Ion-induced electron emission (IIEE) from undoped and B-doped diamond films induced by 1–10 KeV H+ and Ar+

A newly developed experimental system enables measurements of IIEE at a very low ion flux (10⁵ ions/s cm²) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰–10¹³ ions/s cm²). The method overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the IIEE yield. We report on the investigation of the IIEE from hydrogenated undoped and B-doped diamond films as a function of (i) moderate heating in vacuum prior to the measurements, (ii) H⁺ and Ar⁺ energy in the range of 1–10 KeV, and (iii) film thickness and microstructure. An IIEE yield (γ) enhancement was typically detected when the films were heated to 300 °C in vacuum. In the B-doped diamond film heated to 300 °C, γ rose nearly linearly from ∼ 20 to ∼ 100 electrons/ion, for 1 to 10 KeV Ar⁺ ions, respectively. The values of γ obtained with H⁺ showed a more moderate, nonlinear increase from ∼ 8 electrons/ion at 1 KeV up to ∼ 90 electrons/ion at 10 KeV. In heated undoped diamond films of different thicknesses the measured values of γ were similar for all the studied films and somewhat lower than in B-doped film: from ∼ 10–16 to 60–70 electrons per 1 to10 KeV Ar⁺, respectively, and from 14–26 to 50–60 electrons per 1 to10 KeV H⁺, respectively. The experimental results were interpreted using TRIM calculations.     

Nano-particles seeding and its characterization by X-ray photoelectron spectroscopy (XPS)

We present a new procedure for pretreatment seeding by ultrasonic agitation of silicon substrates in diamond nano-powder suspensions to which HF and KOH were added X-ray photoelectron spectroscopy (XPS) was used to measure the surface coverage by diamond nuclei immediately after the pretreatment. Coverage percentages of 70, 40 and 55% were obtained for the HF, KOH and the original diamond slurry, respectively. The seeding density (S_D) was calculated from the known nano-particles size, determined independently from X-ray diffraction of the powder. For nano-particle size of ∼6 nm, we obtain nominal seeding densities of the order ∼10¹² cm⁻². The advantage of the high coverage was most evident for films deposited at low substrate temperature (570 °C). The potential of the new seeding procedure and the XPS characterization method are discussed.     

Microstructure and optical band gap control of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited on silicon and quartz glass substrates by pulsed discharge plasma chemical vapor deposition (CVD), where the plasma was generated by pulsed DC discharge in H₂–CH₄ gas mixture at about 90 Torr in pressure, and the substrates were located near the plasma. The repetition frequency and duty ratio of the pulse were 800 Hz and 20%, respectively. When CH₄ / (CH₄ + H₂) ratio, i.e. methane concentration (Cm), increased from 3 to 40%, C₂ species in the plasma was increased, and corresponding to the increase of C₂, deposition rate of the film was increased from about 0.2 to 2.4 μm/h. The absorption peaks of sp³C–H and sp²C–H structures were observed in the FT-IR spectra, and the peak of sp²C–H structure was increased with increasing Cm, showing that sp² to sp³ bonding ratio was increased when Cm was increased. Corresponding to these structural changes due to the increase of Cm, optical band gap (Eg) was decreased from 3 to 0.5 eV continuously when Cm was increased from 3 to 40%.     

Nano- and micro-crystalline diamond growth by MPCVD in extremely poor hydrogen uniform plasmas

It is well established that argon rich plasmas (> 90% Ar) in Ar/CH₄/H₂ gas mixtures lead to (ultra)nanodiamond nucleation and growth by microwave plasma chemical vapour deposition (MPCVD). Nonetheless, in the present work, both microcrystalline and nanocrystalline diamond deposits developed under typical conditions for ultrananocrystalline (UNCD) growth by MPCVD. Silicon substrates were pretreated by abrasion using two different diamond powder types, one micrometric (< 0.5 μm) and the other nanometric (∼ 4 nm), the latter obtained by detonation methods. Samples characterization was performed by SEM (morphology), AFM (roughness and morphology) and micro-Raman (structure).For all samples, Raman analysis revealed good crystalline diamond quality with an evident ∼ 1332 cm^− 1 peak. The Raman feature observed at ∼ 1210 cm^− 1 is reported to correlate with two other common bands at ∼ 1140 cm^− 1 and ∼ 1490 cm^− 1 characteristic of nano- and ultra-nanocrystalline diamond.A new growth process is proposed to explain the observed morphology evolution from nano- to microcrystalline diamond. Based on this, the microcrystalline morphology is in fact a crystallographically aligned construction of nanoparticles.     

Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–reduction of CH4–CO2 mixed gases

This paper shows the recent results on the development of layered composite promoting two types of electrochemical reactions (oxidation and reduction) in one cell. This cell consisted of porous Ni–Gd-doped (GDC) ceria cathode/thin porous GDC electrolyte (50 μm)/porous SrRuO₃–GDC anode. The external electric current was flowed in this cell at the electric field strength of 1.25 and 6.25 V/cm. The mixed gases of CH₄ (30–70%) and CO₂ (70–30%) were fed at the rate of 50 ml/min to the cell heated at 400–800 °C under the electric field. In the cathode, CO₂ was reduced to CO (CO₂ + 2e^? → CO + O^2?) and the formed CO and O^2? ions were transported to the anode through the pores and surface and interior of grains of GDC film. On the other hand, CH₄ was oxidized in the anode to form CO and H₂ through the reaction with diffusing O^2? ions (CH₄ + O^2? → CO + 2H₂ + 2e^?). As a result, H₂–CO mixed fuel was produced from the CH₄–CO₂ mixed gases (CH₄ + CO₂ → 2H₂ + 2CO). This electrochemical reaction proceeded completely at 800 °C and no blockage of gases was measured for long time (>10 h). Only H₂–CO fuel was generated in the wide gas compositions of starting CH₄–CO₂ gases.     

Effect of prior C,Si and Sn implantation on the etch rate of CVD diamond

Diamond films were implanted with C⁺, Si⁺ or Sn⁺ ions at multiple energies in order to generate a uniform layer of implantation-induced disorder. The implant energies of 60, 180, 330 and 525 keV for C⁺ ions, 200, 500 and 950 keV for Si⁺ ions and 750 and 2000 keV for Sn⁺ ions were selected to give an approximately constant vacancy concentration at depths over the range ∼ 0–0.5 μm. An analysis of the C⁺ implanted surfaces by Raman spectroscopy has shown an increase in non-diamond or sp²-bonded carbon at doses in the range 5 × 10¹³ to 5 × 10¹⁵ cm^− 2. In comparison, a completely non-diamond structure was evident after implantation with either Si⁺ ions at a dose of 5 × 10¹⁵ ions/cm² or Sn⁺ ions at ≥ 5 × 10¹⁴ cm^− 2. For a given dose, the etch rate of the diamond film was shown to increase with the mass of the implanted species in the order of C⁺, Si⁺ and Sn⁺. For a given implant species, the etch rate increased with the implant dose and the ion-induced vacancy concentration. The etch rate of the implanted diamond in various gases decreased in the order of O₂, CF₄/O₂ and CHF₃/O₂ plasmas.     

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.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

Synthesis of diamond at sub 300 °C substrate temperature

Sulfur-assisted hot-filament chemical vapor deposition (HFCVD) was employed to grow nanocrystalline diamond at low substrate temperature, ∼ 300 °C, on molybdenum (Mo), and ∼ 250 °C, on polyimide film. The polyimide films remained flexible and strong after the deposition, clearly indicating that they did not experience temperatures near or above the glass transition temperature at ∼ 360 °C. The relative intensity of the diamond peak in the Raman spectra increases when the substrate temperature is decreased from ∼ 500 to ∼ 300 °C, a result that is inverted with respect to HFCVD without sulfur. This behavior was employed to obtain microcrystalline diamond on Mo at ∼ 270 °C. Profound changes induced to the gas phase chemistry and surface reactions when a trace amount of H₂S is added to the HFCVD process seem to enable these results.     

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH₄-H₂ gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H₂ flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH₄. The highest growth rate, 5 μm h⁻¹, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm⁻¹ K⁻¹. Diamond disks with very low optical absorption (loss tangent tgδ<10⁻⁵) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.     

New insights into the mechanism of CVD diamond growth: Single crystal diamond in MW PECVD reactors

CVD Diamond can now be deposited either in the form of single crystal homoepitaxial layers, or as polycrystalline films with crystal sizes ranging from mm, μm or nm, and with a variety of growth rates up to 100s of μm h^− 1 depending upon deposition conditions. We previously developed a model which provides a coherent and unified picture that accounts for the observed growth rate, morphology, and crystal sizes, of all of these types of diamond. The model is based on competition between H atoms, CH₃ radicals and other C₁ radical species reacting with dangling bonds on the diamond surface. The approach leads to formulae for the diamond growth rate G and average crystallite size <d> that use as parameters the concentrations of H and CH_x (0 ≤ x ≤ 3) near the growing diamond surface. We now extend the model to show that the basic approach can help explain the growth conditions required for single crystal diamond films at pressures of 100–200 Torr and high power densities.     

Formation of bismuth-core-carbon-shell nanoparticles by bismuth immersion during plasma CVD synthesis of thin diamond films

This paper describes an attempt to synthesize bismuth-doped diamond by plasma CVD. Solid bismuth was inserted into the reaction plasma of CH₄ and H₂. Examination by TEM showed that most of the bismuth was included as Bi nanoparticles in the carbon nanospheres, which segregated at the grain boundary of the diamond polycrystals. Diffraction peaks corresponding to the carbon allotrope Chaoite were observed at the grain boundaries. The Raman spectra showed very complex features between 100–1600 cm^− 1, suggesting the existance of molecule-like species.     

Development of a plate-to-plate MPCVD reactor configuration for diamond synthesis

The design and performance of a microwave plasma chemical vapor deposition (MPCVD) reactor based on compressed microwave waveguides and plate-to-plate substrate holders are described. This reactor can be operated at pressures from 10 to 40 kPa with microwave power of 0.4–1.2 kW, and a high plasma power density up to 500 W/cm³ can be obtained. The single-crystal diamond (lower substrate holder) and polycrystalline diamond (upper substrate holder) have been grown by the plate-to-plate MPCVD reactor using high pressure CH₄-H₂ mixture gases. Experimental results show that high quality single-crystal diamond and polycrystalline diamond were simultaneously synthesized at a growth rate of 25 μm/h and 12 μm/h, respectively. The results indicate that our MPCVD reactor is unique for the synthesis of diamond with high efficiency.     

Spectroscopic evaluation of out-of-plane surface vibration bands from surface functionalization of graphite oxide by fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp²/sp³ carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm⁻¹ and 482 cm⁻¹, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the I_D/I_G ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.     

Calibration of boron concentration in CVD single crystal diamond combining ultralow energy secondary ions mass spectrometry and high resolution X-ray diffraction

In this work, we combine ultralow energy secondary ion mass spectrometry (uleSIMS) and high resolution X-ray diffraction (HRXRD) for the analysis of boron doped single crystal diamond. CVD layers of nominal boron concentrations 1.3 · 10²¹ At/cm³, 2.0 · 10²¹ At/cm³, 6.9 · 10²¹ At/cm³ and 3.8 · 10²² At/cm³ have been characterized by uleSIMS at 1 keV, 500 eV and 300 eV using normal incidence O₂⁺ and by HRXRD.We show that the enhancement of ¹²C⁺ signal as a function of boron concentration observed in uleSIMS analysis of heavily doped layers compared to intrinsic diamond is due to a change in the ionization probability for carbon rather than to a change in erosion rate or post-ionization in the gas phase. For this reason, calibrating the boron concentration using a relative sensitivity factor (RSF) derived from an implanted reference material, which will naturally have a different ionization probability for carbon, is not accurate and a calibration curve needs to be obtained from an independent analysis technique such as nuclear reaction analysis (NRA).Simulation and measurement of HRXRD 113 asymmetric and 004 symmetric reflections have confirmed that the layers grew coherently to the substrate. The peak strain in the samples has been obtained from the HRXRD patterns by comparison between simulation and experiments, being ∼ 0.41 · 10^− 3, ∼ 0.43 · 10^− 3 and ∼ 1.30 · 10^− 3 for the first three samples. The peaks in the experimental profiles are broad compared to the simulations of uniform layers, which is explained by the graded composition of the layers, shown by the uleSIMS profiles.     

Optical-quality diamond growth from CO2-containing gas chemistries

From interpretation of the Bachmann diagram, it is conceivable that there may be some advantage to be gained by moving up the H–CO tie line for optical-quality diamond deposition. A convenient system for achieving this are gas chemistries containing CO₂, which, when combined with gases such as CH₄ and C₂H₄ (ethylene), enables the diamond deposition region to be traversed and, with the addition of hydrogen, to move along the H–CO tie line. The fabrication of free-standing diamond wafers using combinations of these feed-stock gases with a high-pressure, 2.45-GHz microwave source reactor (HPMS) able to operate at up to 140 Torr and 6 kW has been investigated. The FWHM line width of the 1332 cm⁻¹ Raman peak is found to be predominately a function of the gas composition. The growth rate is also dependent on the input power and the deposition pressure. The deposition plasmas are bright green in colour, and optical emission spectroscopy (OES) of the plasma reveals distinctive C₂ and Hα peaks. In some cases, it is possible to correlate characteristics of the deposited diamond layer to features in the OES spectra.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

Densification and properties of transition metal borides-based cermets via spark plasma sintering

Engineering borides like TiB₂ and ZrB₂ are difficult to sinter materials due to strong covalent bonding, low self-diffusion coefficient and the presence of oxide layer on the powder particles. The present investigation reports the processing of hard, tough and electrically conductive transition metal borides (TiB₂ and ZrB₂) based cermets sintered with 6 wt.% Cu using spark plasma sintering (SPS) route. SPS experiments were carried out with a heating rate of 500 K/min in the temperature range of 1200–1500 °C for a varying holding time of 10–15 min and the optimization of the SPS conditions is established. A maximum density of ∼95% ρ_th in ZrB₂/Cu and ∼99% ρ_th in TiB₂/Cu is obtained after SPS processing at 1500 °C for 15 min. While the optimized TiB₂/Cu cermet exhibits hardness and fracture toughness of ∼17 GPa and ∼11 MPa m^1/2, respectively, the optimized ZrB₂/Cu cermet has higher hardness of ∼19 GPa and fracture toughness of ∼7.5 MPa m^1/2, respectively. High electrical conductivity of ∼0.20 MΩ⁻¹ cm⁻¹ (TiB₂/Cu) and ∼0.15 MΩ⁻¹ cm⁻¹ (ZrB₂/Cu) are also measured with the optimally sintered cermets.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

Biotribological performance of NCD coated Si3N4–bioglass composites

A nanocrystalline diamond (NCD) coated Si₃N₄–bioglass composite, with potential use for hip and knee joint implants, was tribologically tested in simulated physiological fluids. NCD was deposited using a hot-filament chemical vapour deposition (HFCVD) apparatus in an Ar–H₂–CH₄ gas mixture. Self-mated reciprocating experiments were performed using a pin-on-flat geometry in Hanks' balanced salt solution (HBSS) and dilute fetal bovine serum (FBS). A nominal contact pressure of 25 MPa was applied for up to 500,000 cycles. Very low friction coefficients of 0.01–0.02 were measured using HBSS, while for FBS lubricated tests the values are slightly higher (0.06–0.09), due to a protein attaching effect. AFM assessed wear rates by an approach using the bearing function for volume loss quantification, yielding wear rates of k ∼ 10^− 10 mm³ N^− 1 m^− 1 in HBSS and k ∼ 10^− 9–10^− 8 mm³ N^− 1 m^− 1 for FBS, characteristic of very mild wear regimes.