  Sensing based on Nitrogen-Vacancy (NV) centers in nanodiamonds (NDs) offers
significant potential across various applications. However, optimizing their
quantum-optical properties remains challenging. This study focuses on enhancing
and controlling the optical properties of NV centers in NDs through surface
chemistry tuning and proton beam irradiation. Systematic thermal oxidations
were performed to investigate the evolution of surface chemical groups using IR
spectroscopy and their influence on optical properties using photoluminescence
spectroscopy and PL decay measurements. Proton irradiation was explored over a
wide range of fluences (10^14 to 10^17 cm^-2) to precisely control the NV
center concentration, identifying conditions that maximize creation and
emission intensity. Furthermore, NV center charge state control was achieved by
analyzing the NV-/NV0 ratio with varying surface terminations and NV center
concentrations. A novel predictive mathematical model was developed to evaluate
the efficiencies of forming NV- and NV0. Although tested specifically with
proton-irradiated NDs, this model has broad applicability, representing a
significant advancement in predicting the outcomes of ion-beam-based color
center generation in diamond.
