Abstract : The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (h-BN) and amorphous silicon nitride (Si 3 N 4), under different configurations as support or encapsulation material for WS 2 monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. h-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which h-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.

Fuhui Shao 1, 2 Steffi Woo 1, 2 Nianjheng Wu 1, 2 Robert Schneider 3 Andrew Mayne 2 Steffen Michaelis de Vasconcellos 3 Ashish Arora 3 Benjamin Carey 3 Johann Preuß 3 Noémie Bonnet 1 Mauro Och 4 Cecilia Mattevi 4 Kenji Watanabe 5, 6 Takashi Taniguchi 5, 6 Zhichuan Niu 7 Rudolf Bratschitsch 3 Luiz Tizei 1, 2
1 LPS – Laboratoire de Physique des Solides
2 ISMO – Institut des Sciences Moléculaires d’Orsay
3 WWU – Westfälische Wilhelms-Universität Münster = University of Münster
4 Imperial College London
5 Université de Tsukuba = University of Tsukuba
6 NIMS – National Institute for Materials Science
7 State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences