Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates
Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates
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DOI：
10.1088/2053-1583/aaa4ca
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发表时间：
2018-04-01
期刊：
影响因子：
5.5
通讯作者：
Koch, Norbert
中科院分区：
文献类型：
--
作者：
Park, Soohyung;Mutz, Niklas;Koch, Norbert
了解二维（2D）过渡金属二硫族化合物（TMDCs）激发态的激子性质对于在光电应用中利用其光学和电荷输运特性至关重要。我们通过直接实验测定在两种完全不同的衬底（即绝缘体蓝宝石和金属金）上单层MoS2和WSe2的激子结合能（E-b,E-exc）来促进这一点。通过结合角度分辨的直接光电子能谱和逆光电子能谱，我们测量了电子带隙（E-g），通过反射率测量了光学激子带隙（E-exc）。两者能量之差为E-b，等。蓝宝石上MoS2的Eg值和E-b，E-exc值分别为2.11 eV和240 meV，蓝宝石上WSe2的Eg值和E-b，E-exc值分别为1.89 eV和240 meV。在Au - Eb上，MoS2和WSe2的exc分别降至90 meV和140 meV。显著的E-b、exc降低主要是由于金属增强了屏蔽作用，从而降低了E-g，而E-exc几乎没有因为金属载体而降低。二维TMDCs布里渊区k点处的能级图显示，与蓝宝石相比，MoS2在Au上具有更多的p型特征，而WSe2在两者上都接近本质。这些结果表明，二维TMDCs的介电环境对单个载流子的影响比对相关电子-空穴对（即激子）的影响更为明显。因此，这种二维半导体的适当介电包围设计可用于促进优越的光电器件功能。
Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy (E-b,E-exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (E-g), and by reflectance measurements the optical excitonic band gap (E-exc). The difference of these two energies is E-b, exc. The values of Eg and E-b,E-exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb, exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant E-b, exc reduction is primarily due to a reduction of E-g resulting from enhanced screening by the metal, while E-exc is barely decreased for the metal support. Energy level diagrams determined at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron-hole pair, i.e. the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function.