Depth-resolved imaging of two-dimensional materials via standing-wave photoemission microscopy*

Two-dimensional materials and their heterostructures offer an exciting new avenue for the realization of atomically-thin electronic, photonic, and magnetic devices. This study demonstrates the feasibility of investigating single monolayers of transition-metal dichalcogenides in three dimensions, using depth-resolved standing-wave photoemission microscopy (SW-PEEM) [1,2]. This method is based on excitation with soft x-ray standing waves generated by Bragg reflection from a multilayer (Mo/Si) mirror substrate. Depth-resolved evolution of the WS₂ valence-band electronic structure and chemical bonding (via core-level spectromicroscopy) is studied by translating the x-ray standing wave vertically through the substrate and the transferred monolayer. Angstrom-level depth resolution and sensitivity to different depths within the monolayer are demonstrated.

[1] F. Kronast et al., Appl. Phys. Lett. 93, 243116 (2008); [2] A. X. Gray et al., Appl. Phys. Lett. 97, 062503 (2010).