Surface effects on nitrogen vacancy centers neutralization in diamond
  The performance of nitrogen vacancy (NV$^{-}$) based magnetic sensors strongly depends on the stability of nitrogen vacancy centers near the diamond surface. The sensitivity of magnetic field detection is diminished as the NV$^{-}$ turns into the neutralized charge state NV$^{0}$. We investigate the neutralization of NV$^{-}$ and calculate the ratio of NV$^{0}$ to total NV (NV$^{-}$+NV$^{0}$) caused by a hydrogen terminated diamond with a surface water layer. We find that NV$^{-}$ neutralization exhibits two distinct regions: near the surface, where the NV$^{-}$ is completely neutralized, and in the bulk, where the\ neutralization ratio is inversely proportional to depth following the electrostatic force law. In addition, small changes in concentration can lead to large differences in neutralization behavior. This phenomenon allows one to carefully control the concentration to decrease the NV$^{-}$ neutralization. The presence of nitrogen dopant greatly reduces NV$^{-}$ neutralization as the nitrogen ionizes in preference to NV$^{-}$ neutralization at the same depth. The water layer pH also affects neutralization. If the pH is very low due to cleaning agent residue, then we see a change in the band bending and the reduction of the $2$-dimensional hole gas (DHG) region. Finally, we find that dissolved carbon dioxide resulting from direct contact with the atmosphere at room temperature hardly affects the NV$^{-}$ neutralization. 
Arthur N. Newell, Dontray A. Dowdell, D. H. Santamore
cond-mat.mes-hall cond-mat.mtrl-sci