Type IIa Diamond Research Articles

THE ORIGIN OF LARGE IRREGULAR GEM-QUALITY TYPE II DIAMONDS AND THE RARITY OF BLUE TYPE IIB VARIETIES

Type II (N-poor) diamonds do not constitute a single genetic population with a common paragenesis. Rather, several distinct populations (P-, E-, W- and sub-lithospheric) can be recognized, which formed in contrasting geological environments, with very different factors controlling the N-deficient character of these stones. Mantle-derived silicate and oxide inclusions are absent or extremely rare in large, Irregular Type II stones, which are often of exceptional gem quality. Such characteristics set these diamonds apart from N-poor (Type II) stones with “superdeep” inclusions, as well as the peridotitic and eclogitic suites, both of which include stones with non-detectable N. This in turn points to crystallization of the Irregular Type II stones in an environment markedly different from those in which the other three associations formed. Carbon isotopic signatures and syngenetic inclusions link both framesite and Irregular Type II diamonds to the websterite paragenesis. The latter in turn shows strong chemical affinities to the megacryst suite. From a purely petrographic perspective, the large Irregular Type II stones would be classified as megacrysts on the basis of their size (often >10 mm). The megacryst suite formed as pegmatitic veins from small volumes of kimberlitic liquids, injected into fractures within the thermal aureole surrounding the pooled kimberlite magma in the mantle prior to eruption. Crystallization of Irregular Type II diamonds was initiated when small volumes of evolved, residual megacryst magmas became buffered by highly reduced mantle wall rocks. Visually identified graphite inclusions, often rounded and thus inferred to be protogenetic, are relatively common in these diamonds. This observation is consistent with the inferred P-T conditions of crystallization of megacrysts across the graphite-diamond inversion curve, and thus within the lithosphere. Rapid crystallization of framesites from reduced megacryst magmas was probably triggered by stresses in the mantle immediately prior to kimberlite eruption. The websterite diamond suite is thus directly linked to megacryst crystallization, broadly coeval with the kimberlite magmatic event in the mantle. Experimental studies on the crystallization of granite pegmatites, complemented by processes specific to formation of the megacryst suite, provide a framework to account for the absence or extreme rarity of inclusions of other megacryst phases in the Irregular Type II stones. The elevated boron contents of blue Type IIb diamonds are consistent with the model for crystallization from highly fractionated pegmatitic megacryst magmas. Reduction of boron to the metallic state is ascribed to buffering of the megacryst magma by extremely refractory and highly reduced wallrock harzburgites, such as those found at Premier, which is one of the world’s major kimberlite sources of blue diamonds. Formation of fibrous cubic diamonds and the Premier eclogitic diamonds, which are both broadly coeval with eruption of the host kimberlite, can also be linked to the kimberlite magmatic event in the mantle.

Highly-Ordered Magnéli Ti4O7 Nanotube Arrays as Effective Anodic Material for Electro-oxidation

Pure Magnéli Ti4O7 nanotube arrays (NTA) were successfully fabricated by reducing TiO2 NTA with hydrogen at 850°C for 30minutes. The microstructure, composition and electrochemical behavior of the prepared Ti4O7 NTA were characterized by means of X-ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray Photoelectron Spectroscopy, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy. The as-prepared Ti4O7 NTA had a highly-ordered tubular structure with high crystallinity, large electrochemical window of water electrolysis (2.4V vs. Ag/AgCl, pH=6.0) and low interfacial charge transfer resistance when they were employed as anode for electro-oxidation. Phenol was electro-oxidized on Ti4O7 particles and Ti4O7 NTA with the latter giving 20% more Chemical Oxygen Demand (COD) removal. Pure Ti4O7 NTA also displayed larger degradation coefficient as well as higher COD removal and current efficiency than Boron-doped Diamond and other types of Magnéli NTAs. Cathodic polarization was found to be an effective way of restoring the electrochemical performance of oxidized Ti4O7 NTA as an anode.

Detonation nanodiamonds biofunctionalization and immobilization to titanium alloy surfaces as first steps towards medical application.

Due to their outstanding properties nanodiamonds are a promising nanoscale material in various applications such as microelectronics, polishing, optical monitoring, medicine and biotechnology. Beyond the typical diamond characteristics like extreme hardness or high thermal conductivity, they have additional benefits as intrinsic fluorescence due to lattice defects without photobleaching, obtained during the high pressure high temperature process. Further the carbon surface and its various functional groups in consequence of the synthesis, facilitate additional chemical and biological modification. In this work we present our recent results on chemical modification of the nanodiamond surface with phosphate groups and their electrochemically assisted immobilization on titanium-based materials to increase adhesion at biomaterial surfaces. The starting material is detonation nanodiamond, which exhibits a heterogeneous surface due to the functional groups resulting from the nitrogen-rich explosives and the subsequent purification steps after detonation synthesis. Nanodiamond surfaces are chemically homogenized before proceeding with further functionalization. Suspensions of resulting surface-modified nanodiamonds are applied to the titanium alloy surfaces and the nanodiamonds subsequently fixed by electrochemical immobilization. Titanium and its alloys have been widely used in bone and dental implants for being a metal that is biocompatible with body tissues and able to bind with adjacent bone during healing. In order to improve titanium material properties towards biomedical applications the authors aim to increase adhesion to bone material by incorporating nanodiamonds into the implant surface, namely the anodically grown titanium dioxide layer. Differently functionalized nanodiamonds are characterized by infrared spectroscopy and the modified titanium alloys surfaces by scanning and transmission electron microscopy. The process described shows an adsorption and immobilization of modified nanodiamonds on titanium; where aminosilanized nanodiamonds coupled with O-phosphorylethanolamine show a homogeneous interaction with the titanium substrate.

Quantitative analysis of nitrogen defect N4 in diamond with photoluminescence excited in the 170-240 nm region.

Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range of 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb, and the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.

Microfabrication, characterization and in vivo MRI compatibility of diamond microelectrodes array for neural interfacing.

Neural interfacing still requires highly stable and biocompatible materials, in particular for in vivo applications. Indeed, most of the currently used materials are degraded and/or encapsulated by the proximal tissue leading to a loss of efficiency. Here, we considered boron doped diamond microelectrodes to address this issue and we evaluated the performances of a diamond microelectrode array. We described the microfabrication process of the device and discuss its functionalities. We characterized its electrochemical performances by cyclic voltammetry and impedance spectroscopy in saline buffer and observed the typical diamond electrode electrochemical properties, wide potential window and low background current, allowing efficient electrochemical detection. The charge storage capacitance and the modulus of the electrochemical impedance were found to remain in the same range as platinum electrodes used for standard commercial devices. Finally we observed a reduced Magnetic Resonance Imaging artifact when the device was implanted on a rat cortex, suggesting that boron doped-diamond is a very promising electrode material allowing functional imaging.

Thermally enhanced photoinduced electron emission from nitrogen-doped diamond films on silicon substrates

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on $p$-type silicon substrates. It has been shown that photon-enhanced thermionic emission (PETE) can substantially enhance thermionic emission intensity from a $p$-type semiconductor. An $n$-type diamond/$p$-type silicon structure was illuminated with $400--450$ nm light, and the spectra of the emitted electrons showed a work function less than 2 eV and nearly an order of magnitude increase in emission intensity as the temperature was increased from ambient to $\ensuremath{\sim}400 \ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Thermionic emission was negligible in this temperature range. The results are modeled in terms of contributions from PETE and direct photoelectron emission, and the large increase is consistent with a PETE component. The results indicate possible application in combined solar/thermal energy conversion devices.

Fibrinogen adsorption study on ultrananocrystalline diamond as a biocompatible and antithrombogenic interfacial material for implantable devices

In this work, we have demonstrated that ultrananocrystalline diamond (UNCD®) film, has significantly lower fibrinogen adsorption than the conventional implantable materials and other diamond types, nanocrystalline or microcrystalline diamond films. In addition, we developed a hot hydrogen treatment in a hot filament chemical vapor deposition (CVD) system, to realize a hydrogen terminated (H-terminated) UNCD surface. The change in surface termination of H-terminated surfaces was characterized by contact angle measurements. Fibrinogen adsorption studies were conducted on three different types of UNCD surfaces: as-deposited, oxidized, and hydrogenated using florescence microscopy. The results reveal that H-terminated UNCD surface has the lowest fibrinogen adsorption, which suggests an excellent biocompatibility and anti-thrombogenicity. The authors have also studied the stability of the H-termination UNCD surface in dynamic fibrinogen-NaHCO3 solution for 2 weeks and in air for half year, no surface chemistry change was observed from both cases.

Special features of growing diamond single crystals in the Fe-Co-Zr-C system

The results have been considered of the investigations into the effect of various concentrations of zirconium addition to the Fe-Co alloy solvent on the growth rate and habit of a diamond single crystal grown by the temperature gradient method in the diamond thermodynamic stability region. The dependence of the defect and impurity state of grown crystals on the percentage of the zirconium addition has been established. The maximum zirconium content and temperature of the alloy solvent, which provide the growth of structurally perfect type IIa diamond single crystals, have been defined.

Thermal conductivity changes upon neutron transmutation of 10B doped diamond

10B doped p-type diamond samples were subjected to neutron transmutation reaction using thermal neutron flux of 0.9 × 1013 cm−2 s−1 and fast neutron flux of 0.09 × 1013 cm−2 s−1. Another sample of epilayer grown on type IIa (110) single crystal diamond substrate was subjected to equal thermal and fast neutron flux of 1014 cm−2 s−1. The defects in the diamond samples were previously characterized by different methods. In the present work, thermal conductivity of these diamond samples was determined at room temperature by transient thermoreflectance method. The thermal conductivity change in the samples as a function of neutron fluence is explained by the phonon scattering from the point defects and disordered regions. The thermal conductivity of the diamond samples decreased more rapidly initially and less rapidly for larger neutron fluence. In addition, the thermal conductivity in type IIb diamond decreased less rapidly with thermal neutron fluence compared to the decrease in type IIa diamond subjected to fast neutron fluence. It is concluded that the rate of production of defects during transmutation reaction is slower when thermal neutrons are used. The thermal conductivity of epilayer of diamond subjected to high thermal and fast neutron fluence is associated with the covalent carbon network in the composite structure consisting of disordered carbon and sp2 bonded nanocrystalline regions.

Electron microscopy profiling of ion implantation damage in diamond: Dependence on fluence and annealing

The doping of diamond by ion implantation has been feasible for 25years, but with the proviso that low dose implants can be annealed whereas high dose implants “graphitize”. An understanding of the types of defects, and their depth profiles, produced during the doping/implantation of diamond remains essential for the optimization of high-temperature, high-power electronic applications. This study focuses on investigating the nature of the radiation damage produced during the implantation of carbon ions into synthetic type Ib and natural diamonds using a spread of 4 energies, corresponding to typical doping energies, according to the CIRA (Cold-Implantation-Rapid-Annealing) routine, as well as a single energy implantation at room temperature. Both conventional and high resolution cross-sectional electron microscopies were achieved and used to analyze the implanted diamonds in conjunction with electron energy loss spectroscopy (EELS) and selected area diffraction (SAD). The cross sections were obtained using two different preparation methods.For low fluence implantations, using the CIRA routine, it is confirmed that the damaged diamond regains its crystallinity after annealing at 1600K. However, above the amorphization threshold fluence, followed by rapid annealing at 1600K, the whole implanted layer consisted of primarily amorphous carbon. High resolution TEM shows that the implanted layer consists of nano-regions with bent (002) graphitic planes and regions of amorphous carbon. The interface between the implanted layer and the diamond substrate near end of range shows diamond nanocrystallites, interspersed between regions of amorphous carbon and with bent (002) graphitic planes. There is no evidence for epitaxial regrowth. For high dose single energy ion implantation at room temperature, the unannealed layer shows a high degree of disorder at the maximum ion range, with some alignment of basal planes related to graphitic carbon, but with some of the diamond structure still partially intact. The implanted range included a diamond layer above the damaged region. This diamond layer showed no evidence of amorphous carbon.

High-density fluids and the growth of monocrystalline diamonds

The chemical nature and composition of the growth medium of monocrystalline (MC) diamonds is still a matter of debate, partially because carbonate-bearing high-density fluids (HDFs) that are common in fibrous diamonds have not been found in MC diamonds. Here we report the first finding of HDF microinclusions in a MC octahedral diamond from Finsch, South Africa and in the MC octahedral core of a coated diamond from Kankan, Guinea; both diamonds carry nitrogen in B-centers.Numerous microinclusions in diamond Finsch_2a_cap1 are restricted to two thin layers parallel to the (111) face, ∼20 and 200μm from the diamond rim. Low-Mg carbonatitic HDFs are found along the inner layer while the outer layer trapped saline compositions. The major and trace element compositions of the inclusions and their infrared spectra are highly similar to those of microinclusions found in fibrous diamonds. A few isolated microinclusions of saline compositions are scattered around a sulfide inclusion in the center of the octahedral core of diamond ON-KAN-383.This evidence for the involvement of oxidized fluids in the formation of MC diamonds adds to previous reports on the antiquity of HDFs in fibrous diamonds, the presence of carbonate and halide phases in inclusions in MC diamonds and the similarity of trace element pattern of a MC diamond to those of low-Mg carbonatitic HDF in fibrous diamonds. In addition, we show that the interaction of HDFs with depleted garnets can produce sinusoidal REE patterns which are one of the primary features of lherzolitic and harzburgitic garnet inclusions in MC diamonds. Together, these observations suggest that HDFs are involved in the formation of many types of diamonds from the Archaean to the Phanerozoic. HDFs are trapped in large quantities during rapid, fibrous growth, but must also be present during the growth of many MC diamonds.

Performance of a beam-multiplexing diamond crystal monochromator at the Linac Coherent Light Source.

A double-crystal diamond monochromator was recently implemented at the Linac Coherent Light Source. It enables splitting pulses generated by the free electron laser in the hard x-ray regime and thus allows the simultaneous operations of two instruments. Both monochromator crystals are High-Pressure High-Temperature grown type-IIa diamond crystal plates with the (111) orientation. The first crystal has a thickness of ~100 μm to allow high reflectivity within the Bragg bandwidth and good transmission for the other wavelengths for downstream use. The second crystal is about 300 μm thick and makes the exit beam of the monochromator parallel to the incoming beam with an offset of 600 mm. Here we present details on the monochromator design and its performance.

High pressure structural studies on nanophase praseodymium oxide

The phase stability of nanocrystalline Pr2O3 has been investigated under pressure by in-situ high pressure X-ray diffraction using Mao-Bell type diamond anvil cell. The ambient structure and phase of the praseodymium oxide have been resolved unambiguously using x-ray diffraction, SEM and TEM techniques. Under the action of pressure the cubic phase of the system is retained up to 15GPa. This is unusual as other isostructural rare earth oxides show structural transformations even at lower pressures. From the best fit to the P–V data with the Murnaghan equation of state yields a bulk modulus of 171GPa.

Photoluminescence studies of growth-sector dependence of nitrogen distribution in synthetic Ib diamond

The photoluminescence technology previously employed to investigate the boron distribution of type IIb diamond has now been applied to study the nitrogen distribution of type Ib diamond. All growth sectors were clearly distinguished by the characteristic colors and the brightness of the synthetic Ib diamond's cathodoluminescence topography. As a measure of the concentration of nitrogen impurity, the nitrogen-vacancy luminescence gave relative concentrations in different growth sectors as: the {111} sector was the highest, followed by the {311}, {100} and {511} sectors. The results were reconfirmed by the evidence of the broadened and strengthened zero phonon lines of nitrogen-vacancy center with the increase of nitrogen concentration of type Ib diamond.

New polymorphic types of diamond

The results of calculations by PM3 and LDA-DFT methods of the structure and properties of six new polymorphic types of diamond, in which all atomic sites are crystallographically equivalent, are presented. The structures of LA5 (Cmca), LA7 (Cmcm), and LA8 (I41/amd) phases are obtained as a result of stitching graphene layers and of CA9 \((Fd\bar 3m)\), CA10 \((R\bar 3m)\), and CA11 (P63/mmc) phases by stitching fullerenelike clusters. For these phases the geometrically optimized structures are calculated and the structural parameters, density, sublimation energy, bulk modulus, electron density of states, and X-ray diffraction pattern are measured. It is found that the properties of polymorphic types of diamond depend on the degree of their structure deformation in comparison with the cubic diamond structure.

Comparison of brazed and sintered diamond tools for grinding of stone

The present study was undertaken to compare the characteristics of two typical diamond tools used for the grinding of natural stone materials. Powder metallurgical sintering and brazing were adopted to fabricate two kinds of diamond tools: sintered and brazed tools. The comparison was conducted for the two diamond tools from three aspects: the bonding of metal matrix to diamonds, the evaluation of diamond retention, and tool wear process. It was found that the bonding of brazing alloy was rather different from that in the sintered tools, thereby leading to a totally different method for assessing the bonding of bond materials to diamonds as well as different wear mechanisms for the diamond tools in grinding.

Positron annihilation study for enhanced nitrogen-vacancy center formation in diamond by electron irradiation at 77 K

A compact ensemble of high density nitrogen-vacancy (NV) centers in diamond is essential to sense various external fields with a high precision at the nanoscale. Here, defects in type IIa and type Ib diamonds induced by 28 MeV electron irradiation at 77 K were studied by combining the positron annihilation spectroscopy and first-principles calculations. It is shown that the electron irradiation at 77 K can significantly enhance the NV center formation by directly converting 24% vacancies into the NV centers, indicating that it is an efficient way to produce the high density NV centers in the type Ib diamond.

Diamond electronic devices fabricated using heavily doped hopping p+ and n+ layers

Diamond diodes with extremely low on-resistance are introduced and discussed. Heavily boron-doped p+-type and phosphorus-doped n+-type diamond films with hopping conduction at room temperature are utilized. These diamond films have a unique property, that is, their resistivity decreases drastically even at a high impurity concentration of ∼1020 cm−3 without degrading the crystallinity. This unique property is applied to two kinds of diamond diodes. One is a p+–i–n+ junction diode. The other is a Schottky-pn diode consisting of the combination of the p+–n and n-type Schottky junctions. Current–voltage and capacitance–voltage characteristics are evaluated for both diodes. Switching characteristics are also evaluated for Schottky-pn diodes. The results show a low on-resistance on the order of mΩ cm2 and fast switching on the ns order. These results indicate that the low-loss diamond diodes can be realized by using the hopping p+- and n+-type layers.

Carrier transport in homoepitaxial diamond films with heavy phosphorus doping

Carrier transport in heavily phosphorus (P)-doped n-type (n+-type) diamond films as functions of P concentration and temperature was investigated. It was found that the carrier transport in these diamond films could be explained by summing the band conduction and a nearest-neighbor hopping (NNH) conduction at localized donor levels, even at a high temperature of approximately 500 K. In particular, heavily P-doped samples larger than 7.5 × 1019 cm−3 showed a high conductivity of over 9.8 × 10−2 Ω−1 cm−1 at 300 K by NNH conduction, which can be used for device application.

Identification of the structure of the 3107 cm−1 H-related defect in diamond

A prominent hydrogen-related infrared absorption peak seen in many types of diamonds at 3107 cm−1 has been the subject of investigation for many years. It is present in natural type-Ia material and can be introduced by heat-treating synthetic or CVD diamond. Based upon the most recent experimental data, it is thought that the defect giving rise to this vibrational mode is vacancy-related and is likely to contain nitrogen. Using first-principles simulations we present a VN3H model for the originating centre that simultaneously satisfies the different experimental observations including the strain response.