Edge States And Thermoelectric Effects In Graphene-like Materials

The edge states and thermoelectric effect in graphene-like materials are studied using the non-equilibrium Green's function method combined with the density functional theory?DFT?.The transport properties and spin thermoelectric properties of edge co-doped ferromagnetic zigzag graphene nanoribbons?ZGNRs?are systematically investigated.Besides,the electronic structure of edge states in very narrow nanoribbons of the topological insulator nickel-bis?dithiolene?are briefly addressed where a spin-semi-metal characteristics have been found.In this thesis,we review at first the experimental preparation,the research background,and some novel electronic properties of graphene and nickel-bis?dithiolene?complex nanosheet.Then we describe briefly the ab initio density function theory?DFT?and the nonequilibrium Green's Function?NEGF?method,before the introduction of the software packages used in our computational simulations.Based on these,the results obtained during the master period are presented as follows:?1?The effects of edge co-doping by a pair of boron and nitrogen atoms on electronic structures,linear spin transport,and spin thermoelectric properties of ferromagnetic zigzag graphene nanoribbons?ZGNRs?are investigated.When the two impurity atoms are separated by more than two edge C atoms,they are independent from each other.Near the Fermi energy,an impurity state of the N?B?atom is located just below?above?the spin-up?spin-down?flat bands of edge states.When the two impurity atoms approach to and become the first nearest neighbor of each other,coupling between the impurity states near the Fermi level of the two atoms increases greatly.As a result,the spin-down?spin-up?impurity state of the N?B?atom shifts up?down?to the Fermi level.As the localized impurity states block the electronic transport,one spin-down?spin-up?transmission valley emerges just below?above?the Fermi energy.Since the separation between the two valleys is smaller than the valley width,the transmission spectra of the two spins have opposite slopes at the Fermi energy.These opposite slopes enhance greatly the spin thermoelectric effects according the Mott formula.The energy separation between the two valleys increases with the width of nanoribbons and the distance between the B and N atoms.This changes the transmission spectrum at the Fermi energy and weakens the spin thermoelectric effect.?2?The atomic geometry,the electronic band structure and Bloch states of nickel-bis?dithiolene?complex nanoribbons is simulated.In this case,we focus mainly on narrow nanoribbons of width n=2.When the spin orbit coupling?SOC?is neglected,the electron bands are spin-degenerate.Taking into account the spin orbit coupling?SOC?,we observe spin splitting at energies E1=0.47 eV and E2=0.62 eV measured from the Fermi energy,suggesting coupling between neighboring bands.At the coupling points(K1=0.4635?^-1,E1=0.47 eV and K2=0.135?^-1,E2=0.62 eV),one of the spin bands cross with each other while the opposite spin bands anti-cross,showing characteristics of half-semimetal.