  Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have
recently attracted tremendous interest as potential valleytronic and
nano-electronic materials, in addition to being well-known as excellent
lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low
frequency phonon modes, and how they evolve with the number of layers, are
important for both the mechanical and electrical properties of 2D TMDs. Here we
uncover the ultra-low frequency interlayer breathing and shear modes in
few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman
spectroscopy and first principles calculations. Remarkably, the frequencies of
these modes can be perfectly described using a simple linear chain model with
only nearest-neighbour interactions. We show that the derived in-plane (shear)
and out-of-plane (breathing) force constants from experiment remain the same
from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale
interlayer frictional characteristics of these excellent lubricants should be
independent of the number of layers.
