Positron annihilation studies of γ-irradiated type Ib and IIa diamonds

Cobalt-60 gamma irradiation readily created vacancy-type defects in type Ib diamond whereas none were found in type IIa. The defect is characterized by a positron lifetime of 130 ps and a main annealing stage centered at 270 °C. The vacancy is not a free vacancy, and is suggested to be a vacancy–self-interstitial complex. None of the optical absorption features in the experimentally accessible wavelength range of 400–1100 nm could be correlated with the positron data.Statement of noveltyGamma irradiations of type Ib diamond is shown to produce a vacancy–interstitial complex observable by positron lifetime spectroscopy, and with characteristics very different from those of the isolated monovacancy.     

Characterization of top-quality type IIa synthetic diamonds for new X-ray optics

We have performed a complex analysis of top-quality synthetic diamond HPHT type IIa single crystals in view of their potential application as X-ray optics elements, namely as free electron laser mirrors. We have employed X-ray topography and high-resolution diffractometry along with optical spectroscopy techniques for characterization of our synthetic diamonds.     

The vacancy as a probe of the strain in type IIa diamonds

The width of the vacancy related photoluminescence from diamond is used to qualitatively measure the strain in type IIa diamonds of a range of brown colours and to measure the effect on the strain of high pressure high temperature annealing of the brown diamonds. The results indicate that for untreated type IIa diamonds the strain in colourless diamonds is generally less than that observed in brown ones. Annealing to remove the brown colour brings about a measurable reduction in the strain as assessed via the luminescence line width; but a dependence on the depth of the original brown colour is retained with the value remaining in most cases above 1.70 meV, a width below which 92% of the untreated colourless diamonds lie.     

Positron annihilation study of vacancies in type IIa diamonds illuminated with monochromatic light

Electron irradiated type IIa diamonds were investigated by positron lifetime and Doppler broadening spectroscopies during illumination in the photon energy range 2.2–3.5 eV. Illumination caused neutral vacancies to become negatively charged, and at photon energies close to 2.85 eV the charging was most effective. This suggests that the electronic level for the negatively charged monovacancy is situated 2.85 eV above the valence band. The photon flux dependency of the charging at the photon energy of 2.83 eV depends strongly on temperature and is attributed to recombination between holes and negatively charged monovacancies.     

Broad-band luminescence in natural brown type Ia diamonds

Broad-band luminescence centred at ∼ 1.75 eV (∼ 710 nm) is typically encountered in natural brown type Ia diamonds which show evidence of plastic deformation, particularly when using excitation at 514 nm. The band is comprised of 2 broad sub-bands. One of these bands is clearly a vibronic band, and the Huang–Rhys factor and the one-phonon density of states have been determined. The Huang–Rhys factor of S = 8.0 ± 0.5 is relatively high for diamond, and the one-phonon density of states is composed of 2 moderately sharp peaks at 17 and 44 meV.     

Positron annihilation and optical studies of natural brown type I diamonds

Positron annihilation measurements of natural brown type I diamonds revealed that they contain monovacancy-type defects and large vacancy clusters. The latter defect type was absent in clear diamonds. Both the intensity of the positron vacancy cluster lifetime and the S-parameter were found to increase with the intensity of brown colouration determined by optical absorption measurements. No correlation was found between the nitrogen content and the positron annihilation parameters.     

Vacancy diffusion and trapping in electron-irradiated type IaA diamonds

It has been reported that larger densities of vacancies are formed during electron irradiation of type IaA diamonds than in the case of high purity type IIa diamonds, and that a fraction of these vacancies f decays slower than the rest during annealing [Phys. Rev. B 46 (1992) 13157]. In this paper the latter, experimentally observed, double-decay equation is derived from basic physical postulates. It is assumed that the strain fields, surrounding the A-centers, attract vacancies towards these defects and repel interstitial atoms; in this way reducing the correlated recombinations that would have occurred without such forces being present, and, thus, causing an increase in vacancy density. These forces have been modeled in terms of ‘capture-volumes’ V surrounding the A-centers. Once within such a volume, a vacancy cannot escape and must finally end up at the A-center. The increase in the fast fraction with annealing temperature that had been ascribed to ‘uphill’ diffusion is modeled in terms of smaller ‘catch-volumes’ that also surround the A-centers. Once a vacancy enters the latter, it combines with the A-center to form an H3-center, and this process requires the scaling of a larger activation barrier than in the case of vacancy diffusion. The previously published experimental results are well modeled for capture-volumes V that, when added together, occupy less than 10% of the total volume of the diamond, and for an initial fraction of vacancies larger than 90% within them. This means that outside of these volumes the density of vacancies is far less than would have been the case in a pure diamond that do not contain A-centers. It is, accordingly, postulated that the interstitial atoms that form within the volumes V are, at some stage, ejected from these volumes by strain-assisted diffusion. This increase in density of the interstitials outside of the volumes, enhances vacancy annihilation when the annealing temperature is reached at which these interactions occur.     

Spectroscopic and microscopic characterizations of color lamellae in natural pink diamonds

Nineteen natural, untreated, type IaAB pink diamonds from various localities were studied. They display microscopic (～ 1 μm thick) pink lamellae along {111} in an otherwise colorless diamond. This coloration concentrated in lamellae is commonly referred to as “graining”. The diamonds were examined using high spatial resolution spectroscopic methods and transmission electron microscopy. TEM revealed that a pink lamella consists of a cluster of paired microtwins created under stress by plastic deformation. Raman line shift and broadening associated with the twinned pink lamellae indicate the presence of residual stress. Ultraviolet–visible absorption spectra from each of the samples showed a broad absorption band centered at ～ 550 nm, the source of the pink color. Cathodoluminescence spectra of the pink lamellae are different from those of the bulk, colorless diamond matrix. Within the lamellae only, the H3 center is observed along with a less intense N3 center. In some samples, instead of the N3 center a new center with a zero phonon line at 405.5 nm is observed. This previously unreported 405.5 nm center has phonon sidebands qualitatively identical to the N3 center, and may be an N3 center modified by a specific environment. These results suggest that lattice vacancies were created during twinning resulting from plastic deformation, and that impurity centers (such as those containing nitrogen) trap some of the diffusing vacancies. Since the pink lamellae are still under residual stress, new or modified defect centers are created, e.g. H3 and N3. The color center(s) responsible for the pink color (550 nm absorption) was not identified, but likely is only present in diamonds that experienced plastic deformation. Reported annealing of plastically deformed brown diamonds, which results in a residual pink color, suggests that the pink color is stable under these high pressure, high temperature conditions. The reported observations that annealing plastically deformed brown diamonds results in a residual pink color and that the pink color does not anneal out under similar high pressure, high temperature conditions, suggests that the deformation inducing pink color occurs inside the Earth's mantle, whereas brown coloration might be induced during a more recent event such as the ascent of the diamond to the surface in a kimberlitic/lamproitic eruption.     

Photoluminescence studies of type IIa and nitrogen doped CVD diamond

Photoluminescence (PL) studies were carried out on CVD, type IIa and high purity HPHT diamond samples irradiated with electrons of energies between 150 and 300 KeV; near threshold energies for carbon displacement. The majority of PL spectra are obtained using a 488-nm lasing line, with samples cooled to approximately 7 K. Of particular interest is the behaviour of the self-interstitial related centre, 3H, at 503.5 nm. The centre is particularly sensitive; its formation varies significantly with dose and dose rate and is severely quenched with incident laser power in excess of 10 mW. 3H is the dominant centre in highly doped (50–100 ppm) nitrogen samples, for doses between (10¹⁹–10²⁰) el/cm², but reduces with higher doses. In lower nitrogen (few ppm) samples, the centre is considerably weaker after equivalent doses, comparable to the Raman line. In type IIa crystals, creation of 3H varies considerably from sample to sample. Upon annealing, 3H is at an optimum between 310 and 330 °C for type IIa diamonds and vanishes by 400 °C. Indications show these temperatures increase slightly as nitrogen content is increased. Migration of the centre well outside the irradiated area is frequent, tens of microns after irradiation and hundreds of microns post annealing. Other centres of interest include GR1, the neutral diamond vacancy, which is found to be created linearly with dose and be rate independent. Using 325 and 457.9 nm lines the TR12 centre was studied. It has a strong dose rate dependence, growing as dose rate raised to a power of approximately 2 and is unaffected by annealing up to 700 °C. A 244-nm line was used to study the 5RL centre and contrary to some reports was observed in samples containing approximately 0.1 ppm of nitrogen. PL provides an extremely sensitive way of measuring the nitrogen concentration in diamond, to levels of less than 0.1 ppm. The problem remains how to obtain an accurate measurement.     

A new approach to investigation of nickel defect transformation in the HPHT synthetic diamonds using local optical spectroscopy

The analysis of spatial photoluminescence distribution along a plate from synthetic diamond as grown at 1500 °C allowed us to study a sequence of formation of nitrogen–nickel optically active centers and to build maps in some cases. Several groups of vibronic systems with specific spatial distribution along the sample were separated: nitrogen-vacancy pairs in the thin near- surface layer, low-temperature centers with blue emission, nickel–nitrogen complexes with a single nickel ion and those with several nickel ions.     

Ultrafast photoluminescence spectroscopy of H- and O-terminated nanocrystalline diamond films

We use femtosecond photoluminescence spectroscopy to study the light-induced changes in the sub-gap energy states of nanocrystalline diamond samples (thickness ~ 500 and 1000 nm) prepared on a spectral-grade fused silica substrate by microwave plasma enhanced chemical vapour deposition technique. The decay of photoluminescence in the visible spectral interval excited by blue femtosecond light pulses (405 nm,70 fs) shows that photoexcited charge carrier dynamics depend strongly on the ambient air pressure and on the light irradiation by the laser pulses. Specifically, at lower ambient air pressure (0.5-300 Pa) the irradiation leads to the peak photoluminescence intensity increase and to its faster decay. At higher air pressures (> 600 Pa) the photoluminescence intensity slightly decreases with no change in decay rates. O- and H-termination of nanocrystalline diamond films had negligible effect on their photoluminescence dynamics. The photoluminescence decay curves are described very well by the power-law decay reflecting the importance of the carrier trapping in the dynamics. Based on our results we propose a model of surface and sub-surface structure of nanodiamond films.     

The application of photoluminescence and Raman spectroscopy of synthetic diamond

The properties of the photoluminescent centres H3, 1.945 eV and their associated zero phonon lines (ZPLs) have been used to study various types of synthetic diamond in relation to their growth history. The shapes and widths of the 1.945 eV ZPL have been related to the presence of line and point defects in the different specimens using the theory of Stoneham and correlated with the crystal morphology. Micro-Raman spectroscopy has been used to study the spatial variation of internal stress in synthetic diamond, including single crystal and polycrystalline specimens. Micro-Raman mapping, in which the peak position and width of the allowed Raman peak in diamond are monitored, provides direct information on the state of stress and has been successfully employed to examine the stress distribution around cracks produced by indentation in the diamond.     

Nano and sub-micro inclusions as probes into the origin and history of natural diamonds

Five natural faceted diamond samples, considered to have inherent submicroscopic and nanometric inclusions, were studied using various techniques such as Raman spectroscopy, Raman microscopy, scanning electron microscopy (SEM), and energy dispersive analysis of X-rays (EDAX). The principal focus was to find out the chemical nature of the inclusions and their distribution. As a confirmative tool, laser desorption ionization mass spectrometry (LDI MS) was used for the characterization of the inclusions. Our study suggests that the inclusions in diamond belong to the pyroxene group of silicate minerals with Fe, Mg, Ca, Ru, and Cr as the major elements. As evidenced by the absence of alkali, aluminum, and rare earth elements in the inclusion, the diamond can be of peridotitic origin. Interestingly, we found that chromium scavenges Ru, a platinum group element (PGE), and other metals such as Nb, Co, and Ni, entrained into the pyroxene inclusions. We established that the diamond sample in which inclusions are found is of Kimberlitic origin.     

Determination of metallic impurities in high-purity type IIa diamond grown by high-pressure and high-temperature synthesis using neutron activation analysis

Metallic impurities in high-purity type IIa and conventional type Ib diamond single crystals grown by high-pressure and high-temperature (HPHT) synthesis were determined by neutron activation analysis using thermal neutrons. Metallic impurities of Fe, Co, Cr and other minor elements were detected in the high-purity type IIa diamond crystal. The typical quantities of these metallic impurities were a few ppb. The influence of these metallic impurities on the electrical properties of the type IIa diamond crystal was practically negligible compared with nitrogen and boron impurities behaving as a donor and an acceptor, respectively. In addition to the impurities detected in the type IIa diamond crystal, Ni impurities of several hundreds of ppb were detected in conventional type Ib diamond crystals. A difference in molten metal solvents used in the synthesis of each diamond crystal resulted in the difference in metallic impurities.     

New infrared absorption centres in electron irradiated and annealed type Ia diamonds

New absorption centres having zero phonon lines at 2678 (332 meV), 2913.6 (361 meV), 4392.9 (544 meV) and 4442.2 cm⁻¹ (550 meV) have been produced in heavily electron-irradiated and annealed type Ia diamonds. Their temperature dependence and annealing behaviour have been investigated along with the effect of the radiation dose and isotopic shifts caused by the substitution of ¹⁵N and ¹³C for ¹⁴N and ¹²C, respectively. The results are consistent with the centres being caused by electronic transitions.     

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

The stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce³⁺ ions (YAG:Ce³⁺)/yttrium oxide partially stabilized zirconia double-ceramic-layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce³⁺ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce³⁺ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.     

Photoinduced H1b and H1c centers in some natural treated diamonds

Three natural, treated-color diamonds present photoinduced H1b/H1c centers after UV exposure. As the H1b/H1c absorptions increase, that of the 595 nm center decreases. Recovery occurs by simple exposure to a standard incandescent 100 W lamp. The centers implied are all related to nitrogen and to the existence of a treatment, the formation of H1b/H1c centers coming from the trapping of the 595 nm center. The photoinduced behavior seems to confirm the close relation between the 595 nm center and the H1b/H1c centers, and might help in gemological identification.     

Non-linear optical properties of coloured diamonds: Observations of frequency up conversion and “whispering gallery-like” modes in photoluminescence

Four coloured diamonds, 1.7 MeV e-beam irradiated to a dose of 10¹⁸e per sqcm and annealed around 900–930 °C, were investigated at room temperature using conventional photoluminescence (PL) and excitation spectra, and PL using 514.5 and 785 nm lasers. Interesting new observations are: (i) excitation spectra reveal energy transfer between NV⁰ and NV⁻ centers (ii) frequency up conversion, blue-green emission on excitation in 740–900 nm range, was observed in blue and lemon yellow samples. This appears to be due to two-photon absorption in GR1 centers and energy transfer to N3/H3 centres. (iii) Excitation with 514 nm line of Ar⁺ laser, in pink and purple samples resulted in the appearance of an intense and broad emission at 267 nm with FWHM of 32 nm. This is rather close to 2ν of the exciting line. It is identified as second harmonic generation (SHG) due to a synergistic effect of strain induced birefringence and resonant absorption which facilitated phase matching. The 785 nm excited anti-Stokes PL contained a frequency up converted band in 620 nm region having extensive closely spaced structure like in the case of ‘whispering gallery modes’ due to spheroid inclusions.     

The exfoliation of SWCNT bundles examined by simultaneous Raman scattering and photoluminescence spectroscopy

Simultaneous measurements of Raman scattering and photoluminescence (PL) prove to be a powerful method for quantifying the bundling states of single-walled carbon nanotube (SWCNT). This paper presents physical analysis and experimental evidence to establish that the G-band normalized photoluminescence, which is determined from the simultaneously acquired Raman scattering and PL emission spectra, can serve as a good indicator for quantifying the degree of exfoliation of SWCNT dispersions. Without introducing the complications of sampling geometry and instrumental correction, this indicator directly relates to the intrinsic physical properties of a given SWCNT sample, namely, the absorption cross-section, differential Raman scattering cross-section, and PL quantum yield of SWCNT. An inverse linear relationship between the G-band normalized 267 cm^?1 RBM intensity and the PL emission intensities for SWCNT dispersions with different degrees of exfoliation was experimentally observed, indicating this can be used for quantitative characterization of the degree of exfoliation for a given SWCNT sample. An in-depth analysis of the indicators of the degree of exfoliation in various as-sonicated SWCNT dispersions highlights a two-stage exfoliation mechanism of SWCNT bundles under sonication.     

A novel composite diamond-containing dispersed material of natural and synthetic diamonds powders and abrasive tools made of it

A two-stage process of producing a novel composite material based on fine-dispersed powders of natural diamonds and mesh grinding powders of synthetic diamonds. At the first stage of producing the components mixture is compacted and at the second stage it is sintered in a gas-thermal reactor of the physicochemical synthesis plant at a temperature of 1100–1200 °C and below ambient pressure in the methane atmosphere. The values of the morphometric characteristics and the uniformity of abrasive powders in these characteristics produced from the new diamond-containing material, their physico-mechanical and electrophysical properties were studied. The results of the use of these composite powders in grinding wheels for machining a hard alloy are discussed. A substitution of powders of the new composite material for synthetic diamond mesh powders in the cutting layer of a standard grinding wheel makes it possible to essentially (more than by a factor of 3) increase the wear resistance of the tool.