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Image processing to characterise 2D material heterostructures and graphitic folds Rooney, Aidan


2D materials such as graphene and transition metal dichalcogenides (TMDCs) have demonstrated new properties underpinned by exotic charge carriers with enhanced conductivity and novel light-matter interactions. These properties can be harnessed by ‘stacking’ 2D crystals on top of one another to form a van der Waals heterostructure: a series of atomically thin sheets glued together by weak forces[1]. Heterostructures made in this way allow the new properties of 2D materials to be characterised and applied as transistors[2], optical modulators[3] and light emitting diodes[4]. We demonstrate cross-sectional STEM images of these heterostructures, showing how each 2D crystal interfaces its neighbour at high resolution. Cross sections are fabricated in a dual-beam FIB-SEM instrument using the in situ lift-out method and polished with low energy ions to achieve electron transparency.[5] Cross sections were imaged in high resolution HAADF STEM using a probe-side aberration corrected FEI Titan G2 80-200 kV with an X-FEG electron source. The nature of these van der Waals interfaces not only determines the carrier injection between components, but also affects the bandstructure of the device and its ultimate functionality. However, measurements at these buried interfaces are only possible by cross-sectional STEM. Image processing and principle component analysis were used to determine the width of the interfaces, demonstrating that some 2D materials have cleaner interfaces than others and that fabrication in inert atmosphere produces better interfaces than in air.[6] This analysis is extended to bends and folds which underpin the structure of kink-bands in graphite and other bulk ‘van der Waals’ materials such as hexagonal boron nitride and MoSe2. Extensive image processing allows the bent basal planes to be fitted by functions to reveal their radius of curvature, angle and distance to the nearest neighbour plane. Three broad classes of bend emerge which depend on the bend angle and crystal thickness. [1] A. K. Geim and I. V. Grigorieva, ‘Van der Waals heterostructures’, Nature, vol. 499, no. 7459, pp. 419–425, Jul. 2013. [2] L. Britnell et al., ‘Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures’, Science, vol. 335, no. 6071, pp. 947–950, Feb. 2012. [3] P. A. Thomas et al., ‘Nanomechanical electro-optical modulator based on atomic heterostructures’, Nat. Commun., vol. 7, p. ncomms13590, Nov. 2016. [4] F. Withers et al., ‘Light-emitting diodes by band-structure engineering in van der Waals heterostructures’, Nat Mater, vol. 14, no. 3, pp. 301–306, 2015. [5] M. Schaffer, B. Schaffer, and Q. Ramasse, ‘Sample preparation for atomic-resolution STEM at low voltages by FIB’, Ultramicroscopy, vol. 114, no. 0, pp. 62–71, 2012. [6] A. P. Rooney et al., ‘Observing Imperfection in Atomic Interfaces for van der Waals Heterostructures’, Nano Lett., Jul. 2017.

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