  It has been reported that conversion yield and coherence time of
ion-implanted NV centers improve if the Fermi level is raised or lowered during
the annealing step following implantation. Here we investigate whether surface
transfer doping and surface charging, by UV light, can be harnessed to induce
this effect. We analyze the coherence times and the yield of NV centers created
by ion implantation and annealing, applying various conditions during
annealing. Specifically, we study coating the diamond with nickel, palladium or
aluminum oxide, to induce positive surface transfer doping, as well as
annealing under UV illumination to trigger vacancy charging. The metal coated
diamonds display a two times higher formation yield than the other samples. The
coherence time $T_2$ varies by less than a factor of two between the
investigated samples. Both effects are weaker than previous reports, suggesting
that stronger modifications of the band structure are necessary to find a
pronounced effect. UV irradiation has no effect on yield and $T_2$ times.
