Control of density of self-organized carbon nanotube arrays by catalyst pretreatment through plasma immersion ion implantation

A simple and effective method of controlling the growth of vertically aligned carbon nanotube arrays in a lowerature plasma is presented. Ni catalyst was pretreated by plasma immersion ion implantation prior to the nanotube growth by plasma-enhanced chemical vapor deposition. Both the size distribution and the areal density of the catalyst nanoparticles decrease due to the ion-surface interactions. Consequently, the resulting size distribution of the vertically aligned carbon nanotubes is reduced to 50 ∼ 100 nm and the areal density is lowered (by a factor of ten) to 10 8 cm -2, which is significantly different from the very-high-density carbon nanotube forests commonly produced by thermal chemical vapor deposition. The efficiency of this pretreatment is compared with the existing techniques such as neutral gas annealing and plasma etching. These results are highly relevant to the development of the next-generation nanoelectronic and optoelectronic devices that require effective control of the density of nanotube arrays.

Self-organized vertically aligned single-crystal silicon nanostructures with controlled shape and aspect ratio by reactive plasma etching

The formation of vertically aligned single-crystalline silicon nanostructures via "self-organized" maskless etching in Ar+ H 2 plasmas is studied. The shape and aspect ratio can be effectively controlled by the reactive plasma composition. In the optimum parameter space, single-crystalline pyramid-like nanostructures are produced; otherwise, nanocones and nanodots are formed. This generic nanostructure formation approach does not involve any external material deposition. It is based on a concurrent sputtering, etching, hydrogen termination, and atom/radical redeposition and can be applied to other nanomaterials.

Effect of input power and gas pressure on the roughening and selective etching of SiO2/Si surfaces in reactive plasmas

We report on the application low-temperature plasmas for roughening Si surfaces which is becoming increasingly important for a number of applications ranging from Si quantum dots to cell and protein attachment for devices such as "laboratory on a chip" and sensors. It is a requirement that Si surface roughening is scalable and is a single-step process. It is shown that the removal of naturally forming SiO2 can be used to assist in the roughening of the surface using a low-temperature plasma-based etching approach, similar to the commonly used in semiconductor micromanufacturing. It is demonstrated that the selectivity of SiO2 /Si etching can be easily controlled by tuning the plasma power, working gas pressure, and other discharge parameters. The achieved selectivity ranges from 0.4 to 25.2 thus providing an effective means for the control of surface roughness of Si during the oxide layer removal, which is required for many advance applications in bio- and nanotechnology.

Visible photoluminescence from plasma-synthesized SiO 2-buffered SiN x films : effect of film thickness and annealing temperature

The effect of the film thickness and postannealing temperature on visible photoluminescence (PL) from Si Nx films synthesized by plasma-assisted radio frequency magnetron sputtering on Si O2 buffer layers is investigated. It is shown that strong visible PL is achieved at annealing temperatures above 650 °C. The optimum annealing temperature for the maximum PL yield strongly depends on the film thickness and varies from 800 to 1200°C. A comparative composition-structure-property analysis reveals that the PL intensity is directly related to the content of the Si-O and Si-N bonds in the Si Nx films. Therefore, sufficient oxidation and moderate nitridation of Si Nx Si O2 films during the plasma-based growth process are crucial for a strong PL yield. Excessively high annealing temperatures lead to weakened Si-N bonds in thinner Si Nx films, which eventually results in a lower PL intensity.

Unconventional Ni–P alloy-catalyzed CVD of carbon coil-like micro- and nano-structures

Conventional catalyzed thermal CVD of carbon microcoils commonly suffers from poor control of the coil shape and morphology and rarely reaches the nanoscale size range. This article reports on an unconventional Ni-P alloy-catalyzed, high-throughput, highly reproducible CVD of ultra-long carbon coil-like micro- and nano-structures using acetylene precursor at relatively low process temperatures. Helical carbon microcoils with consistently uniform, circular cross-sections and a high degree of crystallinity have been synthesized at 750 °C. A further reduction of the temperature to 650 °C led to the growth of ultra-long (up to several mm) wave-like carbon nanofibers made of two nanowires with the diameters in the 100-200 nm range. The results of the XRD and Raman analysis reveal that the nanofibers feature only a slightly more disordered structure compared to the microcoils. Our results suggest that morphology and structure of the carbon coil-like micro- and nano-structures can be tailored by the appropriate alloying of the catalyst and the choice of the CVD process parameters.

Nanosphere monolayer-templated, ion-assisted nanofeature etching in dielectric materials : a numerical simulation of nanoscale ion flux topography

The results of numerical simulations of nanometer precision distributions of microscopic ion fluxes in ion-assisted etching of nanoscale features on the surfaces of dielectric materials using a self-assembled monolayer of spherical nanoparticles as a mask are presented. It is shown that the ion fluxes to the substrate and nanosphere surfaces can be effectively controlled by the plasma parameters and the external bias applied to the substrate. By proper adjustment of these parameters, the ion flux can be focused onto the areas uncovered by the nanospheres. Under certain conditions, the ion flux distributions feature sophisticated hexagonal patterns, which may lead to very different nanofeature etching profiles. The results presented are generic and suggest viable ways to overcome some of the limitations of the existing plasma-assisted nanolithography.

Self-assembly of uniform carbon nanotip structures in chemically active inductively coupled plasmas

Self-assembly of carbon nanotip (CNTP) structures on Ni-based catalyst in chemically active inductively coupled plasmas of CH 4 + H 2 + Ar gas mixtures is reported. By varying the process conditions, it appears possible to control the shape, size, and density of CNTPs, content of the nanocrystalline phase in the films, as well as to achieve excellent crystallinity, graphitization, uniformity and vertical alignment of the resulting nanostructures at substrate temperatures 300-500°C and low gas pressures (below 13.2 Pa). This study provides a simple and efficient plasma-enhanced chemical vapor deposition (PECVD) technique for the fabrication of vertically aligned CNTP arrays for electron field emitters.

High-yield atmospheric-pressure CVD of highly-uniform carbon nanocoils using Co–P catalyst nanoparticles prepared by electroless plating

A simple, fast and low-cost atmospheric-pressure chemical vapor deposition technique is developed to synthesize high-yield carbon nanocoils (CNCs) using amorphous Co–P alloy as catalyst and thiophene as nucleation agent. The uniform catalyst pattern with the mean particle size of 350 nm was synthesized using a simple electroless plating process. This uniformity of the Co–P nanoparticles results in a high yield, very uniform size/shape distribution and regular structure of CNCs at the optimum growth temperature of 800 ◦C. The yield of CNCs reaches ∼76%; 70% of the CNCs have fiber diameters approximately 250 nm. The CNC coil diameters and lengths are 450–550nm and 0.5–2mm, respectively. The CNC nucleation and growth mechanism are also discussed.

Annealing-free high-mobility diketopyrrolopyrrole-quaterthiophene copolymer for solution-processed organic thin film transistors

A donor-acceptor polymer semiconductor, PDQT, comprising diketopyrrolopyrrole (DPP) and β-unsubstituted quaterthiophene (QT) for organic thin film transistors (OTFTs) is reported. This polymer forms ordered layer-by-layer lamellar packing with an edge-on orientation in thin films even without thermal annealing. The strong intermolecular interactions arising from the fused aromatic DPP moiety and the DPP-QT donor-acceptor interaction facilitate the spontaneous self-assembly of the polymer chains into close proximity and form a large π-π overlap, which are favorable for intermolecular charge hopping. The well-interconnected crystalline grains form efficient intergranular charge transport pathways. The desirable chemical, electronic, and morphological structures of PDQT bring about high hole mobility of up to 0.97 cm2/(V·s) in OTFTs with polymer thin films annealed at a mild temperature of 100 °C and similarly high mobility of 0.89 cm2/(V·s) for polymer thin films even without thermal annealing.

Application of simulated annealing to data distribution for all-to-all comparison problems in homogeneous systems

Distributed systems are widely used for solving large-scale and data-intensive computing problems, including all-to-all comparison (ATAC) problems. However, when used for ATAC problems, existing computational frameworks such as Hadoop focus on load balancing for allocating comparison tasks, without careful consideration of data distribution and storage usage. While Hadoop-based solutions provide users with simplicity of implementation, their inherent MapReduce computing pattern does not match the ATAC pattern. This leads to load imbalances and poor data locality when Hadoop's data distribution strategy is used for ATAC problems. Here we present a data distribution strategy which considers data locality, load balancing and storage savings for ATAC computing problems in homogeneous distributed systems. A simulated annealing algorithm is developed for data distribution and task scheduling. Experimental results show a significant performance improvement for our approach over Hadoop-based solutions.

Multipurpose nanoporous alumina-carbon nanowall bi-dimensional nano-hybrid platform via catalyzed and catalyst-free plasma CVD

Simple, rapid, plasma-assisted synthesis of large-area arrays of vertically-aligned carbon nanowalls on highly-porous, transparent bare and gold-coated alumina membranes with the two pore sizes is reported. It is demonstrated that the complex patterns of vertically aligned nanowalls can nucleate and form different morphologies in the low-temperature plasmas. The process is stable, and the twofold change in the gas flow (10 and 20 sccm) does not noticeably influence the morphology of the nanowall pattern. Application of a thin (5 nm) gold layer to nanoporous membrane prior to the nanowall growth allows controlling the network morphology.

Time evolution of graphene growth on SiC as a function of annealing temperature

We followed by X-ray Photoelectron Spectroscopy (XPS) the time evolution of graphene layers obtained by annealing 3C SiC(111)/Si(111) crystals at different temperatures. The intensity of the carbon signal provides a quantification of the graphene thickness as a function of the annealing time, which follows a power law with exponent 0.5. We show that a kinetic model, based on a bottom-up growth mechanism, provides a full explanation to the evolution of the graphene thickness as a function of time, allowing to calculate the effective activation energy of the process and the energy barriers, in excellent agreement with previous theoretical results. Our study provides a complete and exhaustive picture of Si diffusion into the SiC matrix, establishing the conditions for a perfect control of the graphene growth by Si sublimation.