Copper-capped carbon nanocones on silicon : plasma-enabled growth control

Controlled self-organized growth of vertically aligned carbon nanocone arrays in a radio frequency inductively coupled plasma-based process is studied. The experiments have demonstrated that the gaps between the nanocones, density of the nanocone array, and the shape of the nanocones can be effectively controlled by the process parameters such as gas composition (hydrogen content) and electrical bias applied to the substrate. Optical measurements have demonstrated lower reflectance of the nanocone array as compared with a bare Si wafer, thus evidencing their potential for the use in optical devices. The nanocone formation mechanism is explained in terms of redistribution of surface and volumetric fluxes of plasma-generated species in a developing nanocone array and passivation of carbon in narrow gaps where the access of plasma ions is hindered. Extensive numerical simulations were used to support the proposed growth mechanism.

Different nanostructures from different plasmas : nanoflowers and nanotrees on silicon

Using the advanced radio-frequency plasma-assisted magnetron deposition system, various nanostructures such as nanoflowers of carbon nanotubes, ZnO nanobelts, and silicon nanotrees were successfully synthesized. In this paper, we present the photographs of ICP and magnetron discharges, the photograph of a complex plasma structure, and the SEM images of various nanostructures synthesized in the system with magnetron and ICP sources operating simultaneously.

Plasma-enabled growth of separated, vertically aligned copper-capped carbon nanocones on silicon

The formation of vertically aligned, clearly separated, copper-capped carbon nanocones with a length of up to 500 nm and base diameter of about 150 nm via three-stage process involving magnetron sputtering, N2 plasma treatment, and CH4 + N2 plasma growth is studied. The width of gaps between the nanocones can be controlled by the gas composition. The nanocone formation mechanism is explained in terms of strong passivation of carbon in narrow gaps, where the access of plasma ions is hindered and the formation of large Cn H2n+2 molecules is possible. This plasma-enabled approach can be used to fabricate nanoelectronic, nanofluidic, and optoelectronic components and devices. © 2010 American Institute of Physics.

Silicon on silicon : self-organized nanotip arrays formed in reactive Ar+H2 plasmas

The formation of arrays of vertically aligned nanotips on a moderately heated (up to 500 degrees C) Si surface exposed to reactive low-temperature radio frequency (RF) Ar+H(2) plasmas is studied. It is demonstrated that the nanotip surface density, aspect ratio and height dispersion strongly depend on the substrate temperature, discharge power, and gas composition. It is shown that nanotips with aspect ratios from 2.0 to 4.0 can only be produced at a higher RF power density (41.7 mW cm(-3)) and a hydrogen content of about 60%, and that larger aspect ratios can be achieved at substrate temperatures of about 300 degrees C. The use of higher (up to 500 degrees C) temperatures leads to a decrease of the aspect ratio but promotes the formation of more uniform arrays with the height dispersion decreasing to 1.5. At lower (approximately 20 mW cm(-3)) RF power density, only semispherical nanodots can be produced. Based on these experimental results, a nanotip formation scenario is proposed suggesting that sputtering, etching, hydrogen termination, and atom/radical re-deposition are the main concurrent mechanisms for the nanostructure formation. Numerical calculations of the ion flux distribution and hydrogen termination profiles can be used to predict the nanotip shapes and are in a good agreement with the experimental results. This approach can be applied to describe the kinetics of low-temperature formation of other nanoscale materials by plasma treatment.

Controlling the surface roughness of epitaxial SiC on silicon

The surface of cubic silicon carbide (3C-SiC) hetero-epitaxial films grown on the (111) surface of silicon is a promising template for the subsequent epitaxial growth of III-V semiconductor layers and graphene. We investigate growth and post-growth approaches for controlling the surface roughness of epitaxial SiC to produce an optimal template. We first explore 3C-SiC growth on various degrees of offcut Si(111) substrates, although we observe that the SiC roughness tends to worsen as the degree of offcut increases. Hence we focus on post-growth approaches available on full wafers, comparing chemical mechanical polishing (CMP) and a novel plasma smoothening process. The CMP leads to a dramatic improvement, bringing the SiC surface roughness down to sub-nanometer level, though removing about 200 nm of the SiC layer. On the other hand, our proposed HCl plasma process appears very effective in smoothening selectively the sharpest surface topography, leading up to 30% improvement in SiC roughness with only about 50 nm thickness loss. We propose a simple physical model explaining the action of the plasma smoothening.

Effect of nano-silica on the mechanical and transport properties of lightweight concrete

This paper investigated the influence of nano-silica (NS) on the mechanical and transport properties of lightweight concrete (LWC). The resistance of LWC to water and chloride ions penetration was enhanced despite strength marginally increased. Water penetration depth, moisture sorptivity, chloride migration and diffusion coefficient was reduced by 23% and 49%, 23% and 10%, 5% and 0%, 22% and 12% compared to the two reference LWC mixes (pure cement and 60% slag blended cement), respectively with 1% NS. Such improvements were attributed to more compact microstructures because the micropore system was refined and the interface between aggregates and paste was enhanced.

Nanolines on silicon surfaces

Although a wide range of periodic surface nets can be grown on low index silicon surfaces, only a few of these have quasi-one dimensional symmetry. If high index silicon surfaces, such as (553) and (557), are used instead, the surface unit cell contains steps. It is possible to fabricate a number of quasi-one dimensional nanoline systems on the terraces and some of these have nested energy bands near the Fermi level. These nano-scale systems may support exotic many-electron states produced by enhanced electron correlations and a reduction in electron screening in one spatial dimension. In this paper, our groups' experimental and theoretical studies of nanolines phases, grown on both low index and vicinal silicon surfaces are reviewed. These studies give us insight into the electronic properties of artificial nanoline structures.

Dual yolk-shell structure of carbon and silica-coated silicon for high-performance lithium-ion batteries

Silicon batteries have attracted much attention in recent years due to their high theoretical capacity, although a rapid capacity fade is normally observed, attributed mainly to volume expansion during lithiation. Here, we report for the first time successful synthesis of Si/void/SiO2/void/C nanostructures. The synthesis strategy only involves selective etching of SiO2 in Si/SiO2/C structures with hydrofluoric acid solution. Compared with reported results, such novel structures include a hard SiO2-coated layer, a conductive carbon-coated layer, and two internal void spaces. In the structures, the carbon can enhance conductivity, the SiO2 layer has mechanically strong qualities, and the two internal void spaces can confine and accommodate volume expansion of silicon during lithiation. Therefore, these specially designed dual yolk-shell structures exhibit a stable and high capacity of 956 mA h g−1 after 430 cycles with capacity retention of 83%, while the capacity of Si/C core-shell structures rapidly decreases in the first ten cycles under the same experimental conditions. The novel dual yolk-shell structures developed for Si can also be extended to other battery materials that undergo large volume changes.

Selective growth of carbon nanotubes on bare silicon : MWNT sinthesys on patterned substrates without predeposition of metal catalyst

Carbon nanotubes (CNTs) are expected to become the ideal constituent of many technologes, in particular for future generation electronics. This considerable interest is due to their unique electrical and mechanical properties. They show indeed super-high current-carrying capacity, ballistic electron transport and good field-emission properties. Then, these superior features make CNTs the most promising building blocks for electronic devices, as organic solar cells and organic light emitting devices (OLED). By using Focused Ion Beam (FIB) patterning it is possible to a obtain a high control on position, relative distances and diameter of CNTs. The present work shows how to grow three-dimensional architecture made of vertical-aligned CNTs directly on silicon. Thanks to the higher activity of a pre-patterned surface the synthesis process results very quick, cheap and simple. Such large area growths of CNTs could be used in preliminary test for application as electrodes for organic solar cells.

PDMS spreading morphological patterns on substrates of different hydrophilicity in air vacuum and water

In paper has been to investigate the morphological patterns and kinetics of PDMS spreading on silicon wafer using combination of techniques like ellipsometry, atomic force microscope (AFM), scanning electron microscope (SEM) and optical microscopy. A macroscopic silicone oil drops as well as PDMS water based emulsions were studied after deposition on a flat surface of silicon wafer in air, water and vacuum. our own measurements using an imaging ellipsometer, which also clearly shows the presence of a precursor film. The diffusion constant of this film, measured with a 60 000 cS PDMS sample spreading on a hydrophilic silicon wafer, is Df = 1.4  10-11 m2/s. Regardless of their size, density and method of deposition, droplets on both types of wafer (hydrophilic and hydrophobic) flatten out over a period of many hours, up to 3 days. During this process neighbouring droplets may coalesce, but there is strong evidence that some of the PDMS from the droplets migrates into a thin, continuous film that covers the surface in between droplets. The thin film appears to be ubiquitous if there has been any deposition of PDMS. However, this statement needs further verification. One question is whether the film forms immediately after forced drying, or whether in some or all cases it only forms by spreading from isolated droplets as they slowly flatten out.

Influence of smectite suspension structure on sheet orientation in dry sediments : XRD and AFM applications

The structure-building phenomena within clay aggregates are governed by forces acting between clay particles. Measurements of such forces are important to understand in order to manipulate the aggregate structure for applications such as dewatering of mineral processing tailings. A parallel particle orientation is required when conducting XRD investigation on the oriented samples and conduct force measurements acting between basal planes of clay mineral platelets using at. force microscopy (AFM). To investigate how smectite clay platelets were oriented on silicon wafer substrate when dried from suspension range of methods like SEM, XRD and AFM were employed. From these investigations, we conclude that high clay concns. and larger particle diams. (up to 5 μm) in suspension result in random orientation of platelets in the substrate. The best possible laminar orientation in the clay dry film, represented in the XRD 0 0 1/0 2 0 intensity ratio of 47 was obtained by drying thin layers from 0.02 wt.% clay suspensions of the natural pH. Conducted AFM investigations show that smectite studied in water based electrolytes show very long-range repulsive forces lower in strength than electrostatic forces from double-layer repulsion. It was suggested that these forces may have structural nature. Smectite surface layers rehydrate in water environment forms surface gel with spongy and cellular texture which cushion approaching AFM probe. This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gel layer, structural linkages are forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing new formed structure apart involves larger adhesion-like forces measured in retrieval. It is also suggested that these effect may be enhanced by the nano-clay particles interaction.

Catalyst-free growth and optical properties of vertically aligned ZnO Nanorod Arrays on Si substrates covered with thin buffer layers

Vertically aligned ZnO nanorods have been grown on silicon substrates pre-coated with thin, less than 10 nm, textured ZnO seeding layers via a vapor-solid mechanism. The ZnO seeding layers, which were essential for vertical alignment of ZnO nanorods without using any metal catalyst, were prepared by decomposing zinc acetate. The structure and the luminescence properties of the ZnO nanorods synthesized onto ZnO seeding layers were investigated and their morphologies were compared with those of single-crystalline GaN substrates and silicon substrates covered with sputtered ZnO flms. Patterning of ZnO seed layers using photolithography allowed the fabrication of patterned ZnO-nanorod arrays.

Hydrogen gas sensing properties of Pt/Ta2O5 Schottky diodes based on Si and SiC substrates

In this paper, we fabricated Pt/tantalum oxide (Ta2O5) Schottky diodes for hydrogen sensing applications. Thin (4 nm) layer of Ta2O5 was deposited on silicon (Si) and silicon carbide (SiC) substrates by radio frequency (RF) sputtering technique. We compared the performance of these sensors at different elevated temperatures of 100 ∘C and 150 ∘C. At these temperatures, the sensor based on SiC exhibited a larger sensitivity while the sensor based on Si exhibited a faster response toward hydrogen gas. We discussed herein, the responses exhibited by the Pt/Ta2O5 based Schottky diodes demonstrated a promising potential for hydrogen sensing applications.

Hardness of silicon nitride thin films characterised by nanoindentation and nanoscratch deconvolution methods

Plasma enhanced chemical vapour deposition silicon nitride thin films are widely used in microelectromechanical system devices as structural materials because the mechanical properties of those films can be tailored by adjusting deposition conditions. However, accurate measurement of the mechanical properties, such as hardness, of films with thicknesses at nanometric scale is challenging. In the present study, the hardness of the silicon nitride films deposited on silicon substrate under different deposit conditions was characterised using nanoindentation and nanoscratch deconvolution methods. The hardness values obtained from the two methods were compared. The effect of substrate on the measured results was discussed.

Hydrogen gas sensing properties of Pt/Ta2O5 Schottky diodes based on Si and SiC substrates

We developed Pt/tantalum oxide (Ta2O5) Schottky diodes for hydrogen sensing applications. Thin layer (4 nm) of Ta2O5 was deposited on silicon (Si) and silicon carbide (SiC) substrates using the radio frequency sputtering technique. We compared the performance of these sensors at different temperatures of 100 °C and 150 °C. At these operating temperatures, the sensor based on SiC exhibited a larger sensitivity, whilst the sensor based on Si exhibited a faster response toward hydrogen gas. We discussed herein, the experimental results obtained for these Pt/Ta2O5 based Schottky diodes exhibited that they are promising candidates for hydrogen sensing applications.