The Optical And Electronic Properties Of Graphene-like Structures: First-principle Study

Following the successful experimental preparation of two-dimensional(2D)graphene in 2004,graphene has rapidly risen to be one of the hottest stars in materials science due to its outstanding physical and chemical properties.However,pure graphene is a zero-gap semiconductor,which is an adverse feature for practical applications.To broaden the application field of graphene and allow it to have a truly effective application in future nanoelectronic devices,researchers are directing their attention into designing and constructing graphene-derived functional materials with properties different from or even superior to those of graphene.Graphene derivatives with many novel properties are greatly expanding the application field of graphene,which is very important for the comprehensive development and utilization of graphene.Because of this background,using the density functional theory combined with the non-equilibrium Green function,we studied the doping effect,and the optical and transport properties of graphene nanoribbons and graphane,which are two common graphene derivatives.The thesis is organized as follows:In chapter 1,we introduce the physical and chemical properties of graphene and its typical derivative structure,and the content summary of this thesis.In chapter 2,we introduce the theoretical basis,calculation method and relevant software.In chapter 3,the modulation effect of Fe adatom on the transport properties of zigzag graphene nanoribbon(ZGNR)is discussed.Our calculations show that the transport properties of the ZGNR are very sensitive to the position of the Fe adatom.When the Fe adatom is at the edge of the ZGNR,its effect on the transport properties is maximum.By moving the adatom from the edge toward the middle of the ZGNR,the impact of adatom on the transport properties is reduced.Furthermore,we find that the effect of Fe adatom on the spin down channel is greater than that of the spin up channel.Therefore,a high spin polarization rate is produced and the spin polarized transport is realized.In chapter 4,we investigate the doping effect of Li atom on graphane and graphene layered systems.The structural models we considered here are increasing graphane layers(from one to four)are placed on four layers graphene.The charge distribution in Li-doped graphane and graphene layered system is calculated by virtue of Bader charge population analysis.The results show that almost all of the transferred charges offered by Li atom distribute on the outmost graphane layer and only a few of them stay in interior graphene layers.Such characteristic of charge transfer indicates that the outmost graphane layer can screen the external charges offered by Li atom so that the interior layers are nearly electrically neutral,which is in good agreement with the classical electrostatic screening effect.Moreover,this phenomenon does not change with the increasing number of graphane layer.By analysis of electrostatic energy and band filling,it is shown that the charge distribution is dominated by both interlayer electrostatic interaction and NFE state of the topmost graphane layer.In chapter 5,we study both the optical properties and transport properties of graphane nanoribbons.With respect to the optical properties,we find that the imaginary parts of the dielectric function for both the zigzag and armchair graphane nanoribbons are very similar to those of 2D graphane,they all show optical anisotropy.Furthermore,the optical properties are independent of the edge shapes and the widths of the graphane nanoribbons,which is mainly due to the carbon atoms of the nanoribbons being bonded together via sp3.The contribution of the orbital to the energy state of the structure is very similar,so they meet the same optical transition rule and show similar optical properties.The special characteristics of this optical property remove the need to prepare a specific width and shape of the boundary of graphane nanoribbons in practical applications.With respect to the transport properties,a magnetic tunnel junction with graphane nanoribbons as the barrier layer is constructed.The current voltage curves of the magnetic tunnel junction in the parallel configuration and the anti-parallel configuration are calculated.We find that the response of the current to the voltage is different under different configurations.In the parallel configuration,the current increases first and then decreases with the increase of the bias voltage,and displays a negative differential resistance phenomenon;in the anti-parallel configuration,the current increases with the increase of the bias voltage.The tunnel magnetoresistances can be obtained from the current voltage curves.The maximum tunnel magnetoresistances is up to 7700%,and we observe the phenomenon of positive and negative magnetic resistance reversal.In addition,the mechanism of this phenomenon is explained by the analysis of the transmission spectrum under different bias voltages.In chapter 6,the work of this thesis is summarized as well as the future research work is proposed.