFT in describing the dynamic magnetic disorder in the paramagnetic state, such local disorder goes beyond the Ising model and has been shown to provide excellent agreement with the measured magnetic transition temperature. 62 Quantum Mechanical Finite Model Calculations. The initial model structures of partially fluorinated graphene, i.e., C18F10, and C18(OH)4F6 for the kinetic stability study were extracted from the PW-DFT-PBC, and link hydrogen atoms were added to saturate any dangling bonds.…”