  The nitrogen-vacancy (NV) color center in diamond has demonstrated great
promise in a wide range of quantum sensing. Recently, there have been a series
of proposals and experiments using NV centers to detect spin noise of quantum
materials near the diamond surface. This is a rich complex area of study with
novel nano-magnetism and electronic behavior, that the NV center would be ideal
for sensing. However, due to the electronic properties of the NV itself and its
host material, getting high quality NV centers within nanometers of such
systems is challenging. Band bending caused by space charges formed at the
metal-semiconductor interface force the NV center into its insensitive charge
states. Here, we investigate optimizing this interface by depositing thin metal
films and thin insulating layers on a series of NV ensembles at different
depths to characterize the impact of metal films on different ensemble depths.
We find an improvement of coherence and dephasing times we attribute to
ionization of other paramagnetic defects. The insulating layer of alumina
between the metal and diamond provide improved photoluminescence and higher
sensitivity in all modes of sensing as compared to direct contact with the
metal, providing as much as a factor of 2 increase in sensitivity, decrease of
integration time by a factor of 4, for NV $T_1$ relaxometry measurements.
