Anomalous magnetic and transport properties of laterally connected graphene quantum dots

We investigate the magnetic, electronic, and transport properties of chemically modified Clar’s goblets and laterally connected graphene quantum dots (GQDs) using density functional theory. The chemical modification includes full or partial edge passivation with 2H, O, and F atoms. We report transformation of antiferromagnetic spin ordering of Clar’s goblet to ferromagnetic ordering with a nonzero net spin such as S = 1 by full passivation with 2H or S = 2.5 by partial passivation with 2H. The full and partial passivation of Clar’s goblet with 2H isolates correspondingly 2 and 5 electrons with the same spin alignment which boosts the ferromagnetism. We also report the spin density localization on one side of the goblet by doping the other side with Li-atom. For laterally connected triangular GQDs, we reveal unconventional net spin. Contrary to Lieb’s rule, the laterally connected triangular GQDs host not S = 0 but S = 4 net spin. This anomalous ferromagnetic state occurs due to the twisting of GQDs with respect to one another. Due to the symmetry breaking between spin up and spin down energy states, the corresponding spin up and down energy gaps are strongly affected by the modification. For instance, the spin down energy gap increased/decreased by partial passivation with F/O atoms leading to tunable transport properties. I-V characteristics indicate that the edge modification increases conduction due to the enhanced dipole moment and the number of conducting charges. Transport through latterly connected GQDs show quantum tunneling effect due to the overlap between spin up and spin down molecular orbitals which makes these systems promising as ferromagnetic tunneling diodes.