The A nitrogen aggregate in diamond-its symmetry and possible structure

The Nitrogen-Vacancy-Nitrogen Color Center: A Ubiquitous Visible and Near-Infrared-II Quantum Emitter in Nitrogen-Doped Diamond

Photoluminescent defects in diamond, such as the nitrogen-vacancy (NV) color center, are at the forefront of emerging optical quantum technologies. Most emit in the visible and near-infrared spectral region below 1000 nm (NIR-I), limiting their applications in photonics, fiber communications, and biology. Here, we show that the nitrogen-vacancy-nitrogen (N2V) center, which emits in the visible and near-infrared-II (NIR-II, 1000-1700 nm), is ubiquitous in as-synthesized and processed nitrogen-doped diamond, ranging from bulk samples to nanoparticles. We demonstrate that N2V is also present in commercially available state-of-the-art NV diamond sensing chips made via chemical vapor deposition (CVD). In high-pressure high-temperature (HPHT) diamonds, the photoluminescence (PL) intensity of both N2V charge states, N2V0 in the visible and N2V- in the NIR-II, increases with increasing substitutional nitrogen concentration. We determine the PL lifetime of N2V- to be 0.3 ns and compare a quantum optical and density functional theory model of the N2V- with experimental PL spectra. Finally, we show that detonation nanodiamonds (DND) exhibit stable PL in the NIR-II, which we attribute to the N2V color center, and use this NIR-II PL to image DNDs inside skin cells. Our results contribute to the scientific and technological exploration and development of the N2V color center and help elucidate interactions with other color centers in diamond.

The NV-N+ charged pair in diamond: a quantum-mechanical investigation

The NV^-N⁺ charged pair in diamond has been investigated by using a Gaussian-type basis set, the B3LYP functional, the supercell scheme and the CRYSTAL code. It turns out that: (i) when the distance between the two defects is larger than 6-7 Å, the properties of the double defect are the superposition of the properties of the individual defects. (ii) The energy required for the reaction NV⁰ + N_s→ NV^- + N⁺ is roughly -1.3 eV at about 12 Å, irrespective of the basis set and functional adopted, and remains negative at any larger distance. (iii) These results support the observation of a charge transfer mechanism through a N_s→ NV⁰ donation occurring in the ground state, through a tunnelling process, without irradiation. (iv) The IR spectrum of the two subunits is characterized by specific peaks, that might be used as fingerprints. (v) Calculation of electrostatic interaction permitted an estimate of the effective charge of the defects.

The CRYSTAL code, 1976-2020 and beyond, a long story

CRYSTAL is a periodic ab initio code that uses a Gaussian-type basis set to express crystalline orbitals (i.e., Bloch functions). The use of atom-centered basis functions allows treating 3D (crystals), 2D (slabs), 1D (polymers), and 0D (molecules) systems on the same grounds. In turn, all-electron calculations are inherently permitted along with pseudopotential strategies. A variety of density functionals are implemented, including global and range-separated hybrids of various natures and, as an extreme case, Hartree-Fock (HF). The cost for HF or hybrids is only about 3-5 times higher than when using the local density approximation or the generalized gradient approximation. Symmetry is fully exploited at all steps of the calculation. Many tools are available to modify the structure as given in input and simplify the construction of complicated objects, such as slabs, nanotubes, molecules, and clusters. Many tensorial properties can be evaluated by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, first and second hyperpolarizabilities, etc. The calculation of infrared and Raman spectra is available, and the intensities are computed analytically. Automated tools are available for the generation of the relevant configurations of solid solutions and/or disordered systems. Three versions of the code exist: serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated on each core, whereas in the third one, the Fock matrix is distributed for diagonalization. All the relevant vectors are dynamically allocated and deallocated after use, making the code very agile. CRYSTAL can be used efficiently on high performance computing machines up to thousands of cores.

Deformation Features of Super-Deep Diamonds

The paper presents new data on the internal structure of super-deep (sublithospheric) diamonds from Saõ-Luiz river placers (Brazil) and from alluvial placers of the northeastern Siberian platform (Yakutia). The sublithospheric origin of these diamonds is supported by the presence of mineral inclusions corresponding to associations of the transition zone and lower mantle. The features of morphology and internal structure have been studied by optical and scanning electron microscopy (SEM), cathodoluminescence topography (CL), and electron backscatter diffraction (EBSD) techniques. Diamonds typically have complicated growth histories displaying alternating episodes of growth, dissolution, and post-growth deformation and crushing processes. Most crystals have endured both plastic and brittle deformation during the growth history. Abundant deformation and resorption/growth features suggest a highly dynamic growth environment for super-deep diamonds. High temperatures expected in the transition zone and lower mantle could explain the plastic deformations of super-deep diamonds with low nitrogen content.

On the Models for the Investigation of Charged Defects in Solids: The Case of the VN- Defect in Diamond

Local charged defects in periodic systems are usually investigated by adopting the supercell charge compensated (CC) model, which consists of two main ingredients: (i) the periodic supercell, hopefully large enough to reduce to negligible values the interaction among defects belonging to different cells; (ii) a background of uniform compensating charge that restores the neutrality of the supercell and then avoids the "Coulomb catastrophe". Here, an alternative approach is proposed and compared to CC, the double defect (DD) model, in which another point defect is introduced in the supercell that provides (or accept) the electron to be transferred (subtracted) to the defect of interest. The DD model requires obviously a (much) larger supercell than CC, and the effect of the relative position of the two defects must be explored. A third possible option, the cluster approach, is not discussed here. The two models have been compared with reference to the VN^- defect; for DD, the positive compensating charge is provided by a P atom. Three cubic supercells of increasing size (containing 216, 512, and 1000 atoms) and up to eight relative VN^--P⁺ defect-defect positions have been considered. The comparison extends to the equilibrium geometry around the defect, band structure, charge and spin distribution, IR and Raman vibrational spectra, and electron paramagnetic resonance constants. It turns out that the CC and DD models provide very similar results for all of these properties, in particular when the P⁺ compensating defect is not too close to VN^-.

Synthesis and characterization of diamonds with different nitrogen concentrations under high pressure and high temperature conditions

In this study, {111}-oriented diamond crystals with different nitrogen concentrations were successfully synthesized in a series of experiments at 5.8 GPa pressure and 1380–1400 °C temperature.

The A-center defect in diamond: quantum mechanical characterization through the infrared spectrum

The A-center in diamond, which consists of two nitrogen atoms substituting two neighboring carbon atoms, has been investigated at the quantum mechanical level using an all-electron Gaussian type basis set, hybrid functionals and the periodic supercell approach.

Nitrogen and luminescent nitrogen-vacancy defects in detonation nanodiamond

An efficient method to investigate the microstructure and spatial distribution of nitrogen and nitrogen-vacancy (N-V) defects in detonation nanodiamond (DND) with primary particle sizes ranging from approximately 3 to 50 nm is presented. Detailed analysis reveals atomic nitrogen concentrations as high as 3 at% in 50% of diamond primary particles with sizes smaller than 6 nm. A non-uniform distribution of nitrogen within larger primary DND particles is also presented, indicating a preference for location within the defective central part or at twin boundaries. A photoluminescence (PL) spectrum with well-pronounced zero-phonon lines related to the N-V centers is demonstrated for the first time for electron-irradiated and annealed DND particles at continuous laser excitation. Combined Raman and PL analysis of DND crystallites dispersed on a Si substrate leads to the conclusion that the observed N-V luminescence originates from primary particles with sizes exceeding 30 nm. These findings demonstrate that by manipulation of the size/nitrogen content in DND there are prospects for mass production of nanodiamond photoemitters based on bright and stable luminescence from nitrogen-related defects.

Luminescence-lifetime mapping in diamond

This paper introduces a new technique to the study of diamonds: mapping the luminescence lifetime of optical centres. The understanding of luminescence lifetimes in diamond is briefly reviewed. Since lifetime mapping involves extended measuring times with focused laser excitation, the stability of the H3 optical centre is investigated. We show that saturation of the H3 luminescence requires excitation power densities in excess of 10 MW cm(-2). The non-radiative energy transfer time from an H3 centre to an A aggregate is found to be equal to that from N3 centres to A aggregates, at ∼3 × 10(-16)r(8) s, where there are r bond lengths between the H3 and A centres. Non-radiative energy transfer is shown to occur from the NV(-) band to the single substitutional nitrogen atoms: the single N atoms may quench luminescence as well as the A aggregates of nitrogen. In contrast, a comparison of the decays from the very similar H3 and H4 centres demonstrates that the B aggregate produces very weak quenching of the visible luminescence from diamond.

Density functional studies of muonium in nitrogen aggregate containing diamond: the Mu(X) centre

Diamond has potential as a wide band-gap semiconductor with high intrinsic carrier mobility, thermal conductivity and hardness. Hydrogen is involved in electrically active defects in chemical vapour deposited diamond, and muonium, via muon spin spectroscopy, can provide useful characterization for the configurations adopted by H atoms in a crystalline material. We present the results of a computational investigation into the structure of the Mu(X) centre proposed to be associated with nitrogen aggregates. We find that the propensity of hydrogen or muonium to chemically react with the lattice makes the correlation of Mu(X) with nitrogen aggregates problematic, and suggest alternative structures.

The annealing of radiation damage in type Ia diamond

The kinetics of the recovery of radiation damage in type Ia diamond has been investigated using isothermal annealing at 600 °C. In diamonds having a reasonably homogeneous distribution of nitrogen the decay of the vacancy concentration with time can be approximately described by a single exponential. Previous investigations have identified 'fast' and 'slow' components in the annealing, and we show that the existence of more than one time constant is associated with inhomogeneous nitrogen concentrations. The measurements show further that, in order to obtain the oscillator strengths of nitrogen-vacancy centres, studies must be restricted to diamonds with moderately high nitrogen concentrations.

Discrimination of metamorphic diamond populations by Raman spectroscopy (Kokchetav, Kazakhstan)

Metamorphic diamond is a powerful but frequently debated indicator for ultrahigh-pressure metamorphic (UHPM) conditions. Because of their small size, their optical identification needs confirmation. Characteristics of chemically extracted microdiamonds from Kokchetav, identified by different analytical methods, are used here for unambiguous in situ identification by Raman microspectroscopy. Differences appear in the diamond spectra and the Raman analytical method is explored as a helpful tool in the discrimination between diamond populations from four different UHPM lithologies of Kokchetav. Not considering the graphite-coated diamond, out of the reach of the laser wavelength used here, the comparison of these Kokchetav Raman spectra may provide additional information in other UHPM studies.

Shallow donor state due to nitrogen-hydrogen complex in diamond

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.

Aggregation of nitrogen in diamond, including platelet formation

Nitrogen occurs in most natural diamonds in concentrations of up to 0.3 at. %. In about 0.1% of diamonds, the nitrogen occurs as single substitutional atoms but in most diamonds aggregates of two or more nitrogen atoms occur. The various types of aggregate have been identified by detailed study of optical absorption spectra and by other methods. This paper describes experiments in which synthetic diamonds, containing substantial quantities of nitrogen in the form of single substitutional atoms, have been heated to cause aggregation. All the aggregates found in natural diamonds have been produced under controlled conditions of temperature and pressure. Of particular interest has been the formation of platelets which are found in type 1 natural diamonds in the {100} planes. Several experiments suggest strongly that these platelets are large aggregates of nitrogen atoms. To obtain aggregation in a reasonable time, the mobility of the nitrogen atoms was increased by irradiating the diamonds with 2 MeV electrons before heating. This irradiation produced numerous vacancies and interstitial atoms in the diamond lattice. Even so, temperatures of up to 2200°C were necessary to produce the required rapid motion of the nitrogen atoms. To prevent graphitization of the diamond at these temperatures, the specimens were subjected to a pressure of 8.5 GPa during the heating. The experiments establish the sequence of the aggregation of nitrogen atoms in diamond and are relevant to the understanding of the conditions under which aggregation occurred in natural diamonds. It seems probable that, for some diamonds at least, the interval between their formation and their ejection to the surface of the earth was very short on a geological time-scale.

Conversion of platelets into dislocation loops and voidite formation in type IaB diamonds

Type la natural diamonds have been heated in the temperature range of 2400-2700°C under stabilizing pressures. The specimens studied are mainly regular type IaB diamonds. Transmission electron microscopy studies of treated speci­mens show that platelets are converted to interstitial ½ a 0 <011> dislocation loops; voidites are also formed. When all the platelets have been converted, the ex­perimental features associated with them also disappear, i. e. the X-ray extra reflections (spikes), the B' local-mode absorption and the lattice absorption in the one-phonon region termed the D spectrum. It is discovered that when diamonds are heated under graphite-stable rather than diamond-stable conditions, the rate of conversion is considerably enhanced; for instance, at 2650°C there is an increase in the rate of about three orders of magnitude. This enhancement is considered to be due to the instability of the diamond structure itself and a reason for this enhancement is suggested.

Uniaxial stress studies of the 2.498 eV (H4), 2.417 eV and 2.536 eV vibronic bands in diamond

The H4 vibronic band is the dominant end product of annealing irradiated diamonds which contain impurity nitrogen in one specific aggregated form, the ‘B’ form. This paper reports uniaxial stress experiments on the H4 zero phonon line, and establishes it unambiguously as a (110) electric dipole transition at a monoclinic I centre. Two independent and previously ignored zero phonon lines at 2.417 and 2.536 eV are also shown to be (110) dipole transitions at monoclinic I centres. The uniaxial stress and vibronic properties of the three centres suggest that their molecular structures are all similar to the commonly observed H3 centre, implying that a family of closely related, but different, optical centres may be produced by irradiating and annealing type l a diamonds The lack of mirror symmetry in the H4 absorption and luminescence bands is shown to be consistent with the existence of two nearly degenerate excited electronic states at the H4 centre, which may interact through modes of vibration of the appropriate symmetry. The model assumes that transitions from the ground state occur readily to one excited electronic state and negligibly to the other. It is found that substantial breakdown of mirror symmetry may occur without any observable complications in the uniaxial stress spectra, as at the H4 centre, or with the appearance of stress induced absorption, as at the H3 centre, with only small differences in the properties of the two centres. Uniaxial stress measurements on a prominent absorption line in the H4 vibronic sideband are consistent with the model.

The kinetics of the aggregation of nitrogen atoms in diamond

Most natural diamonds contain nitrogen as the main impurity. In some rare diamonds (termed type lb) the nitrogen is mainly present as single substitutional atoms. However, the large majority of diamonds (termed type Ia) contain the nitrogen atoms in various forms of aggregate. The different types of aggregate are the A centre (two nitrogen atoms), the N3 centre (three nitrogen atoms) and the B centre (a larger number of nitrogen atoms). These defects give characteristic absorption spectra in the infrared except for the N3 centre which gives a characteristic absorption in the visible region. In this type of diamond there are usually platelets present in the cube planes and these defects can be examined by transmission electron microscopy and infrared absorption techniques. The paper reports work in which synthetic diamonds containing a high concentration of single nitrogen atoms have been heated in a temperature range of 1500 to 2500°C under various pressures. These heat treatments have resulted in the formation of all the types of aggregate that are found in natural type la diamonds. Also some natural diamonds have been heated up to 2700°C under pressure and the ratio of the concentration of the A centres to that of the B centres has been changed. Information has been obtained on the kinetics of the aggregation process. This information has been used to give an approximate estimate of the length of time that natural diamonds spent in the Upper Mantle prior to being ejected to the surface of the Earth. It is suggested that the type I a diamonds spent between about 200 and 2000 Ma in the Upper Mantle at temperatures of between 1000 and 1400°C. Type l b diamonds either spent a comparable time in the Upper Mantle at about 800°C or a considerably shorter period if they encountered temperatures in the same range as the type la diamonds.

The decay time of N3 luminescence in natural diamond

Measurements are reported of the decay time of photoluminescence from the N3 centre: an impurity centre commonly found in natural diamond. The intrinsic decay time at low temperature is found to be 41 ± 1 ns. The decay time is specimen dependent, decreasing with increasing concentrations of pairs of substitutional nitrogen atoms in the diamonds. The data are consistent with an electric dipole, electric-quadrupole coupling of the N3 centres to the nitrogen pairs. In addition, the decay time is reduced by raising the specimen temperature, especially above 450 K. These results are consistent with internal conversion occurring into another excited electronic state of the N3 centre. The properties required for this state agree with deductions from other optical and electron paramagnetic resonance (e. p. r.) data. The radiative lifetime of the N3 luminescence transition is estimated at 150 ns, in agreement with previous luminescence efficiency measurements.