  Inelastic electron tunneling in a scanning tunneling microscope (STM) is used
to generate excitons in monolayer tungsten disulfide (WS$_2$). Excitonic
electroluminescence is measured both at positive and negative sample bias.
Using optical spectroscopy and Fourier-space optical microscopy, we show that
the bias polarity of the tunnel junction determines the spectral and angular
distribution of the emitted light. At positive sample bias, only emission from
excitonic species featuring an in-plane transition dipole moment is detected.
Based on the spectral distribution of the emitted light, we infer that the
dominant contribution is from charged excitons, i.e., trions. At negative
sample bias, additional contributions from lower-energy excitonic species are
evidenced in the emission spectra and the angular distribution of the emitted
light reveals a mixed character of in-plane and out-of-plane transition dipole
moments.
