Recently there has been a lot of interest in conjugated molecular wires self assembled on noble metals due to their potential applications in molecular electronics (1). Electronic transport through organic molecules, in particular the energies of the molecular orbitals involved, the effect of delocalized π electrons and that of specific substituent groups can be probed by scanning tunneling microscopy/spectroscopy (STM/STS). We have synthesized different types of molecular wires and self assembled them on Au(111). In this abstract we report their structural and electronic properties.
The arenethiol studied has a three ring conjugated structure with a central pyridine unit. This molecule was chosen for its conjugated rigid rod structure, with nitrogen substituted for its electron affinity which could lead to interesting transport properties like switching. The self assembly on Au(111)/mica was preformed in a 1mM solution of the molecule in tetrahydrofuran.
The SAM was imaged with high junction impedances (1-10GΩ). On analyzing the images obtained it becomes apparent that there are at least two types of SAM structures. The first type does not show a clear grain formation even in large range scans (300nm) and resembles a layer of film on the Au(111). The steps in the underlying Au film and etch pits are seen and evidence that a monolayer indeed exists comes from spectroscopy. The other type of nanostructure, observed more infrequently, has clusters of closely packed grains (dia. 5-17nm). These two types of grain formation coexist on the same film and are imaged on moving the tip to different locations on the sample. STS on the SAM produce I-V spectra that though not linear, do not show a band gap around the Fermi level. This is very different from alkanethiols which show a finite gap.
In conclusion, inability of imaging the SAM with molecular resolution inspite of using very low tunnel currents and a variety of imaging conditions shows that it does not form an ordered structure. We have observed molecular ordering on alkanethiols. Molecular ordering has also been observed on a similar molecule without the nitrogen (2), which probably indicates that intermolecular forces due to the presence of the nitrogen prevent the formation of order. The fact that no gap is seen in the I-V shows that the molecules indeed act as wires.
38 nm X 37 nm STM image of the SAM. Bias: 50 mV, I: 10 pA.
C. Joachim et al., Nature, Vol. 408, p. 541, 2000.
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