Research / Twistronics
Twistronics is an emerging field that became available once the ability to precisely control of the relative twist angle between adjacent 2D crystals was developed. Introducing twist between 2D crystals with similar lattice constants gives rise to moiré superlattices - a periodic variation of a local atomic registry, which in turn causes modulation of interlayer hybridisation, symmetry and strain, unlocking a new way to tune the electronic and optical properties of the 2D materials heterostructures (2DMH). ​
Our group recently demonstrated that atomic lattices of transition metal dichalcogenides (TMDs) undergo substantial reconstruction for small enough twist angles (θ<2°). As a result, instead of gradually changing atomic displacement between the lattices, perfectly stacked bilayer regions (or domains) form, which possess lower adhesion energy between the layers compared to that of two twisted lattices. Such lattice adjustment is accompanied by the strain which accumulates within narrow domain boundaries, resulting in a periodic domain network separated by a network of dislocations.
Top: Maps of the atomic lattice and the piezoelectric density for anti-parallel stacked WSe2 homo-bilayers with a twist of 0.6°. Bottom: 3D maps show the density of electrons and holes at the K and Γ valleys, respectively.
Top: Maps of the atomic lattice and the dipole moment for parallel stacked WSe2 homo-bilayers with a twist of 0.6°. Bottom: 3D maps show the density of electrons and holes at the K and Γ valleys, respectively.
Unlike graphene, stacking two monolayer of TMDs with parallel and anti-parallel orientation of the unit cells (without twist) produces different polytypes, known as 3R and 2H for bulk TMDs, respectively. Introducing a small twist angle leads to the formation of the domain network which we have recently described using scanning transmission electron microscopy (TEM) and conductive atomic force microscopy (cAFM) in our paper in Nature Nanotechnology.
For the twisted 3R-type heterostructures, a network of triangular domains form corresponding to commensurate MX’ and XM’ stacking (i.e., M: metal atom; X: chalcogen atom). On the other hand, for twisted 2H-type structures a kagome-like patterns emerges, tessellated with MM’ and 2H stacking. This striking difference in the network layout is due to the energy balance between the constituent stacking: the twisted 3R-type configuration has two low and equal in energy stackings (MX’ and XM’) whereas, for the 2H-type configuration, the 2H regions are more energetically favourable than MM and hence occupy large area.
For the twisted 2H-like structures, the strain resulting from the lattice reconstruction produces a texture of piezoelectric charge, which attracts free carriers to the localised regions of the network, behaving like quantum dots for the electrons or holes.
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This effect however is absent in 3R-type structures, where the piezoelectric charge in adjacent layers has opposite sign and cancels out, leaving a small electrical polarisation across the bilayer. Interestingly, the 3R domains lack inversion symmetry which allows for a layer-asymmetric quasiparticle wave-functions to exist. Such asymmetry can be detected using cAFM, reflected through the tunnelling density of states due to reversal of the electron wavefunction weighting on the top TMD layer as we scan across a domain wall boundary:
STEM of triangular moiré lattice on MoS2/MoS2; commensurate domains outlined in white.
cAFM tunnelling map of domains in 3R-MoS2.
This lack of inversion symmetry not only allows for specific states to be layer asymmetric, but also enables total change transfer between the layers. This effect is similar to classical ferroelectricity, but in this case taking precedent in the ultimate 2D limit. In our recent paper, we study this intriguing phenomenon using back-scattered electron channelling contrast imaging (BSECCI) to visualise the two domain types formed in the 3R-like twisted bilayers of MoS2.
Using this technique, we demonstrated that it is possible to manipulate the domains by applying a perpendicular electric field. The domains were found to expand/contract depending on the strength and direction of the field applied. The redistribution of domains allows one polarisation state, where the ferroelectric moment is parallel to the external field, to dominate over the other allowing for effective ‘switching’ between polarisation states. The built-in ferroelectric field also creates variation of the surface potential between the adjacent domains which we quantitatively measured using Kelvin probe force microscopy (KPFM).
Finally, we developed proof-of-concept devices and performed electronic transport measurements demonstrating the hysteretic behaviour of their conductivity with the application of electric field. Similar measurements on the 2H-type heterostructures didn’t show such a response due to their AP stacking and their inversion symmetry. This discovery is quite promising as evidence of RT ferroelectricity in semiconducting films <3 nm was yet to be achieved in ‘traditional’ ferroelectric devices which are most commonly produced from metal-oxide films. Furthermore, TMDs are well-known for their excellent light-matter interaction allowing for the possibility to create memory devices with multi-functionalities.
(a) Example of BSECCI acquired on twisted bilayer MoS2 placed onto a graphite substrate. Light and dark domain contrast corresponds to the two dominant stacking orders referred as MotSb and StMob respectively. (b) Centre - a schematic demonstrating the transition from MotSb to StMob with perfectly stacked bilayer regions separated by a partial dislocation. Side panels show cross-sectional alignment of the MoS2 monolayers along the armchair direction assembled within the double gated device structure. (c-g) Domain switching visualised by BSECCI under transverse electric field applied in situ. Large domains mostly retain their shape when the field is removed and practically disappear when the field is inverted; the arrows in (e) indicate partial dislocations colliding when neighbouring domains of the same orientation try to merge. Micrographs are presented in chronological order. White oval feature in (a) and black ring features in (c-g) are where the intralayer contamination has segregated to form a bubble.
Together our research offers a promising avenue to develop novel nanoscale devices both with memory effect and optoelectronic functionalities, achieved in the ultimate 2D limit.