Electronic State of Molecular Devices at Room Temperature


Touichiro Goto and Yoshiaki Kashimura
Materials Science Laboratory

 Molecular devices with a few molecules have attracted considerable attention as next generation electronic devices because of their high-density and low-power nature. The characteristics of molecular devices can be tuned by controlling molecules. However, there have only been a few reports about molecular devices operating at room temperature. In addition, the instability of the junctions between electrodes and molecules has prevented research on the characteristics of molecular devices.
 In this study, we fabricated gold nanogap electrode devices with conjugated molecules. Conjugated molecules have a functional group at each end, which selectively chemisorbs to gold. First, we measured the electronic properties of molecular devices with terphenyldithiol (TPDT), which is 1.5 nm long. We observed a Coulomb diamond, i.e. the single-electron charging effect, at room temperature [Fig. 1]. A simulation based on single-electron circuits showed that the device characteristics can be explained on the basis of a multi-metallic-island system. We consider that gold islands between the electrodes work as single-electron islands, and TPDT bridging metallic islands and the electrodes work as tunnel junctions [1](In cooperation with Nanodevices Research Group).
 Next, we synthesized rigid conjugated polymer poly(p-phenylene-ethynylene)s (PPEs) with thioacetyl end groups (TA-PPE). TA-PPE self assembled in the gold nanogap electrodes (18-nm gap) and we measured its electronic properties. The room temperature current-voltage (conductance-voltage) characteristics exhibited periodic, repeatable, and identical stepwise features [Fig. 2]. First-principle calculations based on the resonant tunneling effect suggest that the equidistant steps result from the opening of different conducting channels that correspond to the unoccupied molecular orbitals of TA-PPE [2].
 It is intriguing that molecular devices show Coulomb diamond and quantized electronic structures at room temperature. As the next stage, we will study the characteristics of biomolecules in nanodevices.

[1] T. Goto et al., Jpn. J. Appl. Phys. 45 (2006) 4285.
[2] W. Hu et al., Phys. Rev. Lett. 96 (2006) 027801.

Fig. 1. Conductance coutour plot of TPDT at room temperature. The inset shows the TPDT structure.
Fig. 2. TA-PPE structure and current-voltage characteristics at room temperature. The solid lines are experimental results and the broken lines are calculations.

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