Tuning the electronic structure of transition metal dichalcogenidemonolayers via dipole engineering through molecular adsorbates

过渡金属二卤代化合物(TMDC)的单层化合物，如MoS_2或MoSe_2，由于其迷人的电学和光学性质，受到了密集的研究工作。微纳光电子学的可能应用包括场效应管、热电发电机或光电探测器。由于这些材料的准二维结构，沉积在晶体衬底上的TMDC层的几乎所有原子都与衬底直接接触，使得TMDDC的光电性能对表面缺陷和吸附高度敏感。虽然二维TMDCs的合成近年来取得了很大的进展，其中许多化合物现在已经可以在商业上获得高质量的产品，但用于控制表面电子结构的界面激发策略尚不存在。这是这个项目的起点。该项目是基于这样一个假设，即TMDCs的光电性质可以通过吸附有机π-体系和调节以这种方式形成的界面上的偶极来定制。作为π系统，我们使用了含氟金属酞菁和卟啉，这允许与TMDCs的界面性质可调，如界面偶极子、能级位置和电荷转移。通过对酞菁进行化学修饰，如氟化度或金属中心，可以微调这些性质。基于光电发射的技术被用来确定横向平均的电子界面属性。利用空间分辨拉曼显微镜、荧光显微镜和扫描隧道显微镜研究了界面能级排列和偶极对TMDCs光学性质的影响，并阐明了它们以缺陷形式存在的局域偏差。电传输测量提供了有关吸附物对层的电性能的影响的信息。

Individual layers of transition metal dichalcogenides (TMDC) such as MoS2 or MoSe2 are subject to intense research efforts in view of their fascinating electronic and optical properties. Possible applications in micro- and nano-optoelectronics include field-effect transistors, thermoelectric power generators, or photodetectors. As a consequence of the quasi two-dimensional structure of these materials, nearly all atoms of a TMDC layer which is deposited onto a crystalline substrate, are in direct contact with the substrate, rendering the optoelectronic properties of TMDCs highly sensitive to surface defects and adsorbates. While the synthesis of two-dimensional TMDCs has made great progress in recent years and many of these compounds are now commercially available in high quality, an interface-inspired strategy for the controlled adjustment of the electronic structure at the surface is not yet available. This is the starting point for this project. The project is based on the hypothesis that the optoelectronic properties of TMDCs can be tailored via the adsorption of organic π-systems and tuning of the dipole at the interface formed in this way. As π-systems, we utilize fluorinated metal phthalocyanines and and porphyrins, which allow for tunable interface properties with the TMDCs, such as the interface dipole, the position of the energy levels and charge transfer. Through chemical modifications of the phthalocyanines like the degree of fluorination or the metal center, these properties can be fine-tuned. Photoemission-based techniques are used to determine the laterally averaged, electronic interface properties. Using spatially resolved Raman, fluorescence and scanning tunneling microscopy, the effects of energy level alignment and dipoles at the interfaces on the optical properties of the TMDCs are investigated, and their local deviations in the form of defects are clarified. Electrical transport measurements provide information about the effects of the adsorbates on the electrical properties of the layers.