Investigations Of Electronic And Optical Properties Of Surface Functionalized Graphene Nanoribbons

Since graphene was first discovered through mechanical exfoliation of graphite by Andre Geim and Konstantin Novoselov at the University of Manchester in 2004,people are inspired by its unique two-dimensional structure,atomic thickness and attractive physical and chemical properties.However,graphene is gapless,which limits its optical applications,such as luminescence.Various modifications are explored.For example,scientists tried different methods to crack graphene to produce graphene nanoribbons(GNRs).Its quasi-one-dimensional semiconducting properties extend graphene-based materials' applications.GNRs has gained intensive investigations.People not only improve the synthesis methods and explored its electronic,magnetic and optical properties.Furthermore,doping and defects are always used to modify GNRs to extend its future applications.GNRs is one of the most popular semiconducting graphene materials,the controllable system of GNRs is very useful for future optical applications.But the systemic investigations of the effect of surface functionalization on GNRs are limited.In this thesis,electronic and optical properties of different functional groups on GNRs,Stone-Wales defects and hydrogenations on the defects of GNRs are theoretically investigated by first principle theory.The thesis contains five chapters:In Chapter 1,the history and recent progress of different carbon-based materials are simply reviewed,and the structures and properties of GNRs are detailed demonstrated.Recently,different synthesis procedures of GNRs are reported experimentally,and GNRs' field effect transistors,electronic devices and sensors have provided potential valuable applications.In Chapter 2,density functional theory(DFT)is simply introduced,including Hohenberg-Kohn theorem,Kohn-Sham theorem,and several DFT codes such as Dmol3,Quantum ESPRESSO and Yambo codes.In Chapter 3,The optical and electronic properties of differ functional groups on the surface of armchair graphene nanoribbons(W(n)-X AGNRs)is investigated with the use of first-principle calculations.The width index n is chosen 8,9 and 10.The X represents the functional groups(X= CH3,NH2,NO2 and OH).We discover that most of functional groups increase the band gaps of W8 and W9,and decrease the band gaps of W10.The change of band gaps is analyzed in the light of bonding characteristics.In all W10-X structures,most of the exciton wavefunctions locate on the bigger segments distributed by the functional groups.Besides,W10-NO2 possesses great potential for photovoltaic and luminescence devices because of its strong optical absorption and weak exciton binding energy.In Chapter 4,the first-principle calculations is performed in the optic and electronic properties of AGNRs with Stone-Wales(SW)defects and hydrogen adsorption on SW defects.We found that the SW defects increase the band gaps of W8 and decrease the band gaps of W9 and W10.Moreover,the hydrogenations increase the band gaps of W8-and W9-SW,and decrease the band gaps of W10-SW.The distribution of exciton wavefunction locates near one edge in W10-SW-H structures.So there are visible quantum confinement effects on the excitons.Besides,the exciton binding energy difference between W10-SW and W10-SW-H structures is only 0.08 eV,possessing potential optical applications in nanoscale optical switch.Because of the strong optical absorption and weak exciton binding energy of W9-SW,it possesses potential optical applications for luminescence and photovoltaic devices.In Chapter 5,the summary and outlook are provided.Our works provide some theoretical supports for future potential applications of GNRs.