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arxiv 2308.08814 v2 pith:4FHPLFZB submitted 2023-08-17 cond-mat.mtrl-sci cond-mat.mes-hallcond-mat.str-elcond-mat.supr-con

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

classification cond-mat.mtrl-sci cond-mat.mes-hallcond-mat.str-elcond-mat.supr-con
keywords moirtwistangletorsionalforcegraphenesamplestack
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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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.

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