Shallow NV- colour centres in diamond

Negatively charged Nitrogen-Vacancy (NV-) colour centre in diamond is a well-known and characterised point defect with notable properties such as photostable bright fluorescence and spin states that can be initialised and read out, making it of great appeal for quantum technology applications. Specifically, the latter can benefit from forming NV- defects in the proximity of the diamond surface. As an example, for nuclear magnetic resonance (NMR) sensing, it is necessary to have the NV- spins close to the surface as the coupling strength between magnetic dipoles decreases with the cubic distance of the defects from the surface. Shallow NV- defects can also be easily coupled with nanophotonic cavities for photon extraction. Furthermore, shallow NV- can also be beneficial in the biomedical field since the proximity to the surface allows the coupling with biomaterials for sensing applications, such as nano thermometry. However, the fabrication of shallow NV colour centres is challenging since it faces with several issues such as charge stability and low formation efficiency due to the behaviour of the diamond surface as an electron and a vacancy sink. The surface effects (i.e. vacancy sink, band bending and electron tunnelling) tend to convert NV- in NV0 centres which do not possess the same spin properties and are not commonly used for quantum technologies.