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Twistronics

Actualizado: 28 dic 2021

Twistronics is an emerging field that started after achieving control of the relative twist angle between the layers in 2D materials heterostructures (2DMH)




By rotating the layers with respect to each other at specific angles, the interlayer coupling can be tuned, leading to the generation of moiré superlattices, and unlocking a new way to tune the electronic and optical properties of the 2DMH. Initial studies in twisted bi-layer graphene (tBLG) systems showed the potential of twistronics by demonstrating unconventional superconductivity, 1D topological insulating states and a Hofstadter butterfly, among other properties. Further exploration in hBN twisted bi-layers, and TMDs twisted homo- and hetero-bilayers have also shown novel phenomena, and interesting prospects for future applications using controlled twist between the layers. However, even with all these exciting discoveries, the field is still in its infancy and a wide spectrum of phenomena lies undiscovered.



Some of the factors affecting its slow advance include the lack of control on the twisting angle between the layers, the lack of experimental data on how the 2DMH behave at different rotation angles, and the presence of hydrocarbons as a consequence of the 2D materials transfer process relying on polymeric matrices, which spoils the clean arrangement between the interfaces.

A group of researchers at The University of Manchester (UoM) recently demonstrated that earlier assumptions of rigid crystal lattices were incorrect, showing that, in fact, the atomic lattices undergo substantial reconstruction for a wide range of twist angles . This was demonstrated using transmission electron microscopy (TEM) imaging and conductive AFM in homo- and hetero-bilayers of two well-known TMDs, and . The heterostructures were fabricated in Ar environments using a modification of the ‘Tear and Stamp’ method to reliably control the twist angle. The combination of the latter two, the naturally happening lattice reconstruction, and the method for fabrication of heterostructures with control relative twist between the layers, led to the fabrication of new polytypes, showing 3R stacking, which do not occur naturally in these systems. More information can be found here.


Figure 1. (a) STEM of triangular moiré lattice on MoS2/MoS2. Commensurate domains outlined in white. (b) cAFM tunnelling map of domains in 3R-MoS2


Following this initial study, the same group of researchers at the UoM, further investigated the 3R-stacked homobilayer systems. Using a combination of back-scattered electron channelling contrast imaging (BSECCI), Kelvin probe force microscopy (KPFM), and transport measurements, together with theoretical modelling, room-temperature ferroelectricity of a nanoscale semiconductor thinner than 3 nm was observed for the first time. This is an outstanding observation that only happens for 3R-stacked , with relative twist angles between the layers of . More information on this study can be found here.

These discoveries offer a promising avenue to develop novel nanoscale devices both with memory effect and optoelectronic functionalities, both achieved in the ultimate 2D limit

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