Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN.

中文翻译：

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衬底诱导带尾态对 MoS2 电子和光学性质的影响

基材、环境和晶格缺陷对原子级薄过渡金属二硫属化物的局部电子结构和光学性质有很大影响。我们通过使用扫描隧道光谱 (STS) 测量对 SiO2 和六方氮化硼 (hBN) 上的 MoS2 进行比较研究，发现与 hBN 上的 MoS2 相比，SiO2 上的 MoS2 的表观带隙显着降低。对于 SiO2 和 hBN 上的 MoS2，由全光学测量确定的带隙能量以及激子结合能非常相似。发现这种差异是由 SiO2 支持的 MoS2 导带边缘附近的大量带尾态引起的。这些状态的存在会影响 STS 测量中的局部状态密度，并且可以与宽的红移光致发光峰和更高的电荷载流子密度相关联，这些都在使用高质量 hBN 基板时强烈减弱甚至不存在。通过考虑衬底效应，我们获得了与光吸收光谱非常一致的准粒子间隙，我们推导出 SiO2 上的激子结合能约为 0.53 eV，hBN 上约为 0.44 eV。