Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

Simultaneous growth of well-aligned diamond and graphitic carbon nanostructures through graphite etching

A hot filament chemical vapor deposition process based on hydrogen etching of graphite has been developed to synthesize diamond and graphitic carbon nanostructures. Well-aligned diamond and graphitic carbon nanostructured thin films have been synthesized simultaneously on differently pretreated silicon substrates in a pure hydrogen plasma. Graphitic nanocones, diamond nanocones and carbon nanotubes were selectively grown on uncoated, diamond and nickel pre-coated silicon substrates, respectively, in a single deposition process. The nanocones are solid cones with submicron scale roots and nanometer-size sharp tips. The nanotubes are hollow tubes with outer diameter of approximately 50 nm. The orientation of the well-aligned carbon nanostructures depends on the direction of the electric field at the samples surface. Nucleation and growth of diamond on the nanocones were further investigated under similar conditions without plasma. Diamond nanocomposite films have been obtained by depositing a nanocrystalline diamond film on the layer of diamond nanocones.     

Tribological properties and cutting performance of boron and silicon doped diamond films on Co-cemented tungsten carbide inserts

Boron and silicon doped diamond films are deposited on the cobalt cemented tungsten carbide (WC-Co) substrate by using a bias-enhanced hot filament chemical vapor deposition (HFCVD) apparatus. Acetone, hydrogen gas, trimethyl borate (C₃H₉BO₃) and tetraethoxysilane (C₈H₂₀O₄Si) are used as source materials. The tribological properties of boron-doped (B-doped), silicon-doped (Si-doped) diamond films are examined by using a ball-on-plate type rotating tribometer with silicon nitride ceramic as the counterpart in ambient air. To evaluate the cutting performance, comparative cutting tests are conducted using as-received WC-Co, undoped and doped diamond coated inserts, with high silicon aluminum alloy materials as the workpiece. Friction tests suggest that the Si-doped diamond films present the lowest friction coefficient and wear rate among all tested diamond films because of its diamond grain refinement effect. The B-doped diamond films exhibit a larger grain size and a rougher surface but a lower friction coefficient than that of undoped ones. The average friction coefficient of Si-doped, B-doped and undoped diamond films in stable regime is 0.143, 0.193 and 0.233, respectively. The cutting results demonstrate that boron doping can improve the wear resistance of diamond films and the adhesive strength of diamond films to the substrates. Si-doped diamond coated inserts show relatively poor cutting performance than undoped ones due to its thinner film thickness. B-doped and Si-doped diamond films may have tremendous potential for mechanical application.     

Synthesis of carbon nanotubes grown by hot filament plasma-enhanced chemical vapor deposition method

We prepared two kinds of catalytic layers onto n-typed silicon substrate—nickel by r.f.-magnetron sputtering and iron (III) nitrate metal oxide by spin coating. For iron (III) nitrate metal oxide 0.5 mol of ferric nitrate nonahydrate [Fe₂(NO₃)₃·9H₂O] ethanol solution was coated onto silicon by spin coater at different rotation speeds (rev./min). Carbon nanotubes were synthesized on both Ni and iron (III) nitrate metal oxide layers by the HFPECVD (hot filament plasma-enhanced chemical vapor deposition) method. We used ammonia (NH₃) and acetylene (C₂H₂) for the dilution gas and a carbon precursor for the growth of the carbon nanotubes, respectively. We could observe the relationship between the catalytic cluster density and the nanotube density with scanning electron microscopy (SEM) images. The density of carbon nanotubes on iron (III) nitrate metal oxide was controlled by the rev./min of the spin coater. Transmission electron microscopy (TEM) image shows multi-walled carbon nanotube where the catalyst was found in the tip of the carbon nanotube. Electron dispersive X-ray spectrometry (EDS) peaks for CNT's tip show that it was constituted with nickel and iron, respectively. Raman spectroscopy of nanotubes shows D-band and G-band peaks approximately 1370 and 1590 cm⁻¹.     

Nucleation of diamond over nanotube coated Si substrate using hot filament chemical vapor deposition (CVD) system

We studied the use of carbon nanotubes as a seeding layer for the nucleation of diamond on Si (100) substrate by using a hot filament chemical vapor deposition (HFCVD) system. Prior to deposition, substrates were seeded with multi-wall carbon nanotube (MWCNT) powder which was prepared separately. MWCNTs were used as nucleation precursors. The diamond grains grew essentially over the nanotubes with a higher growth density in comparison with the un-seeded substrates. The scanning electron microscopy (SEM) image of surface morphology shows crystallites of cauliflower shaped grains. The micro Raman spectroscopic results show a sharp peak at 1,332 cm^-1 corresponding to diamond phase. X-ray photoelectron spectroscopic study show the presence of carbon (C_1s) phase. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

Carbon nanotubes coated with diamond nanocrystals and silicon carbide by hot-filament chemical vapor deposition below 200 °C substrate temperature

Multi-walled carbon nanotubes (MWCNTs) dispersed onto a silicon substrate have been coated with diamond nanocrystals (DNC) and silicon carbide (SiC) from solid carbon and silicon sources exposed to H₂ activated by hot filament chemical vapor deposition (HFCVD) at around 190 °C substrate temperature. MWCNT coating by DNC initiates during filament carburization process at 80 °C substrate temperature under conventional HFCVD conditions. The hybrid nanocarbon material was analyzed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. The structure of the MWCNTs is preserved during coating and the smooth DNC/SiC coating is highly conformal. The average grain size is below 10 nm. The growth mechanism of DNC and SiC onto MWCNT surface is discussed.     

Friction force microscopy study of diamond films modified by a glow discharge treatment

A glow discharge treatment technique has been developed which enables control of the surface roughness and morphology of diamond films for applications in optical and electrical components. A conventional hot filament chemical vapour deposition (CVD) system was used to deposit the diamond films onto silicon substrates via a three-step sequential process: (i) deposition under normal conditions; (ii) exposure to either a pure hydrogen plasma or 3% methane in an excess of hydrogen using DC-bias; and (iii) diamond deposition for a further 2 h under standard conditions. The frictional characteristics and roughness of the film surfaces were investigated by atomic force microscopy (AFM) and the morphology and the growth rates determined from scanning electron microscope images. Lateral force microscopy (LFM) has revealed significant differences in frictional behaviour between the high quality diamond films and those modified by a glow discharge treatment. Friction forces on the diamond films were very low, with coefficients ∼0.01 against silicon nitride probe tips in air. However, friction forces and coefficients were significantly greater on the DC-biased films indicating the presence of a mechanically weaker material such as an amorphous carbon layer. A combination of growth rate and frictional data indicated that the exposure to the H₂ plasma etched the diamond surface whereas exposure to CH₄/H₂ plasma resulted in film growth. Re-Nucleation of diamond was possible (stage iii) after exposure to either plasma treatment. The resultant friction forces on these films were as low as on the standard diamond film.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Effect of substrate temperature on the selective deposition of diamond films

Selective diamond films on roughened Si(100) substrates with patternings have been achieved by microwave plasma chemical vapor deposition (MP-CVD). The films have been characterized by scanning electron microscopy (SEM) and Raman spectra. The influence of substrate temperature on the selective deposition of diamond films has been discussed in detail: the diamond nucleation density on the SiO₂ mask increased with substrate temperature while the effect of the selective deposition of diamond films deteriorated; the optimized deposition temperature conditions have been concluded.     

Optimizing catalyst nanoparticle distribution to produce densely-packed carbon nanotube growth

The experimental parameters involved in the formation of the Ni catalytic nanoparticles on Si/SiO₂ substrates that seed carbon nanotube growth were investigated. It was found that after deposition of a nickel film on the substrate, the temperature and time of the thermal and reduction catalyst pre-treatment steps are crucial variables for optimized nanoparticle distribution with different average diameters, depending on the initial film thickness. Densely-packed carbon nanotube forests with interesting potential applications have been grown from this nanoparticle distribution.     

Synthesis and characterization of freestanding diamond/carbon nanoflake hybrid films

Freestanding diamond/carbon nanoflake hybrid films have been synthesized by generating variable gas chemistries near a microstructured substrate in a conventional diamond deposition reactor. A multi-cathode direct current plasma enhanced CVD reactor, designed to deposit thick diamond wafers in its conventional configuration, has been used. The deposition conditions are identical to that for diamond films (10%CH₄ in H₂ gas, 100 Torr, 700–800 °C) except for a novel substrate. A sacrificial layer of silica micro-spheres (diameter 10–30 μm) was close-packed through gentle agitation of the spheres on a 100 mm diameter molybdenum disc. The growth of diamond and graphitic phases was observed on the front and back surfaces of the micro-spheres respectively. TEM observation confirmed that they were chemically bonded hybrid films without any distinguishable interlayer between the two phases.     

Enhancement of (100) texture in diamond films grown using a temperature gradient

Diamond films with dominant (100) texture were grown with a temperature gradient across the Si (100) substrates using hot filament chemical vapor deposition technique. Deposition was carried out with 0.8% CH₄ in balance hydrogen at an average substrate temperature of 880 °C. The deposition pressure was varied between 20–120 torr. Films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). XRD shows very strong (400) reflection in all the samples. SEM results show a smooth diamond surface comprised of (100) platelets. As the (100) diamond plates were grown on top of the (100) oriented silicon substrate the faces are more or less aligned parallel with the substrate surface, resulting in a relatively smoother diamond surfaces. FTIR studies show novel features in the films. Quantitative analysis was carried out to measure the H content in the films.     

Investigation of nucleation centres in diamond chemical vapour deposition with spatially resolved X-ray photoelectron spectroscopy

The nucleation of diamond films on silicon substrates deposited by hot filament chemical vapour deposition has been investigated for the first time with an X-ray photoelectron spectroscopy microscope. The distribution of carbon, oxygen, silicon and tungsten on the surface was imaged with a spatial resolution of 10 μm. Significant differences were found between untreated areas of the substrates and scratches created by a diamond tip. Our data reveal that material from the tip abraded in the scratching process is not of key importance for nucleation.     

Graphitized filament plasma enhanced CVD deposition of nanocrystalline diamond

A novel approach to the deposition of polycrystalline diamond is presented. The technique is based on the hot filament chemical vapour deposition technique (HFCVD). While it is similar to a high plasma power “bias enhanced growth” HFCVD, it relies on a graphite filament rather than on a metal one. It was found that with an appropriate choice of the growth parameters, 4–9% CH₄ in H₂, filament temperature > 2200 °C, 25 mBar gas pressure, plasma power > 500 W, a long filament lifetime can be achieved, when a simultaneous deposition of graphitic carbon on the hot graphite filament and of nanocrystalline diamond on a substrate facing the filament assembly is realized. In this paper the growth of nanocrystalline diamond films and their characterization (SEM, XRD, AFM) are presented. While the technique is promising for low cost, large area deposition of nanocrystalline diamond films, also the growth of microcrystalline diamond has been observed.     

Stress study of HFCVD boron-doped diamond films by X-ray diffraction measurements

Stress analysis on chemical vapor deposition (CVD) diamond films has demonstrated an apparent disagreement among various researchers in recent works even for similar deposition conditions. The type and the value of stress have shown a strong dependence on film thickness, which can be attributed to columnar growth and grain size and boundaries. X-Ray diffraction techniques appeared to be more suitable to study these effects and permit the evaluation of the average stress in larger sample areas when compared with micro-Raman spectroscopy, which feels a local strain inside the grains. In the case of boron-doped diamond films, boron incorporation on substitucional or interstitial sites can produce stresses according to the doping level. In order to investigate these effects, a series of diamond films were deposited on silicon (001) substrate in a hot filament (HF)-assisted CVD reactor at 800°C. The CH₄ flow is kept at 0.5 sccm for all experiments and the H₂ and B₂O₃/CH₃OH/H₂ flows are controlled in order to obtain the desired B/C ratios. Stress behavior in HFCVD boron-doped diamond films has been investigated by X-ray diffraction measurements using the sin² ψ technique. Tensile and compressive stresses have been observed and the thermal and intrinsic components have been calculated. The diamond films were characterized by scanning electron microscopy and Raman spectroscopy.     

Low-temperature deposition of optically transparent diamond using a low-pressure flat flame

The radial uniformity and scaleable nature of flat flames make them an attractive technique for diamond deposition. Due to the high temperatures involved in combustion synthesis, typically molybdenum and silicon have been used as substrates. Here we report low-temperature diamond deposition on glass substrates. Diamond deposition was achieved on ordinary sodium silicate glass at substrate temperatures of 500°C; however, film delamination occurred during cooling after deposition. Vycor™ and Pyrex™ are two glasses that have thermal expansion coefficients that are similar to diamond. Continuous, optically transparent films were successfully deposited on both glasses. The diamond films have been characterized by scanning electron microscopy, Raman spectroscopy and secondary ion mass spectroscopy (SIMS). The dependence of hydrogen and sp²-bonded carbon incorporation in the films on reactant composition was quantified. These films were optically transparent and showed good adhesion as measured by a simple tape test.     

Selective deposition of diamond films on insulators by selective seeding with a double-layer mask

Polycrystalline diamond films have been patterned on Si₃N₄/Si and SiO₂/Si substrates by selective seeding with a double-layer mask via hot-filament chemical vapor deposition. High quality in the patterned diamond films and high selectivity were obtained by the process. The diamond films deposited on the insulators at different CH₄/H₂ concentrations were studied by scanning electron microscopy and Raman spectroscopy. The process proved to be far less damaging to the substrates, and yet effective in developing patterns of diamond films on a large and different substrate.     

The crystallographic relationship of heteroepitaxial diamond films on scratched silicon substrates

By using the straight hot filament chemical vapor deposition method with one falt horizonatal filament, diamond films were rapidly grown on a scratched silicon substrate. Observing two kinds of interface structures of the samples by cross-section high-resolution transmission electron microscopy, we found that diamond {111}-oriented films are epitaxially grown on β-SiC{111} planes with a tilt angle of about 7° around the common [110] axis. We also found that diamond771 planes are parallel to silicon111 planes on which diamonds are directly epitaxially grown on silicon substrate. The interface dislocations are of either 60°-type or Schockley partial dislocation in relation to our observations.