Moiré superlattices and their emergent electronic properties
The moiré superlattices that researchers studied are formed by two sheets of graphene (or similar 2D material) put one on top of the other. If one of the sheets is rotated, interference patterns appear as observed in Figure 1.
Figure 1: Interference patterns created in graphene bilayers twisted an angle θ | From Figure 1 of the publication.
Such interference patterns modify significantly the electronic structure of the system: the most famous example is the twisted bilayer graphene becoming superconductor when the twist angle is exactly 1.1 degree, the so-called "magic angle". Other emergent electronic phenomena that appear in these structures include magnetism, topological edge states, exciton trapping or correlated insulator phases.
Until now, the lack of a practical and straightforward method of observation of moiré superlattices has impeded more progress in the field. “That's why it is so important to be able to visualize them in an easy way” says Konstantin Shapovalov, postdoctoral researcher at the Laboratory of Electronic Structure of Materials group at ICMAB.
A simple and straightforward method to visualize moiré superlaticess
Existing methods to visualize moiré superlattices with high resolution, including transmission electron microscopy and scanning tunneling microscopy require some combination of ultra-high vacuum, low temperature, complex setups or specialized sample preparation that makes these methods impractical to apply on a usual basis. Other methods are limited in resolution to length scales above the moiré scales of interest. There is, thus, an urgent need for a facile method to characterize moiré superlattices in these samples.
“Researchers at Columbia University, NY, USA, were the first ones to use Piezoresponse Force Microscopy (PFM) to detect piezoelectric response and visualize moiré patterns in a wide range of twisted bilayer systems including twisted bilayer graphene” explains Shapovalov. Piezoresponse force microscopy (PFM) is an atomic force microscope (AFM) modality that measures local surface deformation under an applied electric field.
"Most likely, the piezoelectric response in these systems comes from the flexoelectric effect, which is responsible for generating electric polarization in other materials whenever strain gradients are present – and strain gradients arise naturally in moiré superlattices" adds Shapovalov and continues “To confirm and show this effect, Massimiliano Stengel performed DFT simulations of bilayer graphene and quantified the expected piezoelectric response due to the flexoelectric effect, and I made continuous theory modelling of the expected PFM patterns for different twist angles.” (Figure 2)
Figure 2: Expected PFM patterns for different twist angles of the bilayer systems. | From Figure 4 of the publication.
PFM is a simple and room-temperature method that has proved capable of revealing the moiré patterns in multiple systems with astounding resolution and, in addition, is able to simultaneously image multiple moiré superlattices. Researchers believe that this technique will be able to accurately map the moiré pattern of any 2D materials system.
Visualization of moiré superlattices
Leo J. McGilly, Alexander Kerelsky, Nathan R. Finney, Konstantin Shapovalov, En-Min Shih, Augusto Ghiotto, Yihang Zeng, Samuel L. Moore, Wenjing Wu, Yusong Bai, Kenji Watanabe, Takashi Taniguchi, Massimiliano Stengel, Lin Zhou, James Hone, Xiaoyang Zhu, Dmitri N. Basov, Cory Dean, Cyrus E. Dreyer & Abhay N. Pasupathy
Nature Nanotechnology 15, 580–584 (2020)