A comprehensive understanding of the energy level alignment mechanisms between two-dimensional (2D) semiconductors and electrodes is currently lacking, but it is a prerequisite for tailoring the interface electronic properties to the requirements of device applications. Here, we use angle-resolved direct and inverse photoelectron spectroscopy to unravel the key factors that determine the level alignment at interfaces between a monolayer of the prototypical 2D semiconductor MoS2 and conductor, semiconductor, and insulator substrates. For substrate work function (Φsub) values below 4.5 eV we find that Fermi level pinning occurs, involving electron transfer to native MoS2 gap states below the conduction band. For Φsub above 4.5 eV, vacuum level alignment prevails but the charge injection barriers do not strictly follow the changes of Φsub as expected from the Schottky-Mott rule. Notably, even the trends of the injection barriers for holes and electrons are different. This is caused by the band gap renormalization of monolayer MoS2 by dielectric screening, which depends on the dielectric constant (εr) of the substrate. Based on these observations, we introduce an expanded Schottky-Mott rule that accounts for band gap renormalization by εr -dependent screening and show that it can accurately predict charge injection barriers for monolayer MoS2. It is proposed that the formalism of the expanded Schottky-Mott rule should be universally applicable for 2D semiconductors, provided that material-specific experimental benchmark data are available.
KAUST Repository Item: Exported on 2021-08-13
Acknowledged KAUST grant number(s): OSR-2018-CARF/CCF-3079
Acknowledgements: This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project No.182087777 - SFB 951. Further support by the National Research Foundation (NRF) of Korea under Grant 2018M3D1A1058793 and Technology Innovation Program
(20012502), funded by the Korean Ministry of Trade, Industry & Energy, is acknowledged. This work was supported by the KIST Institutional Program (Project No. 2 V09108). C.-H.Lee acknowledges the support from the KU-KIST School Project. V.T. and A.A. are indebted to the support from the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award no. OSR-2018-CARF/CCF-3079.
- Physics and Astronomy(all)
- Materials Science(all)