Electrical Conductance of Single Oligothiophene Molecular Wires: Temperature Effect
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Electrical Conductance of Single Oligothiophene Molecular Wires: Temperature Effect See Kei Lee1, Ryo Yamada1, Shoji Tanaka2 and Hirokazu Tada1 1 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan. 2 Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Okazaki, Aichi, Japan. ABSTRACT We investigated temperature dependence of the electrical conductance of single oligothiophene molecular wires with the length of 2.2 nm (5-mer), 5.6 nm (14-mer) and 6.7 nm (17-mer) by using the scanning tunneling microscopy break junction method. Results show that the dominant charge carrier transport for 5-mer molecule is tunneling while for 17-mer molecule is hopping. The carrier transport mechanism of 14-mer are tunneling transport (T ≤ 350 K) and hopping transport (T > 350 K) indicating that hopping and tunnelling transport are competitive process in the molecular junction. INTRODUCTION Since the proposal for the concept of single molecule devices by Aviram and Ratner [1], it have been of great interest to understand the mechanism of charge transport of single molecules bonded between metal electrodes [2]. It had been shown theoretically and experimentally that conductance of a metal/single molecule/metal (MMM) junction shows tunneling behavior [3-5], expressed as:
where G0, is the contact conductivity, β is the pre-exponential factor representing the efficiency of charge transmission through molecules and l is the distance between two electrodes. Theoretical studies have predicted the crossover of tunneling and hopping transport mechanism [6-7]. When the molecule becomes longer, the contribution of tunneling mechanism would get weaker and eventually be replaced by hopping. The transition of charge transport mechanism can be observed as a change of the scaling law because the hopping transport is expected to show linear distance dependence. We have previously reported the change of the scaling law by using oligothiophene molecular wires with the lengths ranging from 2 nm to 9 nm [8,9]. Similar results were also reported by other groups [10-12]. The conductance due to tunneling does not show any temperature dependence whereas the conductance due to hopping obeys the Arrhenius equation given by:
where EA is the activation energy, G0 is the contact conductivity, T is the temperature and kB is Boltzman constant. Since the contribution of the hopping depends on the temperature, the crossover length between tunneling and hopping can change as a function of the temperature. In
the present study, we have measured the temperature dependence of the conductance to clarify the conduction mechanism. EXPERIMENT Figure 1 shows the structure of the oligothiophene molecular wire used in the present study. The molecules are described as (2+3m)T-di-SCN or (2+3m)-mer where m = 1~5, i.e., 5~17-mer. The molecule used in this study was synthesized by using the general synthesis protocols mentioned previously [13-15]. The oligothiophene contains -SCN group attached to both ends of the molecule
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