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As we know that more effective synthesis of diamond combined with physical and chemical properties of hy- drogen termination in aqueous environment as well as device structure design can greatly facilitate the chemical and electrochemical applications of higher cost-performance diamond. For this purpose, the direct-current (DC) characteristics, surface reaction and reparation of a hydrogen-terminated DC arc jet plasma CVD polycrystalline diamond, which has a high cost-performance, were characterized byI-Vexperiments based on a FET-like structure device in multiple aqueous solutions. The variation trends of theI-Vproperties of device based on pH were similar in different aqueous solutions but could be affected by disparate ions (such as K+ions) or organic molecules (such as citric acid radicals or a benzene ring). Especially, owing to the founded replacement of hydrogen termination with hydroxyl (eOH), carboxyl radical (eCOOH) or carbon-oxygen bond (eCeOeCe) in mixed solution, i.e., KHP + H2SO4(and +NaOH) solutions, the resistance of the device was remarkably in- creased from 13.57 kΩto 95.78 kΩ. However, the raised resistance of surface reacted diamond (SRD) can be reduced prominently by repairing hydrogen termination through negative potential sweeps (NPS) at a low ne- gative potential (−1 to−3 V) if the SRD was introduced as an electrode in a strong inorganic acid. What's more, the NPS repaired device, which subsequently stored for four weeks, was more sensitive than the original hy- drogen plasma-treated diamond in aqueous solution environments. This repaired result was coming out of NPS re-formed CeH bonds with higher intensity. Thesefindings will be the references of failure and reparation of diamond hydrogen termination in aqueous environment.

To unfold the promise of diamond as an advanced technical material, single-crystal diamonds (SCDs) and polycrystalline diamonds (PCDs) were smoothed by high-precision three-dimensional movement dynamic friction polishing (3DM-DFP) to achieve the ultra-smooth surface with roughness

The surface conductivity on the H-terminated diamond is still an interesting topic in the field of diamond electronics. Until now, the carrier mobility in the conductive channel has been limited to below 200cm2/Vs due to the various surface scattering mechanisms. In this paper, a high mobility conductive channel on the H-terminated diamond surface was reported. Firstly, the high quality diamond films were deposited on the commercial CVD diamond substrates. After polishing, the H-termination was obtained by hydrogen plasma treatment. The surface morphology of the H-terminated diamond was observed by atomic force microscope (AFM) and scanning electron microscope (SEM). The crystal quality on the diamond surface was characterized by Raman spectroscopy. The impurities in the crystals were tested by photoluminescence spectroscopy. The surface conductivity of H-terminated diamond was monitored comprehensively by Hall test. It can be found that the sheet resistance decreases much and the carrier mobility increases dramatically after the hydrogen plasma treatment and mechanical cleaning. The maximum mobility value is up to 365cm2/Vs with carrier density of 2.9 × 1012 cm−2, which is the highest value reported. Raman spectra of diamond surface show that a peak appears at 1121.4 cm−2 after the hydrogen plasma treatment, which corresponds to the nanocrystal diamond or carbon clusters of sp3 bonded material. The corresponding mobility enhancement mechanism on the H-terminated diamond surface was proposed.

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25 °C, using a single-edge pre-cracked beam method.

Slightly graphitization might be incurred on CVD diamond films when suffering instantaneous high temperature thermal shock. In this paper, phase transitions of CVD free-standing diamond films have been studied experimentally at the temperature range of 1500–1800 °C through a plasma rapid heating process. Raman spectra and X-ray photoelectron spectra (XPS) are compared and discussed for the purpose of measuring the percentage of graphitic carbon component in CVD diamond films after heat treatment at different temperatures. An obvious optical degradation took place in both visible and infrared bands with the increased treating temperature. The result indicates that graphitization is dominated along grain boundaries rather than in internal grains. In addition, the nitrogen related absorption peaks were not covered by the enhanced optical absorption background, which was caused by the opaque graphitic carbon formed at high temperature. Nitrogen concentrations have also been evaluated for these graphitized diamond films and there is no evidence to show the movement or aggregation of nitrogen atoms.

In the present study, two types of diamond films with a diameter of 123 mm were deposited by DC arc plasma jet chemical vapor deposition (CVD) operating in the gas recycling mode. These two diamond films were found to possess peculiar grain family morphologies. The fracture strengths of the samples were 182.65 and 995.58 MPa, respectively. To investigate the possible reason for the large difference in the fracture strength between the samples, the surface and fracture morphologies of the two types of diamond films as well as their grain size were characterized. The results revealed that the sample with low fracture strength suffered from a typical intergranular fracture. Weak grain boundary, impurities distributed in the boundary, and low strength of diamond grains led to a decrease in the fracture strength. However, the sample with high fracture strength mainly suffered from transgranular fracture and the columnar grain exhibited numerous voids. Owing to the impact of twinning, the voids could be occupied, thus increasing the contact area between the adjacent grains. Moreover, large grain family on the growth side contained numerous small grains and twins. These small grains, twins on growth side along with the difference of stress on surfaces inhibit fracture strength from decreasing rapidly. All these features benefitted fracture strength. Moreover, when the sample was loaded, the tips located in the voids probably resulted in increase of risk of stress concentration, which might have led to initiation of crack origin and eventual fracture.

This paper is focused on the in-plane crushing of two-dimensional (2D) porous structures with a special attention on the effect of functionally graded (FG) porosities. The dynamic response and energy absorption of closed-cell metal foams with different porosity distributions are investigated by using finite element (FE) analysis. Two symmetric, two asymmetric and one uniform distributions of internal pores along the impact direction are constructed with Voronoi tessellation. The proposed porous structure is crushed under the impact of a rigid panel with a constant velocity. The deformation of cell walls is simulated using a plastic kinematic material model. The erosion criteria and hourglass control are applied to ensure the accuracy of numerical results, which are validated against the experimental data from open literature. The effects of varying parameters on the energy absorption, deformation pattern, and stress-strain curve of the FG porous structure are discussed. The dynamic response is found to be influenced by different random cell geometries, porosity gradients, cell wall thicknesses, internal pore numbers, and impact velocities. The effective way to improve the energy absorption capability of the porous structure under a constant-velocity impact is proposed, shedding new insights into the deformation mechanism of the FG porous structure for engineering design.

In-situ micro-area analysis of precisely polished polycrystalline diamond (PCD) film by high-speed three-dimensional dynamic friction polishing (3DM-DFP) is carried out in this work, continuing from the previous works focusing on the subsurface cleavage and polishing mechanism. Multi-orientational grains with variable hardness anisotropy are generally considered the reason for the surface irregularity of PCD after smoothing. The unreached surface ultra-smoothness in local region after polishing is mainly resulted from the height difference at grain boundary (GB) area (e.g., a selected GB micro-region is of around Ra roughness of 5.7 nm and altitude intercept of 6.6 nm). However, different individual grains nearby showing (100), (110), (111) orientations are all of roughness

Polycrystalline CVD diamond films were subjected to transient heat treatment at the temperature range of 1500–1900 °C in the Ar/H2 plasma arc environment. The structural changes of the diamond grain boundaries (GB) were indirectly investigated by employing IR optical absorption, Raman spectroscopy, X-ray diffraction (XRD), and fractographic scanning electron microscopy (SEM). The graphitization difference between near-nucleation side and near-growth side is mainly due to the difference in the GBs' content. It is proposed that the bonded‑hydrogen atoms, of which a large amount remained in the diamond GBs, play an active role in the process of graphitization. We recognized the intensity evolution of a peak in Raman spectra located at 1470 cm−1, which was attributed to the trans-polyacetylene at diamond GBs. This result suggests the likelihood that the long-chain hydrocarbon precursor is involved in a mechanism for the sp3-to-sp2 transformation. As the propagation of the graphitization from diamond GB to grain interior, sp2 carbon with compressed state forms in its place due to the limited space for expansion. The graphite along GBs affects the fracture mode of the diamond films, which causes partial GBs to tear, while transgranular fracture occurs instead of directly intergranular fracture.

The H-terminated diamond films, which exhibit high surface conductivity, have been used in high-frequency and high-power electronic devices. In this paper, the surface conductive channel on specimens from the same diamond film was obtained by hydrogen plasma treatment and by heating under a hydrogen atmosphere, respectively, and the surface carrier transport characteristics of both samples were compared and evaluated. The results show that the carrier mobility and carrier density of the sample treated by hydrogen plasma are 15 cm2·V−1·s−1 and greater than 5 × 1012 cm−2, respectively, and that the carrier mobilities measured at five different areas are similar. Compared to the hydrogen-plasma-treated specimen, the thermally hydrogenated specimen exhibits a lower surface conductivity, a carrier density one order of magnitude lower, and a carrier mobility that varies from 2 to 33 cm2·V−1·s−1. The activated hydrogen atoms restructure the diamond surface, remove the scratches, and passivate the surface states via the etching effect during the hydrogen plasma treatment process, which maintains a higher carrier density and a more stable carrier mobility.

The growth process of carbon film on the surface of hydrocooling copper substrate in high-density direct current (DC) arc plasma was studied. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, nuclear magnetic resonance spectroscopy, and UV confocal Raman spectroscopy were used to analyze the structural constituents of the intermediate products generated at different growth periods. Results show that despite the remarkable heating effect of DC arc plasma to the copper substrate, one layer of cauliflower saturated carbon and double carbon bond as main component loose matter are first formed because of water-cooled copper substrate. With the increase in thickness of carbon film, the surface temperature gradually increases, the etching ability of the atomic hydrogen is enhanced and high concentration reactive units are maintained on the surface of carbon film. The cauliflower structure may effectively reduce the diamond nucleation barrier and promote the generation of nano-diamond. The diamond grains gradually grow up to the micron scale by the continuing influence of carbon-based active group and plasma. The growth pattern of carbon film on the hydrocooling copper substrate in high-density DC arc plasma may provide a new way to prepare large crack-free diamond films.

Y 2 O 3 films with different microstructures were deposited through radio frequency magnetron sputtering. The optical properties and the antireflection effects of the cubic and the monoclinic Y 2 O 3 films on diamond substrates were investigated. Results showed that the cubic Y 2 O 3 film had denser and more uniform grains with larger grain size. The band gap energies of the cubic and the monoclinic Y 2 O 3 films were 5.79 and 5.88 eV, respectively. This finding was mainly attributed to the decrease in grain size from 30.1 nm to 20.6 nm. Meanwhile, the reflective indices of the cubic and the monoclinic films were 1.887 and 1.817, respectively. Furthermore, the monoclinic and the cubic Y 2 O 3 films that have the same thickness generated antireflection effects on diamond at the 8–12 μm long-wave infrared (LWIR) range. However, due to the lower reflective index of the monoclinic Y 2 O 3 film compared with that of the cubic Y 2 O 3 film, the maximum antireflection effects of the Y 2 O 3 films on diamond shifted to lower wavelengths.

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films. The evolution of interfacial stress during the cooling process is critical for the fabrication of large-area crack-free diamond films, so it is very worthy of further study. Variable-temperature X-ray diffraction (XRD) tests and finite element simulation were performed to evaluate the stress evolution of a diamond film. The film (with diameter of 150 mm) was prepared on a graphite substrate with Ti interlayer using DC arc plasma jet. Residual compressive stress in the diamond film is gradually increased during the cooling process. Moreover, the value and growth rate of the residual stress from the XRD tests are significantly lower than the finite element simulation results. All XRD peaks of diamond, graphite, and titanium carbide (TiC) generated on both sides of the Ti interlayer shift to a larger angle. By contrast, several XRD peaks of Ti deviate to a low angle. Elastic deformation of the Ti layer may offset the lattice contraction caused by the cooling effect. Elasticity modulus of TiC is significantly greater than that of Ti, which is more inclined to release the stress of the diamond film by self-fracturing. After the variable-temperature tests, residual compressive stress of the diamond film at room temperature is 1.1 GPa, whereas that before the tests is 1.4 GPa. These values are significantly lower than the finite element simulation results (3.05 GPa).

Polycrystalline CVD free-standing diamond films have been heat treated through a rapid heating process using a direct current (DC) arc plasma jet system within the temperature range of 1500–2000 °C. The optical transmission of the diamond film decreases when the treatment temperature exceeds 1500 °C. When the diamond film was annealed to 1800 °C for just 1 min, the infrared (IR) transmittance at λ = 10.6 μm of the film was significantly decreased from 60.5% to 4.0%, due to the transformation from diamond to graphite. Further increase of the temperature to 2000 °C leads to complete loss of optical transmittance in only 30 s, due to the detrimental occurrence of graphitization. XRD patterns indicate that graphite ultimately grows preferentially along the (0002) plane by forming disordered or turbostratic graphite as intermediate product. Raman spectra and SEM images verify that at 1500–1850 °C graphitic carbon transforms from the diamond surface in the form of protruding submicron crystallites, which were still containing a number of sp3 C C bonds. In addition to graphitization on the external surface, the diamond-graphite phase transition also occurs along the grain boundaries and the calculated activation energy of the grain boundary graphitization process is 276 kJ·mol− 1 for the CVD diamond.

The CVD diamond films at different growth stages were prepared on the same substrate by DC arc plasma jet CVD. The surface morphologies and residual stress of diamond films were characterized by scanning electron microscope and Raman spectroscopy. The intrinsic stress of diamond films were obtained by subtracting thermal stress from residual stress. The results showed that the intrinsic compress stress was 0.42 GPa for the precoalescence stage due to the sp 2 carbon inclusions in diamond islands. With the diamond islands growing up and even snapping together to form grain boundary at the coalescence stage the intrinsic compress stress was 0.06 GPa. Eventually, the diamond islands coalesced into a continuous polycrystalline film, the intrinsic compress stress vanished and converted to tensile stress, the intrinsic tensile stress reached 1.41 GPa due to the number of grain boundaries increasing. The evolution of intrinsic stress during diamond film initial growth was mainly attributed to the surface free energy of diamond islands changed to the free energy of grain boundaries which was consistent with the Nix-Clemens model.

Y2O3 anti-reflection films were deposited on freestanding chemical vapor deposited (CVD) diamond substrates by radio frequency magnetron sputtering. The effects of oxygen–argon ratio on microstructure, composition, optical property and mechanical property were analyzed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, Fourier transformed infrared spectroscopy, and nanoindentation. The results showed that the crystal structure of Y2O3 films transformed from cubic to monoclinic with the increase of oxygen–argon ratio. Meanwhile, the degree of non-stoichiometry of the film with the monoclinic phase was higher than the film with the cubic phase. Both monoclinic and cubic Y2O3 film had an anti-reflection effect on diamond in the 8–12 μm long-wavelength infrared (LWIR) range. However, the maximum transmittance of the Y2O3 film-coated diamond was shifted to lower wavelengths, which were mainly attributed to the reflective indexes of the Y2O3 film that decreased with the increase of the oxygen–argon ratio. The Y2O3 film with a cubic phase had high resistance to abrasion and plastic deformation. Therefore, the Y2O3 film with a cubic phase was much more suitable for long-wave infrared application.

The Y2O3 film deposited on chemical oxidation diamond substrate exhibited high adhesion strength and survived thermal shock at 800 °C. Oxygenated functionalization of the diamond surface had been successfully achieved via wet chemical oxidation with the increase in O/C atomic ratio from 0.04 to 0.13. The surface energy of the diamond substrate was significantly improved from 35.7 mJ/m2 to 61.4 mJ/m2. After depositing Y2O3 film on the diamond substrate, the COY bonds were observed at the interface. In addition, the quantity of COY bonds significantly increased with the chemisorbed oxygen concentration increasing. The critical scratch load of Y2O3 film deposited on chemical oxidation diamond substrate increased from 22.5 N to 34.1 N. Furthermore, the Y2O3 film deposited on chemical oxidation diamond substrate can withstand harsh thermal change and protect the diamond substrate from elevated temperature oxidation in atmospheric air without showing degradation of infrared optical performance. Keywords: CVD freestanding diamond, Chemical oxidation, Y2O3 antireflection film, Adhesion strength

Y 2 O 3 anti-reflection films were deposited on freestanding CVD diamond substrates by radio frequency magnetron sputtering. The thermal shock resistance and oxidation resistance of Y 2 O 3 /diamond/Y 2 O 3 samples at 727 °C and 800 °C in atmospheric air were investigated. No delamination of the Y 2 O 3 films occurred after thermal shock, thereby demonstrating extreme adhesion to freestanding diamond substrates. After thermal shock, Y atoms in the films were almost fully oxidized. Moreover, the majority of monoclinic phase in the Y 2 O 3 films transformed into cubic phase, crystallinity was enhanced, and the average grain size significantly increased. The maximum transmittance in the 8–12 μm long-wave IR range of the Y 2 O 3 /diamond/Y 2 O 3 samples increased from 81.3% ± 0.3% to 84.7% ± 0.2% and 83.6% ± 0.4%. These findings indicated that the Y 2 O 3 anti-reflection films displayed good resistance to thermal shock and provided sufficient protection for diamond substrates against elevated temperature oxidation.

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films by self-fracturing and facilitate the harvest of a crack-free free-standing diamond film prepared by direct current (DC) arc plasma jet. In this study, the evolution of the Ti interlayer between large-area diamond film and substrate was studied and modeled in detail. The evolution of the interlayer was found to be relevant to the distribution of the DC arc plasma, which can be divided into three areas (arc center, arc main, and arc edge). The formation rate of titanium carbide (TiC) in the arc main was faster than in the other two areas and resulted in the preferred generation of crack in the diamond film in the arc main during cooling. Sandwich structures were formed along with the growth of TiC until the complete transformation of the Ti interlayer. The interlayer released stress via self-fracture. Avoiding uneven fragile regions that formed locally in the interlayer and achieving cooperatively released stress are crucial for the preparation of large crack-free diamond films.

Molybdenum disulfide (MoS2) in its two-dimensional (2D) morphology has favorable catalytic properties for the hydrogen evolution reaction (HER). To fully take advantage of this capability and in order to enhance this material’s HER performance, viable methods for synthesizing and tuning electronic properties of 2D MoS2 should be developed. Here, we demonstrate a facile and nonhazardous approach to partially intercalate Li+ ions into the 2D MoS2 host structure. We show that such an intercalation is possible in a nonhazardous saline Li+ containing solution using a grinding/sonication-assisted exfoliation method in both dark and simulated sun irradiation conditions. A partial phase transformation from semiconducting (2H) to metallic (1T) phase is observed for the majority of flakes, after the process. We observe a notable enhancement of the HER activity for samples prepared under light illumination in the presence of a Li+ containing solution, in comparison to both pristine 2D MoS2 samples and samples proces...

A large amount of energy consumption has been caused by friction and inefficient heat transfer in the industry. Current research suggests a promising future for graphene nanofluids. In this paper, graphene oxide-deionized water (GO-DI) nanofluids with both friction reduction and high efficiency heat transfer properties was prepared by a two-step method in this paper. The graphene oxide was characterized using FTIR spectroscopy, Raman spectroscopy, XPS and SEM. The dispersion stability of nanofluids was analyzed by dynamic light scattering. An experimental system for measuring the convective heat transfer coefficient of nanofluids was established. The convection heat transfer and friction properties of GO-DI nanofluid were tested. The experimental results show that the addition of GO nanoparticles to the base fluid increases the convective heat transfer coefficient. The proportion of Nunf/Nuf of nanofluids increases from 1.02 to 1.18 as volume fraction rises. For GO-DI nanofluids, the friction coefficient decreased 71% compared to deionized water, when 0.1 vol% of GO and 3 N normal load were applied. The abrasion loss in 0.05 vol% GO-DI nanofluids was 18.5% less than that in deionized water. The association equation for calculating the convective heat transfer coefficient was proposed and the friction reduction mechanism of GO-DI nanofluids was elucidated.

Diamond with superior thermal conductivity would effectively unlock the heat dissipation problem, while the all-diamond microchannel heat sink (AD-MCHS) is immature. In the present work, surface modification of an applied novel AD-MCHS for heat transfer enhancement associate with single-phase (deionized water) coolant was elucidated, for the first time, on an ultra-thick free-standing chemical vapor deposition (CVD) diamond plate. The heat transfer performance of as-processed and post-treated AD-MCHS by surface oxygen-introduction were systematically studied under three heat flux levels, i.e., 40 W/cm2, 80 W/cm2 and 120 W/cm2, respectively. The results demonstrated that the heat transfer performance of the AD-MCHS after acid post-treatment (which was a preferable manner comparing with oxygen plasma and H2O2 bath) was prominently enhanced: 20–50% improvement of heat transfer coefficient (to the maximum of 11917 W/m2·K), 14–28% reduction of thermal resistance and minimum thermal resistance of 0.28 °C/W as well as maximum temperature drop of 11.49 °C for heat source surface. These were the results of hydrophilic diamond surface associated with stronger surface interaction with water molecules, resulting from the reconstructed surface polar carbon-oxygen components, removal of surface graphitic phases as well as the accompanying temperate rising of surface roughness. And the heat transfer acting a more important factor would be enhanced at the expense of acceptable steadily increasing pressure drop (about 1 kPa) and negligible extra of merely

It was predicted that diamond would play an irreplaceable role in thermal management technology at ultra-high heat flux levels owing to its outstanding thermal conductivity. In this work, an ultra-thick diamond plate, which was realized by direct-current (DC) arc jet plasma chemical vapor deposition (CVD) system, was fabricated into an all-diamond microchannel heat sink by laser. The convective heat transfer capacity, thermal resistance, device surface temperature and pressure drop of the all-diamond MCHS were systematically studied under the heat flux of 40–120 W/cm2 based on a single-phase heat transfer system, comparing to the commercially available aluminum MCHS. Under the assumption that the extreme service temperature of the device at 100 °C, the advantage of all-diamond MCHS in reducing fluid flow is evaluated. The results demonstrated that the heat transfer coefficient of the all-diamond MCHS was in the range of 5637.10–11447.20 W/m2‧K, which is 37%–73% higher than that of the aluminum MCHS with the same geometry. The volume flow of the fluid in the diamond MCHS is merely 40% of that in the aluminum MCHS could effectively reduce the equipment load owing to the significant advantage of the all-diamond MCHS in reducing the device surface temperature and thermal resistance.

In this paper, the thermal conductivity and tribological properties of ethylene glycol/water-based nanofluids with hydroxylated short multi-walled carbon nanotubes (SMWCNTs(OH)) were investigated. Long-term stable ethylene glycol/water-based nanofluids were prepared by a two-step method. The morphology and surface functional group characteristics of SMWCNTs(OH) were characterized by field emission scanning electron microscopy and Fourier transform infrared spectroscopy. The kinematic viscosity of the base fluid and nanofluids at different temperatures was measured using a rotary rheometer. The dispersion stability of nanofluids was analyzed by dynamic light scattering. Thermal conductivity test was performed using the transient hot wire method. The reciprocating friction test was carried out by UMT5 test machine, and the appearances of wear scars were analyzed by white light morphology. Results show that a significant enhancement of thermal conductivity could be achieved by increasing the temperature of the nanofluids and adjusting the concentration of the carbon nanotubes compared with the corresponding base. At 60 °C, the thermal conductivity of the nanofluids increases significantly and reaches 6.0%at the volume fraction of 25% for ethylene glycol/water. It is shown that the SMWCNTs(OH) have good self-lubrication antifriction effect in the water base nanofluids for the GCr15 friction pair. Meanwhile, the wear loss of the wear scar is greatly reduced, and the friction coefficient is reduced by 15.4% compared with the pure water, which shows a good application prospect of nanofluids in pump drive fluid loops.

Microwave plasma chemical vapor deposition (MPCVD) was used to grow single-crystal diamonds on two types of single-crystal diamond seed substrates prepared by high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) methods. The quality of diamonds grown on the different seed substrates was compared. Fluorescence characteristics showed that the sectors of the HPHT seed substrates were obviously partitioned. Raman and absorption spectra showed that the CVD seed substrate produced higher-quality crystals with fewer nitrogen impurities. X-ray topography showed that the HPHT seed substrate had obvious growth sector boundaries, inclusions, dislocations, and stacking faults. The polarization characteristics of HPHT seed substrate were obvious, and the stress distribution was not uniform. When etching HPHT and CVD seed substrates using the same parameters, the etching morphology and extent of different growth sectors of the two substrates differed. Although extended defects were inevitably formed at the interface and propagated in the CVD layer, the dislocation density of a 1 mm-thick CVD layer grown on a CVD seed substrate was only half that of a 1 mm-thick CVD layer grown on an HPHT seed substrate. Therefore, the use of CVD seed substrate enabled the growth of a relatively higher-quality CVD single-crystal diamond.

Nd2O3 film is a functional film with great potential application value. In this paper, thinNd2O3 film was prepared on the surface of Si substrates using RF magnetron reactive sputtering. Nd2O3 film with different crystal structures were controlled by changing the deposition temperature. The results show that the Nd2O3 film with a cubic structure can be obtained at 150 °C, while a hexagonal structure appears at 250 °C and above. The refractive indices of the cubic and hexagonal Nd2O3 films were 1.736 and 2.130, respectively. Moreover, the principle of the influence of deposition temperature on the refractive index of the film was illustrated. The hardness and the elastic modulus of the cubic Nd2O3 film were 6.3 GPa and 102 GPa, respectively, which are much lower than the 9.9 GPa and 145 GPa of the hexagonal Nd2O3 film. In addition, with a double-sided cubic Nd2O3 film, the maximum transmittance of the diamond film reached 87.54%. The results show that the Nd2O3 film is an anti-reflection film suitable for optical applications.

The effect of the substrate holder feature dimensions on plasma density (n e), power density (Q mw) and gas temperature (T) of a discharge marginal plasma (a plasma caused by marginal discharge) and homogeneous plasma were investigated for the microwave plasma chemical vapor deposition process. Our simulations show that decreasing the dimensions of the substrate holder in a radical direction and increasing its dimension in the direction of the axis helps to produce marginally inhomogeneous plasma. When the marginal discharge appears, the maximum plasma density and power density appear at the edge of the substrate. The gas temperature increases until a marginally inhomogeneous plasma develops. The marginally inhomogeneous plasma can be avoided using a movable substrate holder that can tune the plasma density, power density and gas temperature. It can also ensure that the power density and electron density are as high as possible with uniform distribution of plasma. Moreover, both inhomogeneous and homogeneous diamond films were prepared using a new substrate holder with a diameter of 30 mm. The observation of inhomogeneous diamond films indicates that the marginal discharge can limit the deposition rate in the central part of the diamond film. The successfully produced homogeneous diamond films show that by using a substrate holder it is possible to deposit diamond film at 7.2 μm h–1 at 2.5 kW microwave power.

Gallium nitride (GaN) has been widely used in electronic and optoelectronic devices because of its unique electrical properties. However, its low thermal conductivity and the high thermal boundary resistance at the interface between GaN and substrates such as Si and Al2O3preventefficient heat dissipation from the heated regions, which limits the further development of GaN-based high power devices. Diamond, with the highest thermal conductivity, has been considered to be one of the most promising heat sink materials. However, it is hard to prepare a diamond film on a GaN substrate because there is a high thermal expansion coefficient difference and also a large lattice mismatch between them. An approach to prepare a nano-diamond film on a GaN substrate by incorporating a Si buffer layer has been proposed. A GaN substrate decomposes significantly from 560 to 680°C when exposed to ahydrogen plasma for 5 min and no adhesive nano-diamond film can be directly grown on it. This decomposition is significantly suppressed by the presence of a Si buffer layer and a nano-diamond film about 2 μm thick can be deposited on a GaN substrate by microwave chemical vapor deposition using CH4 as the carbon source. With an optimum Si layer of 10 nm, the adhesive force between the nano-diamond film and the GaN substrate reaches 10N, which is ascribed to the complete conversion of the Si layer to a silicon carbidetransition layer during the deposition.

The needle-like surface morphology evolution in oxygen plasma in combination with a secondary gas (Cl2, CHF3 or CF4) by inductively coupled plasma reactive ion etching (ICP-RIE) on a free-standing polycrystalline diamond was investigated. The addition of CF4 can produce trans-polyacetylene (t-PA), which is similar to the result when the pure O2 etching takes place, and generate compact needle-tip particles. However, the t-PA disappears with the introduction of Cl or H ions. The optimised etching parameters for the needle-like structure formation are as following: Cl2/O2 ratio 20% and RF-power (RFP) 100 W, where more compact and even nano-needles are realised with an average etching rate of 2 μm/min. The Cl2/O2 plasma etching results indicate that the time-dependent etching mechanism of diamond nano-needles results from (1 1 1) crystal plane selective etching and preferential graphitisation at the twin-plane boundary and dislocation area.

Diamond-based semiconductor with high electrical conductivity is a key point in diamond device development. In this paper, a thin single crystal diamond layer of high quality was epitaxially grown on a commercial tool-grade diamond seed by incorporating active O atoms from the typical growth environment. Subsequently, the H-termination density was enhanced on the diamond surface by exposure to the pure hydrogen plasma, and the surface conductivity of H-terminated diamond was analyzed in detail. The thin epitaxial layers on the high-pressure high-temperature diamond seeds show lower resistance than the ones on the chemical vapor deposition diamond seeds, which could be comparable with the lowest values reported. After the thin diamond layers were grown with and without addition of O2, the carrier mobility in the conductive channel increased to almost 80cm2V−1s−1under O2contained condition, much higher than those without O2incorporation. The ionization scattering is dominant to the carrier mobility compared with the surface scattering. The higher carrier mobility is attributed to the lower impurity density in the epitaxial layer, which is because the active O atoms could purify the epitaxial layer by removing or reducing Si- and N-related impurities.

The ultra-smooth surface and hydrophobic nitrogen-incorporated ultra-nano-crystalline diamond (N-UNCD) was directly synthesized by equilibrating the etching effect of OH radical and growth promotion of CN and CH in the 5% CH4 added H2 plasma environment with additional feeding of constant 0.16% O2 and from 0.3% to 3.3% N2 contents. The initially decreasing and then increasing trend of surface roughness, i.e., from as-grown appearance with pits to smooth and then to worm-like surface, was resulted from the offset and even suppressive effect on OH etched holes by the faster growth rate that under the help of more N2 promoted chemical groups of CH and CN. In addition, chemical composition, i.e., the large amount of sp2 phases (sp2/sp3 ratio up to 1.399) and H termination of N-UNCD surface (proportion was 75.54 ± 3.8%), played an another enhancing function for repelling water (contact angle from 81.3° to 93.8°) although the surface roughness decreased to smoothest of 1.36 nm in Root-Mean-Square (RMS) in the presence of N2 addition from 0.3% to 1%. However, continuing adding N2 to 3.3%, the formed surface (RMS 8.98 nm) with worm-like ultra-nano diamond crystallites together with higher H reconstructed sp2 contents (sp2/sp3 ratio rose to 1.478) further boosted the hydrophobicity, at which the contact angle was finally increased to 110.2°. Therefore, in C-H-O-N gas system, ultra-smooth and uniform N-UNCD surface with excellent hydrophobicity can be obtained by a one-step method without any post-treatment.