Toward Optimized Surface δ-Profiles of Nitrogen-Vacancy Centers Activated by Helium Irradiation in Diamond

The negatively charged nitrogen-vacancy (NV) center in diamond has been shown recently as an excellent sensor for external spins. Nevertheless, their optimum engineering in the near-surface region still requires quantitative knowledge in regard to their activation by vacancy capture during thermal annealing. To this aim, we report on the depth profiles of near-surface helium-induced NV centers (and related helium defects) by step-etching with nanometer resolution. This provides insights into the efficiency of vacancy diffusion and recombination paths concurrent to the formation of NV centers. It was found that the range of efficient formation of NV centers is limited only to approximately 10 to 15 nm (radius) around the initial ion track of irradiating helium atoms. Using this information we demonstrate the fabrication of nanometric-thin (δ) profiles of NV centers for sensing external spins at the diamond surface based on a three-step approach, which comprises (i) nitrogen-doped epitaxial CVD diamond overgrowth, (ii) activation of NV centers by low-energy helium irradiation and thermal annealing, and (iii) controlled layer thinning by low-damage plasma etching. Spin coherence times (Hahn echo) ranging up to 50 μs are demonstrated at depths of less than 5 nm in material with 1.1% of (13)C (depth estimated by spin relaxation (T1) measurements). At the end, the limits of the helium irradiation technique at high ion fluences are also experimentally investigated.

Spectroscopic Observation of Fractal Packing of Oxygen in Variably Modified Glassy Tetrahedral Networks

The spatial distribution of bridging and nonbridging oxygen atoms in silicate glass networks with a wide range of connectivity is studied using (17)O nuclear magnetic resonance spin-lattice relaxation spectroscopy. The results demonstrate a mass fractal spatial distribution of both oxygen species in the network at the nanometer length scale. The fractal dimension increases with increasing relative fraction of these oxygen species and reaches a maximum value of ∼2.6. Such locally fractal character of the structural network is key to the understanding of the apparently anomalous transport, relaxation, and localization properties that are the hallmarks of the glassy state.