Multiscale and Correlative Analysis of 2D Material Heterostructures: Twist domains and Dynamics

Session

Multiscale and Correlative Microscopy Approaches to Microanalysis and Spectroscopy

Authors

Prof Sarah Haigh (1), Dr Astrid Weston (1), Dr Nick Clark (1), Dr Yi-Chao Zou (1), Prof Vladimir Falko (1), Prof Roman Gorbachev (1)

Affiliations

1. University of Manchester

Keywords

2D materials, STEM, domains, in-situ, clay, graphene, TMDCs

Abstract text

Stacking of layered van der Waals materials into increasingly complex heterostructures has led to new discoveries across solid state physics, materials science and chemistry. The possibility to create new ‘designer’ materials by stacking together atomically thin layers extracted from layered materials with different properties has opened up a huge range of opportunities, from new optoelectronic phenomena [1], modifying and enhancing electron interactions in moiré superlattices [2], to creating a totally new concept of designer nanochannels for molecular or ionic transport [3].  Compared to traditional growth methods, van der Waals stacking has an additional degree of freedom since the arrangement of the neighbouring layers can incorporate a controlled twist rotation, which can be exploited   engineer changes to the electronic bandstructure. When the twist angle is less than a few degrees and the adjacent layered materials have similar structure, the heterostructure reconstructs to form domains of perfect stacking separated by disordered partial dislocations.[4]

We have studied reconstructed domains in transitional metal dichalcogenide heterostructures twisted to small angles using conductive atomic force microscopy and scanning transmission electron microscopy (STEM). Below a critical angle,  distinct domain structures emerge with different behaviour seen for crystal alignments close to 3R and 2H type stacking respectively. The dimensions of the domains evolve as a function of twist angle with the 2H stacking structure showing a previously unseen Kagome type domain structure [4]. For the triangular domain structure in the case of the 3R-stacking polytype, these heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of the vertically stacked layers, enables ferroelectric domains with alternating out-of-plane polarisation arranged into a twist-controlled network. Through the mechanism of domain wall sliding, the polarised states can be ‘switched’ by applying out-of-plane electrical fields as visualized in-situ using channelling contrast electron microscopy in the scanning electron microscope (SEM).[5] The polarisation-dependent electrical potential distribution of the observed ferroelectric domains was quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. Our results show that adding a twist between two adjacent monolayers has the potential for room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions. A combination of scanning probe techniques, SEM and (scanning) transmission electron microscopy, (S)TEM, are key to understanding such complex materials. 

We further demonstrate the use of a combination of atomic force microscopy in liquid and scanning transmission electron microscopy annular bright field (ABF) and annular dark field (ADF) imaging to study ion exchange in atomically thin clays and micas [6]. We find a 4 order of magnitude increase in the rate of ion exchange for bilayer vermiculite when compared to bulk flakes. We also observe unusual exchange ion domains at twisted interfaces. The transfer of small, twisted heterostructure devices or ion exchanged clay flakes on to TEM compatible support grids is enabled using our new design of TEM support grids where an MoS2 wetting layer is added to improve adhesion and thereby increase transfer success rates from <10% to >90% [7].

References

[1] J. Zultak et al, Nature Communications 11, 1-6 (2020) 

[2] R. Krishna-Kumar et al, Science, 357, 181-184 (2017) 

[3] B. Radha et al. Nature 538, 222–225 (2016) and Keerthi, et al Nature 558, 7710, 420-424 (2018)

[4] A. Weston et al Nature Nanotechnology, 15 592–597 (2020)

[5] A. Weston et al Nature Nanotechnology, 17, 390–395 (2022)

[6] Y-C Zou et al. Nature Materials, 20, 1677–1682 (2021)

[7] M. Hamer et al, Nano Letters, 20, 9, 6582–6589 (2020)