Negatively charged nitrogen-vacancy (NV-) center in diamond has been paid attention as a highly sensitive detector for magnetic field. The magnetic sensitivity is proportional to the reciprocal of √(NT_2 ), where N is the number of NV centers and T2 is the spin coherence time. To improve the sensitivity, high concentration of NV- centers and extended coherence time are necessary. It is generally known that electron beam irradiation into type-Ib diamond which include high initial concentration of isolated substitutional nitrogen (P1 center) is a useful method to create high concentration of NV- centers. The electron beam irradiation generates vacancies in diamond and the subsequently annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, as a result, NV centers are created. The post annealing has been performed at 1000°C or higher temperature for the purpose of improving the spin properties of NV- centers. In current study, we focused on post-annealing at high temperature to improve magnetic sensitivity of quantum sensing. We investigated annealing temperature and time dependence of the creation behavior of NV centers in type-Ib diamonds to understand appropriate annealing process for enhancing sensitivity of NV- center.
Negatively charged nitrogen-vacancy (NV-) center in diamond has been paid attention as a highly sensitive detector for magnetic field. The magnetic sensitivity is proportional to the reciprocal of √(NT_2 ), where N is the number of NV centers and T2 is the spin coherence time. To improve the sensitivity, high concentration of NV- centers and extended coherence time are necessary. It is generally known that electron beam irradiation into type-Ib diamond which include high initial concentration of isolated substitutional nitrogen (P1 center) is a useful method to create high concentration of NV- centers. The electron beam irradiation generates vacancies in diamond and the subsequently annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, as a result, NV centers are created. The post annealing has been performed at 1000°C or higher temperature for the purpose of improving the spin properties of NV- centers. In current study, we focused on post-annealing at high temperature to improve magnetic sensitivity of quantum sensing. We investigated annealing temperature and time dependence of the creation behavior of NV centers in type-Ib diamonds to understand appropriate annealing process for enhancing sensitivity of NV- center.
NV- center has been paid attention as a quantum sensor for highly sensitive detection of magnetic field, electric field, temperature and strain, etc.. Electron beam irradiation followed by subsequent thermal annealing is known as the effective procedures to create NV- center in diamond. The electron irradiation generates vacancies in diamond and the annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, and as a result, NV centers are created. In addition, substitutional nitrogen atom at a lattice site (P1 center) also acts as a donor for NV center, the charge state of NV centers changes from neutral (NV0) to NV-. Creation high concentration of ensemble NV- centers is needed to achieve high sensitively sensing. The magnetic sensitivity is proportional to the reciprocal of the number of NV centers and T2 (spin coherence time), i.e., high concentration NV- centers with long T2 increase the sensitivity. P1 center has an important role for being a component of NV- centers and acting as donor to the NV center. But it also plays a role as decoherence source (decrease in magnetic field sensitivity). Therefore, it is important to create high concentration of NV- centers considering the amount of P1 centers. In current study, we investigate irradiation fluence dependence of the creation behavior of NV centers in type-Ib diamonds by using electron spin resonance (ESR) and photo-luminescence (PL) to understand the creation mechanism of NV- center. The values of T2 are also evaluated by Hahn-echo measurement, since it is one of the key parameters for highly sensitive magnetometry. Then, we discuss the creation mechanism of NV centers based on concentrations of defects.
Electron irradiation is one of the most powerful methods to create negatively charged nitrogen-vacancy centers (NV-s) in diamonds, which act as qubits and quantum sensors even at room temperature (RT). For quantum sensing, it is important to create ensemble NV-s with long coherence time (T2) since the sensitivity can be affected by both amount of NV- and T2. For high concentration NV-, electron irradiation with higher fluence is needed. However, if electron fluence is too high, neutral NVs (NV0s) are created because of lack of N donors. For T2, since residual defects act as origins of T2 decoherence, it is important to optimize the condition of subsequent annealing as well as irradiation fluence. Thus, we need to consider initial nitrogen (P1) concentration in diamond, electron fluence, and annealing conditions. In this paper, we create NV-s in Ib diamonds by electron irradiation and subsequent annealing with different conditions, and study the amounts of NV-s and their T2 for high-sensitive quantum sensing. The samples used in this study were Ib diamond with P1 concentration around 100 ppm. Those samples were irradiated with 2 MeV-electrons at a fluence of 1x1018/cm2 at RT. After irradiation, the samples were annealed between 600 and 1400oC in Ar. Photoluminescence (PL) measurements were carried out at RT to confirm NV creation. For PL measurements, the laser with a wavelength of 532 nm was used as excitation. PL intensity for samples annealed at 600 oC is about half even after annealing for 600 min, comparing with that for 800 oC-annealed samples although the values increase with increasing annealing time. Thus, the annealing at 650 oC is not enough to create NV. The samples annealed between 800 and 1200 oC show the highest PL intensity even after annealing for 10 min. The 800 and 1000 oC-annealed samples keep the PL intensity after annealing for 1200 min although the PL intensity for 1200 oC-annealed samples decrease with increasing annealing time above 100 min. The PL intensity for the samples annealed at 1400 oC shows smaller value and decreases with increasing annealing time. For T2, the samples annealed at 1400 oC show 2 microsec after annealing for 10 min. However, NV- cannot be observed after annealing at 1400 oC for 30min. Therefore, it can be concluded that annealing at 1400 oC is not suitable for the creation of NV-. For samples annealed between 600 and 1000 oC, the values of T2 are constant (between 0.5 and 0.9 microsec) after annealing up to 1200 min. For samples annealed at 1200 oC, no significant difference in T2 value is observed from those annealed between 600 and 1000 oC up to 30min, and the value increases with increasing annealing time above 30 min. From the point of view of sensing, it is important to consider both the amounts of NV- centers and T2. Therefore, we compare the values of PL intensity times T2. As a result, the samples annealed between 800 and 1200 oC show the highest value although the value for samples annealed at 1200 oC decreases after annealing for 1200 min. This suggests that annealing at 800 oC for 10 min is enough to create NV-s. We also study the creation of NV-s in diamonds by electron irradiation at elevated temperature. The samples with P1 concentration around 60 and 100 ppm were irradiated with 2 MeV-electrons at fluences up to 5x1018/cm2 at 800 or 1000 oC. The samples were annealed at 1000 oC for 120 min after irradiation. The amounts of NV are estimated from electron spin resonance (ESR) signals. As a result, samples irradiated at 800 and 1000 oC show higher NV- concentration than that for RT-irradiated ones, and 800 oC-irradiated ones show higher NV concentration than 1000oC-irradiated ones. Since the T2 values are not affected by irradiation temperature, the elevated temperature irradiation, especially 800 oC irradiation is a useful method to realize quantum sensing with high sensitivity.
Abstract Negatively charged nitrogen-vacancy (NV − ) center ensembles in diamond have proved to have great potential for use in highly sensitive, small-package solid-state quantum sensors. One way to improve sensitivity is to produce a high-density NV − center ensemble on a large scale with a long coherence lifetime. In this work, the NV − center ensemble is prepared in type-Ib diamond using high energy electron irradiation and annealing, and the transverse relaxation time of the ensemble— T 2 —was systematically investigated as a function of the irradiation electron dose and annealing time. Dynamical decoupling sequences were used to characterize T 2 . To overcome the problem of low signal-to-noise ratio in T 2 measurement, a coupled strip lines waveguide was used to synchronously manipulate NV − centers along three directions to improve fluorescence signal contrast. Finally, NV − center ensembles with a high concentration of roughly 10 15 mm −3 were manipulated within a ~10 µ s coherence time. By applying a multi-coupled strip-lines waveguide to improve the effective volume of the diamond, a sub-femtotesla sensitivity for AC field magnetometry can be achieved. The long-coherence high-density large-scale NV − center ensemble in diamond means that types of room-temperature micro-sized solid-state quantum sensors with ultra-high sensitivity can be further developed in the near future.
Nitrogen-Vacancy (NV) centers in diamond are expected as highly sensitive magnetic sensors operable at room temperature. One major challenge in improving sensitivity is controlling the NV axis orientation. In diamond crystals, NV centers are typically randomly oriented along four crystallographic axes. Since only NV centers on a specific axis are utilized, NV centers on other axes act as fluorescence noise. Therefore, selectively forming NV centers along a single crystallographic axis is expected to enhance sensitivity. Perfectly oriented NV centers can be fabricated by nitrogen doping during chemical vapor deposition (CVD) growth, however, achieving a high NV center density remains challenging. In contrast, a high NV center density can be achieved by electron-beam irradiation followed by post-annealing, although orientation control remains difficult. In this study, we attempted to control NV center orientation by applying microwave annealing after electron-beam irradiation. Generally, microwave irradiation of material induces heating via Joule, dielectric and magnetic losses. Diamond has negligible dielectric and magnetic losses, so a boron doped diamond layer was formed as conductive layer and microwave annealing was performed via Joule loss. Because the conductive layer is thin, an electric field is applied throughout the whole diamond, resulting in a drift current flowing across the diamond. We thought that by controlling this current in the [111] direction, it would be possible to preferentially form [111]-oriented NV centers.
To improve the sensitivity of magnetic sensors using negatively charged nitrogen-vacancy (NV−) centers, it is essential to improve the quality of diamond crystals and precisely control the nitrogen concentration in the crystals. We have utilized high-pressure, high-temperature synthesis [1] and chemical vapor deposition [2] for the growth of nitrogen-doped diamond. The concentration of substitutional nitrogen in the neutral charge state [NS0] were evaluated by electron paramagnetic resonance (EPR) measurements. As shown in Fig. 1, [NS0] were precisely controlled by the weight ratio of nitrogen getter material in the metal solvent for the HPHT method (Fig. 1(a)) and by the carbon to nitrogen gas ratio for the CVD method (Fig. 1(b)). Then, the diamond crystals were irradiated with electron beams followed by vacuum annealing so that 10% of the nitrogen at the substitutional site became NV− centers, and the negatively charged NV− centers were successfully formed preferentially [3]. T2 of these diamond crystals was inversely proportional to the nitrogen concentration. The product of T2 and nitrogen concentration of the obtained HPHT diamond crystal is slightly higher than the maximum value reported so far [4], suggesting that the crystal is expected to have high magnetic sensitivity.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.