Theoretical Investigation Of Electronie Transport Properties Based On Fullerene Molecular Devices

With the rapid development of micro-electronics and continuous minimization of electronic devices, using single molecule or molecular cluster, such as single organic molecule, single-or multiple-walled carbon nanotubes, some biomolecules and so on, to construct functional electronic devices has become one recognized developing trend. And the research of electric and optical properties for these molecular-scale devices has become an independent subject gradually, that is molecular electronics. In recent years, advances in micro-fabrication and self-assembly techniques have made it possible to design molecular-scale electronic device. Up to date, special attention has been focused on atomic and molecular systems. With the achievement and application of the involved experimental techniques, such as self-assembled monolayers (SAMs), chemical synthesis, scanning tunneling microscope (STM), mechanically controllable break junction (MCBJ), Langmuir-Blodgett (LB) monolayers, organic molecular beam epitaxy (OMBE), nanopore, electromigration, local oxidative cutting of carbon nanotubes, on-wire lithography (OWL), electrodeposition, shadow mask evaporation, etc, molecular devices with different functionalities have been designed and measured. As new techniques for the atomic-scale manipulation and modification of materials progressed, electron transport properties of nanostructures have attracted considerable interest.The selection of a molecular junction and the accurate control on its coupling to the electrodes are basic prerequisites for designing and manufacturing single molecule electronic devices. Among many types of molecules, fullerene-based molecules is suitable for a molecular bridge compared to other organic molecules because they are expected to be more practical importance owing to their possibilities for novel components in miniaturized electronic devices, and show unique physical and chemical behaviors, and the structure stability different from those of macroscopic systems. Since the first report on the fullerene structure of C60, extensive research has been devoted to understanding the electronic structures and electronic transport properties of fullerene-based nanostructures. Many interesting physical properties based on fullerene molecular devices have been found, such as negative differential resistance (NDR), current rectification, electrical switching, single molecular electromechanical amplifier, superconductivity and so on.The electronic transport properties of fullerene-based molecular devices depend on the number of the molecular and molecular configuration between the electrodes, the relative orientation and the distances of the fullerene molecules with respect to each other. Also, in many cases, the atomistic details of the contact greatly affect the electronic transport properties of the molecular devices. These factors include the relative orientation of the molecule to the electrodes, the distance between the molecule and the electrodes, the size of the electrodes, and the material that is used as the electrodes, etc. Besides, the external conditions are the important factors which determined the electronic transport properties of fullerene-based molecular devices, including the temperate, humidity, pressure and gate voltage, etc. Therefore, it is valuable to investigate the electronic transport properties of fullerene-based molecular devices, control the electronic transport properties of fullerene-based molecular devices and design the functional molecular devices.In the study of this thesis, by using first principles density functional theory (DFT) combined with nonequilibrium Green’s function (NEGF), we focus on the investigations of electronic transport characteristics in fullerene-based molecular devices and mainly discuss the effects of contact distance between the electrodes, contact interface configuration, the different applied positions and positive or negative values of gate voltage on electronic transport properties of fullerene-based molecular devices. The detailed research and main results are given below:1. The electronic transport properties in C60molecular devices with different contact distancesIn the STM experiment, the tunneling electrical current increases approximately exponentially with tip displacement towards to the C60molecule in the tunnel regime. According the experiment results, by applying nonequilibrium Green’s functions in combination with the density functional theory, we investigate the transport behavior of molecular devices composed by metal electrode Ag-C60molecule-metal electrode Ag. Our results show that the electronic transport properties are affected obviously by the different contact distances between the electrodes, and the tunneling current increases approximately exponentially at a certain bias with the decreasing of contact distances. The negative differential resistance behaviors are observed and the peak-to-valley ratio can be tuned by different contact distances. The mechanisms of the contact distance effect and the negative differential resistance behavior are proposed.2. Effect of contact interface configuration on electronic transport in (C20)2-based molecular junctionsUsing first-principles calculations, we study the electronic transport properties in Au-(C20)2-Au molecular junctions with different contact interface configurations: point contact and bond contact. We observe that the transmission through the bond contact is considerably higher than that of point contact, which makes this carbon hybrid system a possible candidate for nanoelectronic switching devices according to the rotation of the fullerenes. Furthermore, the current-voltage (Ⅰ-Ⅴ) characteristics of the point contact and bond contact configuration are rather different. For the bond contact, we get a metallic behavior followed by a varistor-type behavior. While as for the point contact, the current increases very slowly in a nonlinear way and is one order of magnitude smaller than that of bond contact. We attribute these obvious differences to the distinct contact configurations.3. Rectifying behaviors of an Au-(C20)2-Au molecular device induced by the different positions of gate voltageThe electronic transport properties of a gated Au-(C20)2-Au molecular device are studied using nonequilibrium Green’s function in combination with density functional theory. The results show that different applied positions of the external transverse gate voltage can effectively tune the current-voltage (Ⅰ-Ⅴ) characteristic of molecular devices. Rectifying behaviors of the device can be realized when the gate voltage is applied asymmetrically on the left C20molecule, and the rectification directions can also be modulated by the positive or negative value of the gate voltage. These results provide an important theoretical support to experiments and the design of a molecular rectifier.4. Enhanced rectifying performance by asymmetrical voltage gate for BDC20molecular devicesBy applying the asymmetrical gate voltage on the1,4-bis (fullero[c]pyrrolidin-1-yl) benzene BDC20molecule, we investigate theoretically its electronic transport properties using the density functional theory and nonequilibrium Green’s function formalism for a unimolecule device with metal electrodes. Interestingly, the rectifying characteristic with very high rectification ratio,91.7and24.0, can be obtained when the gate voltage is asymmetrically applied on the BDC20molecular device. The rectification direction can be tuned by the different external gate bias applying regions. The rectification behavior is understood in terms of the evolution of the transmission spectra and projected density of states spectra with applied biases combined with molecular projected self-consistent Hamiltonian states analyses. Our finding implies that to realize and greatly promote rectifying performance of the BDC20molecule the variable gate voltage applying positions might be a key issue.