Interest in transition metal dichalcogenides has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. In the few-layer limit, the optical and electronic properties of TMDs are modified by a strongly reduced dielectric screening. As a consequence of the weak screening, these 2D materials are intrinsically susceptible to spatial disorder, which can arise due to defects from the growth or interactions with the substrate. Here, we use a set of complementary imaging techniques - Raman, photoluminescence, Kelvin probe, and photoelectron spectroscopy – to correlate locally the chemical state, electronic structure, and optical properties of 2D transition metal dichalcogenides. 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. 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. We compare these results to combined atomic force and scanning tunneling microscopy studies, where we can unambiguously identify point defects in the films at the atomic level.
Christoph Kastl, Roland Koch, Chris Chen, Johanna Eichhorn, Bruno Schuler, Tev Kuykendall, Simon Moser, Aaron Bostwick, Chris Jowiak, Shaul Aloni, Alexander Weber-Bargioni, Eli Rotenberg, and Adam M. Schwartzberg, "2D materials under the microscope(s): connecting material properties and electronic structure to many-body excitonic phenomena in monolayer WS2 (Conference Presentation)," Proc. SPIE 10534, 2D Photonic Materials and Devices, 1053404 (Presented at SPIE OPTO: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291217.5751529290001.
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