Enhanced Exciton-Exciton Collisions in an Ultraflat Monolayer MoSe₂ Prepared through Deterministic Flattening

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 10¹² cm^-2 μN^-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe₂ at low temperature. We found that a hBN/MoSe₂/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φ_ph ∼ 2.3 × 10¹⁹ cm^-2 s^-1) due to the enhanced collisions between excitons.