  III-VI post-transition metal chalcogenides (InSe and GaSe) are a new class of
layered semiconductors, which feature a strong variation of size and type of
their band gaps as a function of number of layers (N). Here, we investigate
exfoliated layers of InSe and GaSe ranging from bulk crystals down to
monolayer, encapsulated in hexagonal boron nitride, using Raman spectroscopy.
We present the N-dependence of both intralayer vibrations within each atomic
layer, as well as of the interlayer shear and layer breathing modes. A linear
chain model can be used to describe the evolution of the peak positions as a
function of N, consistent with first principles calculations.
