Carbon nanotube growth at 420 °C using nickel/carbon composite thin films as catalyst supports

Nickel/carbon composite (Ni/C) thin films were used as catalyst supports for the growth of vertically aligned multiwalled carbon nanotubes (MWCNTs) at temperature as low as 420 °C. Nickel nanoparticles embedded within the carbon matrix of Ni/C films have served as catalysts for the synthesis of nanotubes by PECVD using acetylene/ammonia plasma. Two different nickel contents (40 at.% and 60 at.%) in the films were used. Analysis indicated a diffusion of nickel atoms in the form of nanoparticles to the film surface upon annealing. This diffusion depends on both annealing temperature and nickel concentration in the films and affects the MWCNT growth at low temperature. The MWCNT synthesis was tested at growth temperature ranging between 335 and 520 °C. The growth of MWCNTs at 420 °C was only achieved by using Ni/C films with a high nickel content (60 at.%). These MWCNTs did not present considerable loss in their growth rate and structural quality compared to MWCNTs grown on classical substrates (Ni catalysts deposited on TiN), at higher temperature (520–600 °C). The results suggest that carbon saturation at the surface and subsurface of nickel catalysts of the Ni/C films is responsible for the improvement of MWCNT growth at low temperature.     

Diffusion coefficient of carbon in Fe–Ni alloy during synthesis of diamond under high temperature and high pressure

Synthetic diamond particles were prepared under high temperature and high pressure using arrayed seeds. A dense Fe–Ni alloy shell covered each diamond seed during synthesis; the growth of diamond particles was controlled by the diffusion of carbon through the metallic shell. The diffusion coefficient of carbon through Fe–Ni melt at 1600 K and 5.5 GPa is about 5×10^?6 cm²/s, with an activation energy for diffusion of 336 kJ/mol.     

High coercivity magnetic multi-wall carbon nanotubes for low-dimensional high-density magnetic recording media

Fe-embedded multi-wall carbon nanotubes (MWCNTs) were fabricated using Fe-catalyst by the chemical deposition method. Microscopic characterizations showed that the well-aligned MWCNTs were ～ 80 mm in length, with outer diameter of 20–50 nm and inner diameter of 10–20 nm. Magnetic properties were characterized in temperatures of 5 K and 305 K, which revealed that the MWCNTs exhibited high coercivity of 2600 Oe at 5 K and 732 Oe at 305 K. These values are much higher than that of bulk iron (～ 0.9 Oe) and Fe/Co/Ni nanoparticles or nano-wire arrays (～ 200–500 Oe) at the room temperature. This high coercivity and the structure of single-domain Fe nanoparticles isolated by anti-ferromagnetic MWCNTs make it a promising candidate for low-dimensional high-density magnetic recording media.     

Nanocrystalline diamond growth in a surface-wave plasma

A surface-wave excited plasma is exploited in a diamond growth process by microwave plasma chemical vapor deposition method. Nanocrystalline diamond films with smooth surfaces are obtained from the plasma. As well as characterizing the deposited diamond films, the electron density and the electron temperature of the plasma are determined by using double-probe measurements. The plasma diagnosis reveals low electron temperatures of 2–3 eV in the process region, which is a distinctive characteristic of the surface-wave plasma. The low electron temperature is essential for the continuous re-nucleation of diamond in a hydrogen-rich plasma during the nanocrystalline diamond growth for a wide range of substrate temperature from under 100 to over 700 ^°C.     

Diamond synthesis by plasma chemical vapor deposition in liquid and gas

The characteristics of diamond synthesis by 2.45 GHz microwave plasma chemical vapor deposition (CVD) under pressures greater than atmospheric pressure were investigated. The deposits on Si substrates were identified by scanning electron microscopy and Raman spectroscopy. The growth rate of diamond was found to be 250 μm/h at 300 kPa, which is ten times greater than that of the conventional low-pressure CVD method. In order to make high-speed deposition of diamond effective, the diamond growth rates for gas-phase microwave plasma CVD were compared to those from the in-liquid plasma CVD method. The growth rate was found to increase as system pressure increased, displaying the same tendency of that in-liquid plasma CVD. The amounts of input microwave energy per unit volume of diamond in the gas-phase and in-liquid plasma CVD methods were also compared. The amount of input microwave energy per unit volume of diamond was found to be 0.6 to 1 kWh/mm³.     

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO₂ phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.     

Synthesis of carbon nanofibers with a polygonal cross section by the catalytic decomposition of C2H2 over bi-metal catalysts

Carbon nanofibers with a polygonal cross section were synthesized using catalytic chemical vapor deposition over Fe–Sn, Ni–Sn or Co–Sn catalysts. Their morphologies were characterized by scanning and transmission electron microscopy. Edges connecting of two walls can be clearly observed and some adjacent walls have a V-shape. The lengths of the sides of the polygonal cross sections range from 150 to 500 nm over Fe–Sn or Co–Sn nanoparticles, increasing to 700–900 nm over Ni–Sn nanoparticles. Polygonal catalyst particles can be seen at the ends of the fibers.     

Carbon isotope fractionation associated with HPHT crystallization of diamond

We have studied the isotope properties of carbon phases including carbon source, diamond, carbon dissolved in metal melt and the gas phase formed at diamond crystallization experiment in the closed system Fe–Ni–C at 1450 °С and 5.5 GPa during 17.5 h. The δ¹³C value of the grown diamond decreases in direction of growth from − 26.5 to − 27.1‰, when the δ¹³C value of initial carbon is − 27.1‰. Carbon isotope composition of the melt is by 3.2‰ lower than that of coexisting diamond. The data obtained agree well with the model calculations of isotope fractionation on crystallization from solution in a closed system.     

Epitaxial nucleation model for chiral-selective growth of single-walled carbon nanotubes on bimetallic catalyst surfaces

An epitaxial nucleation model for single-walled carbon nanotube (SWCNT) growth on bimetallic catalysts surfaces is reported in support of experimental observations of chiral enrichment. We model the bimetallic catalyst surfaces as a 2D (1 1 1) surface consisting of Ni or a combination of Ni and Fe atoms, with varying average bond length between nearest neighbor atoms which corresponds to the crystal structure of the alloys. The energies associated with nanotube cap formation on these various surfaces are calculated using density functional theory (DFT). We find that certain cap chiralities, such as (8, 4), are more stably bound to a surface that resembles a Ni_0.27Fe_0.73 bimetallic catalyst, whereas other chiralities, such as (9, 4), are more stable on a pure Ni surface. These results help explain the predominance of certain chiralities on specific bimetallic catalysts and provide a potential route to controlling the chirality of as-grown SWCNTs.     

Onion-like carbon-encapsulated Co,Ni, and Fe magnetic nanoparticles with low cytotoxicity synthesized by a pulsed plasma in a liquid

We synthesized onion-like carbon-encapsulated Co, Ni, and Fe (Co–C, Ni–C, and Fe–C) magnetic nanoparticles with low cytotoxicity using pulsed plasma in a liquid. The pulsed plasma is induced by a low-voltage spark discharge submerged in a dielectric liquid. The face-centered cubic Co and Ni, and body-centered cubic Fe core nanoparticles showed good crystalline structures with an average size between 20 and 30 nm were encapsulated in onion-like carbon coatings with a thickness of 2–10 nm. Vibrating-sample magnetometer measurements revealed the ferromagnetic properties of as-synthesized samples at room temperature (Co–C = 360 Oe, Fe–C = 380 Oe, and Ni–C = 211 Oe). Raman-spectroscopy analysis found onion-like carbon shells composed of well-organized graphitic structures. Thermal gravimetric analysis showed a high stability of the as-synthesized samples under thermal treatment and oxidation. Cytotoxicity measurements showed higher cancer cell viability than samples synthesized by different methods.     

Aligned diamond nano-wires: Fabrication and characterisation for advanced applications in bio- and electrochemistry

Nano-wires have become promising tools in a vast field of applications. Due to the many unique properties of diamond, the use of diamond nano-wires in biosensors attracts increasing attention. In this paper we introduce the realisation of wires from diamond using self-aligned nickel nano-particles as etching mask in an oxygen ICP dry etching step. With this process it is possible to create wires of high aspect ratios of 50, with diameters as small as 20 nm, and typical lengths of up to 1 μm on a large area in a dense pattern of about 10¹¹ cm^? 2. The Ni nano-particles are formed by thermal annealing at 700 °C for 5 min of a thin (1 nm) Ni film that is deposited onto the diamond surface. The surface enhancement factor due to wires is dependent on the geometrical details of wires and was measured to be 10 to 80. The electrochemical properties of wires have been characterized by cyclic voltammetry using Fe(CN)₆^? 3/? 4 which shows that such topographies act as filter for redox molecules.     

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) substrates using antenna-edge-type microwave plasma assisted chemical vapor deposition

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) was investigated by scanning electron microscopy, reflection high-energy electron diffraction (RHEED) and atomic force microscopy. Bias-enhanced nucleation (BEN) was performed by antenna-edge-type microwave plasma assisted chemical vapor deposition. In BEN, diamond crystallites nucleated and grew along the [−1 1 0] and [1 1 0] directions of iridium. Diamond was likely to nucleate on protruded iridium areas. After BEN, in addition to the diamond diffraction spots, iridium bulk diffraction spots, which were not observed before BEN, were observed by RHEED. The iridium surface appeared to be protruded and changed by the high ion current density in BEN. Under [0 0 1] selective growth conditions, diamond crystallites, which were less than 10 nm in diameter, were etched by H₂ plasma. Diamond nucleated areas corresponded to the surface ridges of iridium along the [−1 1 0] and [1 1 0] directions at 10–40 nm intervals before BEN.     

Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

One-step synthesis of a graphene-carbon nanotube hybrid decorated by magnetic nanoparticles

Graphene-carbon nanotube (G-CNT) hybrids were synthesized by a one-step chemical vapor deposition process using a mixed catalyst of MgO and Fe/MgO. MgO layers acted as templates for the growth of graphene, and Fe particles on the MgO layers catalyzed the growth of single or double-walled CNTs. The G-CNT hybrids had porous structures with hierarchical pore distributions due to the composition of graphene with CNT network. Superparamagnetism with a saturation magnetization of 2.7 emu/g was found in the G-CNT hybrids due to the existence of Fe₃C nanoparticles of size ～3 nm. The graphene to CNT ratio was conveniently changed by varying the MgO to Fe/MgO ratio, as characterized by Raman analysis and specific surface area measurements. Furthermore, a simplified synthesis of G-CNT hybrids was demonstrated by using MgO supported Fe or Ni catalysts with a low metal concentration.     

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.     

Increased CNT growth density with an additional thin Ni layer on the Fe/Al catalyst film

Density of 3 × 10¹¹/cm² and diameter of CNTs of 9–12 nm were successfully controlled by using the multi-layered catalyst film consisting of an additional Ni layer on Fe/Al catalyst film. EDS analysis for the annealed catalyst films revealed that the increase of the density of Fe catalyst particles corresponded with the decrease of Ni in the films, which strongly suggested that the additional thin Ni layer on the Fe/Al multi-layered catalyst films prevented the fine Fe catalyst particles from agglomeration, resulting in the growth of high-density, and uniform diameter of CNTs.     

Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature

Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature (HPHT) was observed with graphite as carbon source. The thickness of the electroplating nickel film which acts as a catalyst/solvent ranges from 54.6 μm to 255.6 μm. The relationship between the Ni film thickness and diamond growth rate is investigated. When the nickel film thickness is from 90 μm to 129 μm, diamond crystals can nearly grow up to three times as large as the original seeds at ∼ 5.8 GPa and ∼ 1460 °C within 14 min. The mechanism of the crystal growth with nickel-plated diamond seeds under HPHT is discussed. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth.     

Diamond growth on Ir/CaF2/Si substrates

A new multi-layered structure of heteroepitaxial (1 0 0) and (1 1 1) Ir grown on CaF₂-buffered (0 0 1) and (1 1 1) Si wafers by UHV electron-beam evaporation was prepared for the deposition of diamond films. A two-step process of bias-enhanced nucleation and a subsequent growth by controlling the α growth parameter was performed to deposit (0 0 1) and (1 1 1) diamond films by chemical vapor deposition, respectively. Scratching or seeding by fine diamond powders was also attempted on the (1 1 1) substrates to enhance the diamond nucleation density. Raman spectroscopy, X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy were used to characterize the Ir/CaF₂/Si substrates as well as the diamond films grown on top of iridium layer. Heteroepitaxial relationship between the deposited diamond grains and (0 0 1) substrates has been observed.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Synthesis of hollow carbon nano-onions and their use for electrochemical hydrogen storage

In this study, we report an efficient method for synthesis of well-graphitized hollow carbon nano-onions (CNOs). CNOs were firstly fabricated by chemical vapor deposition (CVD) method at 850 °C using an Fe–Ni alloy catalyst with diameters of 10–15 nm. Then hollow CNOs were obtained by annealing as-prepared CNOs at 1100 °C for 3 h. It is found that during the CVD growth, the presence of nickel retards the deactivation of Fe–Ni–C austenite, providing the possibility for the growth of up to two hollow CNOs from each alloy particle. The subsequent high-temperature annealing led to the escaping of the Fe–Ni alloy from the graphitic layers, and the re-catalysis of precipitation and graphitization of the carbon atoms previously dissolved in the alloy particle (Fe_0.64Ni_0.36) to form hollow CNOs. The hollow CNOs exhibit good performance as materials for electrochemical hydrogen storage, with a discharge capacity of 481.6 mAh/g under a current density of 500 mA/g, corresponding to a hydrogen storage capacity of 1.76 wt.%. Our results demonstrate that the hollow CNOs are promising materials as a storage medium for hydrogen as a fuel source.