4D-STEM on epitaxially grown vertical heterostructures of WS₂ on Graphene

Abstract text (incl. figure legends and references)

Introduction:

Two-dimensional (2D) materials received a lot of interest over the past decade. Especially van der Waals (vdW) 2D materials, such as transition metal dichalcogenides (TMDCs), and their heterostructures, exhibit semiconducting properties. Mixing and matching these materials with different properties and stacking them under an arbitrary in-plane angle causes structural changes as well as charge redistribution in the heterostructure, highly interesting for novel device applications.¹ For these device structures, precise control of the composition and high uniformity of the layers is desired. Therefore, bottom-up synthesis methods, like metal-organic chemical vapor deposition (MOCVD) are currently being explored.²

Objectives:

In this study, we aim to investigate charge redistributions occurring in 2D vertical heterostructures grown by MOCVD and to identify the epitaxial relationship of the layers within the heterostructure utilizing 4D-STEM.

Materials and Methods:

Vertical heterostructures of mono- to few-layer WS₂ on few-layer graphene were deposited by MOCVD in a 19x2 inch Close Coupled Showerhead (CCS) Aixtron reactor on c-plane sapphire substrates. These structures are then transferred onto a 3 nm carbon film TEM-Grid by the PMMA transfer method.³ The samples are then characterized in a JEOL 3010 TEM and in an aberration-corrected JEOL 2200 FS STEM in conjunction with a fast pixelated pn-CCD detector, opening the possibility to analyze the samples by 4D-STEM.⁴

Results:

The heterostructures of WS₂ on graphene were successfully transferred by PMMA onto the TEM-Grids. The sample exhibits regions of monolayer WS₂ and multiple stacking rotations between the vertically stacked WS₂ and graphene layers. An annular bright field (ADF) image, exhibiting one of the resulting Moiré patterns is shown in fig. 1. The observed pattern aligns very well, to a rotation of about 11° between the lower and upper WS2 layer. In the FFT additional spots originating from the different lattice plane spacing of WS₂ can be observed, which to the {100} lattice spacing of the underlying graphene layers. For the graphene layers in-plane rotations of about 34° and 10° are identified. In addition to the in-plane orientation of the layers, hopping of W and S atoms induced by the electron beam was seen for imaging at 200 keV as well as 80 keV. Studying this process should allow further insights into the crystal formation process. Furthermore, the heterostructures are investigated by 4D-STEM to reveal potential charge redistributions in these structures.

Conclusion:

Our results show a successful growth of high-quality vertical heterostructures of WS₂ on graphene on sapphire and subsequent transfer to TEM-Grids. The first results are very promising to give insights into the nucleation process, the orientation relation of WS₂ on graphene and into the charge redistributions in vertical vdW heterostructures.

Figure 1. a) ABF-STEM image of 2 layers of WS₂ on top of 2-3 layers of graphene recorded at 80 keV. b) Reduced FFT calculated from the STEM image in a). c) Lattice spacings determined from the FFT spots. d) Monolayer of WS₂ on WS₂ rotated by 10.7°.⁵

References:

[1] K. S. Novoselov et al. Science 353, (2016).
[2] S. M. Eichfeld, et al. 2D Materials 3, (2016).
[3] F. Zhang et al. Nanotechnology 29, (2018).
[4] C. Ophus. Microscopy and Microanalysis 25, (2019).
[5] K. Momma et al. J. Appl. Crystallogr. 44, (2011).