Magnetic and electronic crossovers in graphene nanoflakes

Abstract

Manipulation of intrinsic magnetic and electronic structures of graphene nanoflakes is of technological importance. Here we carry out systematic study of the magnetic and electronic phases, and its manipulation in graphene nanoflakes employing first-principles calculation. We illustrate the intricate shape and size dependence on the magnetic and electronic properties, and further investigate the effects of carrier doping, which could be tuned by gate voltage. A transition from nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance, which we identify to be originated from the armchair defect at the junctions of two sublattices on the edge. Electron, or hole doping simultaneously influences the magnetic and electronic structures, and triggers phase changes. Beyond a critical doping, crossover from antiferromagnetic to ferromagnetic phase is observed for the flakes without sublattice imbalance, while suppression of magnetism, and a possible crossover from magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous to magnetic phase changes, a semiconductor to (half) metal transition is observed, upon carrier doping.

Large magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery

“…Figure 4a presents the energetic difference between the open-shell singlet and closed-shell states for a series of [n]-rhombenes, showing a size-dependent onset of magnetism. While for n ≤ 4, the ground state is closed-shell, an open-shell singlet ground state emerges for n ≥ 5, in agreement with previous reports 34,35 and in support of our experimental observations. In particular, for 2, the open-shell triplet (S = 1) and closed-shell states are 100 meV and 122 meV higher in energy, respectively, compared to the open-shell singlet ground state.…”

Quantum Interference Assisted Spin Filtering in Graphene Nanoflakes

“…Instead, intermediate-size graphene nanostructures with ZZ edges present the unique characteristic to host simultaneously QI effects, typical of molecular semiconductors, and a magnetic ordering, which is usually associated with bulk systems. Consistent theoretical predictions [3][4][5]7,27,31,32 and experimental evidence [34][35][36] suggest that graphene nanostructures with ZZ edges are prone to magnetic ordering, which survives up to room temperature. 7,35 In particular, hexagonal ZGNFs display a fully-compensated (i.e., with zero net magnetic moment) antiferromagnetic (AF) order.…”

“…7,35 In particular, hexagonal ZGNFs display a fully-compensated (i.e., with zero net magnetic moment) antiferromagnetic (AF) order. 5,7,31,32 The presence of the edges in the ZGNF determines an inhomogeneous spatial distribution of the ordered local magnetic moments S z i = n i↑ − n i↓ , which are significantly larger at the edges than in the bulk as a consequence of the reduced number of hopping channels for the edge sites. 5,7,31,55 In hexagonal ZGNFs, all the atoms of a given edge of the hexagon belong to the same graphene sublattice, while neighboring edges are connected by an armchair (AC) defect, so that the local magnetic moments are aligned ferromagnetically within the same edge, and antiferromagnetically between neighboring edges, as shown in the inset of Fig.…”

Magnetic, charge, and transport properties of graphene nanoflakes

“…At and close to half-filling the transition to the SDW state was also investigated within the dynamic mean-field theory (DMFT) approach for GNF system with 54 atoms coupled to leads [10][11][12]. The emergence of magnetism in GNF clusters, induced by electron-electron interaction, including formation of finite magnetic moments, was also predicted within the mean-field [4,8,29,30] and density functional theory (DFT) calculations [8,31,32]. The edge magnetization was thoroughly studied, especially for GNF with zigzag edges [4,6,31,[33][34][35][36].…”