Nonlocal PL dynamics near a monolayer/bilayer transition in WS2

Nonlocal PL emission near monolayer to bilayer transitions in the two-dimensional material WS2 is an important indicator of dynamics of the system. For example, we find excitons excited in a bilayer can emit microns away at such a transition. We find that spectral shift of the emission is also important, as it indicates the bandgap in the emission region, and reduced bandgap regions can trap the excitons. We use nonlocal fluorescence measurements in conjunction with position-correlated 2nd harmonic microscopy, which is always local, AFM and Raman spectroscopy to understand the dynamic processes of the carriers. Two complementary nonlocal measurement approaches are applied to detect the Photo Luminescent (PL) emission region around the excitation spot. Quantitative analysis of the spectral and spatial dynamics is discussed.