Jet Chemical Vapor Deposition Research Articles

Accurate comparison of the fracture toughness of ultra-thick polycrystalline diamond plates by ISO standard method

Due to the difficulty in preparing ultra thick diamonds, current research on the mechanical properties of diamonds mostly focuses on thin samples, which cannot represent the true mechanical properties. In this study, DC arc plasma jet chemical vapour deposition method was used to deposit ultra-thick diamond plate measuring Ø125 mm × 6.5 mm. The size of 18 mm × 4 mm × 3 mm sample were prepared. The samples with dimensions of 18 mm × 4 mm × 3 mm were prepared. For the first time, fracture toughness test samples meeting the ISO 15732 requirements were prepared by using the single-sided beam pre-crack method. The accurate fracture toughness of ultra-thick diamond with pre-crack was obtained using the three-point bending method, 6.5 MPa·m1/2. Samples with sharp or blunt notch by laser cutting were used for comparison. The sharp notch sample had a value similar to that of the pre-crack sample, 6.3 MPa·m1/2. The value of the blunt notch sample was relatively large, 7.5 MPa·m1/2. The effects of defects and grain size on fracture toughness and the reasons for the differences in test results for different notches were analyzed. Overall, obtaining accurate fracture toughness values of ultra-thick diamond by ISO standard is crucial for industrial applications.

Structural evolution and self-destructive behavior of Mo/Ti transition layers during free-standing diamond-film preparation

Graphite-based metal transition-layer substrates hold great potential for preparing large-area diamond films and achieving crack-free release. In this study, a five-inch crack-free free-standing diamond film was prepared on a graphite substrate with a sacrificial Mo/Ti double-layer transition layer using direct-current arc plasma jet chemical vapor deposition (DC jet CVD). The interfacial bonding and diffusion behavior between the diamond and the transition layers were investigated. The effect of abundant metal particles and elemental diffusion between layers on film adhesion and stress release are discussed and analyzed. The surface of Ti layer prepared by the multi-arc ion-plating contains numerous metal particles that provide a template for the fine-crystalline Mo layer. Metal particles are encapsulated and embedded in the diamond and act as anchors that ensure the stability of the substrate–diamond bond during high-temperature deposition. After deposition, those metal particles act as weak points and play crucial stress-release roles. Mutual elemental diffusion occurs between the Ti, Mo, and diamond layers. Ti penetrates the Mo layer to form a continuous 140-nm-thick thin layer during the initial deposition period, facilitating diamond nucleation. Carbon from the plasma and diamond diffuses into the metal transition layer. Thermal stress generated between the diamond and the graphite substrate is released by destroying the metal transition layer with high thermal expansion coefficient and low elastic modulus during cooling from the deposition temperature. Cracks rapidly propagate along the interface between destroyable transition layers and graphite substrate, which results in the liftoff of a fully crack-free diamond film.

The uniform and robust 8-inch CVD diamond plate generated by dual-magnetic field controlled DC Jet CVD

Diamond is a highly attractive material for many applications in material science, engineering, chemistry, and biology because of its favorable properties. The preparation of large-area freestanding diamond plate can greatly reduce the production cost of diamond and promote the industrialization of diamond. In this study, the dual-magnetic field mode was introduced to optimize the magnetic field distribution in direct current arc plasma jet chemical vapor deposition (DC Jet CVD) system, and the arc plasma was effectively controlled under the coupling of electric field and magnetic field. The uniformity of the plasma distribution above the substrate surface is significantly improved, resulting in long-term stable and uniform growth of the 8-inch freestanding diamond plate. The influence of dual-magnetic field mode on magnetic field generation, arc feature, and reactive group distribution was studied. 8-inch freestanding diamond plates with an average thickness of 2.8 mm were prepared by dual-magnetic controlled DC Jet CVD. The results showed that the 8-inch diamond plate preferred a high (220) orientation, and the deviation of thickness of which was around 10 % of the averaged values. Raman spectroscopy of diamond revealed a 6.6 cm−1 of full width at half maximum (FWHM) and calculated 1.20 GPa compressive stress in the 8-inch diamond plate. By means of mechanical and thermal property measurements, the large area diamond plate shows a robust structure with the average values of three-point bending fracture strength of 987.2 MPa, and extremely high thermal conductivity of 1797.8 W/(m·k) as well. The deviation of three-point bending fracture strength and the thermal conductivity over the 8-inch diamond plate was around 7.9 % and 12.6 % of the respective averaged values.

Extraordinary Field Emission of Diamond Film Developed on a Graphite Substrate by Microwave Plasma Jet Chemical Vapor Deposition

This work reports both numerical and experimental studies of the reconditioning of a microwave plasma jet chemical vapor deposition (MPJCVD) system for the growth of diamond film. A three-dimensional plasma fluid model is constructed for investigating and conditioning the MPJCVD system and optimizing its operating conditions. The methodology solves electromagnetic wave and plasma dynamics self-consistently using an adaptive finite element method as implemented in COMSOL Multiphysics. The whole system has been modeled under varying parameters, including the reactor geometry, microwave power, and working gas pressure. Using an operating condition identical to the optimized simulation results, the MPJCVD system successfully fabricates a diamond-thin film on a graphite substrate. The SEM image reveals the presence of a diamond film uniformly distributed with particles of a size of ~1 μm. The field emission from the diamond film grown from our homemade MPJCVD system on the graphite substrate presents extraordinary properties, i.e., extremely high current density and relatively low turn-on voltage. The turn-on electric field observed could be as low as ~4 V/μm. This developed model provides valuable physical insights into the MPJCVD system, which guided performance improvements. The work may find applications in surface hardening and provide a better cold cathode for field electron emission.

Exploring three-point-bending fracture toughness of thick diamond films from different directions

In this study, the effect of defects on fracture toughness in different directions was investigated. Two diamond films with a diameter of 122 mm and as-deposited thicknesses of 2.2 and 1.5 mm, respectively, were deposited by DC arc plasma jet chemical vapor deposition. To accurately obtain the fracture toughness, the films were ground and polished to 1.6 and 0.8 mm, respectively. The results indicate that many defects, including pores, are introduced into the films during the growth process, particularly on the side close to the growth surface in thicker film. The size of pores reaches micrometres, which affects the fracture toughness under loading in different directions. Due to the minimum grain size, both films exhibit a maximum toughness of 8.2 and 10.0 MPa·m1/2, respectively, with growth-surface cracks. For the thinner sample, the maximum value is followed by that of the edge-surface cracks. As the grain size of the edge surface falls between that of the growth surface and that of the nucleation surface, the results indicate that fracture toughness is affected by grain size. However, the number of pores near the growth side increases when the thickness exceeds 0.8 mm. Pores reduce the toughness, resulting in the minimum fracture toughness (6 MPa·m1/2) of the thicker diamond film with edge-surface laser cracks. The sample with an edge-surface sharp pre-crack has a similar fracture toughness of 5.6 MPa·m1/2. This study provides guidance for selecting the applied load direction.

Evaluation of the fracture strength of ultra-thick diamond plate by the three-point bending ISO standard method

In this study, standard diamond bars were prepared using an ultra-thick diamond plate, and the bars had sizes consistent with the International Organization for Standardization (ISO) 14704:2016(E) three-point bending method for measuring the fracture strength. Ultra-thick diamond plate with dimensions of 125 mm (diameter) × 6.5 mm (thickness) were deposited using direct current arc plasma jet chemical vapour deposition. After cutting, grinding, and polishing, 12 diamond bars meeting the requirements of the ISO standard were prepared (36 mm × 4 mm × 3 mm) and tested for fracture strength using the cited ISO standard and elastic modulus using deflection measurements. The fracture strength of the ultra-thick diamond plate was found to be 580 ± 66 MPa. In addition, for comparison, the fracture strengths of standard ZnS, sapphire, and alumina were also analysed. Crucially, this is the first report of the fracture strength of standard-sized diamond plate. The effects of the defects and grain size on the crack formation and failure of the diamond film were analysed. The diamond's fracture strength was affected by the sample thickness, which caused the gradual accumulation of defects as this dimension increased, but not by its length and width. The elastic moduli of sapphire and alumina were compared. Overall, the measurement of the fracture strengths of ultra-thick diamond plate via the ISO standard is important for industrial applications.

The new application of boron-doped diamond film: Attenuator for high frequency and high power vacuum electronic devices

Boron-doped diamond film which was used as microwave attenuation material was prepared by DC arc jet chemical vapor deposition (CVD) and the boron-doped diamond attenuator for W-band traveling wave tube (TWT) was fabricated. The effects of boron concentration on microstructure, mechanical property and dielectric property were analyzed. Meanwhile, the performance of boron-doped diamond attenuators was evaluated. The results showed that the fracture strength of boron doped diamond was higher than 400 MPa, although it was slight decreased with the increasing of boron content in the diamond films. Very interestingly, the fracture strength of growth side was larger than that of nucleation side, the twinning band within the grains in the growth side play a critical role for this phenomenon. With the boron concentration increasing from 6.1 × 1018 cm−3 to 23.5 × 1018 cm−3, the average loss tangent of diamond films was increased from 0.05 to 0.32 in the range of 75–110 GHz, which indicated that the complex permittivity of diamond film could be controlled by tailoring boron content. Furthermore, the return loss and insertion loss of the boron-doped diamond attenuator was below −20 dB and −40 dB, respectively. These findings indicated that the boron-doped diamond displayed low reflection and good microwave attenuation. The boron-doped diamond attenuator was promising for using in vacuum electronic device with high performance.

Au@SnO2-vertical graphene-based microneedle sensor for in-situ determination of abscisic acid in plants

For developing electrochemical plant sensors, in-situ detection of hormone levels in living plants is worth attempting. A microneedle array sensor based on Au@SnO2-vertical graphene (VG)/Ta microelectrodes was constructed for analyzing abscisic acid (ABA) in plants. Graphene was vertically grown on Ta wires with a diameter of 0.6 mm by direct current arc plasma jet chemical vapor deposition with SnO2 as the Au catalyst carrier. These VG nanosheets were embedded with core-shell Au@SnO2 nanoparticles, and the formation mechanism of the sensing layer was investigated. Three Au@SnO2-VG microelectrodes, one Ti wire, and one Pt wire were packed into a microneedle array sensor with a three-electrode system. ABA was then quantitatively detected by direct electrocatalytic oxidation, which involves the synergistic catalytic effects of the abundant catalytic active sites of the Au@SnO2 nanoparticles and the excellent conductivity of the VG nanosheets. The microneedle array sensor responds to ABA in the pH range 4–7, the response concentration range was 0.012 (or 0.024)–495.2 μM, and the detection limit varied between 0.002 and 0.005 μM. The small size, wide pH range, low detection limit, and wide linear concentration range allow the microneedle array sensor to be inserted into plants for in-situ detection of ABA.

Non-enzymatic glucose sensor based on micro-/nanostructured Cu/Ni deposited on graphene sheets

The response performance of enzyme-free glucose sensors can be improved by using high conductivity and by loading redox probes with high electrochemical activity onto a graphene skeleton with a large surface area and by employing a design involving a binderless sensor structure. We prepared a Cu/Ni/graphene electrode containing neither a polymeric binder nor biofilm. Instead, graphene was grown vertically on the surface of the current collector by direct current (DC) arc plasma jet chemical vapor deposition (CVD) to form a structure consisting of a high-conductivity three-dimensional network. A large number of Ni and Cu micro-/nanostructured redox probes, which were prepared by square wave pulsed electrodeposition, was loaded onto the graphene skeleton. The reasons for the advantageous effect of these Cu/Ni/graphene electrodes on the performance were investigated by characterizing the morphology, structure, and composition of the sensitive films, and by combining the impedance characteristics of each component and the response characteristics to glucose. The results showed that the fabrication of Cu/Ni/graphene as the sensitive layer combined the performance characteristics of Ni, Cu, and graphene nanomaterials to exhibit excellent electrocatalytic activity toward glucose oxidation; three linear working curves were obtained in a wide concentration range of 0.005–2174μM, with an ultra-low detection limit of 0.0027μM (S/N=3). The glucose sensor exhibited high anti-interference properties against sucrose, fructose, xylose, maltose, lactose, ascorbic acid, dopamine, and uric acid.

Fracture behavior of diamond films deposited by DC arc plasma jet CVD

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.

Optical characterization of single-crystal diamond grown by DC arc plasma jet CVD

Optical centers of single-crystal diamond grown by DC arc plasma jet chemical vapor deposition (CVD) were examined using a low-temperature photoluminescence (PL) technique. The results show that most of the nitrogen-vacancy (NV) complexes are present as NV− centers, although some H2 and H3 centers and B-aggregates are also present in the single-crystal diamond because of nitrogen aggregation resulting from high N2 incorporation and the high mobility of vacancies under growth temperatures of 950–1000°C. Furthermore, emissions of radiation-induced defects were also detected at 389, 467.5, 550, and 588.6 nm in the PL spectra. The reason for the formation of these radiation-induced defects is not clear. Although a Ni-based alloy was used during the diamond growth, Ni-related emissions were not detected in the PL spectra. In addition, the silicon-vacancy (Si-V)-related emission line at 737 nm, which has been observed in the spectra of many previously reported microwave plasma chemical vapor deposition (MPCVD) synthetic diamonds, was absent in the PL spectra of the single-crystal diamond prepared in this work. The high density of NV− centers, along with the absence of Ni-related defects and Si-V centers, makes the single-crystal diamond grown by DC arc plasma jet CVD a promising material for applications in quantum computing.

The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films

Amorphous carbon films were formed on nickel/silicon (100) substrates by plasma jet chemical vapor deposition (PJCVD). The carbon films were deposited at atmospheric pressure using an argon-acetylene and argon-hydrogen-acetylene plasma. The plasma composition was analyzed by optical emission spectroscopy (OES). The results indicated that the dominant species in the argon-acetylene and argon-hydrogen-acetylene plasmas were the CH radical and C2 particles. The emission intensities of CH and C2 lines depended on the hydrogen amount in plasma and the distance. Scanning electron microscopy analysis demonstrated that the surface roughness of the coatings decreased with the addition of hydrogen and the increase of distance from 5mm to 7mm. The oxygen concentration increased with the increase of the deposition distance. The Raman spectroscopy results indicated that the addition of the hydrogen in the argon-acetylene plasma lead to the formation of nano-crystalline graphite films at 5mm distance. The increase of the distance from 5mm up to 7mm stipulated the formation of multiphase SiC/carbon films when the argon-hydrogen-acetylene ratios were 100:2.4:1 and 100:1.2:1.

Sensors made of carbon ceramic composite materials

Carbon nanotubes (CNTs) were deposited on barium titanate (BaTiO3) substrates using the microwave plasma jet chemical vapor deposition (MPJCVD) system. Accelerometer sensors were then made of both as-obtained carbon ceramic (CNTs/BaTiO3) composite materials and barium titanate (BaTiO3). The variations of the output voltages under various accelerating amplitudes were examined on two kinds of sensors at vibrating frequencies of 200Hz, 500Hz, 800Hz, and 2000Hz, respectively. It is found that the output voltages and the sensitivity of CNTs/BaTiO3 sensors were higher than those of BaTiO3 sensors at each vibrating frequency.

A tantalum electrode coated with graphene nanowalls for simultaneous voltammetric determination of dopamine, uric acid, L-tyrosine, and hydrochlorothiazide

The authors describe a voltammetric sensor for simultaneous determination of dopamine (DA), uric acid (UA), L-tyrosine (Tyr), and the diuretic drug hydrochlorothiazide (HCTZ). The assay is based on the use of graphene nanowalls deposited on a tantalum substrate. The nanowalls are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry. The nanowalls are vertically grown on the substrate by direct-current arc plasma jet chemical vapor deposition. The modified electrode is shown to enable simultaneous differential pulse voltammetric determination of DA, UA, Tyr, and HCTZ. The graphene nanowalls display a large specific surface, high conductivity, and a large number of catalytically active sites for oxidation of analytes. Simultaneous detection is performed best at a pH value of 7.0 and at peak potentials of 0.124 V (vs. SCE) for DA, 0.256 V for UA, 0.536 V for Tyr and 0.708 V for HCTZ. The respective detection limits are 0.04 μM, 0.1 μM, 0.6 μM and 0.4 μM. The results show that this graphene wall modified electrode is a promising tool for the design of sensitive, selective, and stable sensors.

Fast growth of amorphous Si nanowires by direct current arc plasma jet chemical vapor deposition

Silicon (Si) nanowires were synthesized by direct current arc plasma jet chemical vapor deposition. It was found that growth temperature and growth time played important roles in the morphologies of the Si nanowires. The possible formation mechanism of Si nanowire was also proposed according to the corresponding results. In addition, the Ni particles are dispersed on the Si substrate with the help of alkaline etching, which enables the lateral growth of Si nanowires on a Si substrate. This offers a simple and practical way of synthesizing and positioning the Si nanowires on a substrate.

Fabrication of graphene/titanium carbide nanorod arrays for chemical sensor application

Vertically stacked graphene nanosheet/titanium carbide nanorod array/titanium (graphene/TiC nanorod array) wires were fabricated using a direct current arc plasma jet chemical vapor deposition (DC arc plasma jet CVD) method. The graphene/TiC nanorod arrays were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. The TiO2 nanotube array was reduced to the TiC nanorod array, and using those TiC nanorods as nucleation sites, the vertical graphene layer was formed on the TiC nanorod surface. The multi-target response mechanisms of the graphene/TiC nanorod array were investigated for ascorbic acid (AA), dopamine (DA), uric acid (UA), and hydrochlorothiazide (HCTZ). The vertically stacked graphene sheets facilitated the electron transfer and reactant transport with a unique porous surface, high surface area, and high electron transport network of CVD graphene sheets. The TiC nanorod array facilitated the electron transfer and firmly held the graphene layer. Thus, the graphene/TiC nanorod arrays could simultaneously respond to trace biomarkers and antihypertensive drugs.

Effect of sputtered titanium interlayers on the properties of nanocrystalline diamond films

Ti interlayers with different thicknesses were sputtered on Si substrates and then ultrasonically seeded in a diamond powder suspension. Nanocrystalline diamond (NCD) films were deposited using a dc arc plasma jet chemical vapor deposition system on the seeded Ti/Si substrates. Atomic force microscopy and scanning electron microscopy tests showed that the roughness of the prepared Ti interlayer increased with increasing thickness. The effects of Ti interlayers with various thicknesses on the properties of NCD films were investigated. The results show nucleation, growth, and microstructure of the NCD films are strongly influenced by the Ti interlayers. The addition of a Ti interlayer between the Si substrate and the NCD films can significantly enhance the nucleation rate and reduce the surface roughness of the NCD. The NCD film on a 120 nm Ti interlayer possesses the fastest nucleation rate and the smoothest surface. Raman spectra of the NCD films show trans-polyacetylene relevant peaks reduce with increasing Ti interlayer thickness, which can owe to the improvement of crystalline at grain boundaries. Furthermore, nanoindentation measurement results show that the NCD film on a 120 nm Ti interlayer displays a higher hardness and elastic modulus. High resolution transmission electron microscopy images of a cross-section show that C atoms diffuse into the Ti layer and Si substrate and form TiC and SiC hard phases, which can explain the enhancement of mechanical properties of NCD.

Synthesis of a ZnO nanorod/CVD graphene composite for simultaneous sensing of dihydroxybenzene isomers

A few-layered graphene nanosheets on a Ta thin film were synthesized by a direct current (DC) arc plasma jet chemical vapor deposition (CVD) process that does not critically depend on cooling rates. The thickness of the graphene layer is about 20 μm, and the graphene sheets superimposed transversely on the outermost layer become a blooming bouquet. This structure is especially suitable as an electrode sensor. The ZnO nanorods have excellent electrochemical performance, and thus we combined ZnO nanorods and CVD graphene for a ZnO/graphene/Ta sensing electrode. The electrochemical oxidation of hydroquinone (HQ), catechol (CC), and resorcinol (RC) were investigated using cyclic voltammetry at the ZnO/graphene/Ta electrode. The results showed that it was feasible to simultaneously measure HQ, CC, and RC. The linear response ranges for HQ, CC, and RC were 0–70, 0–80, and 0–700 μmol L−1. The detection limits (S/N = 3) were 0.1, 0.2, and 1 μmol L−1, respectively. The ZnO/graphene layer shows high electrocatalytic activity toward the redox reaction of HQ, CC, and RC. This activity had high selectivity, high sensitivity and low detection limits.

Synthesis and characterization of MgO nanocrystals for biosensing applications

MgO nanocrystals were prepared using a simple direct current arc plasma jet chemical vapor deposition method. Magnesium nitrate was used as source material and Mo film was used as a substrate and catalyst. The high-temperature plasma produced ensured rapid synthesis of the MgO nanocrystals. The as-prepared nanocrystals were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, Fourier transform infrared spectrometry, ultraviolet–visible spectrophotometry, and photoluminescence measurements. The as-synthesized samples were found to consist of cubic MgO nanobelts and nanosheets with large surface areas and low coordination oxide ions, and contained numerous contacts, rough edges, vacancies, and doping defects. The nanostructures exhibited excellent electrochemical sensing properties with high-sensing sensitivity toward ascorbic acid. Their high electrocatalytic activity was attributed to the effect of defects and the surface electron transfer ability of the one-dimensional MgO nanobelts.

Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid

One-dimensional (1D) MgO nanostructures of various morphologies including tadpole-like nanobelts (tadpoles), nanobelts, and nanorods were synthesized via direct current (DC) arc plasma jet chemical vapor deposition (CVD). The effect of morphology on the biosensing properties of the nanostructures was investigated by comparing their electrochemical properties. Compared with tadpoles and nanorods, the MgO nanobelts had excellent electrocatalytic activity toward ascorbic acid (AA), dopamine (DA) and uric acid (UA). The response of the MgO nanobelts to the analytes was twice that of the tadpoles. A MgO nanobelt-modified electrode was thus fabricated for the simultaneous determination of AA, DA, and UA. The peak separations between AA and DA, DA and UA, and AA and UA for this electrode were 111, 161, and 272mV, respectively. The linear response ranges of the electrodes were 2.5–15 and 25–150μM for AA, 0.125–7.5μM for DA, and 0.5–3 and 5–30μM for UA. The calculated detection limits were 0.2, 0.05, and 0.04μM (S/N=3), respectively. The excellent electrocatalytic activity of the MgO nanobelts can be attributed to various surface defects such as low-coordination anions (O5C2− and O4C2− at the terrace and edge sites, and O3C2− at the corner and kink sites). Additionally, electron tunneling between these surface defects is possible. These defects have a strong adsorption capacity toward AA, DA, and UA. This affinity improves sensitivity and decreases the detection limits of the MgO nanobelt electrodes.