title: Electron Transport and Electrochemical Control of Oligo(phenyleneethynylene) Based Molecular Devices
Creators: name: Fan, Yinqi
Publisher: name: University of Liverpool Repository
Year: 2025

description: The essential goals of molecular electronics are to understand the mechanism of electron transport in molecules and realise the molecular devices with controlled functionality. Many studies have shown that the mechanism of electron transport in molecular junctions is complex. The molecule itself, anchoring group, electrode material and the environment all affect the conductivity of molecular junctions. To facilitate the applications of molecular-scale electronic devices, an in-depth understanding of different factors on the electrical properties of molecular junctions is essential. This project mainly uses oligo(phenyleneethynylene) (OPE) based molecules as a model system to investigate the electrical properties of molecular devices and understand how the chemistry and electronic structure of the molecule and contacts can determine their electrical characteristics. First, the molecular junctions of OPEs with Au/Au electrodes were formed using STM-BJ and STM-I(s) techniques. The experimental results show that the measured conductance of OPEs is different by two different techniques. The contact resistances and attenuation factors of target molecular junctions were calculated to study the length dependence of molecular junctions formed by different techniques. The molecular junctions formed using STM-BJ technique show more obvious length dependence than those formed using STM-I(s) technique. Moreover, anchoring groups used to bind the molecular backbone to the electrical contacts seem to play an important role in the conductance and length dependence of OPEs. For linking long-range OPEs, amine is a more suitable anchor to connect with electrodes than methyl sulfide, which offers a better conductivity. Then, graphene was introduced to replace an Au electrode and formed Au/OPEs/graphene asymmetric molecular junctions. Current results show that asymmetric molecular junctions have lower conductance and attenuation factors than symmetric molecular junctions. The sharp decrease in conductance caused by the destructive quantum interference (DQI) was discussed by comparing the conductance values of para- and meta- connected OPEs with Au/Au electrodes and Au/graphene electrodes. For both Au/Au system and Au/graphene system, the DQI of meta-OPEs leads to much smaller conductance values than para- OPEs. It is worth mentioning that the DQI effect on molecular conductance varies with the electrodes and anchoring groups. For all the studied molecular junctions, the conductance values of ethynyl-anchored OPEs with Au/graphene electrodes show the most significant DQI effect when compared to amine and methyl sulfide terminated molecular junctions. Last, the electrochemical control of methyl sulfide terminated OPEs was achieved using the electrochemical control STM (EC-STM) technique. The experiment results show that the OPEs with DQI exhibit switching characteristics. The conductance slightly increases when the positive potentials are applied to the molecule. When the negative potential is applied, the conductance of meta-connected OPE is significantly reduced. Due to the detection limit of the instrument, the molecular conductance is too small to be detectable at potentials greater than -0.4 V. For the overpotentials used in this study, the difference in conductance values of meta-connected OPEs reaches two orders of magnitude under electrochemical control. In summary, this project shows that electron transport in OPEs is complexed and affected by many factors. The conductivity of OPEs is strongly influenced by the coupling between anchoring groups and electrodes. The conductance of OPEs with the DQI effect can be modulated by changing the electrodes or the anchoring groups. Finally, the conductance of OPEs with the DQI effect shows magnitudes changes under the electrochemical control. Our work holds future technological significance as it could generate new insight towards large switching ratio molecular devices.