Lattice dynamics of the rhenium and technetium dichalcogenides

The rhenium and technetium dichalcogenides are layered van der Waals semiconductors which show a large number of Raman-active zone centre phonon modes as a result of their unusually large unit cells and deviation from hexagonal symmetry. They thus offer the possibility of introducing in-plane anisotropy into composite heterostructures based on van der Waals materials; to determine their in-plane orientation, one can use Raman spectroscopy. We show that first principles calculations can give a good description of the lattice dynamics of this family of materials and predict the phonon frequencies of TcS2. We consider the separation of the phonon modes into two frequency bands and give an interpretation in terms of the atomic displacements, which leads to a simple and unified understanding of the phonon frequencies and Raman spectra of ReS2, TcS2 and ReSe2.