Torsional Force Microscopy of Van der Waals Moir\'es and Atomic Lattices

TL;DR

This paper demonstrates that Torsional Force Microscopy (TFM) can effectively image and analyze the surface and subsurface structures of Van der Waals heterostructures, including moiré patterns and atomic lattices, at room temperature without complex sample prep.

Contribution

The study introduces TFM as a simple, room-temperature technique for mapping twist angles and structural features in Van der Waals heterostructures, offering an accessible alternative to existing methods.

Findings

TFM can image moiré patterns in graphene/hBN heterostructures.

TFM reveals atomic lattice structures of 2D materials.

TFM operates effectively at room temperature without electrical bias.

Abstract

In a stack of atomically-thin Van der Waals layers, introducing interlayer twist creates a moir\'e superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult, hence determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moir\'e, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that Torsional Force Microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of Van der Waals stacks on multiple length scales: the moir\'es formed between bi-layers of graphene and between graphene and hexagonal boron nitride (hBN), and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an AFM cantilever is monitored as it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the sample surface. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moir\'e superlattices and crystallographic orientation of VdW flakes to support predictable moir\'e heterostructure fabrication.