Removing Non-Size-Dependent Electron Spin Decoherence of Nanodiamond Quantum Sensors by Aerobic Oxidation

Surface oxidation of nanodiamonds (NDs) is a primary step of their surface functionalization that is key to the success of their recent emerging applications in nanoscale quantum sensors in biological samples. Here, we investigate how the electron spin coherence of single nitrogen vacancy centers in NDs is extended by two major oxidizing techniques, that is, aerobic oxidation and anaerobic triacid oxidation with various processing parameters. Aerobic oxidation at 550 °C most effectively oxidizes the surface and extends T₂ by a factor of 1.44 ± 0.33 to the original NDs. The ND size dependence of this T₂ extension shows that aerobic oxidation removes a constant decoherence contribution irrespective of the ND size, which clearly separates its origin from the surface-derived decoherence sources. The present results highlight the presence of the ND-specific decoherence sources other than surface termination spin noise and spin-active impurities, thereby improving the spin coherence of ND quantum sensors.