Designer spin order in diradical nanographenes

Abstract

The magnetic properties of carbon materials are at present the focus of intense research effort in physics, chemistry and materials science due to their potential applications in spintronics and quantum computing. Although the presence of spins in open-shell nanographenes has recently been confirmed, the ability to control magnetic coupling sign has remained elusive but highly desirable. Here, we demonstrate an effective approach of engineering magnetic ground states in atomically precise open-shell bipartite/nonbipartite nanographenes using combined scanning probe techniques and mean-field Hubbard model calculations. The magnetic coupling sign between two spins was controlled via breaking bipartite lattice symmetry of nanographenes. In addition, the exchange-interaction strength between two spins has been widely tuned by finely tailoring their spin density overlap, realizing a large exchange-interaction strength of 42 meV. Our demonstrated method provides ample opportunities for designer above-room-temperature magnetic phases and functionalities in graphene nanomaterials.

Renormalization of spin excitations and Kondo effect in open-shell nanographenes

“…This situation has changed dramatically with the advent of on-surface synthesis techniques [9][10][11] combined with surface scanning probe techniques, such as atomic force and scanning tunneling microscopes (AFM and STM). These techniques make it possible now to synthesize an increasing number of open-shell NGs, such as ribbons with zigzag edges [12], the Clar's goblet [13], rhombenes [14], triangulenes [15][16][17][18], heptauthrene [19], and others [20][21][22][23][24][25], and to probe their electronic properties with atomic scale resolution.…”

“…The latter is governed by the strength of the effective exchange interactions to the substrate. In the case of NGs deposited directly on gold [13,14,17,[20][21][22], broadening is definitely larger than Zeeman splitting, making it necessary to rely on the comparison with theory. The situation may be different in the case of the Kondo effect in S = 1 NGs where the narrow Kondo peak splits when the Zeeman energy exceeds the Kondo temperature.…”

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes

“…The formation of such NGs is attributed to the oxidative ring closure of the majority of the methyl groups, together with the dissociation of several of them per NG prior to cyclization. Such removal of methyl moieties was previously reported in the synthesis of several NGs [30,37,[42][43][44][45] and GNRs [22,46] and is concomitant to the annealing step at temperatures where the oxidative ring closure is expected to occur on Au(111), therefore being not possible to achieve the synthesis of E.…”

“…For instance, the nature of the electronic ground state of long pursued members of the acene [14][15][16][17][18][19] and triangulene [20,21] families, widely discussed in theoretical studies, have only recently been untangled. Contemporarily, a successful strategy toward the on-surface formation of open-shell NGs, one-dimensional polymers and graphene nanoribbons (GNRs) is the surface-assisted oxidative ring closure between a methyl group and the neighboring aryl moiety of a properly predesigned molecular precursor, which occurs after thermal activation [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Such synthetic approach, though often successful, presents some limitations related to the cleavage of methyl groups prior to cyclization, which may lead to the formation of topological defects inducing an open-shell ground state to NGs [30,37].…”

Bottom‐Up Synthesized Nanoporous Graphene Transistors

“…[42,43] A Hubbard parameter U = 3.5 eV was used to account for electronelectron interaction. [44] In the implementation of the NEGF procedure, the self-consistent spin Hamiltonians from the MFH model were used as the system was connected to leads through coupling matrices Γ A , Γ B , Γ C , and Γ D , representing the respective rubicene interfaces. These were defined in a wide-band limit (WBL) as Γ mn = (1 eV) δ mn .…”