Behavior of Excited States in 2H and 3R Bilayer WSe₂

Transition metal dichalcogenide bilayers exhibit improved stability and higher carrier mobility compared to their monolayer counterparts, and may be attractive for a variety of applications. Both 2H and 3R bilayers are energetically stable and are expected to exhibit semiconducting behavior. However, 2H has received the bulk of attention due to its ready availability in the form of mechanically exfoliated flakes. Here, we detail the energies and temperature dependent behaviors of the ground and the first excited excitonic states in both 2H and 3R WSe₂ bilayers. Samples are obtained through chemical vapor deposition, encapsulated with hBN, and reflectance contrast (RC) is measured to identify 1s and 2s excitonic states. At cryogenic temperatures, a splitting of approximately 17 meV is experimentally observed in both the 1s and 2s states of 3R bilayers. This splitting is consistent with our DFT calculations and is due to lack of inversion symmetry, with the two peaks corresponding to distinct excitonic transitions in the upper and lower layers of the 3R WSe₂. As temperatures increase, excitonic states broaden and RC intensity decreases, preventing detection of 2s states above 100 K. The 1s state is evident at all temperatures between 4 K and 300 K, and splitting of the 1s state in 3R samples is detectable to approximately 250 K. This work provides much needed insight into bilayer systems and demonstrates that interlayer interactions are strong enough to significantly modify the optical properties in WSe₂ samples.