Single Molecular Conductance Of Benzene-Based Carboxylic Acid And Oligomer Molecules

Single-molecule-based electronics have attracted wide attention due to the intrinsic limitations of silicon-based electronics. At the present stage, the key procedure is to understand and control the electron transport through single molecule. Typically, the electronic property of single molecule is measured through forming metal-molecule-metal junction. The conductance of single molecular junction is influenced by many factors, such as molecular structure, anchoring group, electrode, contact configuration and environment. Among them, anchoring group plays an important role in the electron transport of single molecular junction, and various anchoring groups, including thiol, pyridine, amine and carboxylic acid, have been studied. To date, carboxylic acid has received lots of attention for being an efficient anchoring group which can bind to various electrodes. However, limited researches were paid attention to carboxylic acid group for which the binding site is and how the position and quantity affect the conductance of single molecular junction. On the other hand, comparing with alkyl chain molecules, conjugated oliogmer molecular wires have a higher conductivity, but few researches reported the trend between conductance and the degree of polymerization or aromaticity. Therefore, we applied electrochemical STM-BJ technique (ECSTM-BJ) to measure the conductance of benzene derivatives containing carboxylic acid group using Cu electrode. Meanwhile, we used the STM-BJ to measure the single-molecule conductance of oligofuran and BT using Au electrode. The main work is as follows:1. Using the statistical methods of logarithm and linear, we studied the conductance of carboxylic acid, carbonyl and hydroxy with benzene-based molecules to research the anchoring sites for carboxylic acid group contacting with Cu electrode. The results proved that both-C=O and-COO’can anchor to the Cu.2. Using Cu electrode, the conductance of p-phthalic acid, m-phthalic acid and their derivatives with different -CII2-unit between phenyl and carboxylic acid were studied. It was found that there is no destructive quantum interference in m-phthalic acid with contact between phenyl and carboxylic acid directly, while destructive quantum interference appears with two-CH2-at the both sides of phenyl.3. We measured the conductance of benzenetricarboxylic acid (1,2,4-benzenetricarboxylic acid,1,2,3-benzenetricarboxylic acid and 1,3,5-benzenetricarboxylic acid), and found that benzenetricarboxylic acid contacts to Cu electrode mainly through two anchoring group. In addition, the emergence of additional carboxylic acid group leads to the reduction of molecular conductance. 5-nitro-1,3-benzenedicarboxylic acid and 5-methoxy-1,3-benzenedicarboxylic acid are also used to verify this suspecting. The results proved that the conductance value decreases with electron withdrawing side group, and increases with electron donating group for the current system.4. We systematically explored the relationship between the conductance and length of oligofuran using Au electrode. Gnerally, the results showed that the single-molecule conductance decreased with the increase of molecular length. But there is a unusual situation appear in F5. We believe that this may contribute to the HOMO-1 of F5 which is more close to the Fermi of Au. Meantime, the conductance of oligofuran is always higher than oligothiphene, we attribute it to the length of the oligofuran which is obviously short than oligothiphene.5. We explored the relationship between the conductance and aromaticity for BT. The result showed that there is no obvious correlation between conductance and aromaticity. The conductance of BT may not simply relate to the aromaticity, the electronegativity, and molecular space structure may also cause the phenomenon above.