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Charge disproportionate molecular redox for discrete memristive and memcapacitive switching

Abstract

Electronic symmetry breaking by charge disproportionation results in multifaceted changes in the electronic, magnetic and optical properties of a material, triggering ferroelectricity, metal/insulator transition and colossal magnetoresistance. Yet, charge disproportionation lacks technological relevance because it occurs only under specific physical conditions of high or low temperature or high pressure. Here we demonstrate a voltage-triggered charge disproportionation in thin molecular films of a metal–organic complex occurring in ambient conditions. This provides a technologically relevant molecular route for simultaneous realization of a ternary memristor and a binary memcapacitor, scalable down to a device area of 60 nm2. Supported by mathematical modelling, our results establish that multiple memristive states can be functionally non-volatile, yet discrete—a combination perceived as theoretically prohibited. Our device could be used as a binary or ternary memristor, a binary memcapacitor or both concomitantly, and unlike the existing ‘continuous state’ memristors, its discrete states are optimal for high-density, ultra-low-energy digital computing.

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Fig. 1: Molecular and thin film electronic properties.
Fig. 2: Test structures and electrical characterizations.
Fig. 3: Compact memristor model and state trajectory.
Fig. 4: Identification of molecular states.
Fig. 5: Reaction coordinate diagram and the role of temperature in stabilizing the charge disproportionate state.
Fig. 6: Role of counterions in charge disproportionation.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request.

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Acknowledgements

We thank A. Castiñeiras for solving crystal structures, S. Kumar for helpful discussions on the mathematical modelling, A. Tempez for discussions regarding c-AFM measurements and M. Lal for helping with the nanostructure. T.V. and Sreetosh Goswami acknowledge support from NRF-CRP15-2015-01. Sreetosh Goswami thanks the NGS for funding support. Sreebrata Goswami acknowledges the financial support from SERB, India through grants SR/S2/JCB-09/2011 and EMR/2014/000520. D.T. thanks Science Foundation Ireland (SFI) for support (awards number 15/CDA/3491 and 12/RC/2275_P2) and for computing resources at the SFI/Higher Education Authority Irish Center for High-End Computing (ICHEC). C.A.N. acknowledges the Ministry of Education (MOE) for supporting this research under award no. MOE2015-T2-2-134.

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Authors

Contributions

Sreetosh Goswami devised the project and designed the experiments. Sreetosh Goswami performed the electrical and in situ spectroscopic measurements. S.P.R. and R.P. carried out the chemical synthesis and characterization under the supervision of Sreebrata Goswami. S. Hooda performed the RBS measurements. B.R.I. fabricated the nanopatterns. D.T. performed the theoretical calculations. M.A. performed the c-AFM measurements. R.S.W. developed the mathematical model. Sreetosh Goswami, R.S.W., T.V., Sreebrata Goswami, J.M., D.T., S. Hedström and S.S. performed data analysis. C.A.N. participated in discussions. Sreetosh Goswami, R.S.W., D.T., Sreebrata Goswami, J.M. and T.V. wrote the paper.

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Correspondence to Sreetosh Goswami or Damien Thompson or Jens Martin or Sreebrata Goswami or T. Venkatesan.

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The authors declare no competing interests.

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Peer review information Nature Nanotechnology thanks Alec Talin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Multiple repeated traces.

500 repeated J(V) traces obtained from (a) structure A, (b) structure B and (c) structure C. The translucent cloud is the overlap of all the traces and the solid lines show one representative characteristic. The spread of the cloud indicates the maximum current variation in different traces which are orders of magnitude smaller than the conductance differences between the plateaus.

Extended Data Fig. 2 Non-volatility of the three states.

Grey curve: J(V) for full voltage sweep. Blue curve: J(V) with the forward scan stopped at the positive intermediate plateau (that is, V<V2, see Fig. 2) demonstrating that 11-state is robust and readable. Red curve: J(V) with the backward scan stopped in the negative intermediate plateau (that is, V>V4, see Fig. 2). This proves that the intermediate state is stable, and our device can be used both as binary or ternary memristors, that is both the upper and lower memristive loops can be used for (a) structure A, (b) structure B and (c) structure C. Arrows indicate the direction of the voltage sweep.

Extended Data Fig. 3 Effect of nano structures.

The J(V)s obtained for structure A, B and C are plotted in the same scale to visually illustrate the effect of nano structures in reducing switching voltage by ~30 times and increasing switching current by ~ 6 orders of magnitude.

Extended Data Fig. 4 Endurance.

a Response of structure B to pulses with 30 ns rise time. The current overshoot indicates that the switching time is <30 ns. b Endurance of structure B over ~1010 cycles measured with pulses as shown in (a) showing no indication of any degradation.

Extended Data Fig. 5 Retention and stability.

a, Pulse test to prove retention of structure B at 0 V. After writing, reading operations were performed after 1 hour, 2 hours and 2 days with the written state intact. b, Stability of individual conductance states: Three devices (structure B) with the same dimensions were fabricated and set into three different conductance states: 00 (on), 11 (intermediate), and 31 (off). All devices were measured simultaneously with constant application of read voltage V=500 mV at 350 K.

Extended Data Fig. 6 Top and bottom electrode variation.

The qualitatively similar J(V) response of the film in different electrode combinations, viz. (a) Pt/film/Pt, (b) Au/film/Pt, (c) Au/film/Au proving that the switching phenomenon is not an interface-specific phenomenon. The switching with noble metal electrodes also excludes the role of atomic/ ionic migration of the electrode materials in our observed switching.

Extended Data Fig. 7 Device characteristics with ITO top electrode.

The J(V) of devices with ITO as the top electrode for (a) structure A and (b) structure B. These structures are used for performing in-situ spectroscopic measurements. The current levels are similar to Au/film/ITO structure with small differences in switching voltage values due to the different work function of ITO. Consistent with redox mechanism, because of lower work function of ITO compared to Au, the switch-on voltage values with ITO top electrode increase whereas the switch-off voltages decrease.

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Supplementary Information

Supplementary Figs. S1–S38, Tables S1–S5 and refs. 1–39.

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Goswami, S., Rath, S.P., Thompson, D. et al. Charge disproportionate molecular redox for discrete memristive and memcapacitive switching. Nat. Nanotechnol. 15, 380–389 (2020). https://doi.org/10.1038/s41565-020-0653-1

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