Theoretical Studies On The Electron And Spin-polarized Transport Properties Of Low-dimension Carbon-based Materials

Nano-and molecular devices have aroused the extensive attentions of people, many low dimensional nanomaterials, e.g., carbon nanotube, fullerene, graphene, single-molecule magnet are capable of realizing basic electronic functions, such as negative differential resistance effect, the spin filtering effect, rectifying behaviors, molecular switch, etc. In this thesis, by using the first-principles method based on the density functional theory combined with nonequilibrium Green's function method, we have explored the electron and spin-polarized transport properties of some low-dimension carbon-based materials, including fullerene and grapheme. This thesis mainly includes the following sixth parts:In the first chapter, we briefly introduce the research background of molecular electronics and spintronics, and make a simple review on the development course of them in recent years. Then, we introduce the structures and properties of graphene and fullerene.Finally we summarize the main research content of this paper.In the second chapter, we mainly introduce the method of theoretical calculation adopted by this thesis. First, we introduce the density functional theory method. And then we introduce the non-equilibrium green function(NEGF) method, which is used to calculate the electronic transport problems. Finally we illustrate the first-principle technique based on density functional theory and the nonequilibrium green's function which is adopted to deal with the electron transport properties of molecular devices and one of the basic physical formula, Landauer-Buttiker formula.In the third chapter, using first-principles density functional theory and non-equilibrium Green ' s function formalism for quantum transport calculation, we have investigated the electronic transport properties of BDC60-based molecular junction. We calculate the current-voltage curve of BDC60 molecule, and discuss the effect of magnitude of gate voltage on the transport properties. The results show that the transport properties are strongly modulated by the applied gate voltage, and the current-voltage curve displays an obvious rectifying behavior at much low bias region. But in the case of without gate voltage, system has no rectification. When combined with different sizes of gate voltage, the rectifying properties of the system are also different.The reason is that the gate voltage is an importantapproach to control the electronic transport properties of molecular devices by shifting the molecular levels.In the fourth chapter, we use first principles method which is based on non-equilibrium Green's method and density functional theory, and investigate the spin-polarized transport properties of an endohedral Fe@C60 dimer-based spintronic device. After detailed calculation,we found that system shows obvious magnetic resistance effect at small biases when the structure switches from P to AP spin state. The reason is originated from the fact that the PDOS peaks of left and right Fe@C60 cages are not degenerate again due to the spin asymmetry, and makes the current of P spin state is greater than the current of AP spin state,producing bigger magnetic resistance rate. The spin filtering effect can also be observed in this device, and the spin filtering efficiency reaches more than 90% in the P spin configuration.The reason for this is that when an external voltage is applied, the difference of PDOSs for spin-up and spin-down leads to the spin-filtering effect. When the bias increases further, there will be a negative magnetic resistance and negative spin filtering effects. In addition, the system also shows the negative differential resistance effect. This is produced by the spin-up currents of the system.In the fifth chapter, we study the spin-dependent transport properties of the single-molecule magnet Mn(dmit)2 sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes, by applying the density functional theory and the nonequilibrium Green's function formalism. We have maily discussed the electronic properties and spin transport properties of its coplanar and perpendicular conformations. The system can present spin-filtering when the two dmit in coplanar structure. The spin-filtering efficiency reaches nearly 100% in the P spin state in the whole bias range. However, the current is strongly suppressed when current flowing through its perpendicular conformations.The results show that Mn(dmit)2 is an excellent spin filters or molecular switches. The system can present large rectifying, GMR, and NDR effects with the help of magnetic field modulation. Our results indicate that the system holds great promise in the design of a high-performance multifunctional molecular device.Finally, we make a brief summary and outlook of this paper.