Type IIa Diamond Research Articles

Investigation on the origin of vacancy formation in diamonds by electron-beam irradiation and heat-treatment

In this study, we have attempted to describe the phenomenon of vacancy formation in various types of diamonds (natural type IaAB, synthetic type Ib, synthetic type IIa, and type IaABC suspected of having been pretreated with an unknown method) using electron beam irradiation. After the electron beam irradiation, vacancies were formed in the natural type IaAB and synthetic type IIa diamonds, whereas vacancies were not formed in the synthetic type Ib and pre-treated type IaABC diamonds. From these results, we suggest a model of vacancy creation by electron beam irradiation in various types of diamonds. The irradiation was performed with electron dose densities of 1–6×1017/cm2 at 10 MeV, and with a heat treatment of two hours at a temperature of 900 °C. Electron beam-irradiated and heat-treated samples were analyzed using UV-Vis-NIR, FT-IR, and PL spectroscopy.

Cr in diamond: A first-principles study

We report ab initio pseudopotential DFT calculations on the energetic stability and magnetic ordering of Cr in diamond at various lattice sites and charge states and show that Cr is most stable at the divacancy site in diamond, with the lowest formation energy occurring in $n$- or $p$-type diamond, compared to intrinsic diamond. The incorporation of Cr introduces strongly spin polarized impurity bands into the diamond band gap, with both the spin polarization and magnetic stabilization energies critically dependent on the charge state of Cr and the position of the Fermi level in diamond. In $p$-type diamond, we predict Cr${}^{+2}$ at the divacancy site possesses a ferromagnetic stabilization energy of 17 meV and magnetic moment of 2.5 ${\ensuremath{\mu}}_{\mathrm{B}}$, with Cr in diamond likely to form a diluted ferromagnetic semiconductor, which may successfully be considered for room temperature spintronic applications.

Shape-controlled synthesis of diamond crystal by epitaxial growth under high pressure and high temperature conditions

In this paper, the diamond epitaxial growth mechanism has been studied in detail by employing several types of diamond as a seed in a catalyst—graphite system under high pressure and high temperature (HPHT) conditions. We find that the diamond nucleation, growth rate, crystal orientation, and morphology are significantly influenced by the original seeds. The smooth surfaces of seeds are beneficial for the fabrication of high-quality diamond. Our results reveal that the diamond morphology is mainly determined by the original shape of seeds in the early growth stage, but it has an adjustment process during the growth and leads to well symmetry. Additionally, we have also established the growth model for the twinned diamond grown on several seeds, and proposed the possible growth processes by tracking the particular shapes of seeds before and after treatment under HPHT conditions. These results suggest that the shape-controlled synthesis of diamond with well morphology can be realized by employing certain suitable diamond seeds. This work is expected to play an important role in the preparation of trustworthy diamond-based electronic and photonic devices.

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”.

Time Dependence of Charge Transfer Processes in Diamond Studied with Positrons

We have developed a method called optical transient positron spectroscopy and apply it to study the optically induced carrier trapping and charge transfer processes in natural brown type IIa diamond. By measuring the positron lifetime with continuous and pulsed illumination, we present an estimate of the optical absorption cross section of the vacancy clusters causing the brown color. The vacancy clusters accept electrons from the valence band in the absorption process, giving rise to photoconductivity.

Nanoelectromechanical switch fabricated from single crystal diamond: Experiments and modeling

The nanoelectromechanical system (NEMS) switches based on all single crystal diamond were reported both in experiments and in modeling. In the all-diamond NEMS switch, boron-doped diamond epilayer was utilized to obtain the electrical conductivity, while the type-Ib diamond substrate acted as an excellent insulator. The developed diamond NEMS switches exhibited good controllability, reproducibility, and reliability. Finite element analysis was employed to simulate the operation of the NEMS switch. The experimental data were consistent with the numerical simulation, resulting in a Young's modulus of 1100GPa for diamond. The simulation revealed that the pull-in voltage of the single crystal diamond three-terminal NEMS switches could be controlled by the drain voltage.

The phase composition of crystal-fluid nanoinclusions in alluvial diamonds in the northeastern Siberian Platform

The phase composition of crystal-fluid nanoinclusions in two types of placer diamonds of unknown genesis from the northeastern Siberian Platform (Ebelyakh diamondiferous region) has been first studied by transmission electron microscopy including electron diffraction, analytical electron microscopy (AEM), electron energy loss spectroscopy (EELS), and chromatography. The type I diamonds are transparent dodecahedroids, and the type II ones, widespread in this region, are dark rounded crystals assigned to variety V according to Orlov’s mineralogical classification. Isotopic and IR-Fourie spectroscopic studies showed that the type II diamonds have a strongly light carbon isotope composition (δ 13C av = –22.4‰) and high concentrations of nitrogen admixture (1100–1800 ppm). Nitrogen is present mainly as an aggregate. It is shown that all inclusions no larger than 400 nm are polyphase particles consisting of solid (silicate, oxide, carbonate, salt) and fluid phases. The type I diamonds bear high-Mg carbonatite inclusions (up to 100 nm) consisting of magnesite, dolomite, Fe-spinel, and clinohumite. The fluid phase has high concentrations of K, Cl, and O. The inclusions are similar in composition to the near-solidus melts of saturated carbonatized peridotites; thus, they might have resulted either from the crystallization of the parental melt or from the quenching and crystallization of deep-seated carbonate-silicate melt. The type II diamonds bear low-Mg carbonatite polyphase nanoinclusions consisting of Ba-, Sr-, and Ca,Fe-carbonatites, K,Ba-phosphates, Ti,Si- and Ti,Al-phases, and abundant fluid segregations filled mainly with CO 2, N, and hydrocarbons. These melts/solutions might have been supplied from subducted rocks of the oceanic and, partly, continental Earth’s crust. The enrichment of these inclusions in incompatible elements might evidence the percolation of salt fluids enriched in Ba, Sr, P, Ti, K, and Cl through carbonatized eclogites.

Effects of Two-Step DC Electrochemical Pretreatment of Fine Grinding WC-Co Tool Surface on Morphology and Quality of Diamond Coatings

In the present investigation, diamond coating was deposited on fine grinding cemented carbide substrate by direct current arc discharge chemical vapor deposition. The effect of electrolytic etching time in the two-step electrochemical pretreatment process (firstly using electrolytic etching, and then using acid etching) on morphology and quality of the diamond coating were systemically studied. The surface morphology feature and quality of diamond coatings were characterized by means of Scanning Electron Microscope (SEM), laser Raman spectrometer respectively. The results showed that the electrolytic etching duration has distinctly effect on the quality and crystal features such as morphology, crystal type and grain size of diamond coating. It showed that as electrolytic current is direct current 3A, electrolytic etching time altering from 0.5 min to 7.5min, the surface morphology of diamond films gradually transition from microcrystalline cubic-octahedron to cauliflower type nanocluster, and further increase the electrolytic etching time, will lead to several negative effects on the quality and nucleation of the coatings which is not only retard the diamond nucleation, but also promote the formation of graphite.

Wear Behaviors of PCD Tools in Turning Granite

The performance and wear mechanisms of two types of polycrystalline diamond (PCD) cutting tools (marked as PCD 010 and PCD 030) have been investigated in this paper. Continuous turning of round granite bars has been selected as the test method. The Taylor equation has been developed to describe the tool performance. The experimental results indicate that PCD 010 has a greater thermal wear resistance than PCD 030 at high cutting speeds. Scanning electron microscopy (SEM) was used to evaluate wear mechanisms of PCD tools, our results show that the dominant wear mechanisms for PCD 010 is abrasion wear while that for PCD 030 is inter-granular and cleavage wear.

Analysis of Nitrogen Defects in Diamond with VUV Photoluminescence

Various diamonds were analyzed with photoluminescence (PL) spectra excited with synchrotron radiation in the wavelength range 160-250 nm. The emission of type IaAB diamond begins near 300 nm and extends to 700 nm; two broad lines with maximums about 419 and 469 nm correspond to energies 2.96 and 2.64 eV, respectively. The spectral features observed in the PL excitation spectra show two vibrational progressions, A and B, related to nitrogen defects in diamond. Progression A has a spacing 1266 ± 20 cm(-1) and is associated with the N2 (or A) center of a nitrogen defect, whereas progression B has a spacing 1177 ± 20 cm(-1) related to the N4 (or B) center of a nitrogen defect. These vibrational progressions in PL excitation spectra of N2 and N4 centers in type IaAB diamond are here identified for the first time. We suggest the use of PL spectra excited in the region 170-240 nm to analyze and to identify the type of diamond.

Microfabrication of vacuum-sealed cavities with nanocrystalline and ultrananocrystalline diamond membranes and their characteristics

Vacuum-sealed cavities featuring diamond membranes are fabricated using plasma-activated direct bonding technology. A chemical mechanical polished (CMP) silicon dioxide interlayer, deposited on diamond with a high temperature oxide (HTO) process at 850 °C in a low pressure chemical vapor deposition (LPCVD) furnace, is employed for successful direct bonding and vacuum cavity formation. The circular cavities are defined on the thermally grown oxide of the phosphorus-doped Si wafer (4-in, < 100>, 1.2 Ω/sq) using reactive ion etching (RIE). The same microfabrication steps are applied for low residual stress (i.e. < 50 MPa) nanocrystalline (NCD) and ultrananocrystalline (UNCD) diamonds to determine and compare membrane characteristics. For both diamond types, successful microfabrication of membranes is demonstrated using the optimized process flow. Profilometer measurements of membrane deflection are compared with finite element modeling (FEM), and indicate a Young's modulus of 1000 GPa for NCD and 850 GPa for UNCD. Furthermore, FEM analysis suggests the residual stress of UNCD membrane is approximately 100 MPa tensile, whereas NCD one does not show any significant residual stress (< 50 MPa). Our results show that NCD is a more promising choice than UNCD as a membrane material for electromechanical transducers.

Optically stimulated luminescence (OSL) and laser excited photo luminescence of electron beam treated (EBT) diamonds: Radiation sensitization and potential for tissue equivalent dosimetry

We report the first optically stimulated luminescence (OSL; blue light stimulated luminescence (BLSL) and infrared light stimulated luminescence (IRSL)) results on colored diamonds and present experimental evidence that electron beam treatment (EBT) increases the radiation sensitivity of diamonds to a level that makes them suitable for low level radiation dosimetry. A suite of seven samples was examined. These comprise a white, three brown and three yellow diamond pieces. The FT-IR spectra of these diamonds revealed the nature and concentration of nitrogen impurity. The white diamond was kept as a control. The brown and yellow (with varied saturation) diamonds were irradiated by a 1.7 MeV electron beam. These turned blue/dark green; three of them were then heated in vacuum in the temperature range of 850–900 °C for two hours. Heating turned the irradiated diamonds to lemon yellow, pink, and purple colors. The irradiated and unheated blue samples were designated as 2C and 2D. The control sample, an un-irradiated white type Ia diamond, did not yield any significant IRSL/BLSL with doses up to 100 Gy. The BLSL/IRSL sensitivity of irradiated and heat treated diamonds was very poor, and depended on the heat cycle and hence were not pursued. Sample 2C exhibited significant BLSL and negligible IRSL sensitivity. Sample 2D gave an intense orange red emission under IR excitation as also responded to BLSL. The dose response of the BLSL signal in 2C suggested a minimum detectable dose of ~ 0.1 Gy and its use as a tissue equivalent dosimeter. Based on supporting experiments such as laser excited photoluminescence, we suggest that the BLSL process in 2C is primarily driven by carbon vacancies, which release a mobile hole when excited by GR2 band in the blue region. BLSL intensity exhibited a maximum around 285 °C. Given that TL glow peak also occurs near this temperature and that the nitrogen–interstitial carbon (N–Ci) complex also forms at this temperature (as reported in the literature), and it appears that the e–h recombination at sites with N–Ci complex could be involved in BLSL production. Laser excited photoluminescence (PL) at wavelengths 325, 514 and 785 nm and absorption spectra in UV–Visible range gave insights into the contrasting BLSL/IRSL responses of 2C and 2D. These differences were due to differences in nitrogen impurity complexes and the concentration of carbon vacancies produced by EBT in 2C and 2D. In 2D, the presence of Ni as NE8 center (four nitrogens coordinated to Ni) giving 800 nm emission on 785 nm excitation, appeared to suppress BLSL and sensitize IRSL in the orange window.

Sn4Si{Si(SiMe3)3}4{SiMe3}2]: A Model Compound for the Unexpected First-Order Transition from a Singlet Biradicaloid to a Classical Bonded Molecule

Metalloid cluster compounds of the general formula MnRm (n>m ; M=metal or semi-metal, R= ligand) are ideal model compounds for the system size range encompassed by molecules and the solid state, paving the way for further understanding of element formation from oxidized species on an atomic scale. In the case of tin, metalloid cluster compounds were first synthesized by reductive coupling of Sn compounds, such as SnCl2. [2] Recently it was shown that metalloid cluster compounds of tin can also be synthesized by the disproportionation reaction of tin monohalides. The monohalides are thereby obtained by employing a preparative co-condensation technique. Hence, the reaction of SnBr with LiSi(SiMe3)3 leads to the metalloid cluster compound [Sn10{Si(SiMe3)3}6] (1) in moderate yield of approximately 17%. Because only six of the ten tin atoms in 1 bear a Si(SiMe3)3 ligand, the average oxidation state of the tin atoms is 0.6. Thus, the metalloid cluster 1 is a reduction product of the disproportionation reaction on the way to elemental tin. Because the reaction starts with the monohalide SnBr, oxidized species with an average tin atom oxidation state of greater than 1 must also be present in the reaction solution. Early examples of such compounds were anionic stannylene [Sn{Si(SiMe3)3}3] and cyclotristannene [Sn3{Si(SiMe3)3}4] (2), in which the average oxidation states of the tin atoms is + 2 and + 1.3, respectively. The shortest tin–tin double bond of 258 pm was observed in 2, caused by the steric bulk of the ligands forcing the double bond into a planar arrangement. As 2 is only obtained together with the metalloid cluster compound [Sn10{Si(SiMe3)3}6] (1), subsequent investigations on 2 are always hindered by the presence of 1. Crystallization of 2 from the reaction mixture was attempted to circumvent this problem. During these attempts, another type of black diamond shaped crystals were obtained, and single crystal X-ray diffraction analysis of these crystals revealed a yet unknown crystal system. However, solution of the crystal structure showed that the metalloid cluster compound 1 is present in the crystal lattice, this time crystallizing together with the novel polyhedral cluster compound [Sn4Si{Si(SiMe3)3}4(SiMe3)2] (3). The molecular structure of 3 is best described as a butterfly arrangement of four tin atoms bridged by a Si(SiMe3)2 group (Figure 1). Every tin atom is additionally bound to a Si(SiMe3)3 ligand, with slightly different Sn–Si distances of 261 pm (Sn11–Si7A, Sn13–Si7) and 265 pm (Sn14–Si8, Sn12–Si8A). The capping Si(SiMe3)2 group most likely comes from the degradation of the Si(SiMe3)3 ligand, and a plausible mechanism is given in the supporting information. The tin–tin distances (284 pm) inside the Sn4 butterfly unit in 3 are within the range of a normal single bond (283– 285 pm). The capping silicon atom is bound to two tin atoms, with a Si–Sn distance of 263 pm also within the range of a single bond, leading to a nearly tetrahedral arrangement for these two tin atoms (Sn11, Sn13). In spite of this arrangement,

Phosphorescence in type IIb diamonds

Spectroscopic properties, including phosphorescence spectroscopy, were collected for 354 natural type IIb diamonds, 7 natural type IIa diamonds, along with 24 HPHT-treated diamonds and 18 HPHT synthetic diamonds. The only phosphorescence bands observed in the natural diamonds were centered at 500 and 660 nm. The relative intensity and half-life of these measured bands corresponded well with the boron concentration. None of the treated or synthetic diamonds showed the 660 nm band. However, these samples did have the 500 nm band; the synthetics had much higher intensity than the comparable natural diamonds. The phosphorescence spectra are correlated to data collected from infrared absorption and photoluminescence spectroscopy and these results provide additional evidence of the mechanism responsible for the observed phosphorescence.

Influence of nanofluids on mixed convective heat transfer over a horizontal backward‐facing step

AbstractPredictions are reported for laminar mixed convection using various types of nanofluids over a horizontal backward‐facing step in a duct, in which the upstream wall and the step are considered adiabatic surfaces, while the downstream wall from the step is heated to a uniform temperature that is higher than the inlet fluid temperature. The straight wall that forms the other side of the duct is maintained at constant temperature equivalent to the inlet fluid temperature. Eight different types of nanoparticles, Au, Ag, Al2O3, Cu, CuO, diamond, SiO2, and TiO2, with 5% volume fraction are used. The conservation equations along with the boundary conditions are solved using the finite volume method. Results presented in this paper are for a step height of 4.9 mm and an expansion ratio of 1.942, while the total length in the downstream of the step is 0.5 m. The Reynolds number is in the range of 75 ≤ Re ≤ 225. The downstream wall was fixed at a uniform wall temperature in the range of 0 ≤ ΔT ≤ 30 °C which is higher than the inlet flow temperature. Results reveal that there is a primary recirculation region for all nanofluids behind the step. It is noticed that nanofluids without secondary recirculation region have a higher Nusselt number and it increases with Prandtl number decrement. On the other hand, nanofluids with secondary recirculation regions are found to have a lower Nusselt number. Diamond nanofluid has the highest Nusselt number in the primary recirculation region, while SiO2 nanofluid has the highest Nusselt number downstream of the primary recirculation region. The skin friction coefficient increases as the temperature difference increases and the Reynolds number decreases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20344

Characterization of Fast Switching Capability for Diamond Schottky Barrier Diode

Wide band gap semiconductors have been attracted as the material for fabricating power switching devices to obtain lower power conversion loss in high voltage circuit, and to operate harsh environment of high temperature. This paper focuses on diamond as the wide band gap semiconductor material and elucidates the dynamic characteristics in switching operation. To this end, Schottky barrier diode (SBD) is fabricated with p type diamond semiconductor and static I-V characteristics is evaluated. Then, the switching operation of diamond SBD is demonstrated, and forward current dependency of the recovery phenomena is characterized. The diamond SBDs show superior fast switching capability with low reverse recovery current, which is inherent in uni-polar switching device.

Electrochemical Properties of Two Dimensional Assemblies of Insulating Diamond Particles

The electrochemical properties of two-dimensional assemblies of 500 nm type Ib diamond particles are investigated as a function of their surface oxidation state. High Pressure High Temperature particles are sequentially exposed to a hot strong acid bath and to H(2) plasma in order to generate oxygen (ODP) and hydrogen surface terminations (HDP). Changes in the surface composition following the chemical treatments are confirmed by FTIR. Electrophoretic mobility measurements show that the diamond particles exhibit a negative surface charge at pH above 7 independently of the surface termination. Oxidation in the acid bath and subsequent reduction in the H(2) plasma only affects about 30% of the particle surface charge. The intrinsic negative charge allows the formation of 2D assemblies by electrostatic adsorption on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO(2) electrodes (ITO). The particle number density in the assembly was controlled by the adsorption time up to a maximum coverage of ca. 40%. Cyclic voltammetry in the absence of redox species in solution show that the acid treatment effectively removes responses associated with sp(2) carbon impurities, resulting in a potential independent capacitive signal. On the other hand, HDP assemblies are characterized by a charging process at a potential above 0.1 V vs Ag/AgCl. These responses are associated with hole-injection into the valence band edge which is shifted to approximately -4.75 eV vs vacuum upon hydrogenation. Information concerning the position of the valence band edge as well as hole number density at the HDP surface as a function of the applied potential are extracted from the electrochemical analysis.

Improvement of Crystal Quality of a Homoepitaxially Grown Diamond Layer Using Plasma Etching Treatment for a Diamond Substrate

To realize a practical application of synthetic diamond radiation detectors, an effort to synthesize high-quality diamond single crystals was conducted using microwave plasma assisted chemical vapor deposition (CVD). This study evaluated the influence of etching treatment––a reactive ion-etching technique used for diamond substrates––on the crystal quality of a homoepitaxially grown diamond layer. A type Ib diamond single crystal grown using a high-temperature and high-pressure method was used as the substrate. Homoepitaxial diamond layers with ca. 10 �� m thickness were grown on diamond substrates with different etching depths. Then observations were conducted using a differential interference microscopy and cathode luminescence spectroscopy. Judging from the intensity of free exciton recombination luminescence and other observations, the crystal quality was markedly improved by the etching treatment on the diamond substrates. Furthermore, a macroscopic complex of abnormal growth resulting from a mechanical polished flaw was almost removed by the etching treatment with etching depth of 2.6 �� m.

Effects of Negative Bias Pulse on Characteristics of a-C:H Films Prepared by Plasma-Based Ion Implantation

A series of a-C:H films have been prepared by plasma-based ion implantation (PBII) with acetylene on AISI 321 substrates. The effect of negative bias pulse on the characteristics of these films was investigated. The structures of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The surface hardness was measured by microindentation tests. The results indicated that the characteristics of these films are strongly depended on the negative bias pulse. When the bias pulse ranges from -10kV to -40kV, the films are typical diamond like carbon (DLC) films, while the films deposited at -5kV are polymer films. The peak intensity ratio of the D-band to that of the G-band (ID/IG) of the DLC films changes with the negative bias pulse. The minimum value of ID/IG (1.02) was gotten at -10kV.

An analysis of vacancy clusters and sp2 bonding in natural type IIa diamond using aberration corrected STEM and EELS

The link between brown colour and vacancy structures in natural diamond and has been studied through the techniques of aberration corrected scanning transmission electron microscopy (AC-STEM) and electron energy loss spectroscopy (EELS). Bright field (BF) and high angle annular dark field (HAADF) STEM imaging modes have revealed discrete patches of stronger contrast, of the order of 1nm in size, that are similar to simulated images of vacancy clusters. Core loss spectra show a pre-edge feature for brown diamond corresponding to π* states. This feature is absent for colourless and heat treated brown diamonds. Additional spectra acquired in regions containing dislocations for both brown and white (colourless) diamonds showed a second pre-edge feature indicating that dislocations have electronic states associated with them and are therefore optically active.