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 bilayers 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 the it is
actively driven at a torsional resonance while a feedback loop maintains
contact at a set force with the surface of a sample. 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.Comment: 28 pages, 14 figures including supplementary material