Polyoxometalate (POM) derivatives have been reported to show various specific electronic properties. For example, (NH₄)₆V₁₀O₂₈ and H₇SiW₉V₃O₄₀ show negative differential resistance (NDR) when measured by scanning tunneling spectroscopic (STS) technique in air.^1,2 We have reported that a composite of H₃PMo₁₂O₄₀ with single-walled carbon nanotubes (SWNT) showed complex phenomena depending on the size of the POM nanoparticles. When the size of the POM was smaller than ~6 nm with a semiconducting SWNT, the composite showed rectification with positive bias suppression. By contrast, if the size of the POM was larger, the suppression was observed at negative bias. However, when the SWNT is metallic, the suppression occurred at the opposite bias.³

There are several POM/porphyrin derivatives or analogues reported, saddle-distorted Mo(V)-dodecaphenylporphyrin with Keggin-type H₂SiW₁₂O₄₀,⁴ cyclo[8]pyrrole with H₄SiW₁₂O₄₀,⁵ tetraphenylporphyrin (TPP) with SW₁₀V₂O₄₀.⁶ The electronic properties of these complexes are interesting to measure in macro-scale and in single molecule-scale, because of the versatile electronic behaviors are expected from multi redox characters of POM cores tuned by the surrounding porphyrin derivatives.

The single crystal structure of [H₄TPP][SV₂W₁₀O₄₀] was successfully determined, in which one POM anion is surrounded by four H₄TPP cations. Although this complex is dissociated in solution, the casted sample retains crystalline structures partially as observed by the powder X-ray diffraction study. Devices were made by casting the solution of the complex on Au electrodes with 5 μm gap. The current-voltage curve at ambient conditions showed broad negative differential resistance (NDR) with large hysteresis. The impedance measurements between 10^-1 to 10⁶ Hz at 0.0 to 3.0 V DC bias indicated that in low frequency range (<10 Hz) the impedance is lower at high DC bias voltage. However, in high frequency range (> 10Hz) the impedance is almost independent on voltage. The impedance results indicate that non-linear NDR behavior observed in DC measurement is because of a slow process of the carrier movement, probably by ionic conductance.

References

(1) Ogawa, T.; Handayani, M. In Molecular Architectonics; Ogawa, T., Ed.; Springer Nature: Switzerland, 2017, p 419.

(2) Watson, B. A.; Barteau, M. A.; Haggerty, L.; Lenhoff, A. M.; Weber, R. S. Langmuir 1992, 8, 1145.

(3) Hong, L.; Tanaka, H.; Ogawa, T. J. Mat. Chem., C 2013, 1, 1137.

(4) Yokoyama, A.; Kojima, T.; Ohkubo, K.; Fukuzumi, S. Chem. Commun. 2007, 3997.

(5) Okujima, T.; Matsumoto, H.; Mori, S.; Nakae, T.; Takase, M.; Uno, H. Tetrahedron Lett. 2016, 57, 3160.

(6) Shi, Z. H.; Zhou, Y. S.; Zhang, L. J.; Mu, C. C.; Ren, H. Z.; ul Hassan, D.; Yang, D.; Asif, H. M. RSC Adv. 2014, 4, 50277.