Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. Here, we demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, we reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor 1,3,4,5,7,8-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) and a MoS2 monolayer. Our results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the design of advanced heterostructures, exhibiting tailored electronic properties in (opto-)electronic devices based on two-dimensional semiconductors.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This work was funded by the Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 182087777 - SFB 951. S.P. acknowledges support by the Alexander von Humboldt-Stiftung. V.T. acknowledges the support from KAUST Solar Center Seed fund and under User Proposals (#4420 and #5067) at the Molecular Foundry, Lawrence Berkeley National Lab, supported by the Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work was supported by the KIST Institutional Program (Project No. 2E29850) and the National Research Foundation (NRF) of Korea under Grant 2018M3D1A1058793, funded by the Korean Government. Part of Fig. 4 was created with the software VESTA (Momma, K. and Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 44, 1272–1276 (2011)).