# Electronic bandstructure of ReS2 by high resolution angle resolved photoemission spectroscopy

## TL;DR

This study uses high-resolution ARPES to directly map the valence band structure of bulk ReS2, revealing anisotropic effective masses and a marginally indirect band gap due to inter-layer interactions, clarifying previous controversies.

## Contribution

First direct measurement of ReS2's valence band structure using ARPES, demonstrating anisotropic effective masses and the indirect nature of the bulk band gap.

## Key findings

- ReS2 exhibits in-plane anisotropy with directional effective masses of 1.6 and 2.2 m_e.
- Inter-layer interaction causes a 100-200 meV difference in valence band maxima at Z and Γ points.
- Bulk ReS2 is marginally indirect, explaining conflicting experimental results.

## Abstract

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 ReS$_2$ is of particular interest, due to its comparatively weak inter-planar interaction. However, the degree of inter-layer 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 ReS$_2$ using high resolution angle resolved photoemission spectroscopy (ARPES). 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 $m_e$), in good agreement with density functional theory calculations. An appreciable inter-plane 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 $\Gamma$ 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 $\Gamma$, implying that bulk ReS2 is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the direct or indirect gap nature of this material.