Single Substitutional Nitrogen Research Articles

Diamond crystallization and growth in N–H enriched environment under HPHT conditions

Advances in understanding the state and transformation of impurity-related defects in natural diamond have been achieved by means of high-pressure and high-temperature annealing experiments. However, the existing literature featuring complex behaviors of aggregated nitrogen and hydrogen-related defect (3107 cm−1 center) has remained controversial. Their formation mechanism during diamond crystallization is still unclear, thus further investigation is required. In this work, we have successfully synthesized a variety of high-quality diamonds containing nitrogen impurities with IaA (nitrogen pairs), IaB (group of four nitrogen atoms around a vacancy) and Ib (single-substitutional nitrogen) characteristics ranging from <1 ppm to 3380 ppm at pressures ranging from 5.0 GPa to 6.3 GPa and temperatures of 1300–1650 °C. Our results provide new experimental evidence for the aggregation of nitrogen impurities (A- and B-centers) during diamond growth. We notice that the hydrogen is easily trapped by nitrogen atoms to form nitrogen–hydrogen complexes (–NH, –NH2, –NH3) when the nitrogen tends to form C-centers in hydrogen-enriched environments. Additionally, we observed that the high reaction temperature and formation of A-centers during diamond growth play important roles in the formation of the 3107 cm−1 center. We believe the current results could be helpful for further understanding and constructing a clear model of impurity-related defects, and thus provide us deeper insights into the genesis of natural diamond.

Diamond crystallization with a carbonyl nickel catalyst under high pressure and temperature

For decades, the origin of aggregated nitrogen in diamond has been studied and has led to an important stage in solving the problems of natural diamond genesis. However, clarifying how B-centers (groups of four nitrogen atoms around a vacancy) form in diamond is still one of the most challenging issues in material science. In this work, we apply carbonyl nickel powders as a catalyst to grow diamond over a wide range of pressures, 5.0–6.2 GPa, and temperatures, 1300 to 1800 °C, and we report the first synthesis of high-quality diamonds containing nitrogen impurities with IaB characteristics. Both spontaneous nucleation of diamond and growth of diamond on seeds have been achieved by film growth and temperature gradient growth methods. By Fourier-transform infrared spectrometry, we reveal a novel formation mechanism for aggregated nitrogen impurities in B-centers, that is, B-centers can be readily aggregated by four C-centers (single-substitutional nitrogen atoms) during a diamond growth process under high pressure and temperature. High reaction temperatures can effectively contribute to the formation of nitrogen-vacancy defects in diamond and play a key role in the formation of B-centers during diamond growth. Our current findings will be helpful for further understanding and constructing a clear model of aggregated nitrogen, and thus provide us with deeper insights into the genesis of natural diamond.

Effect of additive zinc on larger diamond crystal growth

The large single crystal diamonds are successfully synthesized in a NiMnCo-C system with the zinc additive in a series of the experiments at temperatures of 1270-1400 ℃ and pressures of 6.2-6.4 GPa by the temperature gradient growth. Morphology and structural properties of the synthesized diamond are characterized by optical microscope and scanning electron microscopy. The Raman spectrum is used to investigate the crystallization of synthesized diamond. The results show that the colors of synthetic diamond crystals change from yellow to light yellow and nearly disappears with the increase of the zinc additive. There are a large number of irregular pits in the surface of diamond crystal when the zinc additive is increased up to 3.0 wt.%. the Fourier transform infrared spectroscopy spectra reveal that the nitrogen impurity in the synthetic diamond crystal is predominantly in the form of C center (single substitutional nitrogen atoms), and the nitrogen concentration decreases with the increase of zinc additive. Two possibilities that the zinc powders can be used as the nitrogen getter are given. the Raman spectrum shows that the diamond crystallization can be improved when the zinc additive is less than 3.0 wt.%. We believe that our work is greatly helpful for deeply understanding the natural diamond genesis, enriching the types of diamonds, and expanding the application areas of synthetic diamond.

Systematic study of defect-related quenching of NV luminescence in diamond with time-correlated single-photon counting spectroscopy

We report on the systematic characterization of photoluminescence (PL) lifetimes in NV- and NV0 centers in 2 MeV H+ implanted type Ib diamond samples by means of a time correlated single photon counting (TCSPC) microscopy technique. A dipole-dipole resonant energy transfer model was applied to interpret the experimental results, allowing a quantitative correlation of the concentration of both native (single substitutional nitrogen atoms) and ion-induced (isolated vacancies) PL-quenching defects with the measured PL lifetimes. The TCSPC measurements were carried out in both frontal (i.e. laser beam probing the main sample surface along the same normal direction of the previously implanted ions) and lateral (i.e. laser beam probing the lateral sample surface orthogonally with respect to the same ion implantation direction) geometries. In particular, the latter geometry allowed a direct probing of the centers lifetime along the strongly nonuniform damage profiles of MeV ions in the crystal. The extrapolation of empirical quasi-exponential decay parameters allowed the systematic estimation of the mean quantum efficiency of the centers as a function of intrinsic and ion-induced defect concentration, which is of direct relevance for the current studies on the use of diamond color centers for photonic applications.

Sidebands in optically detected magnetic resonance signals of nitrogen vacancy centers in diamond

We study features in the optically detected magnetic resonance (ODMR) signals associated with negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) centers coupled to other paramagnetic impurities in diamond. Our results are important for understanding ODMR line shapes and for optimization of devices based on NV${}^{\ensuremath{-}}$ centers. We determine the origins of several side features to the unperturbed NV${}^{\ensuremath{-}}$ magnetic resonance by studying their magnetic field and microwave power dependences. Side resonances separated by around 130 MHz are due to hyperfine coupling between NV${}^{\ensuremath{-}}$ centers and nearest-neighbor ${}^{13}$C nuclear spins. Side resonances separated by approximately ${40,\phantom{\rule{0.16em}{0ex}}260,\phantom{\rule{0.16em}{0ex}}300}$ MHz are found to originate from simultaneous spin flipping of NV${}^{\ensuremath{-}}$ centers and single substitutional nitrogen atoms. All results are in agreement with the presented theoretical calculations.

X-band rapid-scan EPR of samples with long electron spin relaxation times: a comparison of continuous wave, pulse and rapid-scan EPR

X-band room temperature spectra obtained by rapid-scan, continuous wave, field-swept echo-detected and Fourier transform electron paramagnetic resonance (FTEPR) were compared for three samples with long electron spin relaxation times: amorphous hydrogenated silicon (T1 = 11 μs, T2 = 3.3 μs), 0.2% N@C60 solid (T1 = 120–160 μs, T2 = 2.8 μs) and neutral single substitutional nitrogen centres (NS0) in diamonds (T1 = 2300 μs, T2 = 230 μs). For each technique, experimental parameters were selected to give less than 2% broadening of the lineshape. For the same data acquisition times, the signal-to-noise for the rapid-scan spectra was one-to-two orders of magnitude better than for continuous wave or field-swept echo-detected spectra. For amorphous hydrogenated silicon, T2* (∼ 10 ns) is too short to perform FTEPR. For 0.2% N@C60, the signal-to-noise ratio for rapid scan is about five times better than for FTEPR. For NS0 the signal-to-noise ratio is similar for rapid scan and FTEPR.

Entanglement of electron spins and geometric phases in the diamond color center coupled to the P1 center

Impurity spins in semiconductors are potential quantum bits. Entanglement and topological phases are key resources in quantum computation. We prove that the coupled electron spins carried by a diamond nitrogen-vacancy color center (NV${}^{\ensuremath{-}}$) and a single substitutional nitrogen impurity (P1 center) are entangled in the immediate vicinity of the level anticrossing that appears in the Zeeman energy diagram at about 500 G. We also determine the Aharonov-Anandan, Berry, and marginal geometric phases that can be accumulated by the state vectors of this spin system when it is magnetically transported around a closed path. At the resonance where the gap between two energy levels is minimum, the geometric phases undergo discontinuities, and the entanglement of the two electron spins is maximal.

Quantitative characterization of plastic deformation of single diamond crystals: A high pressure high temperature (HPHT) experimental deformation study combined with electron backscatter diffraction (EBSD)

We report the results of a high-pressure high-temperature (HPHT) experimental investigation into the deformation of diamonds using the D-DIA apparatus. Electron backscatter diffraction (EBSD) data confirm that well-defined 300–700nm wide {111} slip lamellae are in fact deformation micro-twins with a 60° rotation around a <111> axis. Such twins formed at high confining pressures even without any apparatus-induced differential stress; mechanical anisotropy within the cell assembly was sufficient for their formation with very little subsequent lattice bending (<1° per 100μm). When apparatus-induced differential stresses were applied to diamonds under HPHT conditions, deformation twin lamellae were generated, and continuous and discontinuous crystal lattice bending occurred (4–18° per 100μm), including bending of the {111} twin lamellae. The {111} <011> slip system dominates as expected for the face-centred cubic (FCC) structure of diamond. Slip occurs on multiple {111} planes resulting in rotation around <112> axes. Deformation microstructure characteristics depend on the orientation of the principal stress axes and finite strain but are independent of confining pressure and nitrogen content. All of the uniaxially deformed samples took on a brown colour, irrespective of their initial nitrogen characteristics. This is in contrast to the two quasi-hydrostatic experiments, which retained their original colour (colourless for nitrogen free diamond, yellow for single substitutional nitrogen, Type Ib diamond) despite the formation of {111} twin lamellae. Comparison of our experimental data with those from two natural brown diamonds from Finsch mine (South Africa) shows the same activation of the dominant slip system. However, no deformation twin lamellae are present in the natural samples. This difference may be due to the lower strain rates experienced by the natural samples investigated. Our study shows the applicability and potential of this type of analysis to the investigation of plastic deformation of diamonds under mantle conditions.

CVD Synthetic Diamonds from Gemesis Corp.

Gemological and spectroscopic properties of CVD synthetic diamonds from Gemesis Corp. were examined. Their color (colorless, near-colorless, and faint, ranging from F to L) and clarity (typically VVS) grades were comparable to those of top natural diamonds, and their average weight was nearly 0.5 ct. Absorption spectra in the mid- and near-infrared regions were free from defect-related features, except for very weak absorption attributed to isolated nitrogen, but all samples were classified as type IIa. Varying intensities of [Si-V]- and isolated nitrogen were detected with UV-Vis-NIR absorption spectroscopy. Electron paramagnetic resonance was used to quantify the neutral single substitutional nitrogen content. Photoluminescence spectra were dominated by N-V centers, [Si-V]-, H3, and many unassigned weak emissions. The combination of optical centers strongly suggests that post-growth treatments were applied to improve color and transparency. PL spectroscopy at low temperature and UV fluorescence imaging are critical in separating these synthetic products from their natural counterparts.

A new defect center in type Ib diamond inducing one phonon infrared absorption: The Y center

The infrared spectra of 68 natural and synthetic diamonds with detectable C centers at about 1130 cm − 1 and 1344 cm − 1 were recorded. After correction and normalization of the spectra there was an attempt to determine the A, B and C center nitrogen content of the samples using the spectral fitting spreadsheet supplied by David Fisher from the DTC. It became clear that for the majority of our samples the results were erroneous for the C center content. In fact, the calculated C center content based on the 1130 cm − 1 absorption was significantly higher than what the 1344 cm − 1 absorption height indicated; in the fitted traces proposed by the spreadsheet the 1344 cm − 1 absorption was always at least 50% higher than in the original spectrum. The overestimated C center content and the residual trace after spectral fitting indicated that some absorption bands were underlying the well-known, single-substitutional nitrogen absorption at 1130 cm − 1 . For this reason all spectra were decomposed again by progressive subtraction of the A, B, C and X center absorptions, in order to visualize any residual absorption not attributed to any of those four, classically-recognized, nitrogen-related one-phonon absorption systems. In the spectra of 38 diamonds the decomposition work resulted in a consistent residual absorption feature with a relatively broad apparent maximum centered at about 1145 to 1150 cm − 1 . For the sake of convenience we use “1145 cm − 1 ” to describe this feature. Two additional samples did not exhibit any 1130 cm − 1 absorption but instead precisely the same one-phonon absorption features as the newly-found residual. Consequently several hundred nominally type Ib diamonds were screened by infrared spectroscopy, and 22 had a one phonon absorption with a dominant 1145 cm − 1 absorption. This confirms that this absorption system was not an artifact of the original spectral decomposition work. Further, this new system is apparently a characteristic component of many natural type Ib diamonds. Based on the fact that this newly described center is clearly related to single substitutional nitrogen and that the last such one-phonon IR absorption had been named “X center” [1] (positively charged single nitrogen), we propose to call this defect “Y center”.

Spin tunneling in the diamond color center coupled to theP1center

The spin-1 carried by the nitrogen-vacancy (NV${}^{\ensuremath{-}}$) color center of a diamond is a promising candidate as a quantum bit (qubit) for the realization of a solid-state quantum computer. The interaction with the $P1$ center, which is a single substitutional nitrogen atom with spin-$\frac{1}{2}$, converts the NV${}^{\ensuremath{-}}$ qubit to a multilevel system. The 18-level energy diagram of the NV${}^{\ensuremath{-}}$-$P1$ coupled defect pair exhibits several level anticrossings and the spin functions change sign at different values of the applied magnetic field. Beyond each resonance, the two electronic spins of the coupled triplet-doublet system flip individually or in concert, offering possibilities to implement quantum logic gates. Interlevel transitions are also possible. The transition probabilities can be predicted rigorously in the framework of the Landau-Zener theory. The spin flip occurs by quantum tunneling. It is also possible to introduce geometric phases, which can be useful for the realization of phase gates.

Single substitutional nitrogen defects revealed as electron acceptor states in diamond using ultrafast spectroscopy

We report on the carrier dynamics and recombination pathways of photogenerated electrons in type Ib synthetic diamond using ultrafast spectroscopic techniques. Samples with controlled amounts of nitrogen defects were grown using the high-pressure high-temperature (HPHT) method. Electrons were excited from single substitutional nitrogen defects into the conduction band via an ultrashort pulse from a frequency-doubled Ti-sapphire laser. Using time-resolved terahertz time-domain spectroscopy, we determined the mobility of the photoexcited electrons and monitored their recombination dynamics, at temperatures ranging from cryogenic temperatures to room temperature. The electron mobility was observed to be limited by scattering with neutral nitrogen impurity defects (N${}_{\mathrm{s}}$${}^{0}$). Electrons were observed to predominantly recombine into neutral nitrogen states rather than their original ionized nitrogen defects, thereby creating negatively charged nitrogen states (N${}_{\mathrm{s}}$${}^{\ensuremath{-}}$). The creation of N${}_{\mathrm{s}}$${}^{\ensuremath{-}}$ states is confirmed experimentally by tracking the localized vibrational modes (LVM) of nitrogen defects during the electron recombination process using visible pump-infrared probe transient spectroscopy. We observe a transient infrared absorption feature at 1349 cm${}^{\ensuremath{-}1}$ that can be assigned to the LVM of N${}_{\mathrm{s}}$${}^{\ensuremath{-}}$. Density functional calculations are carried out to determine the LVMs of nitrogen in various charge states, and we find a \ensuremath{\sim}10 cm${}^{\ensuremath{-}1}$ upward shift of the mode on passing from N${}_{\mathrm{s}}$${}^{0}$ to N${}_{\mathrm{s}}$${}^{\ensuremath{-}}$, in agreement with experimental observations.

Transformation of As-Grown Phosphorus-Related Centers in HPHT Treated Synthetic Diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P–C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073–2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C1 separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073–2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen–phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen–phosphorus centers NP2 and NP3 with the supposed structures C1–N–C–P and N–P–C1, respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C1. Annealing these samples in the temperature range of 2,073–2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen–phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g ≈ 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed.

High pressure and high temperature annealing on nitrogen aggregation in lab-grown diamonds

Diamond single crystals were grown by a high-pressure high-temperature gradient method using a molten Fe–Ni catalyst in a split-sphere apparatus at a pressure of 5.5 GPa and a temperature of 1473–1873 K. The as-grown crystals, ranging in size from 0.2 to 0.6 g, were generally deep yellow in color because of intake of nitrogen impurity during the process. Two different annealing methods were used to change their color to light and vivid yellow. One method involved annealing at 5.5 GPa and 2193–2473 K within the split-sphere apparatus. The other involved annealing of E-beam irradiated crystals at 1773 K in a high-vacuum furnace at 10−6 Torr. Distribution of C-center nitrogen, single substitutional nitrogen, and A-center nitrogen defects, a pair of nearest-neighbor nitrogen, were thoroughly studied by FT–IR spectroscopy. Upon annealing, C-center nitrogen defects decreased by 10–60%, while A-center nitrogen defects increased by 5–70%. It was confirmed that the irradiation process and intake of nickel impurity enhanced the nitrogen aggregation rate, which followed the second order kinetics. Measured activation energies were in the range of 2.88–3.01 eV, which were not strongly affected by the irradiation process. The results indicate that the nitrogen aggregation rate was enhanced by vacancies and interstitials introduced by E-beam irradiation.

Luminescence-lifetime mapping in diamond

This paper introduces a new technique to the study of diamonds: mapping theluminescence lifetime of optical centres. The understanding of luminescence lifetimes indiamond is briefly reviewed. Since lifetime mapping involves extended measuring timeswith focused laser excitation, the stability of the H3 optical centre is investigated. We showthat saturation of the H3 luminescence requires excitation power densities in excess of10 MW cm−2. The non-radiative energy transfer time from an H3 centre to an Aaggregate is found to be equal to that from N3 centres to A aggregates, at∼3 × 10−16r8 s, wherethere are r bond lengths between the H3 and A centres. Non-radiative energy transfer is shown to occur fromthe NV− band to the single substitutional nitrogen atoms: the single N atoms may quenchluminescence as well as the A aggregates of nitrogen. In contrast, a comparison ofthe decays from the very similar H3 and H4 centres demonstrates that the Baggregate produces very weak quenching of the visible luminescence from diamond.

Charge transfer effects, thermo and photochromism in single crystal CVD syntheticdiamond

We report on the effects of thermal treatment and ultraviolet irradiation on the point defectconcentrations and optical absorption profiles of single crystal CVD synthetic diamond. Allthermal treatments were below 850 K, which is lower than the growth temperature and unlikelyto result in any structural change. UV–visible absorption spectroscopy measurementsshowed that upon thermal treatment (823 K), various broad absorption features diminished:an absorption band at 270 nm (used to deduce neutral single substitutional nitrogen (NS0) concentrations) and also two broad features centred at approximately 360 and520 nm. Point defect centre concentrations as a function of temperature werealso deduced using electron paramagnetic resonance (EPR) spectroscopy. Above∼500 K, we observed a decrease in the concentration ofNS0 centresand a concomitant increase in the negatively charged nitrogen–vacancy–hydrogen (NVH) complex(NVH−)concentration. Both transitions exhibited an activation energy between 0.6 and 1.2 eV, which is lower thanthat for the NS0 donor (∼1.7 eV). Finally, it was found that illuminating samples with intense short-wave ultraviolet light recoveredthe NS0 concentration and also the 270, 360 and 520 nm absorption features. From these results, wepostulate a valence band mediated charge transfer process between NVH and singlenitrogen centres with an acceptor trap depth for NVH of 0.6–1.2 eV. Because the loss ofNS0 concentration is greaterthan the increase in NVH− concentration we also suggest the presence of another unknown acceptor existing at asimilar energy to NVH. The extent to which the colour in CVD synthetic diamond isdependent on prior history is discussed.

Study of local concentration of single substitutional nitrogen atoms in microdiamonds from the Kokchetav massif

In this communication we present an estimation of the concentration of single substitutional nitrogen atoms (P1 centre) from EPR data in Kokchetav diamonds. We first performed an analysis of local P1 centre concentrations using van Wyk’s treatment (van Wyk et al ., 1997) of the line width of the resolved EPR spectrum. Also by analysing the dependence of the line-width of the broad exchange line on the P1 concentration in range higher than 1000 ppm in synthetic diamonds, we obtained a calibration curve allowing the estimation of the local P1 centre concentration higher than 1000 ppm and the local volumes occupied by substitutional nitrogen atoms without taking measurements of microcrystalline volume. In addition to the development of a method for calculating high local nitrogen concentrations, the aggregation process in P,T-treated Kokchetav diamonds has been given considerable attention and it was shown that the rate of nitrogen aggregation in these diamonds is consistent with data given in Evans (Evans & Qi, 1982).

Photoionization measurement of deep defects in single-crystalline CVD diamond using the transient-current technique

We have adopted the transient-current technique as a sensitive method to detect small concentrations of charged defects in diamond and to study its photoionization spectrum. It is found that ionized impurity concentrations in the interval ${10}^{9}--{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ can readily be detected in diamond. By continuously measuring the charge concentration, while illuminating the samples with monochromatic light, the evolution of the charge state of the dominating defect can be continuously monitored. We have obtained the photoionization cross-section spectrum from the dominant deep defect in single-crystalline chemical vapor deposition (CVD) diamond using this method. The photoionization spectrum exhibits an onset at photon energies above ${E}_{i}=2.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is also found that the defect is in the positively charged and acts as an electron trap. The observed spectrum can be attributed to the single substitutional nitrogen impurity.

Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN 3 as dopant in metal-carbon system

Diamond crystals have been synthesized from FeNi alloy — carbon system under high pressure and temperature of 5.0 to 5.8 GPa and 1500 to 1750 K. Concentrations of the nitrogen impurities of the crystals have been determined from FTIR spectra, and it is found that the concentrations of single substitutional nitrogen impurities exceeds 1000 ppm, when NaN 3 is added as dopant to the metal-carbon system. The crystals are 0.2–0.5 mm in diameter with cubo–octahedral morphology, although octahedron and cube were also found in the diamond products. They exhibit yellow green to deep green color.

Nickel–nitrogen complexes in synthetic diamond: the 2.429 eV absorption system

The absorption system with zero-phonon line (ZPL) at 2.429 eV is observed in synthetic diamonds grown using a solvent catalyst that contains Ni, after annealing at 1600 °C. The band is only observed on samples with high single substitutional nitrogen concentration, and shows a transitory behaviour in the sense that it anneals out when heat treated at temperatures above 2000 °C. We report the effects of applied uniaxial stresses on this system as well as the temperature dependence of the ZPL. It is shown that stress dependence of both the ZPL and the one-phonon line (OPL) can be understood in terms of an electric dipole transition occurring at a rhombic I centre. The temperature dependence of the 2.429 eV ZPL line and the shape of the vibronic sideband can be explained assuming electron–phonon coupling to a totally symmetric vibrational single mode at ħω=49 meV. The relation between this system and the photo luminescence (PL) centre with ZPL at 1.660 eV was also analysed and, in disagreement with a previously proposed model, we conclude that the observed transitions do not occur at the same centre.