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.
Highlights
The transition metal dichalcogenides (TMD) have been known for a considerable time; the basic properties of about 40 members of the family were reviewed in 1969 [1]
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 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
Summary
The transition metal dichalcogenides (TMD) have been known for a considerable time; the basic properties of about 40 members of the family were reviewed in 1969 [1]. 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.
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