Abstract:
As traditional devices containing silicon transistors begin to approach their physical
limits, new systems composed of organic molecules are being considered for molecularscale
devices of the future. The present work reports on the electrical properties of
molecular diodes, especially observations of electrical rectification from molecular
systems based on donor-(π-bridge)-acceptor molecules. For this purpose three types of
molecular assembly were incorporated and their growth was observed with the quartz
crystal microbalance (QCM) technique.
Covalent self-assembly proved to be the most efficient method of forming well-ordered
molecular films compared to those obtained via LB and ESA techniques. SAMs of
Q3CNQ molecules yielded higher rectification than their LB analogues and achieved
rectification ratio of 30 at ± 1V for every sample. On the other hand ESA films, in
which molecular alignment of the physisorbed cationic dye was controlled by selfassembly
of the anionic component, were probably more disordered, but exhibited
higher (and sample-dependent) rectification ratios with a maximum of 450 at ± 1V.
QCM also showed the phenomena of trapped water molecules within the physisorbed
ESA monolayer that affected molecular order and also the electrical properties of
the samples.
Scanning tunnelling microscopy (STM), incorporated for obtaining current-voltage
(I-V) characteristics from samples, showed that stearic hindrance has to be taken into
consideration when designing donor-(π-bridge)-acceptor rectifiers. Sufficient isolation
of donor and acceptor groups by the π-bridge is essential in order to prevent
delocalisation of molecular orbitals over the entire molecule. Therefore, implementation
of the Aviram-Ratner model of molecular rectification became possible although
molecules investigated here did not possess the proposed σ-bridge. Additionally,
the rectification effect arising from geometrical asymmetry induced by electrode-linking
alkyl chains was shown to be negligible here, which is contrary to other theories of
molecular rectification.
