Exploring excitonic complexes in large angle twisted bilayer WSe₂

Twisted bilayers (tbl) of 2D transition metal dichalcogenides show emergent electronic and optical features. The relative rotation in between the two layers, modifies the band structure, band alignment of momentum transitions and gives rise to emergent moiré phonons [1]. Here, we combine photoluminescence spectroscopy (PL) and first principles density functional theory analysis to study the interplay of charge, layer coupling and band structure in tbl WSe₂ (twist angle > 15⁰) and compare it with the natural bilayer and monolayer [2]. In the tbl, we observe a blue-shift in energy of interlayer excitons, as well as an asymmetric PL spectrum with electrostatic doping, that corroborate well with band structure calculations. The WSe₂ layers show robust higher order excitons (negatively charged biexciton or doubly charged trion) at high temperatures in the heavily doped electron regime. This work provides an improved understanding of complex excitonic states in large angle twisted bilayer WSe₂. 

[1] Debnath, R. et. al., (2020), Nanoscale, 12, 17272-17280.

[2] Debnath, R. et. al., (2021), ACS Nano, Under review.

The authors acknowledge the ITC-PAC, MEITY, Govt. of India and SERC at IISc, Bangalore, India.