Optical properties of impact diamonds from the Popigai astrobleme

Impact diamonds from Popigai astrobleme were found to consist of different carbon phases: cubic and hexagonal diamond with sp³ bonding according to X-ray structural analysis as well as amorphous, crystalline and disordered graphite with sp²-bonding (Raman scattering). The sizes of graphite domains vary from 10 to 100 nm. Fundamental absorption edge for Popigai impact diamonds is shifted ~ 0.5 eV to lower energies in comparison with kimberlite diamonds (5.47 eV) as a result of the lonsdaleite input, in good agreement with ab initio calculations (E_g = 5.34 and 4.55 eV for 3C cubic and 2H hexagonal diamonds, respectively). Yellowish color of impact diamonds is due to Rayleigh light scattering on structural defects whereas graphite is responsible for gray to black coloring. In the mid-IR region there is a multi-phonon absorption of 3C diamond in the 1800 to 2800 cm^− 1 range and some new bands at 969, 1102, 1225, and 1330 cm^− 1 in the one-phonon region. Micro-Raman study shows inclusions of side noncarbon minerals (quartz, magnetite, and hematite) some of which contain Cr^3 + impurity. The vibration modes of cubic diamond and lonsdaleite exhibited in the Raman spectra were elucidated by the first-principles studies. Popigai impact diamonds demonstrate a broad-band luminescence in 2.1, 2.38, and 2.84 eV components similar to that for nanocrystal polycrystalline 3C diamond. All emissions are excited at band-to-band transitions whereas the last two are observed also at excitation into 2.4 and 3.0 bands supposedly as a result of intracenter processes within the H3(NVN) and NV⁰ centers.     

Thermochemistry of nanodiamond terminated by oxygen containing functional groups

The standard enthalpies of formation at 25 °C of nanodiamonds terminated by oxygen containing functional groups have been investigated by high-temperature oxidation calorimetry. Depending on the amount of oxygen containing functional groups, the nanodiamonds (plus oxygen and hydrogen as represented in the surface functional groups) can be up to 52 kJ mol⁻¹ more stable in enthalpy than graphite, which means that less heat is evolved during oxidation of nanodiamonds terminated by oxygen containing functional groups, since their surface carbon is already partially oxidized. The stability of the nanodiamonds terminated by oxygen containing functional groups increases (enthalpy of formation becomes more negative) with increasing surface area within the studied range, reflecting the dominant effect of higher content of surface functional groups over the destabilizing effect of higher surface-to-volume ratio typical for nanoparticles.     

Effective production of fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy centers by ion irradiation

Fluorescence from negatively-charged nitrogen-vacancy centers (NV⁻s) in diamonds has unique optical properties with none of the undesirable effects such as photo-bleaching and photo-blinking. In addition, the spin-dependent fluorescence intensity of NV⁻s allows us to perform optically detected magnetic resonance (ODMR) investigation for evaluating the presence of NV⁻s and for the electronic local environment. In this work, we irradiated H⁺, He⁺, Li⁺ and N⁺ ions to nanodiamonds with a median size of 26 nm at various irradiation energies and doses for improving the NV⁻ concentration. ODMR observations of the nanodiamonds showed that ion irradiation increased the number of nanodiamonds containing NV⁻s up to 200 ppm, whereas without ion irradiation, only few NV⁻s were found. The number of nanodiamonds containing NV⁻s at various ion irradiation doses was not monotonous, but had maxima at certain irradiation doses. These results suggest a competition in two opponent roles of vacancies, effective for pairing with nitrogen atoms and as defects for developing damage in crystalline. We also found that sharp and strong ODMR signals were obtained from nanodiamonds irradiated at the optimal condition for the highest yield of NV⁻s. We concluded that He⁺ ion irradiations with 60 or 80 keV at a dose of 1 × 10¹³ ions cm^–2 are the conditions required for the most efficient production of a high quantity of nanodiamonds containing NV⁻s.     

Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology,CCL-SEM and CL spectroscopy studies

Colorless octahedral diamonds up to 150 μm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na₂Mg(CO₃)₂–graphite and NaKMg(CO₃)₂–graphite systems at pressures of 8–10 GPa and temperatures of 1700–1800 °C. Seeded growth of carbonate–carbon (CC) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic “metal–carbon” (MC) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa.     

Excitation density dependence of luminescence spectrum of electron-hole plasma in diamond

Time-resolved band edge luminescence spectrum in IIa diamond has been measured with the 5th harmonics of a pulsed YAG laser (5.82 eV) and an ICCD image intensifier of 5 ns gate width at 290 K. The time-resolved luminescence spectrum is decomposed into three components of free exciton (FE), excitonic complex (EC) and electron-hole plasma (EHP). The decay times of the FE and EC luminescence are 45 and 27 ns, respectively and that of the EHP luminescence has been seen to be shorter than the gate width, 5 ns. The low energy onset of the EHP luminescence spectrum has been observed to decrease with increasing excitation density and attains the onset of the electron-hole drop luminescence spectrum at the excitation density of 0.6 J/cm², at which the electron-hole pair density is 1.2 × 10²⁰ cm^? 3. Furthermore, the excitation density dependences of the FE, EC and EHP luminescence intensities are explained with the percolation theory.     

Fluorescent nanodiamonds derived from HPHT with a size of less than 10 nm

The fabrication of fluorescent nanodiamonds by the electron irradiation of a high-pressure high-temperature microdiamond followed by annealing and fragmentation has a number of advantages over other fabrication approaches. High energy electron irradiation of micron-sized diamonds is a safe and convenient method to create vacancies within the lattice, thereby allowing for simple reactor designs. Well-defined annealing conditions facilitate vacancy migration and its subsequent capture by substitutional nitrogen (Ns) atoms, while avoiding the formation of unwanted coke on the surface of the diamond. In addition, microdiamonds offer a long vacancy migration path, which significantly increases the probability of vacancy trapping by nitrogen. In this report, we show that the fragmentation of irradiated and annealed microdiamonds creates round ultrasmall nanodiamonds composed of perfectly crystallized cubic-diamond nanocrystals, with fluorescent centers inside the nanocrystal core. Atomic force microscopy and confocal fluorescence microscopy demonstrate that approximately 30% of diamond nanocrystals with a size of less than 10 nm are fluorescent and have a remarkably long spin decoherence time (2.7 μs for a 7 nm diamond nanocrystal). The presence of a high content of non-fluorescent ultrasmall nanodiamonds can be explained by the limited N concentration and its heterogeneous distribution in the initial raw high-pressure high-temperature diamond. The remarkably long spin decoherence time of the ultrasmall fluorescent nanodiamonds may be due to surface cleaning and nanodiamond fabrication procedures, which result in a low number of spin impurities in and around the nanocrystal.     

Measuring Förster resonance energy transfer between fluorescent nanodiamonds and near-infrared dyes by acceptor photobleaching

Fluorescent nanodiamonds (FNDs) containing negatively charged nitrogen-vacancy centers as fluorophores are promising far-red biolables. The fluorophores are perfectly photostable, showing neither photobleaching nor photoblinking, and are useful as Förster resonance energy transfer (FRET) donors. This work demonstrates that it is possible to achieve an average FRET efficiency of up to 30% between a single 23-nm-sized FND (emission maximum ~ 685 nm) and multiple near-infrared dye molecules (absorption maximum ~ 774 nm) co-embedded in a poly-L-lysine matrix. The efficiency was determined by measuring the increases in both fluorescence intensity and lifetime of the FRET donor (i.e. FND) after photobleaching of the FRET acceptor (i.e. IRdye-800CW). Monte Carlo simulations indicate that on average, there are ~ 9 IRDye molecules located in close proximity to each FND particle.     

Disorder in ball-milled graphite revealed by Raman spectroscopy

High-energy ball milling has been used to produce nanocrystalline and amorphous structures in nature graphite, but Raman spectroscopy analysis shows that the structure of the amorphous phase produced by ball milling is less disordered than the amorphous structures in ion-beam bombarded graphite and sputtered amorphous carbon. The band intensity ratio (I_D/I_G) increases first and then drops to 1.1 after 40 h of milling treatment, and remains constant during extended milling up to 100 h. Electron microscopy observation and surface area measurement show that the formation of some nanosized clusters during the extended milling period might be responsible for the less disordered structure in the milled graphite.     

Vacuum or flowing argon: What is the best synthesis atmosphere for nanodiamond-derived carbon onions for supercapacitor electrodes?

We present a comprehensive study on the influence of the synthesis atmosphere on the structure and properties of nanodiamond-derived carbon onions. Carbon onions were synthesized at 1300 and 1700 °C in high vacuum or argon flow, using rapid dynamic heating and cooling. High vacuum annealing yielded carbon onions with nearly perfect spherical shape. An increase in surface area was caused by a decrease in particle density when transitioning from sp³ to sp² hybridization and negligible amounts of disordered carbon were produced. In contrast, carbon onions from annealing nanodiamonds in flowing argon are highly interconnected by few-layer graphene nanoribbons. The presence of the latter improves the electrical conductivity, which is reflected by an enhanced power handling ability of supercapacitor electrodes operated in an organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile). Carbon onions synthesized in argon flow at 1700 °C show a specific capacitance of 20 F/g at 20 A/g current density and 2.7 V cell voltage which is an improvement of more than 40% compared to vacuum annealing. The same effect was measured for a synthesis temperature of 1300 °C, with a 140% higher capacitance at 20 A/g for argon flow compared to vacuum annealing.     

Influence of carbon sources on nitriding process,microstructures and mechanical properties of Si3N4 bonded SiC refractories

The incomplete nitriding and heterogeneity structure of large-size Si₃N₄-bonded SiC refractories limited its application in refractories industry. The objective of this work was to provide a way that adding carbon in matrix with the expectations of carbon could reacted with free-Si and transformed into SiC after nitridation. The effects of carbon sources on the bonded morphologies and nitridation process of Si₃N₄ bonded SiC refractories were investigated. Results indicated the strength and Young’s-modulus of specimen with carbon black increased to 39.4 MPa and 103.89 GPa from 29.8 MPa and 73.43 GPa, respectively. The residual Young’s-modulus also improved from 44.13 GPa to 62.9 GPa after 9-quenching cycles. For specimen with graphite, residual graphite after nitridation resulted in a definite strength decline, but residual strength after quenching was improved. Moreover, analysis results on N-elements indicated surprisingly improvement in the nitriding degree for specimen with carbon black. The microstructure evolution and mechanism associating with the enhanced nitriding process was discussed.     

Effective reduction of AlN defect luminescence by fluorine implantation

We report on the effective reduction of AlN host lattice defect cathodoluminescence by high dose ion implantation of light elements such as fluorine as well as chlorine and neon with peak concentrations of 1 at.%. In order to distinguish between luminescence suppression in the visible to luminescence quenching due to radiation damage, all samples were additionally implanted with europium at fluences of 1 · 10¹³ ions/cm². After annealing the samples at 1373 K under vacuum conditions cathodoluminescence spectra were recorded at room temperature (300 K) and at cryogenic temperature (12 K). These investigations reveal that different light ion species have different influences on the defect luminescence of the AlN host lattice which is likely due to selective passivation of these defects. The best ratio of defect luminescence suppression to radiation damage induced luminescence quenching is achieved in the case of fluorine co-doping.     

The effect of pyrocarbon deposition on the microstructure of graphitic foam

Mesophase-pitch-based graphitic foams (MPGFs) were derived. Pyrocarbon (PyC) has been deposited on the inner surface of some MPGFs. Microstructures in the ligaments and nodes of MPGF, morphology of PyC and the boundary between MPGF and PyC have been investigated by transmission electron microscopy and related techniques. In MPGF, straight and compact graphitic crystallites within the long and thin ligaments extend along the bubble walls, while graphite sheets within the nodes are more disordered and less compact with many defects. A PyC reinforcement uniformly covered the inner surface and filled microcracks between graphitic sheets, and was almost perfectly bonded to the MPGF substrate even at the nanoscale. On the vertical side of MPGF, whose surface is perpendicular to (0 0 0 2) planes of graphite layers, some amorphous carbon was formed between MPGF and PyC on the boundary. On the parallel side with the surface parallel to (0 0 0 2) planes, PyC combines with MPGF surface directly and closely without any amorphous carbon.     

Corrosion behaviour of carbon materials exposed to molten lithium chloride–potassium chloride salt

The corrosion behaviour of four carbon materials namely low density graphite, high density graphite, glassy carbon and pyrolytic graphite were investigated in molten LiCl–KCl electrolyte medium at 600 °C for 2000 h under high pure argon atmosphere. Structural and microstructural changes in the carbon materials after exposure to molten chloride salt were investigated from the weight change and using scanning electron microscopy, atomic force microscopy, X-ray diffraction and laser Raman spectroscopic techniques. Microstructural analysis of the samples revealed the poor corrosion resistance of high density and low density graphite and severe attack was observed at several places on the surface. On the other hand, glassy carbon and pyrolytic graphite were relatively inert, while pyrolytic graphite showed the best corrosion resistance to molten salt attack. In the order of increasing corrosion resistance to molten salt, the carbon materials were found to follow the sequence: low density graphite < high density graphite < glassy carbon < pyrolytic graphite.     

Simple Förster resonance energy transfer evidence for the ultrahigh quantum dot quenching efficiency by graphene oxide compared to other carbon structures

Förster resonance energy transfer (FRET) entails the transfer of energy from a photoexcited energy donor to a close energy acceptor. In this regard, quantum dots (QDs), as donors, are quenched when they are next to an acceptor material. Graphite, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene oxide (GO) were explored as energy acceptors of QD FRET donors in the solid phase. In our setup, the higher estimated values of quenching efficiency for each material are as follows: graphite, 66 ± 17%; CNTs, 71 ± 1%; CNFs, 74 ± 07% and GO, 97 ± 1%. Among these materials, GO is the best acceptor of QD FRET donors in the solid phase. Such an ultrahigh quenching efficiency by GO and the proposed simple mechanism may open the way to several interesting applications in the field of biosensing.     

The production of carbon particles of different shapes produced by the chlorination of Cr(C5H5)2

Carbon particles have been obtained by the chlorination of chromocene (Cr(C₅H₅)₂). Changes in their morphology and micro-nanostructure have been monitored at two different temperatures. At 400 °C, filled materials (tubes and spheres) and agglomerated round particles are formed, whereas at 900 °C closed-end tubes, hollow and solid spheres were produced. Transmission electron microscopy shows that these particles are formed of highly disordered graphene-like layers, which is confirmed by the absence of the 2D and 2G bands in the Raman spectrum. The calculated in-plane correlation length of these graphene-like layers is 1.2 ± 0.1 nm. In all the carbon particles, electron energy-loss spectroscopy shows a very similar sp² carbon bonding content (89–98%) and mass density ranging from 1.6 to 1.8 g/cm³, both below standard graphite. Textural studies performed on the sample prepared at 900 °C show Type II adsorption isotherms with a surface area of 694 m²/g.     

Observation of the H2 defect in gem-quality type Ia diamond

The H2 defect with its zero-phonon line at 986.1 nm (1.257 eV, 10141 cm⁻¹) was first described in 1956 and characterized in detail in 1990–91. In this paper, the observation of strong H2 bands in greenish-yellow natural type Ia gem-quality diamonds is reported. These ‘H2 diamonds’ were apparently subjected to a treatment possibly involving irradiation and clearly involving high temperature annealing. Besides burns on the surface and in fractures, they typically showed strong green luminescence associated with brownish-yellow graining, strong H3/H4 bands, weak to strong H2 bands, and no H1b/H1c lines. Some stones displayed a weak 637 nm line as well. A model explaining the formation of the H2 defect in type Ia diamond is proposed based on the stability and interaction of defects in diamond as a function of temperature and time.     

Nano carbon containing MgO-C refractory: Effect of graphite content

Development of low carbon containing MgO-C refractories has been studied containing a fixed 0.9 wt% of nano carbon and 1–9 wt% of flake graphite. Refractory compositions were prepared and processed as per the conventional manufacturing techniques of MgO-C refractory. Properties of the different compositions were evaluated and also compared against the conventional MgO-C refractory containing 10 wt% graphite prepared under exactly similar conditions. Addition of 3 wt% of flake graphite in combination with 0.9 wt% of nano carbon black was found to be optimum and resulted in better/comparable properties to that of conventional MgO-C refractory.     

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.     

Natural,untreated diamonds showing the A,B and C infrared absorptions (“ABC diamonds”), and the H2 absorption

Unusual brown to yellow diamonds of mixed type Ib/IaAB were analyzed. The occurrence of nitrogen in a combination of A, B and C centers directly detectable by IR spectroscopy in natural diamonds is considered to be extremely rare. We propose to call such stones ABC diamonds for short. These diamonds are characterized by a color, luminescence, and anomalous double refraction distribution which results from mixed growth: a center core formed by cuboid growth, rich in nitrogen, covered by an outer rim of “normal” octahedral growth, with much less nitrogen. Nitrogen in the core is present in a more aggregated state than in the rim, which is practically pure type Ib.In the infrared spectra, besides A, B and C 1-phonon absorptions a large number of previously undescribed, tentatively H-related features were identified in some samples; these proposed H-related features were only present in the periphery of the core and the rim of the diamonds, in the core practically no hydrogen was detected. The low temperature Vis/NIR spectra were mainly characterized by two defect-induced centers: the H2 center, and the apparently new 905 nm vibronic absorption with phonon-side bands at 880, 867, 847 nm plus a feature at 806.5 nm. The 905 nm absorption can possibly be attributed to a hydrogen-related center. Although these diamonds present a combination of spectroscopic characteristics (A + B + C center absorptions, H2 absorptions) deemed characteristic of certain HPHT-treated type Ia diamonds, they cannot be confused for such treated diamonds on the basis of their color, color distribution and some spectral peculiarities.     

Effects of the additive NaN3 added in powder catalysts on the morphology and inclusions of diamonds synthesized under HPHT

In this paper, the effects of the additive NaN₃-added in powder catalysts to synthesize nitric diamond were studied in a cubic anvil high-pressure and high-temperature apparatus (SPD-6 × 1200). Diamond crystals with perfect shape were successfully synthesized using NaN₃-added Fe₉₀Ni₁₀ catalyst under pressure 5.4 GPa and temperature 1600 K for 15 min. The temperature and pressure of crystals growth were increased with an increase of the content of NaN₃. The V-shape section for the diamond's growth, which is the region between the solvent/carbon eutectic melting line and diamond/graphite equilibrium line under pressure and temperature, was moved upwards. The synthetic diamonds exhibited perfect cubo-octahedral shape or octahedral shape with green or densely green in color. However, some orderly accidented lines were observed on the surfaces of most of the diamond crystals synthesized with NaN₃-added in Fe₉₀Ni₁₀. These lines might be formed during the procedure of crystal growth according to the results of the scanning electron microscope images. Moreover, the Mössbauer spectrometry for these diamonds indicated that the concentrations of inclusions formed by iron in diamonds were changed and iron nitride was detected.