Electronic And Magnetism Properties Of Monolayer Triangular Graphene Quantum Dots With Zigzag Edges

The effect of edge reconstruction and vacancies on the electronic and magnetic properties in triangular graphene quantum dots with zigzag edges has been systematically investigated using the Constrained-Path Monte Carlo and ab initio calculations.We reconstruct the triangular graphene quantum dots with zigzag edges including different types of edges, and calculate the energies of triangular graphene quantum dots with regular zigzag edges (ZZ) and reconstructed edges (ZZ5-7-5, ZZ7-6-5, ZZ6-6-5, and ZZ765) in different spin multiplicities under different sizes. The results obtained by the two methods consistently show that:the ground state of ZZ configuration for all sizes lies in the ferromagnetic state, and the magnetic moment of the system increases with an increase of the system size; on the contrary, the ZZ5-7-5ZZ7-6-5, and ZZ6-6-5configurations lie in the antiferromagnetic state, and the ZZ765configuration lies in the antiferromagnetic state except the case with a small size. In addition, the up-and down-spin electronic density and energy spectra near the Fermi level were investigated through the Dmol3package. The results show that the up-and down-spin states are split around the Fermi level, while this splitting is weakened after edge reconstruction, which indicates that the reconstructed edges suppress the spin polarization and magnetism in the triangular graphene quantum dots. The degenerate zero-energy band appears near the Fermi level in the ZZ configuration, and the number of states in the zero-energy band equals the difference between the numbers of atoms in A and B graphene sublattices. An introduction of edge reconstruction smears the distinction between sublattices. Nevertheless, the zero-energy band survives in the reconstructed triangular graphene quantum dots. Furthermore, the number of zero-energy states remains the same, however, the dispersion of this band is increased.We produce triangular-shaped, inverted triangular-shaped, and circular-shaped vacancies in the larger size of graphene quantum dots with zigzag edges, and compute the energies of graphene quantum dots with different vacancies in different spin multiplicities. The results show that:the ground state of all configurations lies in the ferromagnetic state. In addition, the up-and down-spin electronic densities near the Fermi level were investigated through the Dmol3package. The results indicate that the up-and down-spin states are split around the Fermi level. The spin polarization of the graphene quantum dots with triangular-shaped vacancies is reduced, while the spin polarization of the graphene quantum dots with inverted triangular-shaped is enhanced. The spin polarization of the graphene quantum dots with circular-shaped vacancies almost has no change. These results indicate that the triangular-shaped vacancies suppress the magnetism of the system, while the inverted triangular-shaped vacancies promote the magnetism of the system.