Theoretical Study On Negative Differential Resistance And Rectifying Behavior Of Molecular Devices

In recent years, the rapid progresses in micro-fabrication and self-assembly tech-niques have made it possible to control the molecules in nanoscale and to assemble them as the molecular device. These molecular devices are considered the suitable candidate of electronic devices which trend toward the ongoing miniaturization. For this reason, the experimental and theoretical studies on the molecular device have attracted more and more attentions. In this dissertation, we use the first-principles in combination with the density-functional theory to study the transport properties of some molecular devices and mainly discuss the effect of deformation, connected sites, gate voltage, side groups and asymmetric electrodes on the negative differential resistance (NDR) and the rectifying behavior.We investigate the transport properties of a single molecular device constructed by one C60 molecule sandwiched between two Al electrodes. At the same time, we also in-vestigate the effect of deformation and gate voltage on the device's current-voltage prop-erties. The calculated results show that the currents of the device which the C60 connects with the Al electrodes by van der waals force not accord with the traditional Ohm theo-rem. In the special voltage region, the currents decrease with the increase of bias voltages and show the NDR behavior. In addition, we find that the NDR behavior can be enlarged or reduced and shut off by squashing the molecule on the vertical direction. Further study indicates that the gate voltage can also affects the device's transport properties intensively. As a result, we can modulate the device between the high and low conductive states and make it as a gate-controlled current switch. In the same way, the NDR behavior also can be reduced and shut off by the gate voltage.We study the effect of side groups on the transport properties of a linearπconju-gated molecule OPV sandwiched between two Au electrodes. Our calculation explicitly demonstrates that when the molecule modulated by amino, the highest occupied molecu-lar orbitals are localized, while the molecule modulated by nitro, the lowest unoccupied molecular orbitals are localized. The electron transport of device will be enhanced when it modulated by amino or nitro, but will be weakened when it modulated by both of them. More interesting, negative differential resistance is only observed when the molecule modulated by two amino at the same time.Then, we investigate the transport properties of a bimolecular device. The two par- allel OPV molecules are sandwiched between two Au electrodes and the effect of side groups is studied again. Due to the intensive interaction between the two molecules, the molecular orbitals and transport properties of the bimolecular device are more complex than the single molecular device, so does the side groups. The results show that the side groups can modulate the bimolecular device's transport properties by the substituted posi-tion. The current of the device substituted by two amino groups on the same side is bigger than that on the different side. Contrarily, the current of the device substituted by two nitro groups on the same side is smaller than that on the different side. More importantly, the NDR behavior can be observed only when the system is substituted by two amino groups on the same side.We study the transport properties of a single phenalenyl molecular device. Phenalenyl is a well known stable organic radical with high symmetry (D3h) and has two differ-ent sites to connect with the electrodes. The results show that the electronic transport properties are strongly dependent on these contact sites. The negative differential resis-tance behavior with large peak to valley ratio is observed when the molecule contacts the Au electrodes through two second-nearest sites or one second-nearest site and one third-nearest site, while the rectifying behavior is observed only when the molecule contacts the Au electrodes through one second-nearest site and one third-nearest site.We perform a theoretical study of a single C60 sandwiched between Au electrode and nanotube electrode. Due to the huge difference, the matching of orbitals around the Fermi energy among the two electrodes and the molecule is not very well. So the current value of the Au-C60-CNT is much smaller than the C60 device combining the same electrode up to three orders of magnitude. Moreover, the rectifying behavior is observed in this device and the rectification ratio can be modulated by the gate voltage.