  Spin-based applications of the negatively charged nitrogen-vacancy (NV)
center in diamonds require efficient spin readout. One approach is the
spin-to-charge conversion (SCC), relying on mapping the spin states onto the
neutral (NV$^0$) and negative (NV$^-$) charge states followed by a subsequent
charge readout. With high charge-state stability, SCC enables extended
measurement times, increasing precision and minimizing noise in the readout
compared to the commonly used fluorescence detection. Nano-scale sensing
applications, however, require shallow NV centers within a few $\si{\nano
\meter}$ distance from the surface where surface related effects might degrade
the NV charge state. In this article, we investigate the charge state
initialization and stability of single NV centers implanted $\approx
\SI{5}{\nano \meter}$ below the surface of a flat diamond plate. We demonstrate
the SCC protocol on four shallow NV centers suitable for nano-scale sensing,
obtaining a reduced readout noise of 5--6 times the spin-projection noise
limit. We investigate the general applicability of SCC for shallow NV centers
and observe a correlation between NV charge-state stability and readout noise.
Coating the diamond with glycerol improves both charge initialization and
stability. Our results reveal the influence of the surface-related charge
environment on the NV charge properties and motivate further investigations to
functionalize the diamond surface with glycerol or other materials for
charge-state stabilization and efficient spin-state readout of shallow NV
centers suitable for nano-scale sensing.
