CPM Seminar

Logic and memory functions in molecular tunnelling junctions

Hippolyte Astier

AMAT-NUS Corp Lab / Gradečak Group
National University of Singapore

The field of molecular electronics proposes to use molecules as electronic components and was initially envisaged as a route towards extreme circuit miniaturisation. The challenge in integrating these molecules into circuits in a scalable and reliable manner motivated intense research efforts and the proposal of several experimental approaches, notably eutectic GaIn[1], or graphene[2]. These effective platforms have allowed the community to turn towards seeking functionalities beyond standard tunnelling junctions using bespoke molecular structures. One such functionality is that of switches and memory devices, whereby by utilising chemical reactions within the molecule probed, its electronic structure can be changed to produce several distinct conductance states. In this presentation, I will discuss two such structures. Methyl viologen is a molecule that offers two very distinct states depending on the presence of a counter-ion balancing charge in the junction[3]. When biased, this counterion can be caused to migrate, leading to a complete rearrangement of the molecule and a dramatic change in conduction properties, with a ratio between high and low conductance states of 6.7×10³. This functionality is combined with a diode behaviour where the rectification ratio is 2.5×10⁴, thus producing a one-diode-one-resistor (1D1R) component, an attractive combination for logic cross-bar arrays. Interestingly this dual functionality is produced by a single molecular layer, and encoded in its chemical structure. In a second part, I will discuss molecular junctions containing 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA)[4]. These combine electron transport with proton exchange with their environment to produce intricate transport properties including multi-state memory, and negative differential resistance with peak-to-valley ratios of 13.7 ± 3.5 (at 10 mV s^-1). I will explain how these properties make these the first reported molecular-electronic artificial synapses for neuromorphic computing, as they demonstrate modulation plasticity in relation to signal amplitude, duration, and frequency. I will show examples of simplified networks showing their utilisation. Finally, I will present a new experimental approach to probe molecular junctions by manipulating micro-sized EGaIn droplets with a conductive atomic force microscope (AFM)[5]. This approach offers an easy way to probe molecular junctions of known geometry, and produces extremely robust junctions capable of withstanding large electric fields.

[1] Chiechi, R. C., Weiss, E. A., Dickey, M. D., & Whitesides, G. M. (2008). Eutectic Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers. Angewandte Chemie, 120(1), 148–150. https://doi.org/10.1002/ange.200703642
[2] Fruhman, J. M., Astier, H. P. A. G., Ehrler, B., Böhm, M. L., Eyre, L. F. L., Kidambi, P. R., Sassi, U., de Fazio, D., Griffiths, J. P., Robson, A. J., Robinson, B. J., Hofmann, S., Ferrari, A. C., & Ford, C. J. B. (2021). High-yield parallel fabrication of quantum-dot monolayer single-electron devices displaying Coulomb staircase, contacted by graphene. Nature Communications, 12(1), 1–10. https://doi.org/10.1038/s41467-021-24233-2
[3] Han, Y., Nickle, C., Zhang, Z., Astier, H. P. A. G., Duffin, T. J., Qi, D., Wang, Z., del Barco, E., Thompson, D., & Nijhuis, C. A. (2020). Electric-field-driven dual-functional molecular switches in tunnel junctions. Nature Materials, 19(8), 843–848. https://doi.org/10.1038/s41563-020-0697-5
[4] Wang, Y., Zhang, Q., Astier, H. P. A. G., Nickle, C., Soni, S., Alami, F. A., Borrini, A., Zhang, Z., Honnigfort, C., Braunschweig, B., Leoncini, A., Qi, D.-C., Han, Y., del Barco, E., Thompson, D., & Nijhuis, C. A. (2022). Dynamic molecular switches with hysteretic negative differential conductance emulating synaptic behaviour. Nature Materials 2022, 1–9. https://doi.org/10.1038/S41563-022-01402-2
[5] Soh, E. J. H., Astier, H. P. A. G., Daniel, D., Isaiah Chua, J. Q., Miserez, A., Jia, Z., Li, L., O’Shea, S. J., Bhaskaran, H., Tomczak, N., & Nijhuis, C. A. (2022). AFM Manipulation of EGaIn Microdroplets to Generate Controlled, On-Demand Contacts on Molecular Self-Assembled Monolayers. ACS Nano, 16(9), 14370–14378. https://doi.org/10.1021/ACSNANO.2C04667/SUPPL_FILE/NN2C04667_SI_004.MOV