Modeling NV charge stability in nanodiamonds with DFT

The nitrogen-vacancy (NV) defect in diamond is a controllable quantum system with strong fluorescence. When placed in nanodiamonds it provides nanoscale magnetometry and single biological cell tracking. NV must be stable and negatively charged close to the nanodiamond surface. However, NV charge-state fluctuations near surfaces result in intermittent fluorescence; this effect is highly correlated with surface type and structures. Electron affinity has been posited as a cause, but contradictions exist and underlying mechanisms still need to be fully understood. Recent work shows NV charge alternates with pH, suggesting acceptor levels from surface radicals play a role. Using DFT the electronic structure of NVs in nanodiamonds is studied for different, commonly observed terminations, adsorbates and surface radicals. A measure of charge stability is defined and an underlying cause for observed charge instability for certain terminations is proposed. Routes to manipulating NV surfaces to provide stable NV⁻ in nanodiamonds is discussed, thereby allowing more reliable spatiotemporal nanosensors.