posted 15 Feb 2021, 11:54 by Peter Boggild
University of Manchester This talk aims to demonstrate how atomic resolution transmission electron microscope imaging is being used to support and enable the development of 2D materials and their heterostructures. The possibility to create new ‘designer’ materials by stacking together atomically thin layers extracted from layered materials with different properties has opened up a huge range of opportunities, from new optoelectronic phenomena [1], modifying and enhancing electron interactions in moire superlattices [2], to creating a totally new concept of designer nanochannels for molecular or ionic transport [3]. The impressive progress being achieved in the field crucially depends on knowledge of the atomic structure of these heterostructures, which in many cases can only be analysed by transmission electron microscopy (TEM) techniques. In this talk I will try to illustrate this statement with some of our recent work. For example, plan view imaging of point defect dynamics in graphene encapsulated monochalcogenides, GaSe and InSe (Fig. 2) [4].
Cross sectional imaging allows analysis and also prediction of the  microstructures produced when 2D van der Waals material (graphite, boron nitride, MoSe2) are subjected to mechanical deformation [5]. We find that above a critical thickness the materials exhibit numerous twin boundaries and for large bend angles these can contain nanoscale regions of local delamination (Fig.1). Such features are proposed to be important in determining how easily the material can be thinned by mechanical or liquid exfoliation.[5] We finally demonstrate study of twisted bilayer structures of semiconducting transition metal chalcogenides where we see unexpected structural relaxation, different to that observed in bilayer graphene. Complementary scanning tunnelling measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties [6].[1] Zultak, Nature Communications 11 (1), 1-6 (2020)
[2] R. Krishna-Kumar, Science 357, 181-184 (2017)
[3] B. Radha et al. Nature 538, 222–225 (2016) and Keerthi, et al Nature 558 (7710), 420-424. (2018)
[4] Hopkinson et al ASC Nano 13 (5), 5112-5123 (2019)
[5] Rooney et al 9, Nature Communications, 9, 3597, (2018) [6] Weston et al Nature Nanotechnology, in press (2020).
Sarah Haigh is Professor of Materials in the Department of Materials at University of Manchester, Director of the Electron Microscopy Centre (the largest in the UK), and Deputy Director of the BP International Centre for Advanced Materials. Her group applies advanced transmission electron microscopy (TEM) techniques to understand nanomaterial performance and she holds an ERC Starter Grant developing in situ TEM techniques with 2D heterostructures.
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