2D crystals can be layered together to create new van der Waals crystals with bespoke properties. However, the performance of such materials is strongly dependent on the quality of the crystals and the interfaces at the atomic scale. Transmission electron microscopy (TEM) is the only technique able to characterize the nature of buried interfaces in these engineered van der Waals crystals and hence to provide insights into their optical, electronic and mechanical properties. I will report the use of scanning TEM imaging and analysis to aid the development of 2D heterostructures.
For example, it has been shown that confinement between two closely space graphene sheets can have a dramatic effect on the confined material [1,2]. By studying van der Waals structures where encapsulated graphene layers contain channels it is possible to measure the transport behavior of water through such channels. We observe significant enhancement in fluid flow rates for few atomic layer channel heights compared to larger channels . We also observe that confinement in such channels can drive chemical transformations in aqueous salts .
Encapsulation with inert 2D crystals (e.g. graphene or hBN) also provides environmental protection from air or vacuum. We have performed mechanical exfoliation of air sensitive 2D materials in an inert argon atmosphere and used hBN or graphene encapsulation to allow the novel electrical properties of air sensitive 2D crystals to be realized . However we find that even when fabricated in an inert atmosphere, NbSe2 monolayers contain point defects that can be observed by high resolution TEM . Furthermore, cross sectional scanning TEM imaging reveals that even comparatively stable materials like MoSe2 and WSe2 have different interlayer separation when exfoliated in a glove box compared to fabrication in air . Finally, we will demonstrate how 2D heterostructures can be used to fabricate TEM liquid cells with precisely controlled liquid volumes and advanced imaging capabilities.
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