Examining 2D Heterostructure Moiré Patterns Via Atomic Force Microscopy

The cutting-edge properties of graphene were first in the year 2004.1 Since then, there has been wide-ranging research on the potential applications of 2D materials in electrochemical energy storage, nanoelectronics and flexible optoelectronics.2

In a new research field named twistronics, scientists analyzed Van der Waals heterostructures using various types of 2D materials stacked on top of one another to realize a wider range of material properties that can possibly be customized to any application.3

They discovered that superlattices appear because of the difference in periodicity of materials and slight twist angle between the two layers. These superlattices have a larger periodicity compared to the pristine monolayers.4

Moiré Patterns in Graphene

Such superlattices are called Moiré patterns. A Moiré pattern is typically an interference pattern produced by overlaying similar yet slightly offset periodic structures (see Figure 1).

In the case of 2D materials, this pattern can appear either in microstamped Van der Waals heterostructures or layers developed through molecular beam epitaxy (MBE) or plasma-enhanced chemical vapor deposition (CVD).4

Moiré pattern introduced by superposition and 2.5° twist angle of two periodic structures.

Figure 1. Moiré pattern introduced by superposition and 2.5° twist angle of two periodic structures. Image Credit: Park Systems Europe

Imaging Moiré Pattern

For the research and future application of Van der Waals heterostructures in industry, accurate imaging of Moiré patterns is crucial to correlate and adjust the periodicity of the Moiré pattern to material properties like conductive topological channel, superconductivity, or ferromagnetism.

Atomic force microscopy (AFM) is a real-space, high resolution imaging method that not just captures superlattices in the sample topography but also enables the visualization of Moiré patterns in the electromechanical response of samples.5

Typical variations in corrugation on Moiré patterns are about 10 to 50 pm in height. Thus, their visualization necessitates AFMs with a low noise performance in all main imaging modes. Contact mode imaging of Moiré patterns is specifically a standard noise test for any AFM.

This article describes how large sample NX20 AFM from Park Systems resolves Moiré patterns with various periodicities from 11 to 15 nm on a graphene/hexagonal boron nitride (hBN) Van der Waals heterostructure.

In this article, a microstamped graphene layer on top of hBN monocrystalline flake has been used as an example. Such a sample essentially exhibits both bubbles and wrinkles in the sample topography, together with several flat areas that show Moiré patterns (see Figures 2 and 3).

Optical microscopy image of a graphene flake stacked on top of a hBN flake with a large are height image of the graphene topography. Sample courtesy: Dr Ziwei Wang, The University of Manchester (www.artem-lab.com).

Figure 2. Optical microscopy image of a graphene flake stacked on top of a hBN flake with a large are height image of the graphene topography. Sample courtesy: Dr Ziwei Wang, The University of Manchester (www.artem-lab.com). Image Credit: Park Systems Europe

The Moiré patterns were imaged successfully in both contact and tapping modes. The captured in tapping mode clearly shows three main features of microstamped graphene: a bubble, wrinkles and areas with various Moiré patterns (I–III).

The different Moiré patterns arise from slight mismatches in angle between atomic lattices of graphene and hBN that occur while microstamping.

Two different areas of graphene/hBN displaying Moiré patterns. Imaged in contact (a) and tapping modes (b). Moiré pattern periodicities are 11.2 nm (a) and 14.5 nm (Ib), 12.4 nm (IIb) and 10.9 nm (IIIb).

Figure 3. Two different areas of graphene/hBN displaying Moiré patterns. Imaged in contact (a) and tapping modes (b). Moiré pattern periodicities are 11.2 nm (a) and 14.5 nm (Ib), 12.4 nm (IIb) and 10.9 nm (IIIb). Image Credit: Park Systems Europe

Summary

In this article, Moiré patterns on a graphene/hBN heterostructure were resolved in both contact and tapping modes, showing the capabilities of the large sample NX20 AFM for low noise, high resolution imaging needed for precise characterization of superlattices in stacked 2D materials.

Thus, the results demonstrate the potential of AFMs from Park Systems to improve academic and industrial research in the field of twistronics.

References

  1. Novoselov, K. S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science (80-. ). 306, 666 LP – 669 (2004).
  2. Kim, S. J., Choi, K., Lee, B., Kim, Y. & Hong, B. H. Materials for Flexible, Stretchable Electronics: Graphene and 2D Materials. Annu. Rev. Mater. Res. 45, 63–84 (2015).
  3. Carr, S. et al. Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle. Phys. Rev. B 95, 75420 (2017).
  4. He, F. et al. Moiré Patterns in 2D Materials: A Review. ACS Nano (2021). doi:10.1021/acsnano.0c10435
  5. McGilly, L. J. et al. Visualization of moiré superlattices. Nat. Nanotechnol. 15, 580–584 (2020).

This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.

For more information on this source, please visit Park Systems Europe.

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