  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.
