DFT+U Study Of Transitional Metal Doped Graphene

Graphene, a two-dimensional material based on carbon atoms, has been widely studiedby both experimentalists and theorists due to its unqiue Fermi-Diract electronic property.The method to introduce impurities by doping transitional metal has been taken intoaccount to tune some of the properties of graphene-based materials such as its electronicstructure and physicochemical properties. And these graphene-based materials havepotential applications in spintronics and other fields. In this thesis, we present the firstprinciple calculation based on density functional theory and consider the Hubbard U inthese strongly correlately systems to study the geometric, electronic and magneticproperties of a transition metal atom doping in single and double vacancies of graphene.Our calculated results indicate that3d transition metal Mn, Fe, Co and Ni atoms havestrong binding to the vacancy defected graphene. We find that the geometric structurechanges slightly as compared to the results in GGA calculations, while its electronicstructure has been altered a lot. After we consider the effect of Hubbard U, it issemiconducting property for Mn adsorption on the single acancy graphene complex, whliein the absence of Hubbard U it is semimetallic. In the absence of Hubbard U, the Fe andsingle vacancy graphene complex is semiconductor, while it is semimetallic for U in2eVand4eV, and can be further changed to a metal under U equal to8eV. The small andmoderate Hubbard U can’t alter the electronic structure of Co and single vacancy graphenecomplex for U in0eV,2eV and4eV. And it keeps metallic property while it has becomeinto semimetallic property when U increases to8eV. Analysing the partial density of states,we find that the3d spin-up unoccupied orbitals of transitional metals tends to be far awayfrom Fermi level, and the3d spin-down occupied orbitals also shifts to be far away fromFermi level. And as the increasement of U, the degree of shift increases. In theconsideration of Hubbard U, it is found that the magnetic moment of3d transition metalincreases which can be attributed to the bigger difference of quantity of electricity betweenthe spin-up and spin-down orbitals.We further study the effect of Hubbard U in the situation of a single Mn atom indouble vacancies graphene. It is found that the height of Mn atom over the graphene sheetincreases as the increasement of U. And the electronic structure is semiconducting propertyin the absence of Hubbard U, while exhibits semimetallic property in the consideration ofHubbard U. It is also found the magnetic moment of Mn tends to increase in theconsideration of Hubbard U. The reason is also because the difference of quantity ofelectricity becomes bigger when we consider the effect of Hubbard U. Our results in the consideration of Hubbard U indicate that the U plays an importantrole in the study of transitional metal atoms adsorption on single vacancy or doublevacancies graphene. The empirical U is crucial to dominate the electronic and magneticstructures in strongly correlately systems. And our proposed way to determine U willfurther guide to the theoretical and experimental research in transitional metal and vacancygraphene complex.