Room-temperature photoluminescence from nitrogenated carbon nanotips grown by plasma-enhanced hot filament chemical vapor deposition

Nitrogenated carbon nanotips with a low atomic concentration of nitrogen have been synthesized by using a custom-designed plasma-enhanced hot-filament plasma chemical vapor deposition system. The properties (including morphology, structure, composition, photoluminescence, etc.) of the synthesized nitrogenated carbon nanotips are investigated using advanced characterization tools. The room-temperature photoluminescence measurements show that the nitrogenated carbon nanotips can generate two distinct broad emissions located at ∼405 and ∼507 nm, respectively. Through the detailed analysis, it is shown that these two emission bands are attributed to the transition between the lone pair valence and bands, which are related to the sp3 and sp2 C-N bonds, respectively. These results are highly relevant to advanced applications of nitrogenated carbon nanotips in light emitting optoelectronic devices.

Tailoring carbon nanotips in the plasma-assisted chemical vapor deposition: Effect of the process parameters

Carbon nanotips have been synthesized from a thin carbon film deposited on silicon by bias-enhanced hot filament chemical vapor deposition under different process parameters. The results of scanning electron microscopy indicate that high-quality carbon nanotips can only be obtained under conditions when the ion flux is effectively drawn from the plasma sustained in a CH4 + NH3 + H2 gas mixture. It is shown that the morphology of the carbon nanotips can be controlled by varying the process parameters such as the applied bias, gas pressure, and the NH3 / H2 mass flow ratios. The nanotip formation process is examined through a model that accounts for surface diffusion, in addition to sputtering and deposition processes included in the existing models. This model makes it possible to explain the major difference in the morphologies of the carbon nanotips formed without and with the aid of the plasma as well as to interpret the changes of their aspect ratio caused by the variation in the ion/gas fluxes. Viable ways to optimize the plasma-based process parameters to synthesize high-quality carbon nanotips are suggested. The results are relevant to the development of advanced plasma-/ion-assisted methods of nanoscale synthesis and processing.

Analysis of photoluminescence background of Raman spectra of carbon nanotips grown by plasma-enhanced chemical vapor deposition

Carbon nanotips with different structures were synthesized by plasma-enhanced hot filament chemical vapor deposition and plasma-enhanced chemical vapor deposition using different deposition conditions, and they were investigated by scanning electron microscopy and Raman spectroscopy. The results indicate that the photoluminescence background of the Raman spectra is different for different carbon nanotips. Additionally, the Raman spectra of the carbon nanotips synthesized using nitrogen-containing gas precursors show a peak located at about 2120 cm-1 besides the common D and G peaks. The observed difference in the photoluminescence background is related to the growth mechanisms, structural properties, and surface morphology of a-C:H and a-C:H:N nanotips, in particular, the sizes of the emissive tips.

Graphitization of nanocrystalline carbon microcoils synthesized by catalytic chemical vapor deposition

Graphitization, a common process involving the transformation of metastable nongraphitic carbon into graphite is one of the major present-day challenges for micro- and nanocarbons due to their unique structural character and highly unusual thermal activation. Here we report on the successful graphitization of nanocrystalline carbon microcoils prepared by catalytic chemical vapor deposition and post-treated in argon atmosphere at temperatures ∼2500 °C for 2 h. The morphology, microstructure, and thermal properties of the carbon microcoils are examined in detail. The graphitization mechanism is discussed by invoking a model of structural transformation of the carbon microcoils. The results reveal that after graphitization the carbon microcoils are prominently purified and feature a clear helical morphology, as well as a more regular and ordered microstructure. The interlayer spacing of the carbon microcoils decreases from 0.36 to 0.34 nm, whereas the mean crystal sizes in the c - and a -directions increase from 1.64 to 2.04 nm and from 3.86 to 7.21 nm, respectively. Thermal treatment also substantially improves the antioxidation properties of the microcoils by lifting the oxidation onset temperature from 550 to 672 °C. This process may be suitable for other nongraphitic micro- and nanomaterials.

Carbon nanofiber growth in plasma-enhanced chemical vapor deposition

A theoretical model to describe the plasma-assisted growth of carbon nanofibers (CNFs) is proposed. Using the model, the plasma-related effects on the nanofiber growth parameters, such as the growth rate due to surface and bulk diffusion, the effective carbon flux to the catalyst surface, the characteristic residence time and diffusion length of carbon atoms on the catalyst surface, and the surface coverages, have been studied. The dependence of these parameters on the catalyst surface temperature and ion and etching gas fluxes to the catalyst surface is quantified. The optimum conditions under which a low-temperature plasma environment can benefit the CNF growth are formulated. These results are in good agreement with the available experimental data on CNF growth and can be used for optimizing synthesis of related nanoassemblies in low-temperature plasma-assisted nanofabrication. © 2008 American Institute of Physics.

Influence of hydrogen dilution on the growth of nanocrystalline silicon carbide films by low-frequency inductively coupled plasma chemical vapor deposition

Nanocrystalline silicon carbide (nc-SiC) films are prepared by low-frequency inductively coupled plasma chemical vapor deposition from feedstock gases silane and methane diluted with hydrogen at a substrate temperature of 500 °C. The effect of different hydrogen dilution ratios X [hydrogen flow (sccm) / silane + methane flow (sccm)] on the growth of nc-SiC films is investigated by X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). At a low hydrogen dilution ratio X, cubic silicon carbide is the main crystal phase; whereas at a high hydrogen dilution ratio X, hexagonal silicon carbide is the main crystal phase. The SiC crystal phase transformation may be explained by the different surface mobility of reactive Si-based and C-based radicals deposited at different hydrogen dilution ratios X. The FTIR and XPS analyses show that the Si-C bonds are the main bonds in the films and elemental composition of SiC is nearly stoichiometric with almost equal share of silicon and carbon atoms.

Simulation of gas-phase nanoparticle dynamics in the plasma-enhanced chemical vapor deposition of carbon nanostructures

The results of 1D simulation of nanoparticle dynamics in the areas adjacent to nanostructured carbon-based films exposed to chemically active complex plasma of CH4 + H2 + Ar gas mixtures are presented. The nanoparticle-loaded near-substrate (including sheath and presheath) areas of a low-frequency (0.5 MHz) inductively coupled plasma facility for the PECVD growth of the ordered carbon-based nanotip structures are considered. The conditions allowing one to predict the size of particles that can pass through the plasma sheath and softly land onto the surface are formulated. The possibility of soft nano-cluster deposition without any additional acceleration common for some existing nano-cluster deposition schemes is demonstrated. The effect of the substrate heating power and the average atomic mass of neutral species is studied numerically and verified experimentally.

Effect of soil properties on the response of pile to underground explosion

This paper develops and presents a fully coupled non-linear finite element procedure to treat the response of piles to ground shocks induced by underground explosions. The Arbitrary Lagrange Euler coupling formulation with proper state material parameters and equations are used in the study. Pile responses in four different soil types, viz, saturated soil, partially saturated soil and loose and dense dry soils are investigated and the results compared. Numerical results are validated by comparing with those from a standard design manual. Blast wave propagation in soils, horizontal pile deformations and damages in the pile are presented. The pile damage presented through plastic strain diagrams will enable the vulnerability assessment of the piles under the blast scenarios considered. The numerical results indicate that the blast performance of the piles embedded in saturated soil and loose dry soil are more severe than those in piles embedded in partially saturated soil and dense dry soil. Present findings should serve as a benchmark reference for future analysis and design.

E and H regimes of plasma enhanced chemical vapor deposition of diamond-like carbon film in low frequency inductively coupled plasma reactor

This paper reports on the efficient deposition of hydrogenated diamond-like carbon (DLC) film in a plasma reactor that features both the capacitively and inductively coupled operation regimes. The hydrogenated DLC films have been prepared on silicon wafers using a low-frequency (500 kHz) inductively coupled plasma (ICP) chemical vapor deposition (CVD) system. At low RF powers, the system operates as an asymmetric capacitively coupled plasma source, and the film deposition process is undertaken in the electrostatic (E) discharge regime. Above the mode transition threshold, the high-density inductively coupled plasma is produced in the electromagnetic (H) discharge regime. It has been shown that the deposition rate and hardness of the DLC film are much higher in the H-mode deposition regime. For a 2.66-Pa H-mode CH4 + Ar gas mixture discharge, the deposited DLC film exhibits a mechanical hardness of 18 GPa, Young's modulus of 170 GPa, and compressive stress of 1.3 GPa.

Plasma-enhanced chemical vapor deposition of graphene nanostructures

Graphene has received great interest from researchers all over the world owing to its unique properties. Much of the excitement surrounding graphene is due to its remarkable properties and inherent quantum effects. These effects and properties make it a desirable material for the fabrication of new devices. Graphene has a plethora of potential uses including gas and molecular sensors, electronics, spintronics and optics [1-7]. Interestingly, some of these properties have been known about since before the material was even isolated due to a considerable amount of theoretical work and simulations. The material was to some extent a condensed matter modelers "toy" as it was used as a benchmark 2D material Graphene had been used for a long time as the fundamental building block of many other carbon structures...

Effect of plasma environment on synthesis of vertically aligned carbon nanofibers in plasma-enhanced chemical vapor deposition

We present a theoretical model describing a plasma-assisted growth of carbon nanofibers (CNFs), which involves two competing channels of carbon incorporation into stacked graphene sheets: via surface diffusion and through the bulk of the catalyst particle (on the top of the nanofiber), accounting for a range of ion- and radical-assisted processes on the catalyst surface. Using this model, it is found that at low surface temperatures, Ts, the CNF growth is indeed controlled by surface diffusion, thus quantifying the semiempirical conclusions of earlier experiments. On the other hand, both the surface and bulk diffusion channels provide a comparable supply of carbon atoms to the stacked graphene sheets at elevated synthesis temperatures. It is also shown that at low Ts, insufficient for effective catalytic precursor decomposition, the plasma ions play a key role in the production of carbon atoms on the catalyst surface. The model is used to compute the growth rates for the two extreme cases of thermal and plasma-enhanced chemical vapor deposition of CNFs. More importantly, these results quantify and explain a number of observations and semiempirical conclusions of earlier experiments.

Impact of nitrification inhibitor (DMPP) on soil nitrous oxide emissions from an intensive broccoli production system in sub-tropical Australia

Vegetable cropping systems are often characterised by high inputs of nitrogen fertiliser. Elevated emissions of nitrous oxide (N2O) can be expected as a consequence. In order to mitigate N2O emissions from fertilised agricultural fields, the use of nitrification inhibitors, in combination with ammonium based fertilisers, has been promoted. However, no data is currently available on the use of nitrification inhibitors in sub-tropical vegetable systems. A field experiment was conducted to investigate the effect of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N2O emissions and yield from broccoli production in sub-tropical Australia. Soil N2O fluxes were monitored continuously (3 h sampling frequency) with fully automated, pneumatically operated measuring chambers linked to a sampling control system and a gas chromatograph. Cumulative N2O emissions over the 5 month observation period amounted to 298 g-N/ha, 324 g-N/ha, 411 g-N/ha and 463 g-N/ha in the conventional fertiliser (CONV), the DMPP treatment (DMPP), the DMMP treatment with a 10% reduced fertiliser rate (DMPP-red) and the zero fertiliser (0N), respectively. The temporal variation of N2O fluxes showed only low emissions over the broccoli cropping phase, but significantly elevated emissions were observed in all treatments following broccoli residues being incorporated into the soil. Overall 70–90% of the total emissions occurred in this 5 weeks fallow phase. There was a significant inhibition effect of DMPP on N2O emissions and soil mineral N content over the broccoli cropping phase where the application of DMPP reduced N2O emissions by 75% compared to the standard practice. However, there was no statistical difference between the treatments during the fallow phase or when the whole season was considered. This study shows that DMPP has the potential to reduce N2O emissions from intensive vegetable systems, but also highlights the importance of post-harvest emissions from incorporated vegetable residues. N2O mitigation strategies in vegetable systems need to target these post-harvest emissions and a better evaluation of the effect of nitrification inhibitors over the fallow phase is needed.

Measuring, modelling and understanding the mechanical behavior of bagasse

In the Australian sugar industry, sugar cane is smashed into a straw like material by hammers before being squeezed between large rollers to extract the sugar juice. The straw like material is initially called prepared cane and then bagasse as it passes through successive roller milling units. The sugar cane materials are highly compressible, have high moisture content, are fibrous, and they resemble some peat soils in both appearance and mechanical behaviour. A promising avenue to improve the performance of milling units for increased throughput and juice extraction, and to reduce costs is by modelling of the crushing process. To achieve this, it is believed necessary that milling models should be able to reproduce measured bagasse behaviour. This investigation sought to measure the mechanical (compression, shear, and volume) behaviour of prepared cane and bagasse, to identify limitations in currently used material models, and to progress towards a material model that can predict bagasse behaviour adequately. Tests were carried out using a modified direct shear test equipment and procedure at most of the large range of pressures occurring in the crushing process. The investigation included an assessment of the performance of the direct shear test for measuring bagasse behaviour. The assessment was carried out using finite element modelling. It was shown that prepared cane and bagasse exhibited critical state behavior similar to that of soils and the magnitudes of material parameters were determined. The measurements were used to identify desirable features for a bagasse material model. It was shown that currently used material models had major limitations for reproducing bagasse behaviour. A model from the soil mechanics literature was modified and shown to achieve improved reproduction while using magnitudes of material parameters that better reflected the measured values. Finally, a typical three roller mill pressure feeder configuration was modelled. The predictions and limitations were assessed by comparison to measured data from a sugar factory.

Improvement of the mode II interface fracture toughness of glass fiber reinforced plastics/aluminum laminates through vapor grown carbon fiber interleaves

The effects of acid treatment, vapor grown carbon fiber (VGCF) interlayer and the angle, i.e., 0° and 90°, between the rolling stripes of an aluminum (Al) plate and the fiber direction of glass fiber reinforced plastics (GFRP) on the mode II interlaminar mechanical properties of GFRP/Al laminates were investigated. The experimental results of an end notched flexure test demonstrate that the acid treatment and the proper addition of VGCF can effectively improve the critical load and mode II fracture toughness of GFRP/Al laminates. The specimens with acid treatment and 10 g m−2 VGCF addition possess the highest mode II fracture toughness, i.e., 269% and 385% increases in the 0° and 90° specimens, respectively compared to those corresponding pristine ones. Due to the induced anisotropy by the rolling stripes on the aluminum plate, the 90° specimens possess 15.3%–73.6% higher mode II fracture toughness compared to the 0° specimens. The improvement mechanisms were explored by the observation of crack propagation path and fracture surface with optical, laser scanning and scanning electron microscopies. Moreover, finite element analyses were carried out based on the cohesive zone model to verify the experimental fracture toughness and to predict the interface shear strength between the aluminum plates and GFRP laminates.