  TMDCs have attracted a lot of attention in recent years due to their unique
indirect to direct band gap transition from bulk to monolayer thickness. Strong
confinement in the out-of-plane direction enhances the Coulomb potential
between the charged particles (e-h pairs) and thus increases the exciton
binding energy dramatically. The lattice inversion asymmetry in a monolayer
creates two non-equivalent (but degenerate in energy) band edges protected by
time reversal polarisation via pseudo-spin. However, the presence of strong
spin-orbit coupling in the valence band and weak spin-splitting in the
conduction band results in the lowest lying exciton in WX2 (X = S, Se) being
spin forbidden and optically dark. Because of their long life times, dark
excitons (XD) are highly attractive for quantum optics and optoelectronic
applications. To date studying XD emission is limited to cryogenic temperature
or required very complex experimental configurations to observe them at room
temperature (RT). Here, we demonstrate a novel approach of radiative decay of
XD related emissions in 1L-WSe2 studied by micro and nano PL at RT. 1L-WSe2
flakes were sandwiched by noble metal (Au or Ag) substrates and PDMS
nano-patches providing a strong local out-of-plane dipole moment with respect
to the 2D plane. This strong dipole moment not only enhances the XD in WSe2, it
also produces bound excitons due to extrinsic charge defects visible at RT. The
spatial distributions of these XD related emissions were studied by TEPL with a
spatial resolution < 10 nm confirming the confinement of these excitons within
the PDMS nano-patches. Finally, by removing the nano-patches from the top of
the flakes we are able to recover the bright excitons in the 1L-WSe2. Our
approach paves the way for deep understanding and to harness excitonic
properties in low dimensional semiconductors, thus offering a platform towards
quantum optics.
