Near-surface NV centers in diamond with reduced decoherence*

The quantum sensors, based on the NV- centers in diamond, are potential game changers for nuclear magnetic resonance (NMR) spectroscopy. In particular, they are able to supply a structural information on the surrounding spins from micron-scale sample volumes and at millimolar concentrations. In the NMR experiment, the signal-to-noise ratio drastically improves by positioning the target close to the NV- center. However, the near-surface NV- centers usually show poor spin-coherence time which precludes high spectral resolution. Here, we show that for the shallowest NV- centers, surface interferes with the alignment of the triplet sublevels and gives rise to a gradual increase of a rhombic zero-field splitting term. By performing the spin-dynamics simulations, we demonstrate that the newly-developed level anti-crossing enables to maintain the coherence time at close to the bulk values, yet providing a substantial improvement in the STN ratio. By calculating the coherence time in distorted NV- centers with the gCCE method, we evaluate the impact of the electron- and nuclear-spins on the property. We also discuss different types of surface termination and identify most suitable surfaces for quantum sensing.