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Nano Research 2020, 13(5): 1363-1368 https://doi.org/10.1007/s12274-019-2601-7
Research Article | Issue | Published: 17 January 2020

Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures

Show Author's Information Akshay Singh,  Hae Yeon Lee,  Silvija Gradečak( )
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02141, USA
Keywords:
two-dimensional (2D) materials, transition metal dichalcogenides, heterostructures, cathodoluminescence, interfaces
Topics:
Electronic propertiesElectronsHeterojunctionsLayered semiconductorsMolybdenum compoundsMonolayersOptical propertiesTransition metalsTransmission electron microscopyTungsten compounds
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Singh A, Yeon Lee H, Gradečak S. Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures. Nano Research, 2020, 13(5): 1363-1368. https://doi.org/10.1007/s12274-019-2601-7
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Abstract Full text Electronic supplementary material About this article

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.


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About this article

Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures

Show Author's information Akshay Singh, Hae Yeon Lee, Silvija Gradečak( )
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02141, USA

Abstract

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.

Keywords: two-dimensional (2D) materials, transition metal dichalcogenides, heterostructures, cathodoluminescence, interfaces

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Publication history
Copyright
Acknowledgements

Publication history

Received: 28 October 2019
Revised: 02 December 2019
Accepted: 06 December 2019
Published: 17 January 2020
Issue date: May 2020

Copyright

© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Acknowledgements

This work was supported by the Singapore-MIT Alliance for Research and Technology and the National Research Foundation, Prime Minister’s Office, Singapore. The authors acknowledge helpful discussions with Yuan Cao and Kha Tran. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-1419807. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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