Designing Optically Addressable Nitrogen-Vacancy Centers in Ultrasmall Nanodiamonds: Insights from First-Principles Calculations.

Ultrasmall nanodiamonds (USNDs) are promising platforms for fluorescent and quantum-sensing applications. Here we present first-principles electronic structure calculations of nitrogen-vacancy (NV-) centers in USNDs, and we investigate their optical addressability as a function of the surface termination. We consider both isolated nanoparticles and arrays of USNDs with different degrees of packing, and we include quantum vibronic effects in our analysis, using stochastic methods. We find that the NV- can be stabilized in a negative charge state if the nanoparticles are terminated by fluorine, hydroxyl, and ether. While fluorine terminations are useful for fluorescent biotags, we suggest that hydroxyl and ether terminations are beneficial for quantum-sensing applications. We also find that the NV- can be stabilized in arrays of USNDs for interparticle separations larger than the diameter of the nanoparticle. Finally, our results show that in arrays, electron phonon interactions enhance the negative charge stability of NV- centers.