Nanocrystalline Diamond Formation Research Articles

Fast, Efficient Tailoring Growth of Nanocrystalline Diamond Films by Fine-Tuning of Gas-Phase Composition Using Microwave Plasma Chemical Vapor Deposition.

Nanocrystalline diamond (NCD) films are attractive for many applications due to their smooth surfaces while holding the properties of diamond. However, their growth rate is generally low using common Ar/CH4 with or without H2 chemistry and strongly dependent on the overall growth conditions using microwave plasma chemical vapor deposition (MPCVD). In this work, incorporating a small amount of N2 and O2 additives into CH4/H2 chemistry offered a much higher growth rate of NCD films, which is promising for some applications. Several novel series of experiments were designed and conducted to tailor the growth features of NCD films by fine-tuning of the gas-phase compositions with different amounts of nitrogen and oxygen addition into CH4/H2 gas mixtures. The influence of growth parameters, such as the absolute amount and their relative ratios of O2 and N2 additives; substrate temperature, which was adjusted by two ways and inferred by simulation; and microwave power on NCD formation, was investigated. Short and long deposition runs were carried out to study surface structural evolution with time under identical growth conditions. The morphology, crystalline and optical quality, orientation, and texture of the NCD samples were characterized and analyzed. A variety of NCD films of high average growth rates ranging from 2.1 μm/h up to 6.7 μm/h were successfully achieved by slightly adjusting the O2/CH4 amounts from 6.25% to 18.75%, while that of N2 was kept constant. The results clearly show that the beneficial use of fine-tuning of gas-phase compositions offers a simple and effective way to tailor the growth characteristics and physical properties of NCD films for optimizing the growth conditions to envisage some specific applications.

Shear-promoted graphite-to-diamond phase transition at the grain boundary of nanocrystalline graphite

High pressure has traditionally been considered essential for the transformation of graphite into diamond. However, reducing the transition pressure required for this graphite-to-diamond (G2D) conversion holds significant appeal in both scientific research and engineering applications. In this study, we conducted large-scale molecular dynamics (MD) simulations using an environment-dependent interaction potential (EDIP) to examine the shear deformation of nanocrystalline graphite (n-graphite) with a grain size of approximately 6.5 nm. We discovered that the G2D transition pressure in n-graphite can be reduced to 2–3 GPa, significantly lower than the ∼90 GPa uniaxial stress required in crystalline graphite. This reduction is primarily due to concentrated local shear stresses at grain boundaries (GBs), which induce substantial rotations of graphite layers. These rotations facilitate the initial formation of diamond bonds at sites of pre-existing imperfections at the GBs, assisted by shear. Once initiated at the GBs, the G2D transition rapidly propagates within grains aligned parallel to the shear components, resulting in the formation of nanocrystalline diamond. Our findings underscore the critical roles of GBs and shear stress in enabling the G2D transition in n-graphite.

Investigation of Room Temperature Formation of the Ultra-Hard Nanocarbons Diamond and Lonsdaleite.

Diamond is an attractive material due to its extreme hardness, high thermal conductivity, quantum optical, and biomedical applications. There is still much that is not understood about how diamonds form, particularly at room temperature and without catalysts. In this work, a new route for the formation of nanocrystalline diamond and the diamond-like phase lonsdaleite is presented. Both diamond phases are found to form together within bands with a core-shell structure following the high pressure treatment of a glassy carbon precursor at room temperature. The crystallographic arrangements of the diamond phases revealed that shear is the driving force for their formation and growth. This study gives new understanding of how shear can lead to crystallization in materials and helps elucidate how diamonds can form on Earth, in meteorite impacts and on other planets. Finally, the new shear induced formation mechanism works at room temperature, a key finding that may enable diamond and other technically important nanomaterials to be synthesized more readily.

Estimation of Requirements for the Formation of Nanocrystalline Diamond Driven by Electron Beam Ablation

Conditions for the formation of thin films of nanocrystalline diamond by electron-beam ablation of a graphite target are studied. The analysis is based on the predictions of the analytical solutions of two models describing the expansion of the plasma plume into a background gas and toward a substrate. The models allow the calculation of the pressure and temperature of ablated nanoparticles upon impact with the substrate where the film is deposited. The calculation data are reported on the phase diagram of carbon and are used to assess conditions under which the diamond phase is likely to form on the substrate. The results show that decreasing gas pressure and target to substrate distance, over the practical range of accelerating voltage, is conducive to diamond formation.

The effect of nitrogen addition to Ar/CH4 gas mixture on microstructural characterization of nanocrystalline diamond

Abstract In the present study, nanocrystalline diamond (NCD) nanowire films were synthesized on silicon substrates, using microwave plasma-enhanced chemical vapor deposition (PECVD) with a CH4/Ar/N2 gas mixture at moderate temperatures. The influence of nitrogen concentration on the formation of NCD was investigated. The characteristics of NCD films were evaluated using scanning electron microscopy (SEM), Raman spectroscopy, optical emission spectroscopy (OES), and a contact angle meter. NCD nanowire films with 300–500 nm length were grown with the incorporation of nitrogen. Heterostructures of sp3-bonded diamond nanowires and sp2-bonded graphite were synthesized by adding small amounts of nitrogen to the CH4/Ar gas mixture. Surface roughness became smooth and the grain size decreased as the nitrogen was introduced into the CH4/Ar gas mixture. With the increase of nitrogen concentration, the sp2/sp3 ratio of carbon bonds increased. The wettability of the NCD nanowire films was sensitive to the bonding structure. The hydrophobic and non-reactive properties of NCD nanowire films make them highly applicable for biomedical implants.

Nanocrystalline Diamond Matrix Deposited on Copper Substrate by Radical Species Restricted Diffusion

Nanocrystalline diamond matrix (or patterned nanocrystalline diamond) have been grown by hot filament chemical vapor deposition (HFCVD) on copper substrates, which were masked by a copper template filled with through-holes. The influence of the mixing ratio for CH4/H2 source gases, total gas pressure and the aspect ratio (the ratio of hole depth to its diameter) on the morphology, grain size and quality of diamond films were investigated. Continuous diamond films were obtained under 2.0 kpa. When increasing the aspect ratio from 0.67 to 2.0, a gradual reduction of diamond grain size from micrometer to nanometers scale was observed. The formation of nanocrystalline diamond (NCD) matrix can be attributed to the restricted diffusion of radical species and the diamond nucleation kinetics on copper substrates. By through-holes of templates on copper substrates to restrict the diffusion and transport of radical species, NCD matrix was successfully deposited on copper substrates.

One-step CVD-diamond coating process on 3-D titanium substrates using reticulated vitreous carbon as a solid carbon source

Nanocrystalline diamond (NCD), formed on three-dimensional (3D) titanium (Ti) substrates, through the etching of reticulated vitreous carbon (RVC) was investigated. Porous Ti was prepared by powder metallurgy and the RVC was produced at 1300 and 2000 °C graphitization index. In this chemical vapor infiltration/deposition process, the RVC sample was used as the only carbon source that ensured the production of pertinent growth species directly on the Ti surfaces including its inner and bottom (the opposite side of the sample). The films were deposited at 630 °C substrate temperature. NCD scanning electron microscopy images showed agglomerates of nanometer crystallites with a uniform surface texture covering all sample. Raman measurements showed the typical two shoulders at 1150 and 1490 cm − 1 attributed to NCD formation. Electrochemical response by cyclic voltammetry measurements confirmed a wide potential window for such electrodes in addition to its exceptional reversibility response.

Formation of Nanocrystalline Diamond in Polymer Like Carbon Films Deposited by Plasma CVD

Conventional plasma enhanced chemical vapour deposition (PECVD) method is generally not suitable for the growth of nanocrystalline diamond (NCD) films. However, our study shows that conditions favourable for powder formation help to grow large amount of nanocrystallites in conventional PECVD. With CH4 as the carbon source gas, dilution with Ar and moderate (50 W) rf power enhances formations of powders (nanoparticles) and C2 dimers within the plasma. On the other hand, with pure CH4 or with hydrogen diluted CH4, powder formation as also NCD growth is hindered. It is proposed that the nanoparticles formed in the plasma act as the "islands" while the C2 dimers are the "seeds" for the NCD growth. The structure of the films deposited on the grounded anode under different conditions of dilution has been studied. It is observed that with high Ar dilution the films contain NCD embedded in polymer like carbon (PLC) matrix.

The effect of HCl and NaOH treatment on structural transformations in a ball-milled anthracite after thermal and chemical processing

Nanocrystalline diamond (NCD) was observed after reactive ball milling of anthracite coal with cyclohexene, a high-temperature (1400 °C) thermal anneal, and a 4 M HCl treatment followed by a 10 M NaOH treatment. A crystalline carbon region was also observed when the thermal anneal was omitted. This crystalline region is highly unstable and converts to NCD and carbon onions via electron irradiation in the TEM. X-ray diffraction, Raman, ash content, and temperature-programmed oxidation (TPO) data suggest that tetrahedral amorphous carbon is formed during milling, iron carbides are formed during the thermal anneal step, and both the HCl and NaOH purification steps lead to changes in carbon structure. NaOH oxidizes metal carbides and this process may contribute to NCD formation.

Effects of gas flow rate on diamond deposition in a microwave plasma reactor

Diamond thin films have been deposited on mirror-polished silicon substrates in a microwave plasma chemical vapor deposition reactor under fixed pressure but different flow rates of the mixed methane and hydrogen gas supply. The films synthesized were characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction. Microcrystalline diamond (MCD) thin films were obtained at relatively low gas flow rates (30 sccm to 300 sccm), while nanocrystalline diamond (NCD) thin films with cauliflower-like morphology were obtained at higher gas flow rates (above 300 sccm). The formation of NCD at the high flow rates may be attributed to the enhancement of diamond secondary nucleation arising from the increase in the flux rate of carbon containing radicals reaching the diamond growth surface. Measurements of field electron emission from the diamond films have shown that the NCD films exhibit lower field emission thresholds and higher current density compared with the MCD films. Furthermore, the surface roughness of the NCD thin films grown at high gas flow rate is effectively reduced by applying positive biasing on the diamond growth stage.

In vacuo substrate pretreatments for enhancing nanodiamond formation in electron cyclotron resonance plasma

Substrate pretreatment conditions at low pressures have been examined for enhancing nanocrystalline diamond formation on silicon in electron cyclotron resonance (ECR) plasma. Three kinds of pretreatments, (I) exposure to an ECR H2 plasma with application of a substrate bias from −100to+30V, (II) hot-filament heating in H2 gas, and (III) hot-filament heating in vacuum, were used alone or followed by carburization prior to a two-step process of ion-enhanced nucleation in an ECR plasma and subsequent growth in a hot-filament system. The number density of diamond particles after the final growth step was greatly increased up to the order of 107–108cm−2 when applying pretreatment (I) at the bias of 0V corresponding to the ion-bombardment energy of around 10eV. In this treatment, a clean and smooth surface with minimal damage was made by the dominance of anisotropic etching by hydrogen ions over isotropic etching by hydrogen atoms. The number density of diamond particles was still more increased when applying pretreatment (II), but the treated surface was unfavorably contaminated and roughened.

Micro and nanocrystalline diamond formation on reticulated vitreous carbon substrate

High diamond nucleation and a three-dimensional growth on reticulated vitreous carbon substrate are obtained by chemical vapor deposition. Scanning electron microscopy images show continuous films covering the whole substrate including the center of 3.5 mm thick porous samples. It is evident the nanocrystalline diamond (NCD) film formation on deeper substrate regions. The grain size can vary from nano to micro scale for deposition time of 20 h. Raman spectra of sample regions closer to filaments exhibit well-defined diamond line. For central regions of sample (depth between 1.0 and 2.0 mm) Raman spectra also confirm NCD film.

Nanocrystalline diamond formation by using an inductively coupled radio-frequency CH 4/H 2/Ar plasma

Effect of gas-mixing ratio on the formation of nanocrystalline diamond has been investigated in a low electron temperature CH 4/H 2/Ar plasma produced by inductively coupled rf discharge. The gas-mixing ratio is controlled by introducing an orifice between the plasma production region and the deposition region. When the diameter of the orifice is 6 mm, the Raman spectrum indicating a deposition of nanocrystalline diamond is observed when the gas-mixing ratio is less than ∼0.03. The reduction of CH 4 gas-mixing ratio is important for the formation of nanocrystalline diamonds.

Measurement of C 2 radical density in microwave methane/hydrogen plasma used for nanocrystalline diamond film formation

C 2 radical density in a conventional microwave plasma-enhanced chemical vapor deposition (MWPCVD) with CH 4/H 2 mixture was measured using absorption spectroscopy. At the typical growth conditions for MWPCVD used for nanocrystalline diamond formation, C 2 radical density in the plasma was of the order of 10 12 cm −3. Correlation between nanocrystalline diamond growth and C 2 radical density was discussed.

Combined effects of argon addition and substrate bias on the formation of nanocrystalline diamond films by chemical vapor deposition

The article reports combined effects of Ar addition and substrate bias on the grain size, microstructure, and growth rate of the diamond films prepared in microwave plasma-enhanced chemical vapor deposition. The nanocrystalline diamond (NCD) films with grain size of 50–100 nm, characterized by Raman spectra, scanning and transmission electron microscopy, were produced at 90–99 vol % Ar concentration under −50 V substrate biasing. The growth rate of the NCD films was 0.7–0.8 μm h−1, larger apparently than those grown by only Ar addition or by substrate bias effect alone. The NCD formation by various mechanisms is discussed, and a revised C2 insertion mechanism by the promotion of H+ ions is proposed to interpret the higher growth rate of the NCD films.

Nanocrystalline diamond formation during argon ion irradiation of graphite

In the present work, diamond nanoparticles with sizes of ∼2–70 nm have been produced by Ar+ ion irradiation at the fluence of ∼1022 cm−2. With the help of transmission electron microscope, electron diffraction, and Raman spectroscopy, these nanoparticles’ shape and size were determined and their crystal structure was also solved. From these results, our discussion, based on the viewpoint of beam-solid interaction, may be critical for understanding the growth process of nanocrystalline diamonds.

Carbon diffusion and nanocrystalline diamond formation in carbon ion-implanted oxides studied by nuclear elastic scattering

We have shown that MeV implantation of carbon into fused quartz and sapphire followed by thermal annealing in a suitable environment can result in the formation of diamond. Using cross-sectional transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS), we determined (in a previous paper) that, following annealing, there was a redistribution of carbon from the original implantation depth, depending on the annealing environment, annealing time and annealing temperature. In our search, for the optimum implantation and annealing parameters to maximize the yield of diamond, we have used backscattering spectrometry (BS), with MeV hydrogen, to profile the implanted carbon, taking advantage of the large C(p,p)C scattering cross-section at around 1.73 MeV. We studied samples of fused quartz and sapphire implanted with carbon to a range of doses and annealed in forming gas, oxygen and argon. We show that in an oxygen environment, there is significant carbon loss in fused quartz but not in sapphire while in the other environments no significant loss is reported. We conclude that redistribution of carbon, the formation of nanocrystalline diamond (as seen in cross-sectional TEM) and possible carbon loss is determined both by the mobility of carbon in the host matrix at the prevailing annealing temperatures and, most importantly, the annealing ambient.

Synthesis of nanocrystalline diamond by the direct ion beam deposition method

Nanocrystalline diamond has been synthesized on a mirror-polished Si(001) substrate by means of direct ion beam deposition. Low-energy (80–200 eV) hydrocarbon and hydrogen ions, generated in a Kaufman ion source, were used to bombard the substrates. The bombarded samples were characterized by high-resolution transmission electron microscopy and Raman spectroscopy. Nanocrystalline diamond particles of random orientation were observed in a matrix of amorphous carbon film on the Si(001) substrate. The size of the nanocrystalline diamond particles varied in the range of 50–300 Å. The mechanism of ion-induced formation of nanocrystalline diamond is discussed.

Microcrystalline and Nanocrystalline Diamond Film Deposition on Cobalt Chrome Alloy

AbstractThe cobalt chrome alloy Co-28Cr-6Mo is widely used in human joint replacement applications. These joints are highly sensitive to wear and are usually replaced after approximately ten years of use. In order to extend these implants' service lifetimes, a thin film of diamond may be applied to the implant wear surfaces by microwave plasma chemical vapor deposition (MPCVD) following MPCVD nitridation. Diamond films often delaminate from cobalt chrome due to a high thermal expansion mismatch. Additionally, under most conditions diamond films degrade into graphite by dissolution of nuclei into solvents like Co and Cr. By nitriding the cobalt chrome through MPCVD prior to diamond deposition, a usable diamond film may be achieved. It may be possible to merge both the nitriding and deposition steps into a single process, since both nitridation and deposition will be performed by MPCVD. In addition, controlled amounts of nitrogen in hydrogen/methane plasma under CVD conditions result in a nanostructured diamond coating. We have investigated the formation of both micro- and nanocrystalline diamond formation on cobalt chrome.

Formation of nanocrystalline diamond by hydrocarbon plasma beam deposition

Abstract Diamond crystallites up to 40 nm in size have been grown from a highly ionised plasma beam of acetylene for ion energies close to 100 eV per C atom and substrate temperatures above 450°C. This shows that diamond can be grown by physical vapour deposition from an ion-rich plasma as well as by chemical vapour deposition from a radical-rich plasma. The formation mechanism is argued to be one of nucleation and growth rather than a stress-induced transformation from graphite.