Christoph Kastl1 Roland Koch3 Bruno Schuler1 Christopher Chen1 Johanna Eichhorn4 Soren Ulstrup2 Aaron Bostwick3 Chris Jozwiak3 Tevye Kuykendall1 Nicholas Borys1 Francesca Maria Toma4 Shaul Aloni1 Alexander Weber-Bargioni1 Eli Rotenberg3 Adam Schwartzberg1

1, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
4, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
2, Department of Physics, Aarhus University, Aarhus, , Denmark

Interest in transition metal dichalcogenides (TMDs) has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. Due to the strong electronic confinement and the reduced electrostatic screening in two dimensions the electronic and optical properties of 2D materials are generally more susceptible to strain, surface modifications or structural defects than those of their bulk counterparts. While structural defects in semiconductors are usually considered detrimental, the large tunability of 2D materials provide an effective way to create novel functional properties via defect engineering beyond the conventional concept of doping.
Here, we use a set of complementary imaging techniques - photoelectron spectroscopy, photoluminescence, Kelvin probe – to correlate locally the band structure, chemical state, and optical properties of 2D transition metal dichalcogenides.[1] In particular, we employ spatially resolved angle resolved photoemission spectroscopy (nano-ARPES) to map the variations in band alignment, effective mass and chemical composition of CVD-grown monolayer WS2.[2] By correlating the spectroscopic information from nano-ARPES with hyperspectral photoluminescence data, we reveal the interplay between local material properties, such as defect density or chemical composition, and the formation of charged trions, defect-bound excitons and neutral excitons. Finally, we compare these results to combined atomic force and scanning tunneling microscopy, where we can unambiguously identify the occurring point defects and their electronic structure at the atomic level.
[1] Cross-correlating Excitons, Band Structure and Defects in Synthetic WS2, C. Kastl et al., submitted, 2018.
[2] Multimodal Spectromicroscopy of Monolayer WS2 Enabled by Ultra-Clean van der Waals Epitaxy, C. Kastl et al., 2D Materials (accepted) 2018.

Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. S. U. acknowledges funding from VILLUM FONDEN (Grant no. 15375). R.J.K. acknowledges funding from the German Academic Exchange Service (DAAD). B.S. appreciates support from the Swiss National Science Foundation under project number P2SKP2_171770.