Alignment and rectifying properties of donor-electron bridge-acceptor molecules

Abstract

Molecular electronics based on the bottom-up approach appears to be a promising alternative to overcome the limitations of the top-down lithographic fabrication of electronic devices. The ability to manipulate single or small groups of molecules provides a great opportunity to build electronic devices at the molecular level. However, before any device can be constructed, it is vital to understand the parameters that control the device properties such as: molecular structure, conformation and arrangement at the surface, the molecule-substrate and molecule-electrode interactions. This thesis presents an investigation of the alignment of acceptor-electron bridge-donor structures and describes how the molecular structure and arrangement affect rectifying properties of the monolayers. Studies included typical Langmuir-Blodgett (LB), chevron-shaped, and ionically coupled structures that were characterised using various techniques, such as Quartz Crystal Microbalance (QCM), Surface Plasmon Resonance (SPR), Second Harmonic Generation (SHG) and Scanning Tunnelling Spectroscopy (STS). The results obtained showed that to achieve high rectification the molecules must form ordered and stable monolayers that are able to withstand the electric field applied to the junction. It was also shown that due to the disordered monolayer formation and presence of certain ions, it was extremely difficult to state without doubt whether the rectification was a result of the donor-electron bridge-acceptor structure proposed by Aviram and Ratner1. Studies of chevron-shaped molecules confirmed the possibility of depositing them using the LB technique. However, the reduction of long aliphatic chains was very likely balanced by the formation of less ordered or unstable monolayers. The highest rectification ratio of 30 ± 3 at ± 1 V was obtained for 1-butyl-2,6-bis-[2-(4- dibutylamino-phenyl)-vinyl]-pyridinium iodide (dye 7) and the origin of the I-V asymmetry was attributed to back electron transfer from iodide to pyridinium ring. Although dye 1-butyl-2,6-bis-(2-{4-[2-(4-dibutylamino-phenyl)-ethyl]-phenyl}-vinyl)- pyridinium iodide (dye 9) showed electrical asymmetry (RR=16 at plus/minus 1 V) shortly after deposition onto the gold-coated highly oriented pyrolytic graphite (HOPG), it seemed to form an unstable alignment and as a consequence the rectification decayed over a period of a few hours. Improved ordering, stability, and rectification were achieved from ionically coupled structures, where the monolayers were formed using chemisorption and ionic assembly instead of physisorption.