  Stable structures of layered SnS and SnSe and their associated electronic and
vibrational spectra are predicted using first-principles DFT calculations. The
calculations show that both materials undergo a phase transformation upon
thinning whereby the in-plane lattice parameters converge to a pseudo-cubic
phase, similar to the high-temperature behaviour observed for their bulk
counterparts. The electronic properties of layered SnS and SnSe evolve to an
almost symmetric dispersion whilst the gap changes from indirect to direct.
Characteristic signatures in the phonon dispersion curves and surface phonon
states where only atoms belonging to surface layers vibrate can also be
observed for these materials.
