First-principles Study On Electronic Structure Of Graphene Nanoribbons

Graphene nanoribbons, as a novel carbon-based nonmaterial, have been become afocus of physical, chemical, material fields, due to their unique structures andelectronic properties since graphene was discovered by Geim in2004. Researchershave found that electron mobility of graphene is100times faster than that of silicon.This feature makes it promising next-generation semiconductive materials instead ofsilicon, which will be used from high-speed computer chips to biochemical sensorsfor various applications. So the study on the electronic structures of graphene in theapplication of electronics devices provides a means of practical significance. Usingthe first principles density functional theory, the paper explores the electronicproperties of graphene nanoribbons with finite length.In chapter1, we give a simple introduction of graphene and graphene nanoribbonincluding experimental preparation, electronic structure, transport and magneticproperties and so on. Due to its novel properties, such an anomalous quantum halleffect and massless dirac fermions, graphene has been widely studied not only intheory but also in experiment.In chapter2, we introduce the basic idea of DFT and review its recent progress.In the end, we briefly introduce the DFT-based simulation package used in thedissertation.Based on the first-principles density functional theory, we study the electronicproperties of graphene nanoribbons with different width in chapter3. It is found thatthe ground state of graphene nanoribbons we considered are all spin-polarized singletstate and αand β-spin states are highly degenerate. The energy gaps (differencebetween the highest occupied molecular orbital and the lowest unoccupied molecularorbital) of the two spin states are the same. The energy gap oscillations with the widthare observed, and show a decreasing trend in general.Based on the first-principles density functional theory, we consider the singleboron/nitrogen atom substitutive doping effect on the electronic structure of graphenenanoribbon in chapter4. We find that the energetically most favorable doping site is atthe center carbon atom of the zigzag edge. Doping by boron/nitrogen atom destroysthe symmetry of electronic structure of graphenen nanoribbon, leading to the brokenof degeneracy associated with α and β-spin states and the asymmetry of the twogaps. Employing the first-principles density functional theory, we study the lengtheffect on the electronic properties of graphene nanoribbons and boron/nitrogen atomsubstitutive doped graphene nanoribbons in chapter5. As the length increases, it isfound that the energy of graphene nanoribbons is becoming lower and lower. And thetwo spin states are still highly degenerate. The energy gaps of graphene nanoribbonsand boron/nitrogen atom substitutive doped graphene nanoribbons with the length aregetting smaller and smaller. The length does not affect the morphology of density ofstates and the Mulliken charge ranges. But, as the length increases, the spin densitydistribution of boron/nitrogen atom substitutive doped graphene nanoribbons is morelocalized, and the HOMO、LUMO distribution of the two spin states has a significantchange.These results may contribute to the electronic structure designing of the graphenenanoribbons.