Highly intensive emission of the NV⁻ centers in synthetic HPHT microdiamonds at low nitrogen doping

The neutral and negatively charged nitrogen-vacancy (NV) color centers in diamonds (NV⁰ and NV⁻, respectively), consisting of a substitutional nitrogen atom (N_S) and an adjacent vacancy, possess a bright luminescence of zero-phonon and phonon-assisted lines. Normally, the substitutional nitrogen atoms are embedded into diamond lattices during the high-pressure high-temperature (HPHT) synthesis either without any nitrogen dopant by infiltration from atmospheric air with a concentration of about 150-250 ppm (further we will refer to this as a “standard” level of nitrogen concentration) or by using inorganic nitrogen-containing additives,¹ while the vacancies are usually created by irradiation of the diamonds with high energy electrons (2–10 MeV) with fluence up to 10²⁰ e⁻/cm².^2–5 A subsequent thermal annealing of the diamonds provides greater homogeneity of the vacancy distribution and increased probability of NV center formation.

Based on the wealth of photophysics and photonics knowledge, there is considerable interest in potential applications of NV⁻ center diamonds for bioimaging, sensing, and quantum-based technologies.^6–11 Therefore, developing commercial methods of NV⁻ center production and optimizing critical parameters for luminescence are of great importance. Both nitrogen and vacancy defect concentrations are two important parameters to consider.

It has recently been reported that simply increasing both the vacancy⁵ and substitutional nitrogen atom content¹² in Ib-type HPHT diamond microcrystals does not result in the expected increase in photoluminescence (PL) intensity. In the report by Shames et al.,¹² the authors found that although an increase in the electron fluence from 5 × 10¹⁸ to 5.0 × 10¹⁹ e⁻/cm² resulted in a monotonic increase in the number of the NV⁻ defects from 4 to 12 ppm, their PL intensity was maximized at the fluence of 3 × 10¹⁹ e⁻/cm² and dropped quickly at higher electron dosages. This was referred to as a self-quenching of the PL in a tablet of the pressed powder of small diamond particles. In another report,¹² the increasing NS content above the “standard” level of 250 ppm (340 ± 50 ppm and 650 ± 50 ppm) in Ib-type HPHT diamonds irradiated by e-beam at two relatively low irradiation fluencies (2.2 × 10¹⁸ e⁻/cm² and 7.0 × 10¹⁸ e⁻/cm²) resulted in growth of the NV⁻ PL intensity but not as quickly as the number of NV⁻ centers. This slowing was attributed to the appearance of nitrogen-induced structural imperfections within the diamond crystal lattice, which are nonradiative recombination centers responsible for the PL quenching. It is known¹³ that increasing the NS content causes formation of undesirable defects of A-type (close N—N pairs), B-type (4N—V complexes), H3 centers (N—V—N in a neutral charge state), or nitrogen clusters. It follows that the NV⁻ PL intensity is controlled not only by the concentration of NV⁻ centers and vacancies but also by the presence of nitrogen-induced crystal defects and may be optimized by the appropriate choice of nitrogen doping. However, the NV center luminescence of diamond particles with the NS content lower than the “standard” level has not been studied yet.

Here we report the spectral and time-resolved parameters of emission of NV⁻ centers in Ib-type HPHT diamond microcrystals having N_S concentrations ranging from 50 ppm to 600 ppm, with special attention to the concentration region below 250 ppm. The N_S and NV⁻ concentrations were determined by the analysis of Electron Paramagnetic Resonance (EPR) spectra of the samples, while the structural quality of the diamond microcrystals and the presence of the nonradiative recombination were determined by Raman spectroscopy and the NV⁻ PL intensity and decay measurements.

Monocrystalline diamonds were synthesized from carbon precursors by a tightly controlled, proprietary high pressure, high temperature process, as described in Ref. 13. Briefly, a mixture of graphite as the carbon precursor, diamond “seeds,” and a metal catalyst that reduces the temperature and pressure conditions necessary for diamond growth is used. The HPHT process begins at a pressure of 5-6 GPa and a temperature between 1300 and 1600 °C when the catalyst melts. The new diamond crystals are formed on the diamond seeds from graphite dissolved in the metal catalyst. The catalyst and unconverted graphite are removed from the mixture after the end of synthesis by treatment in concentrated acid, leaving behind diamond crystals containing a “standard” level of substitutional nitrogen of about 250 ppm as was measured later by an EPR technique. The amount of N_S in the crystals can be increased by doping the catalyst with nitrogen containing compounds such as iron nitrides or reduced by adding a nitrogen getter into the catalyst, which chemically binds nitrogen contained in the mixture of graphite powder and catalyst. In order to act as a nitrogen getter, the element must have a strong affinity for nitrogen. An essential part of nitrogen in the growth capsule is therefore trapped in this case and forms nitrides. Materials such as metal oxides, silicon, and aluminum usually work well as getters of nitrogen.

For this study, we fabricated four samples of HPHT diamond crystals, samples #1, #2, #3, and #4, with N_S concentrations of 50, 90, 250, and 600 ppm, respectively, with concentrations determined from the analysis of the samples’ EPR spectra. Sample #3 was obtained without any additives to the graphite powder, while samples #1 and #2 were obtained using a nitrogen getter in the form of one of the most common and available oxides of 3d-metal—titanium dioxide. The as-grown diamond crystals varied in size between 200 and 400 μm, as measured using a Nikon NIS-Elements software package available on a Nikon Eclipse Ti—S epifluorescence microscope. As-grown diamond crystals contained low levels of catalyst inclusions. To remove undesirable catalyst and other metallic ferromagnetic inclusions, the as-grown coarse diamonds were milled to mid-size powders (∼12 μm) and, to a lesser extent, to smaller 0.100 μm diamond crystals then extensively cleaned in concentrated acid. This effectively removed metal inclusions (mainly iron) from the milled diamonds. Removal of metal contaminants was monitored by the EPR method and verified by the absence of the related specific EPR signal (g-factor ≈ 4).

Then, the diamonds were spread across a cooling plate (774 cm²) as a layer of 3-4 mm thickness and irradiated with an electron beam (energy ∼ 7 MeV). A beam current of approximately 25 mA was applied for 16 h. Based on capturing at least 60% of the total electrons within the area, we estimated a dosage of approximately 7 × 10¹⁸ e⁻/cm². Following irradiation, the diamonds were annealed in an inert gas atmosphere at 800 °C for 6 h and then air oxidized at 450 °C for 1 h. At this point, the diamonds appeared as a light purple powder.

For subsequent Raman and luminescence studies, we specifically selected diamond microcrystals with a size of about 2 μm from synthesized samples #1–#4 with different nitrogen contents. This was done in order to exclude the possible influence of the crystal size on the optical characteristics of diamond microcrystals, especially the PL relaxation times. A representative SEM image of diamond microcrystals, selected from sample #1 for optical measurements, is shown in Fig. 1. The crystals of ∼2-3 μm size, analogous to those for which Raman and luminescent spectra were subsequently obtained, are marked with red hexagons. The SEM images were obtained with a Merlin (Zeiss) scanning electron microscope. Raman and luminescence spectra were obtained by the use of an “inVia” (Renishaw) micro-Raman spectrometer equipped with a Leica microscope with a micro-objective to focus a laser radiation of 488 nm into a spot with a diameter of about 1 μm, a CCD camera cooled to −70 °C, and a 3000 l/mm diffraction grating. The spectrometer allowed us to obtain Raman and luminescence spectra simultaneously from the same volume of diamond microcrystals with a spectral resolution of 1 cm⁻¹. The obtained secondary emission spectra of diamond microparticles were normalized to the spectral sensitivity of the spectrometer, determined with a black-body radiation unit, and to the 1332 cm⁻¹ diamond Raman band integrated intensity. The latter provides assurance that the measured luminescence from different samples comes from equal volume of the diamond microcrystals and that any changes in the PL intensity are related to the number of NV centers in the diamond crystal lattice. The Raman and PL spectra were averaged over 7 microcrystals from each sample. All measurements were performed at room temperature. PL decay curves at room temperature were obtained by using a laser scanning confocal microscope MicroTime 100 (PicoQuant) equipped with a 100× objective (NA = 0, 9) and a 80 ps pulsed diode laser head with a pulse repetition rate of 5 MHz at 405 nm, which implements the method of time-correlated single photon counting. A spectral filter at 650 nm with 20 nm bandpass width was used to suppress the NV⁰ center photoluminescence. The PL decay curves were fitted by a biexponential function I(t) = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂). The average PL lifetime was calculated as $⟨τ⟩=∑Iiτi2∑Iiτi$⁠. PL decay of at least 20 diamond particles was recorded for each sample. All data showed good reproducibility. Concentrations of N_S atoms and the negatively charged NV⁻ color centers were determined from X-band EPR spectra of the samples. The EPR spectra were registered with JEOL JES EPR spectrometer (X-band and frequency ν ≈ 9.40 GHz) at room temperature. The weight of microcrystalline diamond powder loaded on the bottom of a 4 mm o.d. quartz capillary tube was about 20 mg. The intensive narrow EPR signals (with g-factor g ∼ 2) were registered under the following experimental conditions: microwave power level P_MW = 1 μW, 100 kHz magnetic field modulation with the amplitude A_mod = 0.035 mT, receiver gain G = 50, and number of coherent acquisitions n = 3. In turn, very weak EPR signals (with g-factor g ∼ 4) from triplet centers in the half magnetic field region were detected at higher microwave power, greater modulation of the magnetic field, and receiver gain: P_MW = 10 μW, A_mod = 0.25 mT, and G = 800, n = 4. The time constant was 30 ms in both cases, and the full duration of one acquisition was 120 s. As a representative example, Fig. 2 shows the spectra of sample #2. The spectrum in Fig. 2(a) shows the narrow EPR line with g-factor g = 2.0024 from N_S atoms (so-called P1 centers in the classification of paramagnetic defects in diamonds). The symmetrical satellites on the left and right sides around the central signal are related to the hyperfine structure of this EPR signal caused by interaction of unpaired antibonding nitrogen electronic orbitals (S = 1/2) with a nitrogen nucleus having magnetic moment I = 1.¹⁴ 

Following the approach by van Wyk et al.,¹⁵ the concentrations of the N_S atoms were determined from the linewidth (peak-to-peak distance, ΔH_pp) of the main EPR signal (g = 2.0024) related to paramagnetic nitrogen. It is illustrated in Fig. 2(a), where the EPR spectrum of a low-N sample having very small linewidth of central line is shown (ΔH_pp = 0.103 mT). This linewidth corresponds to an N_S concentration of 90 ± 20 ppm. Note here that the N_S concentration determined from EPR data in the studied samples well match those found by means of the FTIR spectroscopy. The concentrations of NV⁻ centers can be obtained from the EPR signal (g = 4.27) in the half magnetic field region, which is a unique characteristic feature of triplet NV⁻ centers (S = 1) related to the so-called “forbidden” (ΔM_s = 2) microwave transitions in them. Here we need to elucidate that the weak EPR signal from “forbidden” transitions in triplet NV⁻ centers is located at resonance magnetic field H_res = hν/gµ_B ≈ 158 mT, where h is the Plank constant, the microwave frequency, μ_B is the Bohr magneton, and g is the Lande factor, which equals g = 4.27 for 9.443 GHz microwave radiation.^16,17 This signal therefore is located at magnetic field H_res twice smaller than that for an intensive EPR signal from paramagnetic nitrogen having g = 2.0024.¹⁴ The example of this EPR signal for sample #3 is shown in Fig. 2(b). Using the approach developed in Ref. 16, where a direct relation between the double integrated intensity of this EPR line per unit weight of the sample and the actual NV⁻ concentration has been established, we can determine, by non-optical methods, the NV⁻ content in all studied samples, e.g., for sample #2 with an N_S concentration of 90 ppm, the NV⁻ content is about 3.3 ppm. We did not observe any broad EPR signals due to iron-related and/or other 3d-metal complexes in the half magnetic field region. This suggests that the diamond samples we studied were free from ferromagnetic contaminations/inclusions. This includes both inside the diamond crystallites and on their surface, despite their use as catalyst components during HPHT synthesis.

PL spectra of HPHT diamonds with different contents of the N_S atoms in the range of 50–600 ppm are presented in Fig. 3(a). They show characteristic NV⁰ and NV⁻ colour center zero-phonon lines at 575 nm and 637 nm, respectively, and corresponding broad phonon-assisted sidebands with evident domination of the NV⁻ center luminescence. It is seen that the relative PL intensities of the NV⁰ and NV⁻ centers, as well as the integral NV⁻ PL intensity, strongly depend on the N_S content. At a minimal concentration of the N_S atoms of 50 ppm, a contribution of PL from the neutral NV⁰ centers is maximal and close to that from negatively charged NV⁻ centers. An increase in the N_S atom concentration to 600 ppm outstrips the effect of the decrease in NV⁰ PL intensity that is achieved by transformation of neutral color centers to negatively charged ones, as noted in Ref. 18. For such a recharge to be effective, it is necessary that the amount of N_S atoms is much greater than the number of vacancies induced by radiation, which is satisfied for our samples with nitrogen doping exceeding 150-200 ppm.

The PL spectra show that the N_S content controls the integral PL intensity of NV⁻ centers to a large extent. This is demonstrated in Fig. 3(b) where dependencies of the NV⁻ PL intensity and concentration on the N_S content in the range of 50-600 ppm are presented. The figure shows that increasing the N_S concentration from 50 ppm to about 90-100 ppm results in a practically parallel growth of both the concentration and PL intensity of the NV⁻ centers. Although a slight increase in the concentration of luminescent NV⁻ centers is observed with a further increase in the N_S concentration, the intensity of their luminescence decreases sharply, forming a maximum in the range of 90-100 ppm. The most unexpected feature here is a strong increase in the NV⁻ PL intensity at an N_S content that is much below the 250 ppm concentration for diamonds of standard HPHT synthesis, which has never been reported before.

Based on our PL results with increasing concentrations of N_S atoms in HPHT diamonds, we speculate that the luminescence intensity of the NV⁻ centers is the net result of competition between the PL intensity due to formation of the centers and the quenching of their luminescence due to the nonradiative recombination of photoexcited centers involving structural defects in the diamond crystal lattice induced by the N_S atoms. In other words, the luminescence intensity of negatively charged nitrogen-vacancy color centers in HPHT diamonds is determined not only by the concentration of the centers but, above all, also by the quality of the crystallographic structure of the diamond which, for a relatively small number of vacancies, is determined by the number of N_S atoms.¹² That is why the PL intensity of NV⁻ centers decreases despite an increase in their concentration due to the increase in the number of N_S atoms and the transformation of NV⁰ centers to NV⁻ ones.

This conclusion is supported by our data on the structural quality of the diamond microcrystals which were obtained by Raman spectroscopy, the NV⁻ PL intensity and decay measurements. In the first case, we used the fact¹⁹ that the linewidth of the characteristic diamond Raman band at 1332 cm⁻¹ is directly related to the number of defects induced by nitrogen. Moreover, a linear relation between the nitrogen content and the linewidth was reported. Figure 4(a) presents examples of the 1332 cm⁻¹ Raman band measured for samples #1 and #4 with the lowest and highest N_S concentrations, and the inset shows the dependence of the FWHM of the 1332 cm⁻¹ band on the N_S concentration. It is seen that, at low N_S concentration of 50 and 90 ppm, the FWHM of the 1332 cm⁻¹ band has a smallest value of about 3.4 cm⁻¹ and that this increases with the N_S content, reflecting the onset of deterioration in the quality of the diamond crystal structure. To clarify the origin of the reduction in the NV⁻ PL intensity in the series of samples #2–#4, the corresponding PL decay times have been measured. The N_S content dependencies of both NV⁻ PL intensity and decay time are shown in Fig. 4(b). A simultaneous decrease in the decay time and the intensity of the PL indicates the emergence of a nonradiative relaxation channel for the luminescent state of the NV⁻ centers with growth in the N_S content above 90 ppm due to, most likely, the appearance of nitrogen induced defects in the diamond crystal lattice. All obtained data on N_S atom and NV⁻ center concentrations, integral NV⁻ PL intensity, and PL decay time, as well as on a FWHM of 1332 cm⁻¹ diamond Raman line for the four HPHT diamond samples are summarized in Table I.

The NV color center light emission in synthetic Ib-type high-pressure high-temperature (HPHT) diamond microcrystals in a wide range of N_S concentrations from 50 ppm to 600 ppm were studied by steady state and time-resolved fluorescence techniques and Raman spectroscopy as a function of concentrations of substitutional nitrogen atoms (N_S) and negatively charged nitrogen-vacancy color centers (NV⁻), which was directly determined from the analysis of the electron paramagnetic resonance spectra of the samples. The analysis of experimental data allows one to conclude that the luminescence intensity of the NV⁻ centers is controlled by the competition between the growth of the PL intensity with formation of the NV⁻ centers and quenching of their luminescence by the nonradiative recombination of photoexcited centers involving structural defects induced by the N_S atoms in the crystal lattice. This results in maximal intensity of the NV center light emission at an N_S atom concentration of about 90-100 ppm, which is much below that for N_S atoms embedded into diamond lattices during the standard HPHT synthesis by infiltration from air atmosphere (200-250 ppm) when the quality of the crystallographic structure and optical quality of the diamonds are very good. It is suggested that the PL intensity of NV colour centers in synthetic Ib-type HPHT diamonds at non-perturbing irradiation fluence may be optimized by the choice of nitrogen doping with an appropriate getter.

The work was financially supported by the JSPS/RFBR Research Program including RFBR Project No. 17-52-50004. V.Yu.O. thanks JSPS Fellowship No. L17526 for financial support. K.T. was supported by JSPS KAKENHI Grant Nos. 16K05758 and 26107532. Financial support was provided to A.R. to produce NV⁻center diamonds by the National Heart, Lung and Blood Institute, National Institutes of Health, Department of Health and Human Services, under Contract No. HHSN268201500011C.