  Recently discovered Ising superconductors have garnered considerable interest
due to their anomalously large in-plane upper critical fields ($B_{c2}$).
However, the requisite strong spin-orbital coupling in the Ising pairing
mechanism generally renders these superconductors heavy-element dominant with
notably low superconducting transition temperatures ($T_c$). Here, based on the
Migdal-Eliashberg theory and the mean-field Bogoliubov-de Gennes Hamiltonian,
we demonstrate a significant enhancement of Ising superconductivity in
monolayer NbSe$_2$ through surface fluorination, as evidenced by concomitant
improvements in $T_c$ and $B_{c2}$. This enhancement arises from three
predominant factors. Firstly, fluorine atoms symmetrically and stably adhere to
both sides of the monolayer NbSe$_2$, thereby maintaining the out-of-plane
mirror symmetry and locking carrier spins out-of-plane. Secondly, fluorination
suppresses the charge density wave in monolayer NbSe$_2$ and induces a van Hove
singularity in the vicinity of the Fermi level, leading to a marked increase in
the number of carriers and, consequently, strengthening the electron-phonon
coupling (EPC). Lastly, the appearance of fluorine-related, low-frequency
phonon modes further augments the EPC. Our findings suggest a promising avenue
to elevate $T_c$ in two-dimensional Ising superconductors without compromising
their Ising pairing.
