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Humidity-controlled rectification switching in ruthenium-complex molecular junctions

Abstract

Although molecular rectifiers were proposed over four decades ago1,2, until recently reported rectification ratios (RR) were rather moderate2,3,4,5,6,7,8,9,10,11 (RR ~ 101). This ceiling was convincingly broken using a eutectic GaIn top contact12 to probe molecular monolayers of coupled ferrocene groups (RR ~ 105), as well as using scanning tunnelling microscopy-break junctions13,14,15,16 and mechanically controlled break junctions17 to probe single molecules (RR ~ 102–103). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 100 and RR ≥ 103) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current–voltage (IV) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF6 ). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.

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Fig. 1: Experimental set-up and molecular systems studied.
Fig. 2: Experimental current–voltage characteristics of the 1-Ru-N and 2-Ru-N molecular junctions.
Fig. 3: RR data for all molecular junctions and tip-radius dependent data for 2-Ru-N.
Fig. 4: Theoretical model of the electronic states of the 2-Ru-N molecule for dry and humid conditions.
Fig. 5: Theoretical IV characteristics for 1-Ru-N and 2-Ru-N molecules.

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Acknowledgements

We thank F. Galli for technical support and advice. We acknowledge financial support from the Swiss National Science Foundation (grant no. 200020-144471), as well as from the JSPS KAKENHI (grant no. JP17H05383; Coordination Asymmetry, Japan) and the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan, and from the Netherlands Organisation for Scientific Research (NWO) via FOM programme no. 141.

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H.O. and M.H. designed and synthesized the molecules. H.A., V.K., P.B., H.O. and M.H. characterized the SAMs. H.A. and V.K. performed the C-AFM measurements. H.A., V.K. and S.J.v.d.M. designed the experiment and performed data analysis. J.A.C.G. and J.M.T. performed the calculations. H.A., V.K., J.A.C.G., J.M.T. and S.J.v.d.M. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Veerabhadrarao Kaliginedi or Sense Jan van der Molen.

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Atesci, H., Kaliginedi, V., Celis Gil, J.A. et al. Humidity-controlled rectification switching in ruthenium-complex molecular junctions. Nature Nanotech 13, 117–121 (2018). https://doi.org/10.1038/s41565-017-0016-8

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