Enhanced exciton-exciton collisions in an ultra-flat monolayer MoSe2 prepared through deterministic flattening

TL;DR

This study demonstrates that ultra-flat MoSe2 monolayers prepared via a deterministic nano-squeezing method exhibit significantly enhanced exciton-exciton collisions, leading to biexciton formation at very low excitation powers.

Contribution

The paper introduces a modified nano-squeezing technique to produce ultra-flat MoSe2 monolayers with minimal impurities, enabling observation of enhanced excitonic interactions and biexciton formation.

Findings

Ultra-flat MoSe2 shows negligible trion peaks.

Biexciton formation occurs at extremely low excitation power.

Residual linewidth of exciton peak is only 2.2 meV.

Abstract

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 ultra-flat 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 n ~ 2 x 10^12 cm^-2 uN^-1, resulting in strong intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "modified nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual linewidth 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 formation of biexciton occurs even at extremely low excitation power (Phi ~ 2.3 x 10^19 cm^-2 s^-1) due to the enhanced collisions between excitons.