First-principles Study Of The Electron Transport Properties Of Optical Molecular Switches

In recent years, with continuous development of micro-electronics and continuous minimization of electronic device, using single molecule or molecular cluster, such as single- and multiple-walled carbon nanotubes, single organic molecule, macromolecules and so on, to construct functional electronic devices has become one developing trend, and the research of electric and optical properties for these molecular-scale devices has become an independent subject gradually, i.e., molecular electronics. The occurrence and application of the involved experimental techniques, such as Langmuir-Blodgett (LB) monolayers, self-assembled monolayers (SAMs), OMBE, scanning tunneling microscope (STM), nanopore, mechanically controllable break junction (MCBJ), electromigration, local oxidative cutting of carbon nanotubes, electrodeposition, shadow mask evaporation, on-wire lithography(OWL), etc., has made it possible to design and measure molecular devices with different functionalities. Among these devices, molecular switches have attracted great attention because they are basic elements of any modern design of logic and memory circuits.The on (high conductance) and off state (low conductance) of a molecular switch can be reversibly converted into each other in response to an external trigger, such as electric field, tip of STM, redox process, and so on. However, these triggering means are not ideal since they may interfere greatly with the function of a nano-size circuit and limit the real applications. Besides, all these means are relatively slow. On the contrary, light is a very attractive external stimulus for such switches because of the ease of addressability, fast response time, and compatibility with a wide range of condensed phases.In this thesis, we will investigate the electron transport properties of three different kinds of photochromic molecules and discuss their feasibility as a molecular switch. The outline and the main conclusions of the studies are as follows.1.15,16-Dinitrile dihydropyrene/cyclophanediene-based (DDP/CPD) optical molecular switch1.1 It is found that the current through the DDP is larger than that through the CPD, indicating a switching characteristic. The reason is attributed to the change in the effective conjugation length and the change in the HOMO-LUMO gap of the two isomers.1.2 The adsorption sites of the molecule on the gold electrode surface can make much influence on the molecular conductance, whereas not much on the on-off ratio.1.3 The chirality of SWCNT electrodes can affect stongly the electron transport properties. Under small biases, the current through the switch with zigzag SWCNT electrodes is bigger than that through the switch with armchair SWCNT electrodes, due to the sharp resonance close to the Fermi Level Ef.2. Phenoxynaphthacenequinone-based (PNQN) optical molecular switch2.1 It is found that the current through the ana-PNQN is larger than that through the trans-PNQN, indicating a switching characteristic. 2.2 The switching performance can be further improved to some extent through suitable donor and acceptor substituents.2.3 The joint manner of molecule-SWCNT electrodes can affect strongly the electron transport properties, and further influence the switching characteristic.3. Anthracene-based (AT) optical molecular switch3.1 It is found that the current through the AT monomers is significantly larger than that through the AT dimmer, indicating a switching characteristic. The lack of any significant peak in between-2.0 and 1.9 eV accounts for the low conductivity in the AT dimmer form.3.2 Since the molecule-electrode coupling strength gets weaker with the increase in bias, a large negative differential resistance effect in the AT monomer form appears, i.e., the current through the AT monomer form fdecreases significantly when the bias exceeds 2.0 V.3.3 The terminal atoms on the end of SWCNT electrodes can affect strongly the electron transport properties, and further influence the switching characteristic. There is a significant transmission peak around the Ef due to the presence of nitrogen atoms at the CNT ends. In existence of such states close to the Ef, the mechanism of electronic transport at low biases is dominated by the quasi-resonant tunneling, which leads to a bigger current. However, there is no any transmission peak around the Ef in the H-termination case, and then the electronic transport at low biases can only take place through the non-resonant tunneling, which cause a significant suppression of current.