Posted: June 7, 2007

Surface transforms self-organization

(Nanowerk News) Future organic electrical and optical devices could be based on thiophenes, including oligothiophenes, because of their potential for improved and tunable properties. Devices based on these materials also offer the incentive of easy fabrication via self-organization. However, the assembly of these molecules depends not only their individual properties, but also on their arrangement on and interaction with a surface. A research team led by WAN Lijun from the Chinese Academy of Scienecs (CAS) Key Laboratory of Molecular Nanostructure and Nanotechnology affiliated to the CAS Institute of Chemistry has used scanning tunneling microscopy (STM) to investigate two oligothiophenes on Au and highly oriented pyrolytic graphite (HOPG) surfaces. Their work has been reported in a recent issue of Proceedings of the National Academy of Sciences (PNAS).
Surface transforms self-organization
"Oligothiophenes have a well-defined chemical structure and are used as a model molecule for the study of self-organization," explains Wan. The molecules also have novel properties that are promising for electronic and optical devices. Both types of oligothiophene form adlayers on HOPG and Au(111) but the surface has a marked effect on the self-organization of the molecules. On HOPG, both oligothiophenes form highly ordered adlayers. One molecule forms both linear and quasi-hexagonal adlayers, while the other forms a wave-like adlayer and, very occasionally, a lamella structure. In contrast, both molecules randomly adsorb onto Au(111) surfaces.
"Although the adlayer structure of oligothiophenes has been investigated previously, the structural transition with thermal annealing, substrate effect, and molecular properties are rarely reported," says Wan. So the researchers also observed the adlayers after thermal annealing. For the oligothiophene that exhibits both linear and quasi-hexagonal adlayers on HOPG, thermal annealing results in the disappearance of the quasi-hexagonal adlayers. Ultimately, thermal annealing results in a disordered structure. The affect that temperature has on adlayer structure could be very important for the stability and efficiency of devices.
The researchers explain the differences in adlayer structure in terms of the interaction between the oligothiophene molecule and the surface. There is a weaker interaction between the HOPG surface and the molecules compared with Au(111). Molecules can, therefore, move more easily on the HOPG surface and self-organize into ordered structures. In contrast, the stronger interaction between the Au(111) surface and the molecules reduces their mobility and leads to a more random adsorption.
"The work is a very important step from adlayer observation to adlayer application," says Wan.
Source: CAS