Shallow Nitrogen-vacancy Centers In Diamond Research Articles

Charge Stability and Charge-State-Based Spin Readout of Shallow Nitrogen-Vacancy Centers in Diamond.

Spin-based applications of the negatively charged nitrogen-vacancy (NV) center in diamonds require an efficient spin readout. One approach is the spin-to-charge conversion (SCC), relying on mapping the spin states onto the neutral (NV0) and negative (NV-) charge states followed by a subsequent charge readout. With high charge-state stability, SCC enables extended measurement times, increasing precision and minimizing noise in the readout compared to the commonly used fluorescence detection. Nanoscale sensing applications, however, require shallow NV centers within a few nanometers distance from the surface where surface related effects might degrade the NV charge state. In this article, we investigate the charge state initialization and stability of single NV centers implanted ≈5 nm below the surface of a flat diamond plate. We demonstrate the SCC protocol on four shallow NV centers suitable for nanoscale sensing, obtaining a reduced readout noise of 5-6 times the spin-projection noise limit. We investigate the general applicability of the SCC for shallow NV centers and observe a correlation between the NV charge-state stability and readout noise. Coating the diamond with glycerol improves both the charge initialization and stability. Our results reveal the influence of the surface-related charge environment on the NV charge properties and motivate further investigations to functionalize the diamond surface with glycerol or other materials for charge-state stabilization and efficient spin-state readout of shallow NV centers suitable for nanoscale sensing.

Advances in Stabilization and Enrichment of Shallow Nitrogen-Vacancy Centers in Diamond for Biosensing and Spin-Polarization Transfer.

Negatively charged nitrogen-vacancy (NV-) centers in diamond have unique magneto-optical properties, such as high fluorescence, single-photon generation, millisecond-long coherence times, and the ability to initialize and read the spin state using purely optical means. This makes NV- centers a powerful sensing tool for a range of applications, including magnetometry, electrometry, and thermometry. Biocompatible NV-rich nanodiamonds find application in cellular microscopy, nanoscopy, and in vivo imaging. NV- centers can also detect electron spins, paramagnetic agents, and nuclear spins. Techniques have been developed to hyperpolarize 14N, 15N, and 13C nuclear spins, which could open up new perspectives in NMR and MRI. However, defects on the diamond surface, such as hydrogen, vacancies, and trapping states, can reduce the stability of NV- in favor of the neutral form (NV0), which lacks the same properties. Laser irradiation can also lead to charge-state switching and a reduction in the number of NV- centers. Efforts have been made to improve stability through diamond substrate doping, proper annealing and surface termination, laser irradiation, and electric or electrochemical tuning of the surface potential. This article discusses advances in the stabilization and enrichment of shallow NV- ensembles, describing strategies for improving the quality of diamond devices for sensing and spin-polarization transfer applications. Selected applications in the field of biosensing are discussed in more depth.

Generation of shallow nitrogen-vacancy centers in diamond with carbon ion implantation

The shallow nitrogen-vacancy center of diamond exhibits excellent sensitivity and resolution in the magnetic detection and quantum sensing areas. Compared with other methods, low-energy carbon ion implantation does not need high-purity diamond nor introduce new impurity atoms, but the formation mechanism of nitrogen-vacancy center is not clear. In this work, shallow nitrogen-vacancy centers are created in the diamond by low energy carbon ion implantation and vacuum annealing, and the transformation mechanism of nitrogen-vacancy centers in diamond is studied by Raman spectroscopy, X-ray photoelectron spectroscopy, and positron annihilation analysis. The results show that shallow nitrogen-vacancy centers can be obtained by carbon ion implantation combined with vacuum annealing. After implantation, superficial layer of diamond shows the damage zone including lattice distortion and amorphous carbon, and carbon-vacancy cluster defects (carbon atoms are surrounded by vacancy clusters) are generated. In the vacuum annealing process, the damaged area gradually transforms into the diamond structure through the recovery of the distortion area and the solid-phase epitaxy of the amorphous carbon area, accompanied by the continuous dissociation of carbon-vacancy cluster defects. When samples are annealed at 850 and 900 ℃, the structure of the damaged area is partially repaired. While annealing at 950 ℃, not only the damaged layer is basically recovered, but also nitrogen atoms capture the single vacancy obtained by the dissociation of carbon vacancy clusters, forming the nitrogen-vacancy centers.

Charge state dynamics and optically detected electron spin resonance contrast of shallow nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in diamond can be used for nanoscale sensing with atomic resolution and sensitivity; however, it has been observed that their properties degrade as they approach the diamond surface. Here we report that in addition to degraded spin coherence, NV centers within nanometers of the surface can also exhibit decreased fluorescence contrast for optically detected electron spin resonance (OD-ESR). We demonstrate that this decreased OD-ESR contrast arises from charge state dynamics of the NV center, and that it is strongly surface-dependent, indicating that surface engineering will be critical for nanoscale sensing applications based on color centers in diamond.

Distribution of shallow NV centers in diamond revealed by photoluminescence spectroscopy and nanomachining

We performed nanomachining combined with photoluminescence spectroscopy to understand the depth distribution of nitrogen-vacancy (NV) centers formed by low energy nitrogen ion irradiation of the diamond surface. NV− and NV0 fluorescence signals collected from the surface progressively machined by a diamond tip in an atomic force microscope (AFM) initially rise to a maximum at 5 nm depth before returning to background levels at 10 nm. This maximum corresponds to the defect depth distribution predicted by a SRIM simulation using a 2.5 keV implantation energy per nitrogen atom. Full extinguishing of implantation produced NV− and NV0 zero phonon line peaks occurred beyond 10 nm machining depth, coinciding with the end of easy surface material removal and onset of significant tip wear. The wear ratio of for NV active, ion irradiated diamond compared to the single-crystal diamond tip was surprisingly found to be 22:1. The reported results constitute the first integrated study of in-situ machining and wear characterization via optical properties of the diamond surface containing shallow formed NV centers. We discuss possible metrology applications for diamond tools used in precision manufacturing.

Enhancing fluorescence of shallow nitrogen-vacancy centers in diamond by surface coating with titanium oxide layers

We present an enhancement of the fluorescence of shallow (<10 nm) nitrogen-vacancy (NV−) centers by using atomic layer deposition to deposit titanium oxide layers on the diamond surface. In this way, the shallow NV− center charge states were stabilized, leading to the increasing fluorescence intensity of about 2 times. This surface coating technique could produce a protective layer of controllable thickness without any damages to the solid-state quantum system surface, which might be an approach to the further passivation or packaging techniques for the solid-state quantum devices.

Lithographically engineered shallow nitrogen-vacancy centers in diamond for external nuclear spin sensing

The simultaneous control of the number and position of negatively charged nitrogen-vacancy (NV) centers in diamond was achieved. While single near-surface NV centers are known to exhibit outstanding capabilities in external spin sensing, trade-off relationships among the accuracy of the number and position, and the coherence of NV centers have made the use of such engineered NV centers difficult. Namely, low-energy nitrogen implantation with lithographic techniques enables the nanoscale position control but results in degradation of the creation yield and the coherence property. In this paper, we show that low-energy nitrogen ion implantation to a 12C (99.95%)-enriched homoepitaxial diamond layer using nanomask is applicable to create shallow NV centers with a sufficiently long coherence time for external spin sensing, at a high creation yield. Furthermore, the NV centers were arranged in a regular array so that 40% lattice sites contain single NV centers. The XY8-k measurements using the individual NV centers reveal that the created NV centers have depths from 2 to 12 nm, which is comparable to the stopping range of nitrogen ions implanted at 2.5 keV. We show that the position-controlled NV centers are capable of external spin sensing with a ultra-high spatial resolution.

Effect of ultraprecision polishing techniques on coherence times of shallow nitrogen-vacancy centers in diamond

We investigate the correlation between surface roughness and corresponding T2 times of near-surface nitrogen-vacancy centers (~10 nm/5 keV implantation energy) in diamond. For this purpose we compare five different polishing techniques, including both purely mechanical as well as chemical mechanical approaches, two different substrate sources (IIa Technologies and Element Six) and two different surface terminations (O- and H-termination) during nitrogen-vacancy forming. All coherence times are measured and compared before and after an oxygen surface treatment at 520 °C.We find that the coherence times of shallow nitrogen-vacancy centers are surprisingly independent of surface roughness.

Effect of a radical exposure nitridation surface on the charge stability of shallow nitrogen-vacancy centers in diamond

A nitridation process of a diamond surface with nitrogen radical exposure far from the radio-frequency plasma for the stabilization of a negatively charged nitrogen-vacancy (NV−) centers near the surface is presented. At a nitrogen coverage of as high as 0.9 monolayers, high average Rabi contrasts of 0.40 ± 0.06 and 0.46 ± 0.03 have been obtained for single NV− centers formed by shallow nitrogen implantation with acceleration voltages of 1 and 2 keV, respectively. This indicates that nitrogen termination by a radical exposure process produces an electric charge state suitable for single NV− centers near the surface compared with the states obtained for alternatively terminated surfaces.

Mie resonance-enhanced pumping and detection efficiency for shallow nitrogen-vacancy centers in diamond.

We investigate Mie resonances of a diamond nano-resonator as a means to enhance the pumping and detection efficiency of shallow nitrogen-vacancy color centers. We show it is possible to tune a couple of high-order modes of a single resonator to each absorption and emission spectrum of the color center, and thereby the resonator plays a dual role of pump field concentration and emission field guiding. Furthermore superposition of the resonator field and the uncoupled near field results in even stronger pump intensity in the shallow top layer of the resonator. We also examine possible coupling between adjacent resonators when they form a periodic array. This approach allows us to achieve lower excitation power and higher signal intensity at local sites defined by resonators providing a way to enhance wide-field metrology in the sampled region of shallow color centers.

NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond

We demonstrate a robust experimental method for determining the depth of individual shallow Nitrogen-Vacancy (NV) centers in diamond with $\sim1$ nm uncertainty. We use a confocal microscope to observe single NV centers and detect the proton nuclear magnetic resonance (NMR) signal produced by objective immersion oil, which has well understood nuclear spin properties, on the diamond surface. We determine the NV center depth by analyzing the NV NMR data using a model that describes the interaction of a single NV center with the statistically-polarized proton spin bath. We repeat this procedure for a large number of individual, shallow NV centers and compare the resulting NV depths to the mean value expected from simulations of the ion implantation process used to create the NV centers, with reasonable agreement.

Effect of low-damage inductively coupled plasma on shallow nitrogen-vacancy centers in diamond

Near-surface nitrogen-vacancy (NV) centers in diamond have been successfully employed as atomic-sized magnetic field sensors for external spins over the last years. A key challenge is still to develop a method to bring NV centers at nanometer proximity to the diamond surface while preserving their optical and spin properties. To that aim we present a method of controlled diamond etching with nanometric precision using an oxygen inductively coupled plasma process. Importantly, no traces of plasma-induced damages to the etched surface could be detected by X-ray photoelectron spectroscopy and confocal photoluminescence microscopy techniques. In addition, by profiling the depth of NV centers created by 5.0 keV of nitrogen implantation energy, no plasma-induced quenching in their fluorescence could be observed. Moreover, the developed etching process allowed even the channeling tail in their depth distribution to be resolved. Furthermore, treating a 12C isotopically purified diamond revealed a threefold increase in T2 times for NV centers with &amp;lt;4 nm of depth (measured by nuclear magnetic resonance signal from protons at the diamond surface) in comparison to the initial oxygen-terminated surface.

Effect of oxygen plasma and thermal oxidation on shallow nitrogen-vacancy centers in diamond

We investigate the effect of two different surface treatments on shallow nitrogen-vacancy (NV) centers in diamond. Short duration oxygen plasma exposure is found to damage near-surface NV centers, resulting in their disappearance in fluorescence images. Subsequent annealing creates large numbers of new NV centers, attributed to plasma-induced vacancy creation. By tracking individual NV centers during thermal oxidation, we show that oxidation at 550 °C results in modest improvement of spin coherence. Higher temperature oxidations correlate with gradual decline in spin coherence and eventual instability of NV centers before ultimate disappearance. This is indicative of a reduction of the NV-to-surface distance due to oxidative etching. Thermal oxidation can offer controlled access to near-surface NV spins at the nanometer scale, an important requirement for many applications of NV-based nanomagnetometry.