Distinguishing different stackings in WSe2 bilayers grown Using Chemical Vapor Deposition

TL;DR

This paper introduces a combined optical and spectroscopic approach to accurately identify stacking configurations in WSe2 bilayers, overcoming limitations of previous methods and enabling high-throughput, nondestructive analysis.

Contribution

It proposes a new methodology using SHG and Raman spectroscopy to distinguish stacking orders in WSe2 bilayers, providing more reliable identification than edge orientation alone.

Findings

SHG response varies with stacking order

SHG vanishes in certain stacking phases

Low-frequency Raman features depend on stacking

Abstract

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 AAprime (2H) and AB (3R) configurations uses the relative edge orientation of each triangular layer (theta) 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 (theta = 0) deg layers is more intense than the signal from the single layer structure. However, the SHG totally vanishes in the AAprime and ABprime phases (theta = 60 deg) and 0 deg respectively) of homo-bilayer WSe2. Also, several optical features of homo-bilayer WSe2 are found to depend on the details of the stacking order, with the difference being the clearest in the low frequency (LF) Raman frequencies, as confirmed by DFT simulation. This allows unambiguous, high-throughput, nondestructive identification of stacking order in TMDs, which is not robustly addressed in this emerging research area.