  We investigate the ultralow-frequency Raman response of atomically thin ReS2,
a special type of two-dimensional (2D) semiconductors with unique distorted 1T
structure. Bilayer and few-layer ReS2 exhibit rich Raman spectra at frequencies
below 50 cm-1, where a panoply of interlayer shear and breathing modes are
observed. The emergence of these interlayer phonon modes indicate that the ReS2
layers are coupled and stacked orderly, in contrast to the general belief that
the ReS2 layers are decoupled from one another. While the interlayer breathing
modes can be described by a linear chain model as in other 2D layered crystals,
the shear modes exhibit distinctive behavior due to the in-plane lattice
distortion. In particular, the two shear modes in bilayer ReS2 are
non-degenerate and well separated in the Raman spectrum, in contrast to the
doubly degenerate shear modes in other 2D materials. By carrying out
comprehensive first-principles calculations, we can account for the frequency
and Raman intensity of the interlayer modes, and determine the stacking order
in bilayer ReS2.
