Atomic resolution dynamics imaged using 2D heterostructure liquid cells

Abstract text (incl. figure legends and references)

Stacking 2D materials to build van der Waals (vdW) heterostructures can result in artificial crystals with perfect interfaces, enabling investigation and exploitation the unique electronic and optical properties of 2D crystals [1]. Transmission electron microscope (TEM) imaging has provided essential input on the nature of local defects and interface structure during the development of such vdW heterostructures [2]. For example, we have used scanning TEM (STEM) imaging to observe the atomic reconstruction that occurs when two transition metal dichalcogenide crystals are stacked with a small twist angle (Fig 1a) [3,4]. We have also used vdW heterostructures to study chemical degradation for air sensitive 2D materials such as CrBr3 TaS2 and GaSe, where mechanical exfoliation in an argon glove box and graphene encapsulation is used to protect the 2D material before TEM imaging [5-7]. VdW heterostructures also enable pioneering study of confined gases and liquids. Nanochannels with precisely controlled dimensions can be integrated in to the heterostructure by lithographic etching of specific layers but our TEM imaging is needed to ensure reliable, contamination free fabrication (Fig. 1b) [8,9].

Nanosized pockets of fluid are also highly desirable for TEM studies of atomic behavior in liquids. Commercial in-situ liquid cells enable TEM of liquid samples but the thick silicon nitride windows often prevent atomic resolution imaging and can also limit the sensitivity of spectroscopic analysis. Graphene liquid cells overcome these restrictions but the original fabrication method, suffers from poor control of the liquid dimensions and results in cells with limited stability. We have pioneered a new approach to fabrication of liquid cells based on 2D heterostructure stacking technology [10]. Our 2D heterostructure liquid cell approach also enables liquid mixing, triggered by the electron beam (Fig. 1c), allowing studies of the earliest stage of chemical synthesis at the atomic scale; something that was not previously possible by any technique [11]. Our experimental set-up contains two pockets of liquid separated by an atomically thin membrane, where mixing is induced by nanofracture of the separation membrane with the electron beam [11]. Furthermore, our platform enables the first studies of adatom dynamics at solid liquid interfaces (Fig. 1d)[12] and provides a route to understanding the enormous differences in ionic diffusion behavior for interplanar spaces as a result of changes to local crystal structure.[13]

References:

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Figure 1. (a) Lattice reconstruction in a twisted WS2-WS2 bilayer (from [3]) (b)STEM imaging of 2D nanochannels,(from [9]). (c) Our TEM mixing cell platform (from [11]). (d) STEM frame from a liquid cell movie (ref [12]).