Ion-assisted precursor dissociation and surface diffusion : enabling rapid, low-temperature growth of carbon nanofibers

Growth kinetics of carbon nanofibers in a hydrocarbon plasma is studied. In addition to gas-phase and surface processes common to chemical vapor deposition, the model includes (unique to plasma-exposed catalyst surfaces) ion-induced dissociation of hydrocarbons, interaction of adsorbed species with incoming hydrogen atoms, and dissociation of hydrocarbon ions. It is shown that at low, nanodevice-friendly process temperatures the nanofibers grow via surface diffusion of carbon adatoms produced on the catalyst particle via ion-induced dissociation of a hydrocarbon precursor. These results explain a lower activation energy of nanofiber growth in a plasma and can be used for the synthesis of other nanoassemblies. © 2007 American Institute of Physics.

Plasma-controlled adatom delivery and (re)distribution: Enabling uninterrupted, low-temperature growth of ultralong vertically aligned single walled carbon nanotubes

Large-scale (∼109 atoms) numerical simulations reveal that plasma-controlled dynamic delivery and redistribution of carbon atoms between the substrate and nanotube surfaces enable the growth of ultralong single walled carbon nanotubes (SWCNTs) and explain the common experimental observation of slower growth at advanced stages. It is shown that the plasma-based processes feature up to two orders of magnitude higher growth rates than equivalent neutral-gas systems and are better suited for the SWCNT synthesis at low nanodevice friendly temperatures. © 2008 American Institute of Physics.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p-i-n junction solar cells.

Simulation and visualization of self-assembled nanodevice networks synthesized via plasma–surface interaction

Using advanced visualization techniques, a comprehensive visualization of all the stages of the self-organized growth of internetworked nanostructures on plasma-exposed surface has been made. Atomistic kinetic Monte Carlo simulation for the initial stage of deposition, with 3-D visualization of the whole system and half-tone visualization of the density field of the adsorbed atoms, makes it possible to implement a multiscale predictive modeling of the development of the nanoscale system.

Surface science of plasma exposed surfaces : a challenge for applied plasma science

This article introduces a deterministic approach to using low-temperature, thermally non-equilibrium plasmas to synthesize delicate low-dimensional nanostructures of a small number of atoms on plasma exposed surfaces. This approach is based on a set of plasma-related strategies to control elementary surface processes, an area traditionally covered by surface science. Major issues related to balanced delivery and consumption of building units, appropriate choice of process conditions, and account of plasma-related electric fields, electric charges and polarization effects are identified and discussed in the quantum dot nanoarray context. Examples of a suitable plasma-aided nanofabrication facility and specific effects of a plasma-based environment on self-organized growth of size- and position-uniform nanodot arrays are shown. These results suggest a very positive outlook for using low-temperature plasma-based nanotools in high-precision nanofabrication of self-assembled nanostructures and elements of nanodevices, one of the areas of continuously rising demand from academia and industry.

Catalyst size effects on the growth of single-walled nanotubes in neutral and plasma systems

The results of large-scale (∼109 atoms) numerical simulations of the growth of different-diameter vertically-aligned single-walled carbon nanotubes in plasma systems with different sheath widths and in neutral gases with the same operating parameters are reported. It is shown that the nanotube lengths and growth rates can be effectively controlled by varying the process conditions. The SWCNT growth rates in the plasma can be up to two orders of magnitude higher than in the equivalent neutral gas systems. Under specific process conditions, thin SWCNTs can grow much faster than their thicker counterparts despite the higher energies required for catalyst activation and nanotube nucleation. This selective growth of thin SWCNTs opens new avenues for the solution of the currently intractable problem of simultaneous control of the nanotube chirality and length during the growth stage.

Effect of doping with Co and/or Cu on electronic structure and optical properties of ZnO

This paper reports on ab initio numerical simulations of the effect of Co and Cu dopings on the electronic structure and optical properties of ZnO, pursued to develop diluted magnetic semiconductors vitally needed for spintronic applications. The simulations are based upon the Perdew-Burke-Enzerh generalized gradient approximation on the density functional theory. It is revealed that the electrons with energies close to the Fermi level effectively transfer only between Cu and Co ions which substitute Zn atoms, and are located in the neighbor sites connected by an O ion. The simulation results are consistent with the experimental observations that addition of Cu helps achieve stable ferromagnetism of Co-doped ZnO. It is shown that simultaneous insertion of Co and Cu atoms leads to smaller energy band gap, redshift of the optical absorption edge, as well as significant changes in the reflectivity, dielectric function, refractive index, and electron energy loss function of ZnO as compared to the doping with either Co or Cu atoms. These highly unusual optical properties are explained in terms of the computed electronic structure and are promising for the development of the next-generation room-temperature ferromagnetic semiconductors for future spintronic devices on the existing semiconductor micromanufacturing platform.

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.

Effects of ions and atomic hydrogen in plasma-assisted growth of single-walled carbon nanotubes

The growth of single-walled carbon nanotubes (SWCNTs) in plasma-enhanced chemical vapor deposition (PECVD) is studied using a surface diffusion model. It is shown that at low substrate temperatures (≤1000 K), the atomic hydrogen and ion fluxes from the plasma can strongly affect nanotube growth. The ion-induced hydrocarbon dissociation can be the main process that supplies carbon atoms for SWCNT growth and is responsible for the frequently reported higher (compared to thermal chemical vapor deposition) nanotube growth rates in plasma-based processes. On the other hand, excessive deposition of plasma ions and atomic hydrogen can reduce the diffusion length of the carbon-bearing species and their residence time on the nanotube lateral surfaces. This reduction can adversely affect the nanotube growth rates. The results here are in good agreement with the available experimental data and can be used for optimizing SWCNT growth in PECVD.

Plasma heating effects in catalyzed growth of carbon nanofibres

A theoretical model describing the plasma-assisted growth of carbon nanofibres (CNFs) that accounts for the nanostructure heating by ion and etching gas fluxes from the plasma is developed. Using the model, it is shown that fluxes from the plasma environment can substantially increase the temperature of the catalyst nanoparticle located on the top of the CNF with respect to the substrate temperature. The difference between the catalyst and the substrate temperatures depends on the substrate width, the length of the CNF, the neutral gas density and temperature as well as the densities of the ions and atoms of the etching gas. In addition to the heating of the nanostructure, the ions and etching gas atoms from the ionized gas environment also strongly affect the CNF growth rates. Due to ion bombardment, the CNF growth rates in plasma enhanced chemical vapour deposition may be much higher than the rates in similar neutral gas-based thermal processes. The CNF growth model, which accounts for the nanostructure heating by the plasma-generated species, provides the growth rates that are in better agreement with the available experimental data on CNF growth than the models in which the heating effects are ignored.

High-rate, low-temperature synthesis of composition controlled hydrogenated amorphous silicon carbide films in low-frequency inductively coupled plasmas

It is commonly believed that in order to synthesize high-quality hydrogenated amorphous silicon carbide (a-Si1-xCx : H) films at competitive deposition rates it is necessary to operate plasma discharges at high power regimes and with heavy hydrogen dilution. Here we report on the fabrication of hydrogenated amorphous silicon carbide films with different carbon contents x (ranging from 0.09 to 0.71) at high deposition rates using inductively coupled plasma (ICP) chemical vapour deposition with no hydrogen dilution and at relatively low power densities (∼0.025 W cm -3) as compared with existing reports. The film growth rate R d peaks at x = 0.09 and x = 0.71, and equals 18 nm min-1 and 17 nm min-1, respectively, which is higher than other existing reports on the fabrication of a-Si1-xCx : H films. The extra carbon atoms for carbon-rich a-Si1-xCx : H samples are incorporated via diamond-like sp3 C-C bonding as deduced by Fourier transform infrared absorption and Raman spectroscopy analyses. The specimens feature a large optical band gap, with the maximum of 3.74 eV obtained at x = 0.71. All the a-Si1-xCx : H samples exhibit low-temperature (77 K) photoluminescence (PL), whereas only the carbon-rich a-Si1-xCx : H samples (x ≥ 0.55) exhibit room-temperature (300 K) PL. Such behaviour is explained by the static disorder model. High film quality in our work can be attributed to the high efficiency of the custom-designed ICP reactor to create reactive radical species required for the film growth. This technique can be used for a broader range of material systems where precise compositional control is required. © 2008 IOP Publishing Ltd.

Catalytic probes with nanostructured surface for gas/discharge diagnostics : a study of a probe signal behaviour

Catalytic probes are used for plasma diagnostics in order to quantify the density of neutral atoms. The probe response primarily depends on the probe material and its surface morphology. Here we report on the design, operation and modelling of the response of niobium pentoxide sensors with a flat and nanowire (NW) surfaces. These sensors were used to detect neutral oxygen atoms in the afterglow region of an inductively coupled rf discharge in oxygen. A very different response of the flat-surface and NW probes to the varying densities of oxygen atoms was explained by modelling heat conduction and taking into account the associated temperature gradients. It was found that the nanostructure probe can measure in a broader range than the flat oxide probe due to an increase in the surface to volume ratio, and the presence of nanostructures which act as a thermal barrier against sensor overheating. These results can be used for the development of the new generation of catalytic probes for gas/discharge diagnostics in a range of industrial and environmental applications.

Reactive oxygen plasma-enabled synthesis of nanostructured CdO : tailoring nanostructures through plasma–surface interactions

Plasma-assisted synthesis of nanostructures is one of the most precise and effective approaches used in nanodevice fabrication. Here we report on the innovative approach of synthesizing nanostructured cadmium oxide films on Cd substrates using a reactive oxygen plasma-based process. Under certain conditions, the surface morphology features arrays of crystalline CdO nano/micropyramids. These nanostructures grow via unconventional plasma-assisted oxidation of a cadmium foil exposed to inductively coupled plasmas with a narrow range of process parameters. The growth of the CdO pyramidal nanostructures takes place in the solid-liquid-solid phase, with the rates determined by the interaction of plasma-produced oxygen atoms and ions with the surface. It is shown that the size of the pyramidal structures can be effectively controlled by the fluxes of oxygen atoms and ions impinging on the cadmium surface. The unique role of the reactive plasma environment in the controlled synthesis of CdO nanopyramidal structures is discussed as well.

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.

Plasma-assembled carbon nanotubes : electric field–related effects

The paper presents results of comparative investigation of carbon nanotubes growth processes in dense low-temperature plasma and on substrate surface. Hybrid/Monte-Carlo numerical simulations were used to demonstrate the differences in the ion fluxes, growth rates and kinetics of adsorbed atoms re-distribution on substrate and nanotubes surfaces. We show that the plasma parameters significantly affect the nanotubes growth kinetics. We demonstrate that the growth rates of the nanotubes in plasma and on surface can differ by three orders, and the specific fluxes to the nanotube in the plasma can exceed the flux to surface-grown nanotube by six orders. We also show that the metal catalyst used for the nanotubes production on surface and in arc is a subject to very different conditions and this may be a key factor for the nanotube growth mode. The obtained dependencies for the ion fluxes to the nanotubes and nanotubes growth rates on the plasma parameters may be useful for selection of the production methods.