Raman spectroscopic studies on the evolution of interlayer coupling and stacking order in twisted bilayers and polytypes of WSe2

Twisted 2D bilayers of van der Waals materials, a new class of quantum materials, offer pioneering advances in the field of nanoelectronics and photonics. As these layered materials can have various preferential stacking configurations with varying electronic behavior, it is important to have a characterization technique that can unambiguously probe the stacking order and interlayer interactions in 2D materials and twisted 2D homobilayers. In this work, we show that by using Raman spectroscopy, we can probe variations in the interlayer coupling of bilayer WSe2 stacked at different twist angles. The interlayer interactions are weakest at a twist angle of 30°, and the twisted bilayer system is almost equivalent to two decoupled monolayers of WSe2. Also we demonstrate Raman mapping as a quick imaging tool with capabilities of clear distinction between 2H and 3R polytypes of bilayer WSe2 and can be used to study various kirigami structures and bilayer nucleation centers commonly observed during chemical vapor deposition-based growth of WSe2. This work proves to be beneficial in the characterization of twisted bilayers of 2D materials and offer key insights into the optoelectronic properties of 2D materials and heterostructures.