Electronic band structure of ReS₂ by high-resolution angle-resolved photoemission spectroscopy

The rhenium-based transition metal dichalcogenides (TMDs) are atypical of the TMD family due to their highly anisotropic crystalline structure and are recognized as promising materials for two-dimensional heterostructure devices. The nature of the band gap (direct or indirect) for bulk, few-, and single-layer forms of ReS₂ is of particular interest, due to its comparatively weak interplanar interaction. However, the degree of interlayer interaction and the question of whether a transition from indirect to direct gap is observed on reducing thickness (as in other TMDs) are controversial. We present a direct determination of the valence band structure of bulk ReS₂ using high-resolution angle-resolved photoemission spectroscopy. We find a clear in-plane anisotropy due to the presence of chains of Re atoms, with a strongly directional effective mass which is larger in the direction orthogonal to the Re chains (2.2 m_e) than along them (
1.6 m_e). An appreciable interplane interaction results in an experimentally measured difference of ≈100−200 meV between the valence band maxima at the
Z point (0,0,1/2) and the Γ point (0,0,0) of the three-dimensional Brillouin zone. This leads to a direct gap at  Z and a close-lying but larger gap at Γ, implying that bulk ReS₂ is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the nature of the band gap in this material.