NV0 Center Research Articles

Photoluminescence studies of the charge states of nitrogen vacancy centers in diamond after arsenic ion implantation and subsequent annealing

In this work, nitrogen vacancy (NV) centers of high nitrogen diamond implanted with arsenic ions were investigated by photoluminescence spectroscopy. The transition of the NV center charge state was discussed by the regularly changing laser excitation power and measurement temperature following high-temperature annealing. After high-temperature annealing, the amorphous layer generated by arsenic ion implantation is transformed into a graphitization layer, resulting in a decrease in the NV yield. The electric neutral NV (NV0) center and negatively charged NV (NV−) center are affected by both radiation recombination and Auger recombination with increasing laser power. Accompanied by the increasing measurement temperature, the intensities of NV centers gradually decreased and eventually quenched. In addition, the charge states of NV− and NV0 centers were undergoing a transition. The zero phonon line positions of NV centers were also red shift, it was attributed to the dominant role of electron–phonon interaction in the temperature-dependent displacement of diamond energy gaps. The full width at half maxima of NV center were broadened significantly at higher temperatures.

Excitation lifetime extracted from electron–photon (EELS-CL) nanosecond-scale temporal coincidences

Electron–photon temporal correlations in electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopies have recently been used to measure the relative quantum efficiency of materials. This combined spectroscopy, named cathodoluminescence excitation (CLE) spectroscopy, allows for the identification of excitation and decay channels, which are hidden in average measurements. Here, we demonstrate that CLE can also be used to measure excitations' decay time. In addition, the decay time as a function of the excitation energy is measured, as the energy for each electron–photon pair is probed. We used two well-known insulating materials to characterize this technique, nanodiamonds with NV0 defects and hexagonal boron nitride (h-BN) with 4.1 eV defects. Both also exhibit marked transition radiations, whose extremely short decay times can be used to characterize the instrumental response function. It is found to be typically 2 ns, in agreement with the expected limit of the EELS detector temporal resolution. The measured lifetimes of NV0 centers in diamond nanoparticles (20–40 ns) and 4.1 eV defect in h-BN flakes (<2 ns) match those reported previously.

Change in charge state of NV center caused by monovacancy formation

Effects of monovacancy formation on the charge states of nitrogen vacancy (NV) centers in nitrogen-doped diamond single crystal were investigated. Monovacancies are formed using high-energy electron beam irradiation. The ratio of the concentration of negatively charged nitrogen vacancy (NV−) centers to the total concentration of NV− centers and neutral nitrogen vacancy (NV0) centers decreases with increased concentration of monovacancies. This decrease occurs because of the reduction of the concentration of neutral substitutional nitrogen [Ns0] donating electrons to NV0 centers. Findings indicate that numerous electrons of Ns0 are transferred preferentially to monovacancies, not to NV0 centers.

Imaging Extreme Ultraviolet Radiation Using Nanodiamonds with Nitrogen-Vacancy Centers.

Extreme ultraviolet (EUV) radiation with wavelengths of 10-121 nm has drawn considerable attention recently for its use in photolithography to fabricate nanoelectronic chips. This study demonstrates, for the first time, fluorescent nanodiamonds (FNDs) with nitrogen-vacancy (NV) centers as scintillators to image and characterize EUV radiations. The FNDs employed are ∼100 nm in size; they form a uniform and stable thin film on an indium-tin-oxide-coated slide by electrospray deposition. The film is nonhygroscopic and photostable and can emit bright red fluorescence from NV0 centers when excited by EUV light. An FND-based imaging device has been developed and applied for beam diagnostics of 50 nm and 13.5 nm synchrotron radiations, achieving a spatial resolution of 30 μm using a film of ∼1 μm thickness. The noise equivalent power density is 29 μW/(cm2 Hz1/2) for the 13.5 nm radiation. The method is generally applicable to imaging EUV radiation from different sources.

Stimulated emission cross sections and temperature‐dependent spectral shifts of neutral nitrogen‐vacancy centers in diamonds

AbstractThe neutral nitrogen‐vacancy (NV0) defects in diamond are photostable color centers, suitable for a wide range of applications in science and engineering. However, the photophysical properties of the centers have not yet been fully characterized. This work measured the stimulated emission cross sections of NV0 in a single‐crystal diamond by two‐photon excitation of its matrix at 344 nm. From the measured photoluminescence spectrum and the fluorescence lifetime of 20 ± 1 ns, we determined a peak stimulated emission cross section of 1.43 ± 0.07 × 10−17 cm2 at 650 nm for the NV0 centers. In addition, we have also examined the thermal shifts of the zero‐phonon line of NV0 centers in nanoscale diamonds (~100 nm in diameter) over the temperature range of 30–120°C. A temperature measurement sensitivity of 0.2 K·Hz−1/2 was achieved, which is about two‐fold better than that of NV−, despite that the fluorescence intensity of NV0 is about six‐fold lower than that of NV− in the same nanoparticles. The result is attributed to the smaller electron–phonon coupling strength of the neutral center, compared with its negatively charged counterpart.

Nitrogen related paramagnetic defects: Decoherence source of ensemble of NV− center

We investigated spin-echo coherence times T2 of negatively charged nitrogen vacancy center (NV−) ensembles in single-crystalline diamond synthesized by either the high-pressure and high-temperature and chemical vapor deposition methods. This study specifically examined the magnetic dipole–dipole interaction (DDI) from the various electronic spin baths, which are the source of T2 decoherence. Diamond samples with NV− center concentration [NV−] comparable to those of neutral substitutional nitrogen concentration [Ns0] were used for DDI estimation. Results show that the T2 of the ensemble NV− center decreased in inverse proportion to the concentration of nitrogen-related paramagnetic defects [NPM], being the sum of [Ns0], [NV−], and [NV0], which is a neutrally charged state NV center. This inversely proportional relation between T2 and [NPM] indicates that the nitrogen-related paramagnetic defects of three kinds are the main decoherence source of the ensemble NV− center in the single-crystalline diamond. We found that the DDI coefficient of NVH− center was significantly smaller than that of Ns0, the NV0 center, or the NV− center. We ascertained the DDI coefficient of the NV− center DNV− through experimentation using a linear summation of the decoherence rates of each nitrogen-related paramagnetic defect. The obtained value of 89 μs ppm for DNV− corresponds well to the value estimated from the relation between DDI coefficient and spin multiplicity.

UV-induced charge-state conversion from the negatively to neutrally charged nitrogen-vacancy centers in diamond

Under ultraviolet (UV) excitation with photon energy larger than 4.5 eV, a charge-state conversion from negatively to neutrally charged nitrogen-vacancy (NV− to NV0) centers in diamond samples is realized. The UV-induced charge-state conversion is found to strongly depend on the N concentration in the sample and the irradiation fluence of the electron beam. For the samples with high N concentrations, low-fluence (2.5 × 1017 cm−2) 10-MeV electron beam irradiation usually leads to UV-induced charge-state conversion efficiency higher than that of the samples irradiated with high fluences (≥2.5 × 1018 cm−2). For the samples with a few ppm N, however, the charge-state conversion efficiency is relatively low in the cases of irradiation fluences in this work. Meanwhile, UV-induced NV0 luminescence exhibits temperature dependence different from that of visible-light excited NV0 or NV− centers; that is, the photoluminescence intensity does not reach saturation at temperatures lower than 135 K but decreases with the decrease in temperature. Based on the photoluminescence excitation spectra of NV0 centers in the UV region, the UV-induced charge-state conversion is suggested to involve free-hole generation, diffusion, and the capture by ground-state NV− centers.

Photoluminescence of Nitrogen-Vacancy Centers by Ultraviolet One- and Two-Photon Excitation of Fluorescent Nanodiamonds.

Fluorescent nanodiamonds contain nitrogen-vacancy (NV) centers as quantum defects. When exposed to a continuous-wave 325 nm laser or a femtosecond 344 nm laser, the particles emit red fluorescence from NV0 centers at ∼620 nm. Power dependence measurements of the emission strength revealed a predominantly linear behavior at the laser peak intensity lower than 1 GW·cm-2, contributed mainly by photoexcitation of electrons from the valence band of diamond to the NV0 centers, followed by relaxation via electron-hole recombination. In the higher power regions, however, nonresonant two-photon interband excitation of the diamond matrix dominates the photoluminescence processes. Best fits of the experimental data to semiempirical models revealed an ionization coefficient of ∼1 cm-1 for the one-photon valence-to-defect excitation and a saturation intensity of 180 ± 60 GW·cm-2 for the two-photon interband excitation. The study provides new insight into the photoionization of NV0 centers and the interband excitation properties of diamond in the UV region.

X‐Rays in Diamond Photonics: A New Way to Control Charge States of Color Centers

This work is focused on the investigation of the X‐ray's interaction with the color centers in diamond. X‐rays have the high penetrating power of radiation, which allows performing unhindered modifications deeply in the bulk of diamond crystals. Herein, it is shown that X‐rays irradiation of diamond can change the charge states of its defects, including silicon‐vacancy (SiV) and nitrogen‐vacancy (NV) color centers. By studying low‐temperature absorption spectra, it is shown that negatively charged SiV− and NV− centers partially transform into neutrally charged SiV0 and NV0 centers, accordingly. In addition, new absorption lines are registered, which may belong to other charge states of color centers. The results open a new way for the study of charge states for defects in various crystals (not limited to diamond), as well as allow the control over the charge state of color centers for diamond‐based quantum optics.

Divergent Effects of Laser Irradiation on Ensembles of Nitrogen-Vacancy Centers in Bulk and Nanodiamonds: Implications for Biosensing.

Ensembles of negatively charged nitrogen-vacancy centers (NV−) in diamond have been proposed for sensing of magnetic fields and paramagnetic agents, and as a source of spin-order for the hyperpolarization of nuclei in magnetic resonance applications. To this end, strongly fluorescent nanodiamonds (NDs) represent promising materials, with large surface areas and dense ensembles of NV−. However, surface effects tend to favor the less useful neutral form, the NV0 centers, and strategies to increase the density of shallow NV− centers have been proposed, including irradiation with strong laser power (Gorrini in ACS Appl Mater Interfaces. 13:43221–43232, 2021). Here, we study the fluorescence properties and optically detected magnetic resonance (ODMR) of NV− centers as a function of laser power in strongly fluorescent bulk diamond and in nanodiamonds obtained by nanomilling of the native material. In bulk diamond, we find that increasing laser power increases ODMR contrast, consistent with a power-dependent increase in spin-polarization. Conversely, in nanodiamonds we observe a non-monotonic behavior, with a decrease in ODMR contrast at higher laser power. We hypothesize that this phenomenon may be ascribed to more efficient NV−→NV0 photoconversion in nanodiamonds compared to bulk diamond, resulting in depletion of the NV− pool. A similar behavior is shown for NDs internalized in macrophage cells under the typical experimental conditions of imaging bioassays. Our results suggest strong laser irradiation is not an effective strategy in NDs, where the interplay between surface effects and local microenvironment determine the optimal experimental conditions.Supplementary InformationThe online version contains supplementary material available at 10.1186/s11671-022-03723-2.

Diamond nitrogen-vacancy center charge state ratio determination at a given sample point

A new, all-optical, relatively simple, and fast method for determining the concentration ratio of negatively charged (NV−) to neutral (NV0) nitrogen-vacancy centers at a given microdiamond point is proposed. The photoluminescence spectra of NV centers were measured at various excitation powers exploiting the photoinduced interconversion effect. An isoemission point was found, and a range of incident laser intensity was determined, under which the total concentration of NV centers of both charge states was conserved, justifying decomposition of the set of photoluminescence curves into NV− and NV0 emission spectra by the standard non-negative matrix factorization (NNMF) procedure. Based on these results, the relative photoluminescence rate of NV− to NV0 centers for HPHT microdiamond was obtained with high accuracy. Possible applications of the new method are discussed.

Катодолюминесценция азотсодержащих алмазных образцов при температурах 80-800 К

The spectral characteristics of the cathodoluminescence of several synthetic nitrogen-containing diamond samples upon excitation by an electron beam with an energy of up to 300 keV in the temperature range of 80-800 K have been studied. Data on temperature quenching of the cathodoluminescence of diamond samples with various impurity-defect compositions at elevated temperatures (more than 400 K) have been obtained. It has been found that the rate of thermal quenching of NV0 centers is higher than that of NV centers at temperatures of ~500 K and higher.

Equilibrium charge state of NV centers in diamond

We investigated charge states of nitrogen vacancy (NV) centers in diamond single crystals. The charge states were evaluated using concentrations of both negatively charged (NV−) and neutral-state (NV0) NV centers. This study specifically examines diamond crystals containing mainly neutral substitutional nitrogen (N0s), NV− centers, and NV0 centers. Results show that the charged states of NV centers evaluated for this study depended on concentrations of both neutral substitutional nitrogen (N0s) and NVT centers, which is a sum of [NV−] and [NV0]. The NV centers were fully negatively charged when the ratio of [NVT] to [N0s] was 1%–20% in our diamond crystals. When [NVT]/[N0s] is larger than 20%–30%, NV0 centers appeared gradually. The concentration ratio between NV− centers and NV0 centers was described using an equilibrium equation.

Long-Lived Ensembles of Shallow NV– Centers in Flat and Nanostructured Diamonds by Photoconversion

Shallow, negatively charged nitrogen-vacancy centers (NV–) in diamond have been proposed for high-sensitivity magnetometry and spin-polarization transfer applications. However, surface effects tend to favor and stabilize the less useful neutral form, the NV0 centers. Here, we report the effects of green laser irradiation on ensembles of nanometer-shallow NV centers in flat and nanostructured diamond surfaces as a function of laser power in a range not previously explored (up to 150 mW/μm2). Fluorescence spectroscopy, optically detected magnetic resonance (ODMR), and charge-photoconversion detection are applied to characterize the properties and dynamics of NV– and NV0 centers. We demonstrate that high laser power strongly promotes photoconversion of NV0 to NV– centers. Surprisingly, the excess NV– population is stable over a timescale of 100 ms after switching off the laser, resulting in long-lived enrichment of shallow NV–. The beneficial effect of photoconversion is less marked in nanostructured samples. Our results are important to inform the design of samples and experimental procedures for applications relying on ensembles of shallow NV– centers in diamond.

Luminescence spectra of diamonds containing nitrogen-vacancy and interstitial photoactive centers

The spectra of pulsed cathodoluminescence and photoluminescence of synthetic diamonds subjected to radiation-thermal treatment were investigated. The luminescence spectra showed vibronic bands of NV0 centers, interstitials, nickel-nitrogen complexes, as well as previously unidentified bands at 370–380 nm and paired bands at 510 and 532 nm. On the basis of spectral analysis of peaks positions, the 510 and 532 nm bands are assigned to transitions in <100> split self-interstitials (vibronic system 3H) upon interband excitation.

Luminescence of brown CVD diamond: 468 nm luminescence center

Detailed study of the luminescence of multiple brown CVD diamonds was performed. It has been found that the well-known optical center with zero-phonon line at 468 nm is a characteristic of brown color. It has been found that the defects responsible for 468 nm center are located within brown striations suggesting close relation of the 468 nm center and the vacancy clusters. Simultaneous reduction of the intensity of 468 nm center and brown color during annealing support the assumption of their close relation. Identical spectroscopic parameters of the 468 nm center and the radiation center with ZPL at 492 nm suggest that the former relates to an intrinsic defect probably containing vacancies. The distribution of intensity of the 468 nm center in some brown diamonds follows the distribution of the NV− center while being opposite to that of the NV0 center and the dislocation-related A-band. This observation suggests the negative charge state of the 468 nm center. Due to its high luminescence efficiency, the 468 nm center can be used as a highly sensitive indicator of the traces of vacancy clusters. We found that the 468 nm center is detected practically in every as-grown CVD diamond including colorless CVD diamonds of high structural perfection and high purity.

Resonance Light Scattering by Optical Phonons in Diamond Crystal with Nitogen-Vacancy Centres

Observation of the resonance enhancement of the intensity of light scattering by optical phonons in a diamond crystal with nitogen-vacancy centres is reported. It is established that the unusual resonance with electronic transitions for optically active nitrogen impurities with both zero-phonon-lines plays the determining role in the increase in intensity of such scattering: at 575.468 nm for a neutral NV0 center and at 637.874 nm for a negatively charged NV– center. The obtained experimental data indicate the detection of a strong resonant-intensity amplification in the input scattering channel for the NV0 center and in the output channel for the NV– center.

Adjustable charge states of nitrogen-vacancy centers in low-nitrogen diamond after electron irradiation and subsequent annealing

In this work, we investigate the photoluminescence spectra of nitrogen-vacancy (NV) centers in low-nitrogen diamond under 200 keV electron irradiation. We discuss the dependence of NV center charge states on annealing temperature, laser excitation power, and measurement temperature. The results show that the NV centers in low-nitrogen diamond are more likely to exist in the form of NV0 centers. Annealing breaks the charge balance between two charged NV centers, and, as the annealing temperature increases (300–800 °C), NV− centers are converted into NV0 centers. Meanwhile, the NV intensities significantly increase when the nitrogen atoms capture the vacancies after electron irradiation and subsequent annealing. With increases in laser power, the NV− centers are more prone to Auger recombination, and thus, the NV− centers are converted into NV0 centers. In addition, the NV centers are quenched by increases in measurement temperature, but the measurement temperature does not affect their intensity ratio. This result indicates that low-nitrogen diamond becomes more likely to form stable NV0 and NV− centers at different measurement temperatures.

Study on synthesis of diamond by adding magnesium silicate pentahydrate in Fe–Ni–C system

Abstract In this paper, diamond crystals are successfully synthesized by adding magnesium silicate pentahydrate (Mg2Si3O8·5H2O) into the Fe–Ni–C system at 5.8–6.3 GPa pressure and 1300–1420 °C temperature. Our results show that the decomposition products of Mg2Si3O8·5H2O under high temperature and high pressure have significant inhibitory effects on the nucleation and growth of diamond. Moreover, with add the amount, the temperature and pressure of synthetic diamond also gradually increased. The SEM shows that the surface defects of the crystal also increase with the increase of the add amount. The IR spectroscopy results show that the content of nitrogen decreases with the increase of the add amount of Mg2Si3O8·5H2O in the crystals. The Raman spectrum shows that the Raman peak of the crystal shifts to the right as the added amount of Mg2Si3O8·5H2O increases. The PL spectrum results show that the increase in magnesium silicate pentahydrate reduces the intensity of NV− and NV0 centers in the crystal. Therefore, our research results provide important ways for understanding the formation mechanism of natural diamond.

Impact of positive space charge depletion layer on negatively charged and neutral centers in gold–diamond Schottky junctions

It has been widely accepted that conversion between negatively charged nitrogen-vacancy (NV−) center and neutral nitrogen-vacancy (NV0) center can be realized with the Fermi level crossing the transition level corresponding to the ground state of NV−. In this study, a suppressing effect was observed in the emission of NV− center; however, emission of NV0 center was not observed to be increasing as expected and was stable when lowering of the Fermi level was induced by a Schottky junction between gold and oxygen-terminated diamond containing NV centers. The relationship between the thickness of the depletion layer as well as that of the suppression layer, appearing when the Fermi level shifted below the ground state level of NV−, and the donor concentration was derived. Moreover, a model based on the assumption that an intermediate transition level (nonfluorescence state) existed between the ground state energy levels of NV− and NV0, was proposed to explain why suppressed NV− did not convert to NV0. The results of this study will aid in further understanding of the characteristics of NV centers. In addition, a Schottky junction can be developed to determine the charge state of unknown color centers and facilitate study of defects in diamonds.