Anomalous twin boundaries in two dimensional materials

By Dr A. P. Rooney and Prof. S. J. Haigh from School of Materials, University of Manchester
Published in Materials
Anomalous twin boundaries in two dimensional materials

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1. Could you briefly outline the key findings of your paper?

Twin boundaries are well-known defect structures in crystalline materials, forming atomically sharp boundaries between parent and twin crystalline regions at a specific twin-angle. We have found that in van der Waals materials such as graphite, hBN and MoSe2 these twin boundaries look and form very differently from most materials. Primarily, they are not atomically sharp but instead the boundary is gradual and curved across several nanometres. Nevertheless it is a twin boundary as the twin-angle is still respected (it can be first calculated from the material's lattice constants), albeit with a large degree of tolerance thanks to the non-directional nature of van der Waals forces. Further experiments revealed that the formation of these defects depends on the thickness of the material and the angle it is bent to. We also show that through twinning we can induce highly unfavourable stacking in graphite, which reveals the armchair and zig-zag crystal directions in the Raman 2D peak.


2. What is your role in this work?

I stumbled upon these defect structures by complete accident. We had a new focused ion beam instrument and I wanted to make a test specimen, and on a whim I cross-sectioned some highly defective hBN. Even the first images I took of the atomic planes on the transmission electron microscope (TEM) were quite striking. My supervisor (Prof Sarah Haigh) happened to show them to Prof Bob Young, who identified them as twin boundaries and got very excited. I then designed and carried out a series of experiments to understand these structures in greater detail, developing ways of analysing the TEM images along the way. 


3. What was the genesis of this paper?  How did you come to this particular problem?

60 years ago, Kelly and Freise observed twin boundaries in graphite using the best optical microscopes available at the time. They assumed these boundaries were atomically sharp, which is the case for almost all other crystalline materials like metals. In 2001 the atomic structure they produced to explain their observations was shown to be unstable by computational methods and so the true structure remained a mystery. 60 years later we have re-examined these structures under an electron microscope capable of routinely resolving atomic columns. So mystery solved! When viewed at the atomic scale the structures are not sharp planes but gradual bends with the expected twin-angle.

4. What is the most empowering implication of your results?

Our paper goes on to explore the conditions under which twin boundaries form in van der Waals materials. Their presence and population depend on the thickness of the crystal (i.e. number of basal planes or stacked monolayers) and the angle it is bent to. In the immediate future, the synthesis of 2D materials from their bulk crystals relies on a poorly understood process by which a material is repeatedly cleaved via shear forces. Our paper goes some way to explain why exfoliation (liquid phase or scotch tape) produces many sheets of material >30 basal planes thick, and yet so few below this. Our findings will also be crucial to designing novel devices from 2D materials, particularly when we consider integrating them into non-planar architectures. 


5. How have 2D materials been uniquely instrumental to enabling these results?

Without strong in-plane bonds and weak van der Waals forces holding the sheets together, these structures are not favourable and have not been observed in any other materials. However we expect them to be common to all stacked 2D materials, even black phosphorus.

6. Can you describe the main challenges associated to the preparation of this manuscript? Any anecdotes you’d like to share with us?

There was too much to talk about! Choosing one narrative to publish was very difficult, especially as we were stumbling across some new quirk with these structures every other week. I think we found a good balance by focusing on 3 different materials and mapping the conditions important for them to be present or absent.


7. Anything that struck you as particularly surprising, unexpectedly pleasant/unpleasant during the peer review process?

Some interesting questions came up about the arrangment of atoms at the boundary. This required many more experiments to get atomic imaging in this region and prove the atoms to be incommensurately stacked.

8. What is your favourite 2D paper published in 2016/2017, and why?

This paper from Cory Dean's group is a favourite. Chari et al. Nano Lett., 2016, 16 (7), pp 4477–4482. The design of the experiment and the results are very nice. It demonstrates how we can dramatically change the properties of 2D materials with only small, reversible modifications. 

9. Which is the development in the field of 2D materials that you would like to see in the next 10 years?

I would like to see the synthesis refined for controlled scale-up. This really is the main bottleneck to development and integration. There are lots of applications that need these materials, so the sooner the better.

10. And now, what’s next?

We know from silicon transistors that strain in crystal structures allows the properties to be finely tuned. We anticipate similar things for 2D material-based transistors, and possibly we can induce this with 3D structures such as twin boundaries or even by folding them. So hopefully folding devices and finding more weird things!

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