Study On Transport Properties And Regulation Of Low-dimensional Carbon-based Materials

In recent years,the electronic device miniaturization aroused people to study the molecular electronics.At the same time,carbon molecular devices are considered to be the ideal replacement for silicon semiconductor devices.The dissertation combing with electronic structure theory and transport theory based on density functional theory?DFT?and the non-equilibrium green function?NEGF?respecivly,studied the electronic transport properties and its regulation mechanism of molecular devices composed of low dimensional carbon-based materials,especially on the atom doped,metal adsorption and reverse methods.The main points of this paper are as follows:In the 1^st chapter,we introduce the background of the topic,purpose and meaning of the study are discussed.Then,the structure of several typical low dimensional carbon-based materials and the development progress of properties on low dimensional carbon-based materials are introduced.Finally,the main contents and structure of the dissertation has been presented.In the 2^nd chapter,we introduce the calculation methods of the electronic structure and their related transport characteristics in low-dimensional carbon-based materials.First,we briefly introduce the framework of density functional theory of modern electronic structure calculation method,and clarify the key point is how to deal with electronic correlation.Then the non-equilibrium Green function method is introduced.On this basis,we combined density functional theory with the non-equilibrium Green function method and applied to the correlation structure and transport characteristic calculation.At the same time,the ATK software package used in the calculation is also briefly described.Finally,several typical transport characteristics are expounded,which laid a solid foundation for the follow-up study.In 3^rd chapter,the properties of two polyphenyls doped with nitrogen and boron atoms,which are connected by an alkane chain,are investigated by the non-equilibrium Green's function method combined with the density functional theory.It has been found that the doped sites have significant effects on the current–voltage characteristics.For models with the N?B?near the alkane chain,the rectification ratio is smaller,but the rectifying performance of models with the N?B?far away from the alkane chain is tremendously enhanced and rectification ratios can reach 287,alongside negative differential resistance behavior.The mechanisms for these phenomena are explained by transmission spectra,the molecular projected self-consistent Hamiltonian eigenstates,electrostatic potential distribution,and projected density of states.In 4^th chapter,The First-principles calculation is used to investigate the transport properties of a carbon chain connected with N-and/or B-doped caped carbon nanotube acting as electrodes.The I-V curves of the carbon chain are affected by the N/B doping sites,and rectifying behavior can be obtained distinctly when the carbon chain is just connected onto two doping atom sites?N-chain-B?,and a weak rectification occurs when N?B?doping at other sites.Interestingly,the spin-filtering effects exist in the junction when it is doped at other sites,undoped system,or N-terminal carbon chains.However,no this behavior is found in N-chain-B and B-chain-B systems.The analysis on the transmission spectra,PDOS,LDOS,spins density,and the electron transmission pathways give an insight into the observed results for the system.In 5^th chapter,based on density functional theory,we studied the structural and electronic properties of seven different metal atoms adsorbed graphene?M+graphene?.The geometries,adsorption energies,density of states?DOS?,band structures,electronic dipole moment,magnetic moment and work function?WF?of graphene and M+graphene were calculated.Li,Na,K,Ca and Fe adsorbed on graphene were tending to form stable structures.However,diffusion would occur very possible on Cu and Ag adsorbed graphene.In addition,the characteristic electronic structure of graphene was significantly altered by Fe,Cu and Ag,but was preserved by Li,Na,K and Ca near the Fermi level.The electronic dipole moment and magnetic moment of each M+graphene was very sensitive to the adsorbed metal atoms.We found the WF of Ag+graphene and Fe+graphene was increasing.In 6^th chapter,we theoretically investigate the thermoelectric properties of twisted armchair graphene nanoribbons?TAGNRs?with various rotation angles.We find that the twist engineering applied to AGNRs can alter the thermoelectric transport properties by modifying the electronic structures and phonon dispersion relations.With twist angle increasing,the thermal conductance tends to decrease,and the max ZT can tunable with different twist angle.Our calculation results suggest a possible route to increase the ZT values of AGNR-N for potential thermoelectric applications.In 7^th Chapter,we have briefly summarized the above work and pointed out the openness of the existing problems and the next step.This dissertation taken the low dimensional carbon materials as the main research object,through doping,adsorption and torsion on carbon based material,and discovered the molecular rectifier,negative differential resistance and spin filtering and other phenomena,and the causes are analyzed in detail.This provides an effective method for the design of carbon based nano devices,and provides a feasible way to control its structure and performance,provides multiple possibilities for the application of low dimensional carbon materials in molecular electronics.