Crystal defects in diamond have emerged as unique objects for a variety of applications, both because they are very stable and because they have interesting optical properties. Embedded in nanocrystals, they can serve, for example, as robust single-photon sources or as fluorescent biomarkers of unlimited photostability and low cytotoxicity. The most fascinating aspect, however, is the ability of some crystal defects, most prominently the nitrogen-vacancy (NV) center, to locally detect and measure a number of physical quantities, such as magnetic and electric fields. This metrology capacity is based on the quantum mechanical interactions of the defect's spin state. In this review, we introduce the new and rapidly evolving field of nanoscale sensing based on single NV centers in diamond. We give a concise overview of the basic properties of diamond, from synthesis to electronic and magnetic properties of embedded NV centers. We describe in detail how single NV centers can be harnessed for nanoscale sensing, including the physical quantities that may be detected, expected sensitivities, and the most common measurement protocols. We conclude by highlighting a number of the diverse and exciting applications that may be enabled by these novel sensors, ranging from measurements of ion concentrations and membrane potentials to nanoscale thermometry and single-spin nuclear magnetic resonance.

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We investigate spin and optical properties of individual nitrogen-vacancy centers located within 1-10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic resonance spectrum down to lowest depth. We also find a small, but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.

We present nanoscale nuclear magnetic resonance (NMR) measurements performed with nitrogenvacancy (NV) centers located down to about 2 nm from the diamond surface. NV centers were created by shallow ion implantation followed by a slow, nanometer-by-nanometer removal of diamond material using oxidative etching in air. The close proximity of NV centers to the surface yielded large 1 H NMR signals of up to 3.4 µT-rms, corresponding to ∼ 330 statistically polarized or ∼ 10 fully polarized proton spins in a (1.8 nm) 3 detection volume.The proposal of diamond magnetometry [1,2] and its subsequent demonstration [3,4] has received considerable attention for potential applications in nanoscale magnetic resonance imaging and spectroscopy with single nuclear spin resolution [5]. Recently, diamond-based magnetic sensors have enabled detection of 1 H nuclear magnetic resonance (NMR) from organic molecules deposited on the surface of a diamond chip with a sensitivity of 10 4 − 10 6 proton nuclei [6][7][8], which is a roughly one-million-fold improvement compared to conventional NMR [9] and on par with magnetic resonance force microscopy [10,11]. Recent advances with diamond sensors were made possible by the controlled positioning of nitrogen-vacancy (NV) centers within less than 20 nm from the diamond surface [8,12,13]. In order to eventually detect single nuclear spins, NV centers must be moved even closer to the surface in order to pick up the rapidly decaying dipolar field of a single magnetic moment. Here, we discuss nanoscale NMR measurements performed with NV centers down to 2 nm from the diamond surface. These NV centers were created by shallow implantation followed by controlled removal of a few nanometers of diamond material by oxidative etching in air. The close proximity of NV centers to the surface allowed us to detect as few as 330 statistically polarized hydrogen nuclei in an organic calibration sample as well as in the adsorbate layer naturally present on the diamond surface.The diamond chip used in this study was a commercially available single crystal of electronic grade purity and with a (100) surface orientation [14]. The two-side polished chip had dimensions of 2 × 2 × 0.5 mm 3 . The as-received diamond was briefly etched by ArCl plasma [15] to remove the first few hundred nanometers of material that were possibly compromised by the polishing. NV centers were then created by implantation with 15 N + ions using an energy of 5 keV and a fluence of 10 11 cm 2 [16] and by subsequent annealing at 850 • C in high vacuum (p < 2 × 10 −7 mbar) for two hours. The peak depth of created NV centers is about 8 nm with a straggling * Electronic address: degenc@ethz.ch~8 of ±3 nm according to stopping-range-of-ions-in-matter calculations [13,17,18]. A photoluminescence measurement, shown in Fig. 1(b), confirmed that a large number of NV centers (∼ 5 NV − per µm 2 ) was formed by this procedure.To realize shallower NV centers we have exploited the slow oxidative etching of diamond at ∼ 600 • C in ambient air [19][20][21]. ...

We report successful introduction of negatively charged nitrogen-vacancy (NV(-)) centers in a 5 nm thin, isotopically enriched ([(12)C] = 99.99%) diamond layer by CVD. The present method allows for the formation of NV(-) in such a thin layer even when the surface is terminated by hydrogen atoms. NV(-) centers are found to have spin coherence times of between T2 ~ 10-100 μs at room temperature. Changing the surface termination to oxygen or fluorine leads to a slight increase in the NV(-) density, but not to any significant change in T2. The minimum detectable magnetic field estimated by this T2 is 3 nT after 100 s of averaging, which would be sufficient for the detection of nuclear magnetic fields exerted by a single proton. We demonstrate the suitability for nanoscale NMR by measuring the fluctuating field from ~10(4) proton nuclei placed on top of the 5 nm diamond film.