Negatively-charged Nitrogen-vacancy Centers Research Articles

Ensemble Negatively-Charged Nitrogen-Vacancy Centers in Type-Ib Diamond Created by High Fluence Electron Beam Irradiation

Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV−) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV− centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV− centers in type-Ib diamond, with an initial P1 concentration of 40–80 ppm by electron beam irradiation up to 8.0 × 1018 electrons/cm2. P1 concentration and NV− concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 1018 electrons/cm2. The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 1018 electrons/cm2 especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV− centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV− center around 12–19%.

Optimized single-crystal diamond scanning probes for high sensitivity magnetometry

The negatively-charged nitrogen-vacancy center (NV) in diamond forms a versatile system for quantum sensing applications. Combining the advantageous properties of this atomic-sized defect with scanning probe techniques such as atomic force microscopy (AFM) enables nanoscale imaging of e.g. magnetic fields. To form a scanning probe device, we place single NVs shallowly (i.e. < 20 nm) below the top facet of a diamond nanopillar, which is located on a thin diamond platform of typically below 1 μm thickness. This device can be attached to an AFM head, forming an excellent scanning probe tip. Furthermore, it simultaneously influences the collectible photoluminescence (PL) rate of the NV located inside. Especially sensing protocols using continuous optically-detected magnetic resonance benefit from an enhanced collectible PL rate, improving the achievable sensitivity. This work presents a comprehensive set of simulations to quantify the influence of the device geometry on the collectible PL rate for individual NVs. Besides geometric parameters (e.g. pillar length, diameter and platform thickness), we also focus on fabrication uncertainties such as the exact position of the NV or the taper geometry of the pillar introduced by imperfect etching. As a last step, we use these individual results to optimize our current device geometry, yielding a realistic gain in collectible PL rate by a factor of 13 compared to bulk diamond and 1.8 compared to our unoptimized devices.

Tutorial: Magnetic resonance with nitrogen-vacancy centers in diamond—microwave engineering, materials science, and magnetometry

This tutorial article provides a concise and pedagogical overview on negatively charged nitrogen-vacancy (NV) centers in diamond. The research on the NV centers has attracted enormous attention for its application to quantum sensing, encompassing the areas of not only physics and applied physics but also chemistry, biology, and life sciences. Nonetheless, its key technical aspects can be understood from the viewpoint of magnetic resonance. We focus on three facets of this ever-expanding research field, to which our viewpoint is especially relevant: microwave engineering, materials science, and magnetometry. In explaining these aspects, we provide a technical basis and up-to-date technologies for research on the NV centers.

Fluorescent nanodiamonds as a robust temperature sensor inside a single cell.

Thermometers play an important role to study the biological significance of temperature. Fluorescent nanodiamonds (FNDs) with negatively-charged nitrogen-vacancy centers, a novel type of fluorescence-based temperature sensor, have physicochemical inertness, low cytotoxicity, extremely stable fluorescence, and unique magneto-optical properties that allow us to measure the temperature at the nanoscale level inside single cells. Here, we demonstrate that the thermosensing ability of FNDs is hardly influenced by environmental factors, such as pH, ion concentration, viscosity, molecular interaction, and organic solvent. This robustness renders FNDs reliable thermometers even under complex biological cellular environment. Moreover, the simple protocol developed here for measuring the absolute temperature inside a single cell using a single FND enables successful temperature measurement in a cell with an accuracy better than ±1°C.

Nanomechanical Sensing Using Spins in Diamond.

Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy. For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step toward combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nanospin-mechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to not only detect the mass of a single macromolecule but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale.

Spin coherence and 14N ESEEM effects of nitrogen-vacancy centers in diamond with X-band pulsed ESR

Pulsed ESR experiments are reported for ensembles of negatively-charged nitrogen-vacancy centers (NV−) in diamonds at X-band magnetic fields (280–400mT) and low temperatures (2–70K). The NV− centers in synthetic type IIa diamonds (nitrogen impurity concentration <1 ppm) are prepared with bulk concentrations of 2 ⋅ 1013 cm−3 to 4 ⋅ 1014 cm−3 by high-energy electron irradiation and subsequent annealing. We find that a proper post-radiation anneal (1000°C for 60min) is critically important to repair the radiation damage and to recover long electron spin coherence times for NV−s. After the annealing, spin coherence times of T2=0.74ms at 5K are achieved, being only limited by 13C nuclear spectral diffusion in natural abundance diamonds. By measuring the temperature dependence of T2 in the under-annealed diamonds (900°C) we directly extract the density (1014−16 cm−3) and activation energy (2.5meV) of unannealed defects responsible for the faster NV− decoherence. At X-band magnetic fields, strong electron spin echo envelope modulation (ESEEM) is observed originating from the central 14N nucleus, and we extract accurate 14N nuclear hypefine and quadrupole tensors. In addition, the ESEEM effects from two proximal 13C sites (second-nearest neighbor and fourth-nearest neighbor) are resolved and the respective 13C hyperfine coupling constants are extracted.

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.

All-optical thermometry and thermal properties of the optically detected spin resonances of the NV(-) center in nanodiamond.

The negatively charged nitrogen-vacancy (NV(-)) center in diamond is at the frontier of quantum nanometrology and biosensing. Recent attention has focused on the application of high-sensitivity thermometry using the spin resonances of NV(-) centers in nanodiamond to subcellular biological and biomedical research. Here, we report a comprehensive investigation of the thermal properties of the center's spin resonances and demonstrate an alternate all-optical NV(-) thermometry technique that exploits the temperature dependence of the center's optical Debye-Waller factor.

Effective production of fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy centers by ion irradiation

Fluorescence from negatively-charged nitrogen-vacancy centers (NV−s) in diamonds has unique optical properties with none of the undesirable effects such as photo-bleaching and photo-blinking. In addition, the spin-dependent fluorescence intensity of NV−s allows us to perform optically detected magnetic resonance (ODMR) investigation for evaluating the presence of NV−s and for the electronic local environment. In this work, we irradiated H+, He+, Li+ and N+ ions to nanodiamonds with a median size of 26nm at various irradiation energies and doses for improving the NV− concentration. ODMR observations of the nanodiamonds showed that ion irradiation increased the number of nanodiamonds containing NV−s up to 200ppm, whereas without ion irradiation, only few NV−s were found. The number of nanodiamonds containing NV−s at various ion irradiation doses was not monotonous, but had maxima at certain irradiation doses. These results suggest a competition in two opponent roles of vacancies, effective for pairing with nitrogen atoms and as defects for developing damage in crystalline. We also found that sharp and strong ODMR signals were obtained from nanodiamonds irradiated at the optimal condition for the highest yield of NV−s. We concluded that He+ ion irradiations with 60 or 80keV at a dose of 1×1013ions cm–2 are the conditions required for the most efficient production of a high quantity of nanodiamonds containing NV−s.

Engineering of nitrogen-vacancy color centers in high purity diamond by ion implantation and annealing

The negatively-charged nitrogen-vacancy (NV) center is the most studied optical center in diamond and is very important for applications in quantum information science. Many proposals for integrating NV centers in quantum and sensing applications rely on their tailored fabrication in ultra pure host material. In this study, we use ion implantation to controllably introduce nitrogen into high purity, low nitrogen chemical vapor deposition diamond samples. The properties of the resulting NV centers are studied as a function of implantation temperature, annealing temperature, and implantation fluence. We compare the implanted NV centers with native NV centers present deep in the bulk of the as-grown samples. The results for implanted NV centers are promising but indicate, at this stage, that the deep native NV centers possess overall superior optical properties. In particular, the implanted NV centers obtained after annealing at 2000 °C under a stabilizing pressure of 8 GPa showed an ensemble linewidth of 0.17 nm compared to 0.61 nm after annealing at 1000 °C. Over the same temperature range, the ensemble NV−/NV0 ratio increased by a factor of ∼5, although this was accompanied by an overall decrease in the NV count.

A composite microcavity of diamond nanopillar and deformed silica microsphere with enhanced evanescent decay length

We report the experimental realization of a composite microcavity system, in which negatively-charged nitrogen vacancy (NV) centers in diamond nanopillars couple evanescently to whispering-gallery modes (WGMs) in a deformed, non-axisymmetric silica microsphere. We show that the deformed microsphere can feature an evanescent decay length four times larger than that of a regular silica microsphere. With the enhanced evanescent coupling, WGMs can in principle couple to NV centers that are 100 to 200 nm beneath the diamond pillar surface, providing a promising avenue for exploring evanescently-coupled cavity QED systems of NV centers in ultrahigh purity diamond.