Theoretical Investigation On Charge Transport In Coupled-Molecular Devices

Recently, along with development of Nanotechnology and molecular synthesis technology, molecular electronics has become a promising research area as well as an important field in Nano-electronics. Comparing with inorganic materials, organic molecules have some distinguished advantages, such as small-density, light-weight, hard to be oxygenated, easy to be modified and so on. And they also have abundant properties in electronics, optics and magnetics, with good applications in OLED and display device and so on. Since 1980s, it has become possible to measure the current through single molecule and molecular cluster by relevant experimental technology, such as Langmuir-Blodgett (LB) film, chemical synthesis technology of Self-Assemble Molecular (SAM), Organic Molecular Beam Epitaxy (OMBE) and Scan Tunnel Microscopy (STM). At the same time, both experimental and theoretical researchers are becoming very interested in the emerging series of non-linear I-V properties of molecular device including Negative Differential Resistance, Conducting Switch and Rectification discovered in molecular devices, which indicate the promising potential application of molecular devices in the future. In that, how to understand and utilize charge transport properties of molecule is becoming one of the most popular research areas in molecular electronics.We find that the I-V curves are not measured under the condition of single molecule through lots of experimental facilities. It is not reasonable just to consider single molecular chain between two electrodes. The interchain coupling effect between molecular chains should not be neglected. What's the role of interchain coupling during the process of charge transport is still not very clear. Therefore, it is very important to further understand the existence of interchain coupling effect and its influence on charge transport in molecular device.Based on formal theoretical research on organic molecular devices in our group and taking the interchain coupling effect in a double-chain system into account, we did some theoretical research on charge transport properties of coupled-molecular devices in this thesis. According to electrode—organic molecule—electrode sandwich structure, we adopted the tight binding SSH model and Green's Function theory during our study. We built up our coupled-molecular devices model firstly. We also considered the different types of connection between molecule and electrodes. Then we calculated the charge transport properties of such Coupled-Molecular devices.The first part of this thesis concentrates on the uniform dimerization configuration of the molecule. We studied the effects of connection between molecule and electrode, the area of interchain coupling, the strength of interchain coupling, the interface between molecule and electrode on the transport properties. According to the result, we can see the following conclusions: the turn-on current is larger due to better connection with electrodes; the turn-on voltage is smaller due to larger coupling area; the existence of interchain coupling gives rise to new stairs in the I-V curve and reduces the turn-on voltage, larger interchain coupling results in smaller turn-on voltage and larger area on the first stair; better interface is favorable for charge transport between electrodes. The second part of this thesis is mainly on the transport properties of charged states. We investigated the single/double electron(s) charged states of organic molecule respectively and discussed the length effect on charged states. The results illustrate that net charge in the molecular can destroy the uniform dimerization because of strong electron-lattice coupling in organic material and reduce the gap in transmission spectra. The channels near the gap of transmission spectra are favorable for charge transport in short chain and unfavorable for charge transport in long chain due to the localization of corresponding orbitals.