Monolayers of 2D transition metal dichalcogenides (TMDCs) provide excellent semiconducting
counterparts to insulating hexagonal boron nitride (hBN) and conductive graphene.[1] Combining all
three materials in a Van der Waals vertical heterostructure allows the electronic, photovoltaic and
electroluminescent properties of the TMDCs to be studied.[2] Whilst transport and optoelectronic
measurements can probe the properties of exotic charge carriers, and ARPES can map the
bandstructure such systems, direct high resolution imaging of the buried interfaces is only possible
via high resolution cross sectional (S)TEM imaging.[3] The nature of these van der Waals interfaces
not only determines the carrier injection between components[4], but also affects the bandstructure
of the device and its ultimate functionality[5].

Here we present a novel strategy to denoise high resolution HAADF STEM images of cross sections
using principal component analysis (PCA).[6] 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.[7] 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. By treating each line
profile perpendicular to the fringes as a signal, components associated with noise and scattering can
be separated by their variance. Removing the noise components allows us to accurately determine
the separation between dissimilar crystals at these unique interfaces. More widely, the approach
developed here also has application in a variety of layered material systems.

[1] X. Duan, C. Wang, A. Pan, R. Yu, and X. Duan, ‘Two-dimensional transition metal dichalcogenides
as atomically thin semiconductors: opportunities and challenges’, Chem. Soc. Rev., vol. 44, no.
24, pp. 8859–8876, Nov. 2015.
[2] F. Withers, O. Del Pozo-Zamudio, A. Mishchenko, A. P. Rooney, A. Gholinia, K. Watanabe, T.
Taniguchi, S. J. Haigh, A. K. Geim, A. I. Tartakovskii, and K. S. Novoselov, ‘Light-emitting diodes by
band-structure engineering in van der Waals heterostructures’, Nat Mater, vol. 14, no. 3, pp.
301–306, 2015.
[3] S. J. Haigh, A. Gholinia, R. Jalil, S. Romani, L. Britnell, D. C. Elias, K. S. Novoselov, L. A.
Ponomarenko, A. K. Geim, and R. Gorbachev, ‘Cross-sectional imaging of individual layers and
buried interfaces of graphene-based heterostructures and superlattices’, Nat Mater, vol. 11, no.
9, pp. 764–767, 2012.
[4] L. Britnell, R. V. Gorbachev, R. Jalil, B. D. Belle, F. Schedin, M. I. Katsnelson, L. Eaves, S. V.
Morozov, A. S. Mayorov, N. M. R. Peres, A. H. Castro Neto, J. Leist, A. K. Geim, L. A.Ponomarenko, and K. S. Novoselov, ‘Electron Tunneling through Ultrathin Boron Nitride
Crystalline Barriers’, Nano Lett., vol. 12, no. 3, pp. 1707–1710, Mar. 2012.
[5] A. M. van der Zande, J. Kunstmann, A. Chernikov, D. A. Chenet, Y. You, X. Zhang, P. Y. Huang, T.
C. Berkelbach, L. Wang, F. Zhang, M. S. Hybertsen, D. A. Muller, D. R. Reichman, T. F. Heinz, and
J. C. Hone, ‘Tailoring the Electronic Structure in Bilayer Molybdenum Disulfide via Interlayer
Twist’, Nano Lett., vol. 14, no. 7, pp. 3869–3875, Jul. 2014.
[6] D. Rossouw, P. Burdet, F. de la Peña, C. Ducati, B. R. Knappett, A. E. H. Wheatley, and P. A.
Midgley, ‘Multicomponent Signal Unmixing from Nanoheterostructures: Overcoming the
Traditional Challenges of Nanoscale X-ray Analysis via Machine Learning’, Nano Lett., vol. 15, no.
4, pp. 2716–2720, Apr. 2015.
[7] 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.

Figures:

A) Schematic showing the arrangement of 2D materials in the heterostructure: Monolayer TMDC encapsulated by thick hBN. B) Scree plot of the first 50 principal components. C) Raw HAADF image of a monolayer TMDC encapsulated by hBN. The red line denotes the location of the line profile shown in D). E) Shows the same HAADF image after processing. The line profile in F) is almost completely denoised. The peaks correspond to the locations of the 2D crystal planes, as shown by the overlaid schematic model. From this the interlayer separation can be accurately determined.

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EMC Abstracts - https://emc-proceedings.com/abstract/principle-component-analysis-applied-to-high-resolution-cross-sectional-stem-imaging-quantitative-analysis-of-2d-heterostructures/