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Disputation Asieh Yousofnejad

31.03.2022 | 14:00
Thema der Dissertation:
Structural and Electronic Properties of Organic Molecules on Monolayer Molybdenum Disulfide on Ag (111)
Abstract: Molecular charge transfer complexes are an interesting class of materials due to their promising applications in molecular electronic devices. Understanding the charge transfer between donor and acceptor molecules at the nanoscale is crucial for the ongoing development and optimization of those devices. In this thesis, we investigate charge-transfer processes within prototype mixed molecular systems on the single layer molybdenum disulfide (SL-MoS2) on Ag(111). The characterization of molecules on SL-MoS2 is motivated by the fact that SL-MoS2 electronically decouples molecules from the metal substrate and enables us to resolve the electronic properties (vibronic states and charge transfer processes) of an almost isolated molecule. Spectroscopic measurements of SL-MoS2 on Ag(111) reveal the semiconducting bandgap. We also observe a moiré pattern due to the lattice mismatch between SL-MoS2 and Ag(111). The decoupling properties of MoS2/Ag(111) are challenged for tetracyanoquinodimethane (TCNQ) molecules. Deposition of TCNQ molecules on MoS2/Ag(111) results in the formation of self-organized molecular islands. STS measurements reveal that the lowest unoccupied molecular orbital (LUMO) is significantly narrower than on a metal substrate. The LUMO is accompanied by additional characteristic spectroscopic features which are identified as vibronic states with strong electron-phonon coupling using Franck-Condon picture.
After investigating the electron acceptor TCNQ, an electron donor counterpart tetrathiafulvalene (TTF) is added to the system. The deposition of both TTF and TCNQ on MoS2/Ag(111) leads to the formation of a mixed monolayer. Local STS measurements show electronic modifications in the mixed structure. The onset of conductance is closer to the Fermi level compared to the pure molecular islands. The modification is attributed to a charge-transfer process between TTF and TCNQ. Additionally, one can charge/discharge the molecules by gating them with the STM tip.

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31.03.2022 | 14:00