  Many classes of two-dimensional (2D) materials have emerged as potential
platforms for novel electronic and optical devices. However, the physical
properties are strongly influenced by nanoscale heterogeneities in the form of
edges, grain boundaries, and nucleation sites. Using combined tip-enhanced
Raman scattering (TERS) and photoluminescence (TEPL) nano-spectroscopy and
-imaging, we study the associated effects on the excitonic properties in
monolayer WSe2 grown by physical vapor deposition (PVD). With <15 nm spatial
resolution we resolve nonlocal nanoscale correlations of PL spectral intensity
and shifts with crystal edges and internal twin boundaries associated with the
expected exciton diffusion length. Through an active atomic force tip
interaction we can control the crystal strain on the nanoscale, and tune the
local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV
shift) fashion. This allows us to distinguish the effect of strain from the
dominant influence of defects on the PL modification at the different
structural heterogeneities. Hybrid nano-optical and nano-mechanical imaging and
spectroscopy thus enables the systematic study of the coupling of structural
and mechanical degrees of freedom to the nanoscale electronic and optical
properties in layered 2D materials.
