  Two-dimensional (2D) layered materials provide an ideal platform for
engineering electronic and optical properties through strain control because of
their extremely high mechanical elasticity and sensitive dependence of material
properties on mechanical strain. In this paper, a combined experimental and
theoretical effort is made to investigate the effects of mechanical strain on
various spectral features of bilayer MoTe2 photoluminescence (PL). We found
that bilayer MoTe2 can be converted from an indirect-to direct-bandgap material
through strain engineering, resulting in a photoluminescence enhancement by a
factor of 2.24. Over 90% of the PL comes from photons emitted by the direct
excitons at the maximum strain applied. Importantly, we show that strain
effects lead to a reduction of the overall linewidth of PL by as much as 36.6%.
We attribute the dramatic decrease of linewidth to a strain-induced complex
interplay among various excitonic varieties such as direct bright excitons,
trions, and indirect excitons. Our experimental results on direct and indirect
exciton emission features are explained by theoretical exciton energies that
are based on first-principle electronic band structure calculations. The
consistent theory-experimental trend shows that the enhancement of PL and the
reduction of linewidth are the consequences of the increasing direct exciton
contribution with the increase of strain. Our results demonstrate that strain
engineering can lead to a PL quality of the bilayer MoTe2 comparable to that of
the monolayer counterpart. The additional benefit of a longer emission
wavelength makes the bilayer MoTe2 more suitable for Silicon-photonics
integration due to the reduced Silicon absorption.
