Sarah Haigh - Video Lecture

Sarah Haigh - Video Lecture

IVS-IPSTA 2020 Online Conference December 13, 2020

Nanomaterials, Thin films, and Surface Science
Afternoon Session

Session chair:
Maya Bar-Sadan

Ben Gurion University of the Negev

Enabling 2D heterostructure development with atomic resolution imaging: studies of twist reconstruction and degradation

Sarah Haigh

Materials Engineering, Department of Materials, National Graphene Institute, FSE Research Institutes,
The University of Manchester, UK


2D materials and their heterostructures provide an exciting playground for pioneering science. In this talk I will demonstrate how atomic resolution scanning transmission electron microscopy (STEM)
imaging and analysis is being used to support the advancement of these systems. For example, I will discuss new results revealing the structural relaxation that occurs when two transition metal dichalcogenide (TMDC) monolayers are misoriented by a small twist (rotation angle). Many high profile experimental measurements of the exciting physics present in twisted TMDCs have necessarily neglected the potential for local structural relaxation. We have used atomic resolution STEM to reveal that TMDCs bilayers twisted to a small angle, less than ~3˚, reconstruct into energetically favourable stacking domains separated by a network of stacking faults, and that this behaviour is more complex than is observed in graphene. For crystal alignments close to 3R stacking, we find that a tessellated pattern of mirror reflected triangular 3R domains while for alignments close to 2H stacking, stable 2H domains dominate, with nuclei of an earlier unnoticed metastable phase limited to ~ 5nm in size. This appears as a kagome-like pattern at twist angles less than 1 ˚, transitioning to a hexagonal array of screw dislocations separating large-area 2H domains as the twist angle approaches 0 ˚. Tunneling spectroscopy measurements reveals that such reconstruction creates strong piezoelectric textures and pseudo-magnetic fields, opening new avenues for engineering of 2D material properties on the nanometre scale. As the range of 2D materials increases rapidly, many of those with the most the promising magnetic or optoelectronic properties are found to be unstable in air or moisture, which makes fabrication challenging and hampers their use. Even relatively stable 2D crystals like MoSe2 and WSe2 are found to have perfect interfaces only when processed in an inert environment. These materials can be stabilised to ambient conditions by encapsulating with stable 2D layers, such as graphene and hexagonal boron nitride. For example, we have applied the encapsulation approach to study the presence and behaviour of point and extended defects in the 2D monochalcogenides, GaSe and InSe which are of interest due to their relatively high electron mobilities, strong second harmonic generation and quasi-direct bandgap when reduced to the monolayer limit. Nonetheless the properties of InSe and GaSe crystals often degrade on exposure to air, light, and/or moisture, thought to be the result of oxidation induced defects. We have used atomic resolution STEM together with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to characterise individual point and extended defects in InSe and GaSe. We observe that both materials contain multiple point and extended defects, even when exfoliated in argon and encapsulated in graphene. Point vacancies are observed to be healable under the electron beam, while extended defects and stacking faults have symmetries not found in bulk samples and which could be used to tune the properties of 2D post-transition-metal monochalcogenide materials for optoelectronic applications. We also demonstrate the nanopatterning of black phosphous using electron beam lithography via etching protocols developed in the STEM, and recent work revealing oxidation induce improvement of superconductivity in few layer TaS2.