Characterisation of buried interfaces in van der Waals materials by cross sectional scanning transmission electron microscopy

Abstract

Graphene and other two-dimensional materials can be stacked together to form vander Waals heterostructures: synthetic crystals composed of different atomically thinlayers with a bespoke electronic band structure. Structural characterisation of vander Waals heterostructures is difficult using conventional methods as the propertiesare almost entirely defined by the nature of the buried interfaces between dissimilarcrystals. These methods also fall short of resolving the atomic structure of burieddefects in van der Waals materials such as graphite. This work demonstrates therefinement and successful application of ion beam specimen preparation to producecross sectional slices through these unique crystals so that they can be characterisedby high resolution scanning transmission electron microscopy (STEM).Cross sectional specimen were prepared using in situ lift-out in a focused ion beam(FIB) dual-beam instrument. The fine polishing steps were optimised to preventdamage to the core of the specimen.High resolution STEM imaging of twin defects in graphene, hexagonal boron ni-tride and MoSe2 revealed that the boundaries are not atomically sharp but extendedacross many atoms. Advanced processing and analysis of these images uncoveredfundamental mechanics which govern their geometry. This technique was further ap-plied to complex transition metal dichalcogenide heterostructures to quantitativelydetermine the properties of buried interfaces between atomically thin crystals.

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Sarah Haigh (Supervisor)