Distinguishing different stackings in WSe2 bilayers grown using chemical vapor deposition

The stacking order of two-dimensional transition metal dichalcogenides (TMDs) is attracting tremendous interest as an essential component of van der Waals heterostructures. A common and fast approach to distinguish between the AA′ (2H) and AB (3R) configurations uses the relative edge orientation of each triangular layer (angle θ) from optical images. Here, we highlight that this method alone is not sufficient to fully identify the stacking order. Instead, we propose a model and methodology to accurately determine the bilayer configuration of WSe2 using second harmonic generation (SHG) and Raman spectroscopy. We demonstrate that the SHG response of the AB phase (θ=0°) layers is more intense than the signal from the single-layer structure. However, the SHG totally vanishes in the AA′ and AB′ phases (θ=60° and 0°, respectively) of homobilayer WSe2. Also, several optical features of homobilayer WSe2 are found to depend on the details of the stacking order, with the difference being the clearest in the low-frequency Raman frequencies, as confirmed by density functional theory simulation. This allows unambiguous, high-throughput, nondestructive identification of stacking order in TMDs, which is not robustly addressed in this emerging research area.