  The paper explores the optical properties of an exfoliated MoSe$_2$ monolayer
implanted with Cr$^+$ ions, accelerated to 25 eV. Photoluminescence of the
implanted MoSe$_2$ reveals an emission line from Cr-related defects that is
present only under weak electron doping. Unlike band-to-band transition, the
Cr-introduced emission is characterised by non-zero activation energy, long
lifetimes, and weak response to the magnetic field. To rationalise the
experimental results and get insights into the atomic structure of the defects,
we modelled the Cr-ion irradiation process using ab-initio molecular dynamics
simulations followed by the electronic structure calculations of the system
with defects. The experimental and theoretical results suggest that the
recombination of electrons on the acceptors, which could be introduced by the
Cr implantation-induced defects, with the valence band holes is the most likely
origin of the low energy emission. Our results demonstrate the potential of
low-energy ion implantation as a tool to tailor the properties of 2D materials
by doping.
