Charge Transport Mechanisms in Oligothiophene Molecular Junctions Studied by Electrical Conductance and Thermopower Measurements
Understanding charge transport mechanism of single-molecule–metal–molecule junctions is important in the field of molecular electronics. Till now, most of the reported works focused on small molecules, where tunneling transport dominates the charge transport. As the length of the molecule increases, the charge transport is expected to show a transition from tunneling to hopping. In this work, we performed a comprehensive investigation on oligothiophene molecules. We have measured the temperature dependence of electrical conductance and thermopower of oligothiophene molecular junctions with molecular lengths ranging from 2.2 nm (5T-di-SCN) to 7 nm (17T-di-SCN) using the homebuilt scanning tunneling microscope. The conductance measurement results reveal that the dominant charge transport for oligothiophene changed from tunneling to hopping transport at molecular length of ca. 5 nm. In addition, the thermopower for all the oligothiophene molecules was found to be positive, indicating the transport of charge carrier through the highest occupied molecular orbital level.
KeywordsElectrical conductance Thermopower Molecular junctions Oligothiophenes Charge transport
This work was supported by JSPS KAKENHI Grant Numbers JP20343741, JP15K13673 and JP25110012. L.S.K. would like to thank Henrique Rosa Testai for assistance in the thermoelectric experiments.
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