  We study the Raman spectrum of CrI$_3$, a material that exhibits magnetism in
a single-layer. We employ first-principles calculations within density
functional theory to determine the effects of polarization, strain, and
incident angle on the phonon spectra of the 3D bulk and the single-layer 2D
structure, for both the high- and low-temperature crystal structures. Our
results are in good agreement with existing experimental measurements and serve
as a guide for additional investigations to elucidate the physics of this
interesting material.

Materials Science