A variety of van der Waals homo- and hetero- structures assembled by stamping
monolayers together present optoelectronic properties suitable for diverse applications.
Understanding the details of the interlayer stacking and resulting coupling is crucial for
tuning these properties. In that context, twisted bilayer transition metal dichalcogenides
offer a great platform for developing a precise understanding of the structure/property
relationship. Here, we study the low-frequency interlayer shear and breathing Raman
modes (<50 cm-1) in twisted bilayer MoS2 by Raman spectroscopy and first-principles
modeling. Twisting introduces both rotational and translational shifts and significantly
alters the interlayer stacking and coupling, leading to notable frequency and intensity
changes of low-frequency modes. The frequency variation can be up to 8 cm-1 and the
intensity can vary by a factor of ~5 for twisting near 0º and 60º, where the stacking is a
mixture of multiple high-symmetry stacking patterns and is thus especially sensitive to
twisting. For twisting angles between 20º and 40º, the interlayer coupling is nearly
constant since the stacking results in mismatched lattices over the entire sample. It
follows that the Raman signature is relatively uniform. Interestingly, unlike the breathing
mode, the shear mode is extremely sensitive to twisting: it disappears between 20º and
40º as its frequency drops to almost zero due to the stacking-induced mismatch. Note that
for some samples, multiple breathing mode peaks appear, indicating non-uniform
coupling across the interface. In contrast to the low-frequency interlayer modes,
high-frequency intralayer Raman modes are much less sensitive to interlayer stacking and
coupling, showing negligible changes upon twisting. This research demonstrates the
effectiveness of low-frequency Raman modes for probing the interfacial coupling and
environment of twisted bilayer MoS2, and potentially other two-dimensional materials
and heterostructures.
