Layer-dependent electronic, vibrational and optical properties of 2D AlSe crystals

We present a detailed theoretical study of the structural, electronic and dynamical properties of ϵ-stacked bulk and mono-, bi- and tri-tetralayers AlSe. We employ density functional theory and density functional perturbation theory using several exchange-correlation functionals. The Se-Al-Al-Se tetralayer is found as very stable atomic structure with minor variations of the bond lengths with the number of tetralayers. The latter one, however, influences the energy gain per tetralayer because of their attractive interaction. We show that all studied layered AlSe systems are indirect semiconductors, whose fundamental gaps are substantially widened after approximate inclusion of quasiparticle corrections. The phonon branches show very similar behavior. The increasing number of tetralayers gives rise to a corresponding large number of branches with small splitting near the Brillouin zone center. They clearly indicate also the dynamical stability of all systems. In-plane E and out-of-plane A zone-center lattice vibrations dominate the Raman and IR spectra. The spectra are rather similar for the different layered structures. Bulk ϵ-AlSe spectrum, however, differs from those of its 2D counterparts, exhibiting variations in peak intensities, peak splitting and additional peaks.