title: Lattice Dynamics of the Rhenium and Technetium Dichalcogenides.
abstract: 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, and Raman spectroscopy is generally used to determine their in-plane orientation. We show that first-principles calculations give a good description of the lattice dynamics of this family of materials and thus predict the zone-centre phonon frequencies and Raman activities of TcS2. We consider the distribution of the phonon modes in frequency and their atomic displacements and give a unified understanding of the phonon frequencies and Raman spectra of ReS2, TcS2 and ReSe2 in terms of the scaling of Raman frequency with the chalcogen mass.
authors.affiliation: Department of Physics, University of Bath, Claverton Down, Bath, UK. d.wolverson@bath.ac.uk.
authors.forename: Daniel
authors.initials: D
authors.lastname: Wolverson
authors.affiliation: Department of Physics, University of Bath, Claverton Down, Bath, UK.
authors.forename: Lewis S
authors.initials: LS
authors.lastname: Hart
keywords: Raman spectroscopy; Rhenium selenide; Rhenium sulphide; Technetium sulphide; Transition metal dichalcogenide