Low Nucleation Density Research Articles

Investigation on Nanodiamond and Carbon Nanotube‐Diamond Nanocomposite Synthesized using RF‐PECVD

AbstractNanodiamond (ND) and carbon nanotube‐diamond (CNTD) nanocomposite films are synthesized on silicon(100) substrates, discharging hydrogen‐based gas mixture (methane and hydrogen), using radio frequency plasma‐enhanced (RF‐PE)CVD. Substrate pretreatment conditions are examined for enhancing microdiamond (MD), ND, CNTD nanocomposite, or carbon nanotube (CNT) nucleation. Well‐faceted MD with a low nucleation density, on a silicon substrate, is achieved using a solution of NaOH in water to roughen the silicon substrate, while ND, CNTD nanocomposite, and CNTs are acquired using Co(NO3)2/Mg(NO3)2.6H2O or Fe(NO3)3.9H2O/Mg(NO3)2.6H2O or Ni(NO3)2.6H2O/Mg(NO3)2.6H2O solution in alcohol dripped onto the silicon substrate, which means that pretreatments have a significant influence on grain size, nucleation density, and microstructure of the obtained films. The growth mechanisms for ND and CNTD nanocomposite are discussed.

Modelling the Influence of the Nucleation Density on Ultra Nano Crystalline Diamond Growth Morphology: Preliminary Results

AbstractThe morphology of ultra nanocrystalline diamond (UNCD) films is modeled by a pure geometrical approach. Diamond spheres with a fixed radius are added randomly spread on a smooth substrate surface covered with diamond seeds in varied primary nucleation density. The secondary nucleation rate and the nucleation density on the bare substrate define the distribution of diamond spheres during each cycle. In preliminary results, this simple geometrical model leads to good comparison with published experimental data; low primary nucleation densities yield to the growth of separated diamond islands with a rough surface, the smoothest films are obtained with the highest primary nucleation densities.

Simultaneous formation of silicon carbide and diamond on Si substrates by microwave plasma assisted chemical vapor deposition

The effects of several process parameters, such as substrate temperature, nucleation density, and substrate surface pretreatment, on the simultaneous formation of SiC and diamond under typical growth conditions of diamond by microwave plasma assisted chemical vapor deposition (MPCVD), have been investigated by scanning electron microscopy (SEM), X-ray diffraction, and Raman and Fourier-transfer infrared (FTIR) spectroscopy. Results show that no SiC can be detected in the diamond films grown with a high nucleation density, whereas, SiC is detected in the thick diamond films grown with a low nucleation density, with or without surface pretreatment of the Si substrates. SEM micrographs and FTIR spectra illustrate that SiC is formed on the Si substrate not covered by diamond nuclei or in void regions between diamond nuclei. The formation of SiC and diamond on Si substrates under the growth conditions of diamond by MPCVD is a concurrent competitive deposition process, especially at the initial stage of diamond nucleation and growth. This is an alternative method for the synthesis of diamond-SiC composites by MPCVD.

Monolayers of Twisted Binaphthyls for Aromatic Crystallization at Low Nucleation Densities and High Growth Rates

Monolayers of 1,1'-bi-2-naphthol (BN) derivatives, of which the two naphthalene rings are twisted along the carbon(1)-carbon(1') single bond, were studied for their conformational effect on the growth of pentacene crystals on their monolayer surface. BN monolayers with H and Br at 6,6'-positions (H-BN and Br-BN) were prepared by immersion-coating in toluene solution of the corresponding BNSiCl2. Pentacene was thermally evaporated onto the H-BN and Br-BN monolayers, silica, octadecylsilyl (ODTS) SAM, and a micropattern of H-BN and ODTS SAM. Pentacene crystals were also grown on the SAMs of 1-naphthylsilyl(NPh), phenylsilyl(Ph), and diphenylsilyl (DPh) groups, which are aromatic and have contact angle values similar to those of the the BN monolayers. AFM images of the crystals at the early stage of growth indicated that the BN monolayers suppressed the nucleation while facilitating the growth of nuclei to larger crystals. The low nucleation density and high growth rate are accounted for by the amorphous nature of the twisted BN monolayer surface where the intermolecular interaction between neighboring adsorbates is likely to be suppressed. The results offer new insights into designing surfaces for controlling the crystallization kinetics of organic materials.

Epitaxial Growth of Large Pentacene Crystals on Si(001) Surfaces Functionalized with Molecular Monolayers

The nucleation and growth of pentacene thin films are controlled largely by the energies associated with the interfaces. We have used low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) to investigate the nucleation and growth of pentacene thin films on Si(001) surfaces modified with two different molecular monolayers. Clean Si(001)-(2 × 1) surfaces were modified with either 1,5-cyclooctadiene or 1-dodecene prior to pentacene growth to study the effects of exposed π bonds at the interface, orientation of those π bonds relative to each other, and rigidity of the molecular layer on pentacene nucleation, growth, and crystalline orientation. Both molecular monolayers weaken the substrate−pentacene interaction sufficiently to allow for low pentacene nucleation density and good pentacene diffusion, leading to the growth of pentacene grains as large as 100 μm. Pentacene grows epitaxially on both functionalized surfaces, adopting an orthorhombic unit cell that follows the orientatio...

Morphology and structure of the diamond film synthesized from carbon monoxide gas

Diamond film was deposited in CO and H2 gas mixture by microwave plasma assisted chemical vapor deposition (MPCVD). Bias enhanced nucleation and two-step growth method were employed. Scanning electron microscopy (SEM), Raman spectroscopy and transmission electron microscopy (TEM) were used to characterize the film. It was found that twins and stacking faults were mainly produced at the stages of nucleation and growth. In the second growth stage, the production of defects in the film was reduced considerably, resulting in the final smooth-faceted diamond grains with five-fold symmetry or parallel twins with a small amount of stacking faults. At the edge of the sample, there was a region with low nucleation density, which might be the result of non-uniform plasma existing there. Meanwhile, a non-diamond structure of carbon was found in the edge region.

Role of Nucleation on the Growth of Nanocrystalline Diamond (NCD) Films: A Particular Study in the Fabrication of Micro-cantilevers

AbstractThe effect of ultrasonic method of seeding on the growth of nanocrystalline diamond (NCD) films has been studied by investigating the nucleation densities of two nanodiamond slurries- 1) nanodiamond powder suspended in acetone 2) mixture of titanium nanopowder and nanodiamond powder suspended in acetone. The former has resulted in a lower nucleation density on the order of 108 cm−2 while the later has increased the nucleation density by over two orders (>1010 cm−2) of magnitude. NCD films were grown on Si(100) substrates in a Cyrannus I Iplas microwave plasma enhanced chemical vapor deposition (MPECVD) reactor using 0.5% CH4, 1% H2 and 98.5% Ar gas chemistry at a substrate temperature of ∼ 750°C, pressure of 135 T and a microwave power of 1.8 kW. The double anchored cantilever structures with NCD as structural material have been fabricated by conventional photolithography. It was observed that the films grown after pre-treating the substrates in pure nanodiamond slurry have not provided a complete coverage at the anchor regions of the cantilevers due to the lower nucleation density. Hence during the sacrificial oxide etch, the anchor regions did not adhere well to the substrate and the entire structures got detached from the substrate. But on the other hand, the films grown by seeding the substrates in a suspension of titanium nanopowder and diamond nanopowder showed complete film coverage and consequently very good bonding to the Si substrate at the anchor pads. In this case, the cantilevers have been successfully released without any problems at the anchor pads. Further the mechanical properties of the films were conducted by evaluating the Young's modulus and the hardness by nano-indentation technique.

Improved Nucleation Behavior of Ru Thin Films Prepared by MOCVD on TiCl[sub 4] Pretreated Substrates

Ru films were deposited on SiO 2 and TiN substrates by low-pressure metallorganic chemical vapor deposition (MOCVD). To improve the low nucleation density of Ru on the two types of substrates and the resultant rough surface morphologies of the deposited films, the surface was pretreated with TiCl 4 molecules using a repeated cyclic pulse and purge process. In this way, the nucleation density of Ru on the pretreated substrates was increased and continuous and smoother Ru films were obtained. X-ray photoelectron spectroscopy revealed that the improvement of the nucleation property is related to the change in the binding characteristics of the substrate surfaces.

Epitaxial structures of self-organized, standing-up pentacene thin films studied by LEEM and STM

Growth of pentacene (Pn) thin films has been studied in situ by means of low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). A very low nucleation density of Pn grains has been observed on Bi(0 0 0 1)/Si(1 1 1) template, resulting in formation of large, monolayer-high Pn grains with diameter exceeding several hundreds of micrometers. We determined that formation of self-organized, standing-up Pn epitaxial layers was stabilized by a weak interaction between the substrate and Pn molecules and by the presence of the commensurate structure between the oblique Pn lattice and trigonal substrate surface lattice. The ‘point-on-line’ commensurability has been found along a -axis of Pn and one of the primitive vectors of substrate surface lattice. Strong ‘point-on-line’ commensurability in Pn/Bi(0 0 0 1)/Si(1 1 1) system resulted in a bulk-like epitaxial thin film growth, starting from the first layer. The presence of twins, often having a mirror line parallel to the direction of the ‘point-on-line’ matching, has been also detected using an asymmetric dark-field imaging mode in LEEM experiments, which, we believe, is the first LEEM demonstration of molecular tilt imaging.

Poly-Si Thin Film Transistors: Effect of Metal Thickness on Silicon Crystallization

In this work metal induced crystallization (MIC) using nickel (Ni) was employed to obtain poly-Si by crystallization of amorphous films for application as active layer in TFTs. Ni layers with thicknesses of 0.5 nm, 1 nm and 2 nm were used to crystallize the silicon. The TFTs were produced with a bottom gate configuration using a multi-layer Al2O3/TiO2 insulator produced by atomic layer deposition (ALD) as gate dielectric. The best performances of the TFT produced were obtained when using very thin Ni layers for the crystallization. This is attributed to a lower metal contamination and to the enhancement of grain size, as a result of the lower nucleation density achieved, when using the thinnest Ni layer. Devices that exhibit effective mobility of 45.5 cm2V-1s-1 and an on/off ratio of 5.55×104 were produced using a 0.5 nm Ni layer to crystallize the active channel area.

A Study of Diamond Synthesis by Hot Filament Chemical Vapor Deposition on Nc Coatings

Deposition of diamond films onto various substrates can result in significant technological advantages in terms of functionality and improved life and performance of components. Diamond is hard, wear resistant, chemically inert, and biocompatible. It is considered to be the ideal material for surfaces of cutting tools and biomedical components. However, it is well known that diamond deposition onto technologically important substrates, such as co-cemented carbides and steels, is problematic due to carbon interaction with the substrate, low nucleation densities, and poor adhesion. Several papers previously published in the relevant literature have reported the application of interlayer materials such as metal nitrides and carbides to provide bonding between diamond and hostile substrates. In this study, the chemical vapor deposition (CVD) of polycrystalline diamond on TiN/SiN x nc (nc) interlayers deposited at relatively low temperatures has been investigated for the first time. The nc layers were deposited at 70 or 400 °C on Si substrates using a dual ion beam deposition system. The results showed that a preliminary seeding pretreatment with diamond suspension was necessary to achieve large diamond nucleation densities and that diamond nucleation was larger on nc films than on bare sc-Si subjected to the same pretreatment and CVD process parameters. TiN/SiN x layers synthesized at 70 or 400 °C underwent different nanostructure modifications during diamond CVD. The data also showed that TiN/SiN x films obtained at 400 °C are preferable in so far as their use as interlayers between hostile substrates and CVD diamond is concerned.

Effects of Ti- and Zr-Based Interlayer Coatings on Hot-Filament Chemical Vapor Deposition of Diamond on High-Speed Steel

The prospect of obtaining good adhesion of diamond films onto steel substrates is highly exciting because the achievement of this objective will open up numerous new applications in industry. However, a major problem with depositing diamond onto steel is high diffusion of carbon into steel at chemical vapor deposition (CVD) temperatures leading to a very low nucleation density and cementite (Fe 3 C) formation. Therefore, the study of the nucleation and growth processes is timely and will yield data that can be utilized to get a better understanding of how adhesion can be improved. This work focuses on the adhesion of thin diamond films onto high speed steel previously coated with various interlayers such as ZrN, ZrC, TiC, and TiC/Ti(C,N)/TiN. The role of seeding on nucleation density and the effect of diamond film thickness on stress development and adhesion has been investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and Raman spectroscopy (RS). The main emphasis in this study lies with TiC, which for the first time proved to be a suitable layer for diamond CVD on high-speed steel (HSS). In fact, different from other interlayer materials investigated, no delamination was observed after 3 h of CVD at 650 °C when TiC was used. Nevertheless, the increase of diamond film thickness on TiC-coated HSS substrates led to delamination of small areas. This occurrence suggests that there was a distribution of adhesive toughness values at the diamond/TiC interface with the stress development being dependent on film thickness.

Effects of Ti- and Zr-based interlayer coatings on the hot filament chemical vapour deposition of diamond on high speed steel

The prospect of obtaining good adhesion of diamond films onto steel substrates is highly exciting because the achievement of this objective will open up applications in the cutting and drilling industry. However, a major problem with depositing diamond onto steel is high diffusion of carbon into steel at chemical vapour deposition (CVD) temperatures leading to very low nucleation density and cementite (Fe 3C) formation. Therefore, the study of the nucleation and growth processes is timely and will yield data that can be utilised to get a better understanding of how adhesion can be improved. This work focuses on investigating the adhesion of thin diamond films on high speed steel previously coated with various interlayers such as ZrN, ZrC, TiC and TiC/Ti(C,N)/TiN. The role of seeding on nucleation density and the effect of diamond film thickness on stress development and adhesion has been investigated using SEM, XRD and Raman spectroscopy. The main emphasis in this study is the TiC interlayer which for the first time proved to be a suitable layer for diamond CVD on high speed steel (HSS). In contrast from other interlayer materials investigated here, no delamination was observed even after 3 h of CVD at 650 °C only when TiC was employed. Nevertheless, the increase of diamond film thickness on TiC coated HSS substrates led to the delamination of small areas in various regions of the substrate. This occurrence suggests that there was a distribution of adhesive toughness values at the diamond/TiC interface with stress development being dependent on film thickness.

Growth of free-standing diamond films of hemispheric shells

Free-standing diamond films of hemispheric shells were fabricated in a hot filament CVD system by using positive substrate bias. The graphite mold was sputtered with a stainless steel layer of 0.5 um on the surface. During the diamond growth, a thin carbon soot layer with loose structure was first formed on the iron surface and then the growth of a thick diamond film followed. The diamond film could be easily delaminated from the graphite mold due to the weak bonding between the iron-containing carbon soot layer and the graphite substrate. Without the positive substrate bias, the varied distance between the flat hot filaments and the hemispherical substrate in the hot filament CVD system resulted in low nucleation density and non-uniform diamond film. The electron bombardments induced by the positive substrate bias were shown to enhance the diamond nucleation density on the soot-covered surface and thus improved the uniformity of diamond film thickness over the non-flat graphite mold.

Electrochemical Templating of Metal Nanoparticles and Nanowires on Single-Walled Carbon Nanotube Networks

The use of single-walled carbon nanotube (SWNT) networks as templates for the electrodeposition of metal (Ag and Pt) nanostructures is described. Pristine SWNTs, grown on insulating SiO2 surfaces using catalyzed chemical vapor deposition, served as the working electrode. In the simplest case, electrical contact was made by depositing a gold strip on the SWNT substrate (device 1). Deposition of Ag and Pt over extensive periods (30 s) resulted in a high density of particles on the SWNTs, with almost contiguous nanowire formation from the Au/SWNT boundary moving to isolated nanoparticles at further distances from the contact. For direct electrochemical studies of Ag and Pt nucleation, the assembly was coated in a resist layer and a small window opened up to expose only the electrically connected SWNTs to solution (device 2). In this case, the electrochemical signature in voltammetric and amperometric studies of metal deposition was due solely to processes at the SWNTs. Coupled with high-resolution microscopy measurements (atomic force microscopy and field emission scanning electron microscopy), this approach provided detail on the nucleation and growth mechanisms of Ag and Pt on SWNTs under electrochemical control. In particular, Ag growth was found to be rapid and progressive with an increasing nanoparticle density with time, whereas Pt deposition was characterized by lower nucleation densities and slower growth rates with a tendency for larger particles to be produced over long times.

A study of diamond synthesis by hot filament chemical vapour deposition on nanocomposite coatings

Deposition of diamond films onto various substrates can result in significant technological advantages in terms of functionality and improved life and performance of components. Diamond is hard, wear resistant, chemically inert and biocompatible and considered as the ideal material for surfaces of cutting tools and biomedical components. However, it is well known that diamond deposition onto technologically important substrates such as Co-cemented carbides and steels is problematic due to carbon interaction with the substrate, low nucleation densities and poor adhesion. Several papers previously published in the relevant literature have reported the application of interlayer materials such as metal nitrides and carbides to provide bonding between diamond and hostile substrates. In this study, we have investigated for the first time the chemical vapour deposition (CVD) of polycrystalline diamond on TiN/SiN x nanocomposite interlayers deposited at relatively low temperatures. The nanocomposite layers were deposited at 70 °C or 400 °C on silicon substrates by using a dual ion beam deposition system. The results showed that a preliminary seeding pre-treatment with diamond suspension was necessary to achieve large diamond nucleation densities and that diamond nucleation was larger on nanocomposite films than on bare sc-Si submitted to same pre-treatment and CVD process parameters. TiN/SiN x layers synthesized at 70 °C or 400 °C underwent different nanostructure modifications during diamond CVD. The data also showed that TiN/SiN x films obtained at 400 °C are preferable so far as their use as interlayers between hostile substrates and CVD diamond is concerned.

Solvent & second component influence on spherulitic morphology in PHB/PAZO blends

“Polyhydroxybutyrate, (PHB) and poly 1-[4-(3carboxy-4-hydroxy-phenylazo)benzene sulphonamido-1,2-ethanediyl, sodium salt)], (PAZO), provide a new and exciting range of blend materials, Fig. 1. On harmonization these create the perfect economically viable ecofriendly blend material, with optical data storage and biodegradable ability—a so called “designer photonic material”. The PAZO is an azobenzene derivative containing an N N bond. This N N bond is able to cis-trans isomerise, with the isomerisation type dependent on the wavelength of light. The trans type is favorable at 440 nm visible light, whereas the cis type is favorable at 360 nm ultra violet light [1]. PHB is a naturally occurring semi-crystalline biological polymer, forming large banded spherulites when crystallized from the melt, owing to the low nucleation density [2, 3]. Spherulitic studies on PHB blends demonstrate that the second component is initially engulfed in the interlamellar and interfibrillar regions, hindering the lamellar layer process [4–6]. The final spherulitic morphology is then dependent on the second components rejection from these regions, controlled by the interactions arising between the homopolymers. The area of azobenzene derivative-based blends to produce novel materials is a relatively new research field with only a few papers published [7–9], since achieving miscibility with these highly non-polar groups is extremely difficult. However, we have demonstrated from our previous studies that PHB/PAZO blends are in fact miscible. This is accomplished by solvent blending using acetone, chloroform, and toluene, solvents. We have proposed, using intense DSC and FTIR studies [10, 11], that a complex is formed via a hydrogen bonded “bridge” between the two structures. This “bridge” can only be formed at high PHB composition. Earlier reported calculations have shown PHB/PAZO must be present in a 2–4:1 ratio (dependent on solvent type) for the complex to form. However, nothing is known of this blend morphology and the way in which this complex formation will affect the spherulitic structure. These PHB/PAZO blends have immense potential as a new generation of designer biophotonic materials, but critical to this role is identifying and understanding the PAZO and solvent participation in the resultant morphology. In this paper, by observing the spherulite growth rates we will establish the

Investigation of the growth mechanism and structure of nanocrystalline diamond films by rapid thermal annealing

Nanocrystalline diamond (NCD) films in an amorphous matrix have been deposited by standard microwave plasma chemical vapour deposition from CH 4/N 2 mixtures, and subsequently characterized comprehensively with respect to their structure and morphology, their composition, their crystalline properties, and their bonding environment. Thereafter, some of the coatings have been subjected to rapid thermal annealing (RTA) at 1100 and 1400 °C, respectively. Characterization of the annealed films by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) revealed the loss of material upon annealing, while the bonding structure remained almost unchanged at least after the 1100 °C treatment. RTA at 1400 °C leads to the formation of crystalline SiC x N y O z material, very probably by reaction with the residual gas, whereas crystalline graphite is not observed. FTIR measurements indicate that hydrogen containing species from the residual gas may contribute to the loss of material. Finally, the morphological changes observed after the annealing give some insight into the growth mechanisms of these NCD films which are discussed in terms of a spherulitic growth caused by a rather low primary nucleation density but very high secondary nucleation rate.

Enhancing nucleation density and adhesion of polycrystalline diamond films deposited by HFCVD using surface treaments on Co cemented tungsten carbide

The deposition of diamond films by chemical vapour deposition onto tungsten carbide is an attractive proposition since it can lead to improvements in the life and performance of cutting tools. However, deposition of diamond onto cemented tungsten carbide (WC-Co) dental burs and inserts are problematic due to the cobalt binder in the WC that provides additional toughness to the tool but it causes poor adhesion and low nucleation density. A number of surface treatments can be used to overcome these problems including chemical etching, ion implanting, interlayer coating and bias treatment. Negative biasing of the substrate is attractive because it can be controlled precisely; it is carried out in-situ, gives good homogeneity and results in improved adhesion. On flat substrates, such as copper and silicon, biasing has been shown to give better adhesion, improved crystallinity and smooth surface. In this study, we have used a modified hot filament chemical vapour deposition (HFCVD) system to coat complex shaped tools such as dental burs with polycrystalline diamond films, which have good adhesion and crystallinity. By applying a negative bias to the substrate, we show that the nucleation density, adhesion and surface properties can be improved. The effects of various process parameters such as bias time, emission current, bias voltage and the filament arrangement on the film properties are reported. For machining applications CVD diamond coatings must be hard, wear resistance and having a good quality film.

Two-dimensional simulations of temperature fields of the reactor wall during hot-filament CVD diamond film growth over a large area

In order to study the growth of diamond films over a large area in a traditional hot-filament chemical vapour deposition (HFCVD) reactor, two-dimensional mathematical models were first developed to investigate the temperature fields of the reactor walls, which made significant contributions to thermal round-flow of the reactant gases under different energy transfer systems. The set of partial differential equations involved in the thermal conduction system was solved with different boundary conditions by the finite control volume method. Numerical simulations showed that the temperature space distributions were heterogeneous when thermal radiation was assumed to be the only mechanism in heat transfer from the filaments to the reactor walls. However, taking into account the effects of thermal conduction under adiabatic and different isothermal temperature boundary conditions, the temperature uniformities improved greatly. In addition, thermal convection did not affect the temperature distributions but only increased the total temperature of the reactor wall. These results not only give insight into the dominant reasons resulting in low nucleation density and low growth rate of diamond films, but also provide a basis for the design of industrial HFCVD reactors to obtain high-quality diamond films over a large area.