Fast Ion Exchange, Chemical Synthesis and Atomic Motion in Liquids Studied Using 2D Heterostructures and Scanning Transmission Electron Microscopy

This talk will present recent work showing atomic resolution scanning transmission electron microscopy (STEM) imaging and analysis can provide new information to guide the development and applications of novel 2D materials. It will then go on to show how 2D heterostructure technology has enabled in situ STEM investigations with atomic resolution and uncompromised elemental analysis, providing new insights into surface dynamics and early-stage chemical synthesis.<br/>In part 1 the talk will demonstrate how we have used a STEM-based snap-shot approach to investigate ion exchange in atomically thin clays and micas. We find that in vermiculite clay samples the speed of ion exchange in the 2D interlayer space is many orders of magnitude faster for samples that consist of only 2 aluminosilicate layers compared to the bulk [1]. By studying &gt;60 samples of different thickness we find that this behaviour tends towards the bulk for samples more than ~6 atomic layers. The behaviour is explained by the increased swelling measured in atomically thin samples via liquid phase atomic force microscopy. These atomic scale observations have been exploited to produce membranes composed of few-layer vermiculite which demonstrate ion exchange in minutes compared to the days needed for bulk samples, suggesting their potential use for optimisation of membrane applications [2]. We also observe ion exchange islands are formed in the twisted interlayer space for biotite samples [1], with potential application in novel quantum systems[3].<br/>Secondly, I will how we are applying 2D heterostructure to advancing our understanding of solid-liquid interfaces and liquid phase chemical reactions. Liquid-phase transmission electron microscopy offers a unique combination of nanometer spatial resolution and millisecond temporal resolution. It has provided many useful insights into nanoscale reaction dynamics but achieving sub-nanometer resolution has proved difficult due to limitations in the current liquid cell designs. In this talk I will present results showing atomic resolution liquid phase scanning transmission electron microscopy investigations enabled using insights from 2D materials nanochannel fluidics [4,5] and achieved with our two-dimensional heterostructure-based graphene liquid cells [6]. I will demonstrate that the approach facilitates in situ atomic resolution imaging and elemental analysis to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment [7]. I will also demonstrate the latest application of the technique to studying the dynamic motion and preferred resting sites of single atom metallic species on surfaces in aqueous salt solution.<br/>[1] Ion exchange in atomically thin clays and micas. Zou et al. Nature Materials (2021) https://www.nature.com/articles/s41563-021-01072-6<br/>[2] Atomically thin micas as proton-conducting membranes. L. Mogg, et al, Nature Nanotechnology, 14, 962–966 (2019)<br/>[3] Atomic reconstruction in twisted bilayers of transition metal dichalcogenides, A. Weston, et al. Nature Nanotechnology, 15, 592–597 (2020)<br/>[4] Capillary condensation under atomic-scale confinement, Q Yang et al, Nature 588 (7837), 250-253 (2020);<br/>[5] Ballistic molecular transport through two-dimensional channels, A Keerthi et al, Nature 558 (7710), 420-424 (2018);<br/>[6] Nanometer resolution elemental mapping in graphene-based TEM liquid cells, Kelly et al Nano Letters (2018) 18 (2), 1168-1174<br/>[7] In situ TEM imaging of solution-phase chemical reactions using 2D-heterostructure mixing cells Kelly et al, Advanced Materials, (2021) https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202100668