First-principles Study On Electromagnetic Properties Of Bilayer Graphene Nanoribbons

The graphenes have attracted widespread attention due to the fantastic physical and chemical properties,such as the high strength,high conductivity and high electron mobility.It is also considered as a new generation of nanoelectronic device materials after silicon materials.However,the graphene is hardly to apply to nanoelectronic device because of the zero-band gap.Therefore,it is a hot spot that modulating the band gap of Graphene.Cutting the graphene into nanoribbons can make the graphene with non-zero band gap characteristics,which are related to the width and shape of the graphene nanoribbons,and enrich the electromagnetic characteristics of graphene.In addition,the bilayer graphene has more flexible properties due to the breaking of the symmetry in vertical to the plane.Considering the above two factors,we take the bilayer graphene nanoribbons?BGNRs?as the research object.Using the first principle method based on density function theory,we systematically study the effect of the transition metal monatomic chains intercalated bilayer graphene nanoribbons,the geometry,electrical,magnetic and optical properties of BGNRs in external electric field.Some important conclusions are summarized as follows:?1?For the systems of transition metal adatom chains?TM=V,Cr,Mn,Fe,Co,Ni,Cu?intercalated AA-stacked BAGNRs with different widths?the w from 4 carbon atoms to 9 carbon atoms?.The combining processes of all the[GTMG]_w compounds are exothermic except the[GCuG]_w?w=5,6,7,8,9?compounds,accompanying with larger formation energies and charges transfer.The formed TM–C ionic bonds are contributed to enhancing the stability of[GTMG]_w compounds.The most stable intercalated structures,are the[GVG]_w compounds for the different TM chain,and are the[GTMG]₄ compounds for the different widths of bilayer graphene nanoribbons?w=4,5,6,7,8,9?.The[GMnG]_w compounds have the maximum magnetic moment,but the[GVG]_w,[GNiG]_w and[GCuG]_w compounds have magnetic quenching phenomenon with zero magnetic moment.There are different magnetic properties for the even?w=4,6,8?and odd?w=5,7,9?widths of bilayer graphene nanoribbons in[GTMG]_w compounds.Therefore,the TM chains intercalated bilayer graphene nanoribbons exhibit tunable magnetic properties for potential spintronics applications.?2?For the systems of Zigzag transition metal adatom chains?TM=V,Cr,Mn,Fe,Co,Ni?intercalated AB-stacked BAGNRs with the ten carbon atoms width,those are four different adsorption position due to the symmetry.The binding energy shows that all adsorption systems are stable,and the V adsorption systems have the most stable structures.In the same atomic chains,the stability varies with different adsorption sites.As the adsorption position move on center of BAGNRs,the stability became weaker.The charges still transferred from TM atom to C atom of BAGNRs,forming an ionic bond that contributes to increased stability,which have the similar conclusion with the system of 3d transition metal chains intercalated AA-BAGNRs.In adsorption systems,[GCrG]₁₀-2,[GCrG]₁₀-4,[GMnG]₁₀-2,[GMnG]₁₀-3,[GFeG]₁₀-1 and[GCoG]10-2 have the half-metal properties,and magnetic quenching still occurs in Ni adsorption systems.It's show that the magnetic quenching always exists in adsorption system of Ni and graphene,regardless of shape and layers of graphene.In the same adsorption position,the order of the magnetic moments of magnetic systems is:M_Mn>M_Fe>MCr>MCo,and the magnetic moments come from the transition adatom chains.Due to the edge effect of BAGNRs,the magnetic moment of adsorption systems has a significant different,and the different decreases gradually and disappears as the adsorption position move to the center.There is obvious edge effect for the magnetic moment of two atoms with different position?A and B?in the same TM chain.The research results show that the structural stability,electron and magnetic properties are related to the edge effect of graphene nanoribbon which can bring the rich electromagnetic properties and extend the application of graphene.?3?The electronic structures and optical properties of AA-stacked BAGNRssubject to the influences of a perpendicular electric field are investigated by using first-principles calculations based on density functional theory.We find that the BAGNRs have semiconductive characteristics,and the band gaps decrease oscillationally with the increase of the nanoribbon widths?4 to 12 carbon atoms?of BAGNRs.When the electric fields are applied on the BAGNRs,the band gaps of BAGNRs decrease with the increase of electric field intensity,and the bigger band gap is more sensitive to the electric field intensity.All the band gaps of different width BAGNRs are zero when the electric field intensity reaches to 0.5v/A.The dielectric functions of BAGNRs are of semiconductor in the vertical polarization direction while of metal in the parallel polarization direction showing that the BAGNRs have anisotropic optical properties.For the dielectric functions,absorption coefficients,electron energy loss spectra,optical conductivities,reflectivities and the refraction indexes of BAGNRs,under the external electric field their spectrum peaks move to lower energy zone,indicating red-shifted spectra.The electric field enhances the interband transition.These optical property parameters are influenced differently by the widths of BAGNRs.The research results explain the microscopic mechanism of the electronic structures and optical properties of BAGNRs in the external electric field.