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 ${\mathrm{ReS}}_2$ 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 ${\mathrm{ReS}}_2$ 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 ($2.2m_e$) than along them ($1.6m_e$). An appreciable interplane interaction results in an experimentally measured difference of $\approx 100-200$ meV between the valence band maxima at the $Z$ point (0,0,$\frac{1}{2}$) and the $\mathrm{\Gamma }$ 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 $\mathrm{\Gamma }$, implying that bulk ${\mathrm{ReS}}_2$ 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.

• Received 20 April 2017
• Revised 26 July 2017

DOI:https://doi.org/10.1103/PhysRevB.96.115205

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