  A van der Waals heterostructure containing an atomically thin monolayer
transition-metal dichalcogenide as a single-photon emitting layer is emerging
as an intriguing solid-state quantum-photonic platform. Here, we report the
utilization of spin-coating of silica nanoparticles for deterministically
creating the spectrally isolated, energetically stable, and narrow-linewidth
single-photon emitters in ML-WS$_2$. We also demonstrate that long-duration
low-temperature annealing of the photonic heterostructure in the vacuum removes
the energetically unstable emitters that are present due to
fabrication-associated residue and lead to the emission of single-photons in a
<25 nm narrowband visible spectral range centered at $\sim$620 nm. This work
may pave the way toward realizing a hybrid-quantum-photonic platform containing
a van der Waals heterostructure/device and an atomic-vapor system
emitting/absorbing in the same visible spectral range.
