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Nano Lett. 2017 Sep 13;17(9):5222-5228. doi: 10.1021/acs.nanolett.7b01248. Epub 2017 Aug 1.

Observing Imperfection in Atomic Interfaces for van der Waals Heterostructures.

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School of Materials, University of Manchester , Manchester M13 9PL, United Kingdom.
Manchester Centre for Mesoscience and Nanotechnology, University of Manchester , Manchester M13 9PL, United Kingdom.
School of Physics and Astronomy, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.
National Graphene Institute, University of Manchester , Manchester M13 9PL, United Kingdom.
Institute for Molecules and Materials, Radboud University , 6525 AJ Nijmegen, Netherlands.
College of Engineering, Mathematics, and Physical Sciences, University of Exeter , Exeter, Devon EX4 4SB, United Kingdom.


Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the first systematic study of interfaces in van der Waals heterostructure using cross-sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS2 or WS2 for stacks prepared by mechanical exfoliation in air. However, for two technologically important transition metal dichalcogenide (TMDC) systems, MoSe2 and WSe2, our measurement of interlayer separations provide the first evidence for impurity species being trapped at buried interfaces with hBN interfaces that are flat at the nanometer length scale. While decreasing the thickness of encapsulated WSe2 from bulk to monolayer we see a systematic increase in the interlayer separation. We attribute these differences to the thinnest TMDC flakes being flexible and hence able to deform mechanically around a sparse population of protruding interfacial impurities. We show that the air sensitive two-dimensional (2D) crystal NbSe2 can be fabricated into heterostructures with pristine interfaces by processing in an inert-gas environment. Finally we find that adopting glovebox transfer significantly improves the quality of interfaces for WSe2 compared to processing in air.


2D Materials; FIB; STEM; TMDC; defects

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