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

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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.