It is important to design molecular junctions with molecules as active electrical
components, which may be applicable in future electronic devices. This thesis reports the electrical properties of molecular diodes, as well as molecular wires. The work is concentrated, to a large extent, on observations of molecular rectification from monolayer and bilayer assemblies based on sterically hindered A⁺-π-D dyes, and protonated molecular wires of different lengths, which are then ionically coupled with anionic strong donors. The second part presents the studies of the electric properties of various single-molecule wires. The growth of the molecular assembly process was observed with the quartz crystal microbalance technique (QCM).
Scanning tunnelling microscopy (STM) was used to analyse the electron transport through the molecule. I-V characteristics of these new types of organic rectifying junctions were obtained, with the high current being observed in the negative quadrant of the I-V plots. These devices exhibited rectification ratios of 20-500 at ±1 V, and even in excess of 3000 at ± 1 V for a particular type of A⁺-π-D¦D^- compound. This significant enhancement of the rectification behaviour was achieved by using ionically coupled structures; it may provide a solution to the ultimate challenge of electronic device miniaturisation.
Organic rectifying junctions which incorporated protonated forms of arylene-ethynylene molecular wires were seen to exhibit rectification with current ratios of 15-80 at ± 1 V, whereas, self-assembled monolayers (SAMs) of the neutral forms of these wires, on gold substrates, exhibited symmetrical I-V curves when contacted by gold and PtIr tips. The 7 nm and the 10 nm wires presented in this thesis are the longest to date to be used for single-molecule electrical studies.
The spontaneous formation of stable molecular wires between the tip and the sample was monitored using I(t) and I(s) methods, for the measurements of single-molecule current. Electrical contact between the molecule and the gold probe was achieved by the use of thiol groups present at each end of the molecules. Histograms of the measured current jumps at a constant sample bias of -0.3 V were prepared to assess the single molecule current.