Hydrogen in plasma-nanofabrication : selective control of nanostructure heating and passivation

The possibility of independent control of the surface fluxes of energy and hydrogen-containing radicals, thus enabling selective control of the nanostructure heating and passivation, is demonstrated. In situ energy flux measurements reveal that even a small addition of H2 to low-pressure Ar plasmas leads to a dramatic increase in the energy deposition through H recombination on the surface. The heat release is quenched by a sequential addition of a hydrocarbon precursor while the surface passivation remains effective. Such selective control offers an effective mechanism for deterministic control of the growth shape, crystallinity, and density of nanostructures in plasma-aided nanofabrication. © 2010 American Institute of Physics.

Disentangling fluxes of energy and matter in plasma-surface interactions : effect of process parameters

The possibility to discriminate between the relative importance of the fluxes of energy and matter in plasma-surface interaction is demonstrated by the energy flux measurements in low-temperature plasmas ignited by the radio frequency discharge (power and pressure ranges 50-250 W and 8-11.5 Pa) in Ar, Ar+ H2, and Ar+ H2 + CH4 gas mixtures typically used in nanoscale synthesis and processing of silicon- and carbon-based nanostructures. It is shown that by varying the gas composition and pressure, the discharge power, and the surface bias one can effectively control the surface temperature and the matter supply rates. The experimental findings are explained in terms of the plasma-specific reactions in the plasma bulk and on the surface.

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.

Low-temperature plasma-assisted growth of optically transparent, highly oriented nanocrystalline AlN

Optically transparent, highly oriented nanocrystalline AlN(002) films have been synthesized using a hybrid plasma enhanced chemical vapor deposition and plasma-assisted radio frequency (rf) magnetron sputtering process in reactive Ar+ N2 and Ar+ N2 + H2 gas mixtures at a low Si(111)/glass substrate temperature of 350 °C. The process conditions, such as the sputtering pressure, rf power, substrate temperature, and N2 concentration were optimized to achieve the desired structural, compositional, and optical characteristics. X-ray diffractometry reveals the formation of highly c -oriented AlN films at a sputtering pressure of 0.8 Pa. Field emission scanning electron microscopy suggests the uniform distribution of AlN grains over large surface areas and also the existence of highly oriented in the (002) direction columnar structures of a typical length ∼100-500 nm with an aspect ratio of ∼7-15. X-ray photoelectron and energy dispersive x-ray spectroscopy suggest that films deposited at a rf power of 400 W feature a chemically pure and near stoichiometric AlN. The bonding states of the AlN films have been confirmed by Raman and Fourier transform infrared spectroscopy showing strong E2 (high) and E1 transverse optical phonon modes. Hydrogenated AlN films feature an excellent optical transmittance of ∼80% in the visible region of the spectrum, promising for advanced optical applications.

Thermophoretic control of building units in the plasma-assisted deposition of nanostructured carbon films

The role of the plasma-grown nanoparticles in the plasma-enhanced chemical vapor deposition (PECVD) of the nanostructured carbon-based films was investigated. The samples were grown in the low-pressure rf plasmas of CH 4+H2+Ar gas mixtures. The enhanced deposition of the building units from the gas phase was found to support the formation of polymorphous nanostructured carbon films. The results reveal the crucial role played by the thermophoretic force in controlling the deposition of the plasma-grown fine particles.

Aluminum-assisted crystallization and p-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH 4+Ar+H2 gas mixture at a low substrate temperature of 300∈°C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si-Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×1018 cm-3. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300∈°C is proposed.

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.

Low-temperature PECVD of nanodevice-Grade nc-3C-SiC

In this work, we report a plasma-based synthesis of nanodevice-grade nc-3C-SiC films, with very high growth rates (7-9 nm min-1) at low and ULSI technology-compatible process temperatures (400-550 °C), featuring: (i) high nanocrystalline fraction (67% at 550 °C); (ii) good chemical purity; (iii) excellent stoichiometry throughout the entire film; (iv) wide optical band gap (3.22-3.71 eV); (v) refractive index close to that of single-crystalline 3C-SiC, and; (vi) clear, uniform, and defect-free Si-SiC interface. The counter-intuitive low SiC hydrogenation in a H2-rich plasma process is explained by hydrogen atom desorption-mediated crystallization.

Electrostatic nanoparticle filter for atomic scale fabrication in low-temperature plasmas

The means of reducing nanoparticle contamination in the synthesis of carbon nanostructures in reactive Ar + H2 + CH4 plasmas are studied. It is shown that by combining the electrostatic filtering and thermophoretic manipulation of nanoparticles, one can significantly improve the quality of carbon nanopatterns. By increasing the substrate heating power, one can increase the size of deposited nanoparticles and eventually achieve nanoparticle-free nanoassemblies. This approach is generic and is applicable to other reactive plasma-aided nanofabrication processes.

Nanoparticle manipulation in plasma-assisted nanofabrication of electron field emitters based on single crystalline carbon nanotip patterns

Nanoparticle manipulation by various plasma forces in near-substrate areas of the Integrated Plasma-Aided Nanofabrication Facility (IPANF) is investigated. In the IPANF, high-density plasmas of low-temperature rf glow discharges are sustained. The model near-substrate area includes a variable-length pre-sheath, where a negatively charged nanoparticle is accelerated, and a self-consistent collisionless sheath with a repulsive electrostatic potential. Conditions enabling the nanoparticle to overcome the repulsive barrier and deposit onto the substrate are investigated numerically and experimentally. Under certain conditions the momentum gained by the nanoparticle in the pre-sheath area appears to be sufficient for the driving ion drag force to outbalance the repulsive electrostatic and thermophoretic forces. Numerical results are applied for the explanation of size-selective nanoparticle deposition in the Ar+H2+CH4 plasma-assisted chemical vapor deposition of various carbon nanostructure patterns for electron field emitters and are cross-referenced by the field emission scanning electron microscopy. It is shown that the nanoparticles can be efficiently manipulated by the temperature gradient-controlled thermophoretic force. Experimentally, the temperature gradients in the near-substrate areas are measured in situ by means of the temperature gradient probe and related to the nanofilm fabrication conditions. The results are relevant to plasma-assisted synthesis of numerous nanofilms employing structural incorporation of the plasma-grown nanoparticles, including but not limited to nanofabrication of ordered single-crystalline carbon nanotip arrays for electron field emission applications.

PECVD of carbon nanostructures in hydrocarbon-based RF plasmas

Different aspects of the plasma-enhanced chemical vapor deposition of various carbon nanostructures in the ionized gas phase of high-density, low-temperature reactive plasmas of Ar+H2+CH4 gas mixtures are studied. The growth techniques, surface morphologies, densities and fluxes of major reactive species in the discharge, and effects of the transport of the plasma-grown nanoparticles through the near-substrate plasma sheath are examined. Possible growth precursors of the carbon nanostructures are also discussed. In particular, the experimental and numerical results indicate that it is likely that the aligned carbon nanotip structures are predominantly grown by the molecular and radical units, whereas the plasma-grown nanoparticles are crucial components of polymorphous carbon films.

Optical emission characteristics and mode transitions in low-frequency inductively coupled plasmas

An attempt was made to investigate the optical emission spectra of atomic, molecular, and ionic species in low-frequency, high-density ICP discharges in pure nitrogen, ar con gases, and gas mixtures Ar+H2, N2+Ar, and N2+H2. The excited species were identified by in situ optical emission intensity (OEI) measurements in the discharge chamber. In general, significant results were obtained.

Triphasic pattern in the ex vivo response of human proliferative phase endometrium to oestrogens

The aim of this study was to evaluate the ex vivo oestrogen responsiveness of human proliferative phase endometrium using short-term explant cultures. The effects of oestrogen (17beta-E2) on proliferation and the expression of oestrogen-responsive genes known to be involved in regulating endometrial function were evaluated. Three distinct response patterns could be distinguished: (1) the menstrual (M) phase pattern (cycle days 2-5), which is characterised by a complete lack in the proliferative response to 17beta-E2, while an increased expression of AR (2.6-fold, P<0.01), PR (2.7-fold, P<0.01) and COX-2 (3.5-fold, P<0.01) at the mRNA level was observed and a similar upregulation was also found for AR, PR and COX-2 at the protein level; (2) the early proliferative (EP) phase pattern (cycle days 6-10) with 17beta-E2 enhanced proliferation in the stroma (1.7-fold, P<0.05), whereas the expression of AR, PR and COX-2 were not affected at the mRNA and protein levels and ER-a mRNA and protein levels were significantly reduced by 17beta-E2; (3) the late proliferative (LP) phase pattern (cycle days 11-14), which is characterised by a moderate stimulation of proliferation (1.4-fold, P<0.05) and PR mRNA expression (1.7-fold, P<0.01) by 17beta-E2. In conclusion, three distinct response patterns to 17beta-E2 could be identified with respect to proliferation and the expression of known oestrogen-responsive genes in human proliferative phase endometrium explant cultures.

Optical gas sensing properties of nanoporous Nb2O5 films

Nanoporous Nb2O5 has been previously demonstrated to be a viable electrochromic material with strong intercalation characteristics. Despite showing such promising properties, its potential for optical gas sensing applications, which involves the production of ionic species such as H+, has yet to be explored. Nanoporous Nb2O5 can accommodate a large amount of H+ ions in a process that results in an energy bandgap change of the material, which induces an optical response. Here, we demonstrate the optical hydrogen gas (H¬2) sensing capability of nanoporous anodic Nb2O5 with a large surface-to-volume ratio prepared via a high temperature anodization method. The large active surface area of the film provides enhanced pathways for efficient hydrogen adsorption and dissociation, which are facilitated by a thin layer of Pt catalyst. We show that the process of H2 sensing causes optical modulations that are investigated in terms of response magnitudes and dynamics. The optical modulations induced by the intercalation process and sensing properties of nanoporous anodic Nb2O5 shown in this work can potentially be used for future optical gas sensing systems.

Modelling CO2 adsorption and separation on experimentally-realized B40 fullerene

Searching for efficient solid sorbents for CO2 adsorption and separation is important for developing emergent carbon reduction and natural gas purification technology. This work, for the first time, has investigated the adsorption of CO2 on newly experimentally realized cage-like B40 fullerene (Zhai et al., 2014) based on density functional theory calculations. We find that the adsorption of CO2 on B40 fullerene involves a relatively large energy barrier (1.21 eV), however this can be greatly decreased to 0.35 eV by introducing an extra electron. A practical way to realize negatively charged B40 fullerene is then proposed by encapsulating a Li atom into the B40 fullerene (Li@B40). Li@B40 is found to be highly stable and can significantly enhance both the thermodynamics and kinetics of CO2 adsorption, while the adsorptions of N2, CH4 and H2 on the Li@B40 fullerene remain weak in comparison. Since B40 fullerene has been successfully synthesized in a most recent experiment, our results highlight a new promising material for CO2 capture and separation for future experimental validation.