Synthesis Of Novel Molecular Wires And Investigation On Their Molecular Conductance

The pursuit of molecular electronics has created an increasing interest in the electrical measurement of single molecule in past decades. Understanding the electron transport properties in metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. However, the fabrication and measurement of single molecule junction are still very challenging today due to many technical difficulties.This thesis aims to understand the electron transport mechanism in various organic molecular wires by studying their molecular conductance in metal/molecule/metal junctions. We firstly set up two measurement systems capable of conducting electrical measurements in nano-scale junctions, including STM break junction and crossed-wire junction. Meanwhile, we design two series of novel molecular wires and investigated the electron transport properties in these molecular wires.The main results of the thesis are listed as following:●We have successfully setup two highly sensitive electrical measurement systems, including a STM break junction system for measuring single molecule conductance and a crossed-wire junction system for measuring 1-V curves of self-assembled monolayers (SAMs).●An systematic electron transport investigation on a series of "core-shell" structured oligo(phenylene ethynylene)s (OPEs) molecular wires has been conducted. By using dendrimers of different generations as insulating "shells", the intermolecularπ-πinteractions between the OPE "cores" can be precisely controlled in single component monolayers. Three techniques were used to evaluate the electron transport properties of the Metal/Molecule/Metal molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break junction techniques. STM break junction measurement reveals that the electron transport pathways are strongly affected by the size of the side groups. We demonstrated that using different generation dendrimer groups as shells not only allows tailoring the molecular packing density, but also enables a good control to the intermolecular interaction and electron transport pathways.●A study of electron transport in a series of conformation controlled OPE molecular wires by STM break junction has been reported. We find that the conductance for these molecular wires decreases with the increasing twist angle consistent with a cosine-squared relation. To gain further understanding of the electron transport in these molecular wires, we applied an ab-initio calculation with the first-principle package ATK to simulate the molecular wires electrical properties, which is based on DFT and NEGF technology. The calculated results are in good agreement with the experimental results.