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.

中文翻译：

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高分辨率角分辨光发射光谱法研究ReS2的电子能带结构

基于their的过渡金属二硫化碳（TMD）由于其高度各向异性的晶体结构而非典型的TMD系列，并且被认为是用于二维异质结构器件的有前途的材料。体，层和单层形式的带隙（直接或间接）的性质${\mathrm{稀土}}_{\mathrm{2个}}$由于其相对较弱的平面间相互作用，它特别受关注。然而，层间相互作用的程度以及在减小厚度时是否观察到从间接间隙过渡到直接间隙的问题（与其他TMD一样）是有争议的。我们提出了直接的价带结构的直接确定${\mathrm{稀土}}_{\mathrm{2个}}$使用高分辨率角度分辨光发射光谱法。由于存在Re原子链，我们发现了明显的面内各向异性，具有强方向有效质量，该质量在与Re链正交的方向上更大（$2.2{米}_{Ë}$），而不是沿他们（$1.6{米}_{Ë}$）。明显的平面间相互作用会导致实验测量到的差异$\approx 100-200$ 价带最大值之间的meV $ž$ 点（0,0，$\frac{\mathrm{1个}}{\mathrm{2个}}$）和 $\mathrm{\Gamma }$三维布里渊区的点（0,0,0）。这导致直接的差距$ž$ 还有一个差距悬而未决的地方 $\mathrm{\Gamma }$，表示体积庞大 ${\mathrm{稀土}}_{\mathrm{2个}}$是间接的。这可能解释了最近冲突的传输和光致发光测量结果，以及由此产生的关于该材料中带隙性质的不确定性。