Uniaxially aligned ceramic nanofibers obtained by chemical mechanical processing

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

A one-step technique to prepare aligned arrays of carbon nanotubes

A simple effective pyrolysis technique has been developed to synthesize aligned arrays of multi-walled carbon nanotubes (MWCNTs) without using any carrier gas in a single-stage furnace at 700 °C. This technique eliminates nearly the entire complex and expensive machinery associated with other extensively used methods for preparation of CNTs such as chemical vapour deposition (CVD) and pyrolysis. Carbon source materials such as xylene, cyclohexane, camphor, hexane, toluene, pyridine and benzene have been pyrolyzed separately with the catalyst source material ferrocene to obtain aligned arrays of MWCNTs. The synthesized CNTs have been characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Raman spectroscopy. In this technique, the need for the tedious and time-consuming preparation of metal catalysts and continuously fed carbon source material containing carrier gas can be avoided. This method is a single-step process where not many parameters are required to be monitored in order to prepare aligned MWCNTs. For the production of CNTs, the technique has great advantages such as low cost and easy operation.

Preparation of ferrite MFe2O4 (M = Co, Ni) ribbons with nanoporous structure and their magnetic properties

Spinel ferrite, MFe2O4 (M = Co, Ni), ribbons with nanoporous structure were prepared by electrospinning combined with sol-gel technology. The ribbons were formed through the agglomeration of magnetic nanoparticles with PVP as the structure directing template. The length of the polycrystalline ribbons can reach millimeters, and the width of the ribbons can be tuned from several micrometers to several hundred nanometers by changing the concentration of precursor. The nanoporous structure was formed during the decomposition of PVP and inorganic salts.

Preparation of one-dimensional CoFe2O4 nanostructures and their magnetic properties

Cobalt ferrite one-dimensional nanostructures (nanoribbons and nanofibers) were prepared by electrospinning combined with sol-gel technology. The nanoribbons and nanofibers were formed through assembling magnetic nanoparticles with poly(vinyl pyrrolidone) (PVP) as the structure-directing template. Nanoribbons and nanofibers were obtained after calcining the precursor nanoribbons at different temperatures. Successive Ostwald ripening processes occur during the formation of CoFe2O4 nanoribbons and nanofibers. The sizes of nanoparticles varied with calcination temperatures, which leads to different one-dimensional structures and variable magnetic properties. These novel magnetic one-dimensional structures can potentially be used in nanoelectronic devices, magnetic sensors, and flexible magnets.

Tailoring the microstructure and mechanical properties of arrays of aligned multiwall carbon nanotubes by utilizing different hydrogen concentrations during synthesis

We synthesize vertically aligned arrays of carbon nanotubes (CNTs) in a chemical vapor deposition system with floating catalyst, using different concentrations of hydrogen in the gas feedstock. We report the effect of different hydrogen concentrations on the microstructure and mechanical properties of the resulting material. We show that a lower hydrogen concentration during synthesis results in the growth of stiffer CNT arrays with higher average bulk density. A lower hydrogen concentration also leads to the synthesis of CNT arrays that can reach higher peak stress at maximum compressive strain, and dissipate a larger amount of energy during compression. The individual CNTs in the arrays synthesized with a lower hydrogen concentration have, on average, larger outer diameters (associated with the growth of CNTs with a larger number of walls), but present a less uniform diameter distribution. The overall heights of the arrays and their strain recovery after compression have been found to be independent of the hydrogen concentration during growth. (C) 2011 Elsevier Ltd. All rights reserved.

Bending and bundling of metal-free vertically aligned ZnO nanowires due to electrostatic interaction

Bending and bundling was observed from vertically aligned arrays of ZnO nanowires with flat (0001) top surfaces, which were synthesized using a vapor-phase method without metal catalysts. Sufficient evidence was found to exclude electron-beam bombardment during scanning electron microscopy as a cause for bending and bundling. We attribute the bending and bundling to electrostatic interactions due to charged (0001) polar surfaces, and also discussed the threshold surface charge densities for the bending and bundling based on a simple cantilever-bending model. Some growth features were indicative of the operation of electrostatic interactions during the growth.

Studies towards synthesis, evolution and alignment characteristics of dense, millimeter long multiwalled carbon nanotube arrays

We report the synthesis of aligned arrays of millimeter long carbon nanotubes (CNTs), from benzene and ferrocene as the molecular precursor and catalyst respectively, by a one-step chemical vapor deposition technique. The length of the grown CNTs depends on the reaction temperature and increases from similar to 85 mu m to similar to 1.4 mm when the synthesis temperature is raised from 650 to 1100 degrees C, while the tube diameter is almost independent of the preparation temperature and is similar to 80 nm. The parallel arrangement of the CNTs, as well as their tube diameter can be verified spectroscopically by small angle X-ray scattering (SAXS) studies. Based on electron diffraction scattering (EDS) studies of the top and the base of the CNT films, a root growth process can be deduced.

Patterning of crystalline organic materials by electro-hydrodynamic lithography.

The control of semi-crystalline polymers in thin films and in micrometer-sized patterns is attractive for (opto-)electronic applications. Electro-hydrodynamic lithography (EHL) enables the structure formation of organic crystalline materials on the micrometer length scale while at the same time exerting control over crystal orientation. This gives rise to well-defined micro-patterned arrays of uniaxially aligned polymer crystals. This study explores the interplay of EHL structure formation with crystal alignment and studies the mechanisms that give rise to crystal orientation in EHL-generated structures.

Electrical and optical studies on carbon nanotube arrays

Single-walled carbon nanotubes (SWNTs) have been studied as a prominent class of high performance electronic materials for next generation electronics. Their geometry dependent electronic structure, ballistic transport and low power dissipation due to quasi one dimensional transport, and their capability of carrying high current densities are some of the main reasons for the optimistic expectations on SWNTs. However, device applications of individual SWNTs have been hindered by uncontrolled variations in characteristics and lack of scalable methods to integrate SWNTs into electronic devices. One relatively new direction in SWNT electronics, which avoids these issues, is using arrays of SWNTs, where the ensemble average may provide uniformity from device to device, and this new breed of electronic material can be integrated into electronic devices in a scalable fashion. This dissertation describes (1) methods for characterization of SWNT arrays, (2) how the electrical transport in these two-dimensional arrays depend on length scales and spatial anisotropy, (3) the interaction of aligned SWNTs with the underlying substrate, and (4) methods for scalable integration of SWNT arrays into electronic devices. The electrical characterization of SWNT arrays have been realized by polymer electrolyte-gated SWNT thin film transistors (TFTs). Polymer electrolyte-gating addresses many technical difficulties inherent to electrical characterization by gating through oxide-dielectrics. Having shown polymer electrolyte-gating can be successfully applied on SWNT arrays, we have studied the length scaling dependence of electrical transport in SWNT arrays. Ultrathin films formed by sub-monolayer surface coverage of SWNT arrays are very interesting systems in terms of the physics of two-dimensional electronic transport. We have observed that they behave qualitatively different than the classical conducting films, which obey the Ohm’s law. The resistance of an ultrathin film of SWNT arrays is indeed non-linear with the length of the film, across which the transport occurs. More interestingly, a transition between conducting and insulating states is observed at a critical surface coverage, which is called percolation limit. The surface coverage of conducting SWNTs can be manipulated by turning on and off the semiconductors in the SWNT array, leading to the operation principle of SWNT TFTs. The percolation limit depends also on the length and the spatial orientation of SWNTs. We have also observed that the percolation limit increases abruptly for aligned arrays of SWNTs, which are grown on single crystal quartz substrates. In this dissertation, we also compare our experimental results with a two-dimensional stick network model, which gives a good qualitative picture of the electrical transport in SWNT arrays in terms of surface coverage, length scaling, and spatial orientation, and briefly discuss the validity of this model. However, the electronic properties of SWNT arrays are not only determined by geometrical arguments. The contact resistances at the nanotube-nanotube and nanotube-electrode (bulk metal) interfaces, and interactions with the local chemical groups and the underlying substrates are among other issues related to the electronic transport in SWNT arrays. Different aspects of these factors have been studied in detail by many groups. In fact, I have also included a brief discussion about electron injection onto semiconducting SWNTs by polymer dopants. On the other hand, we have compared the substrate-SWNT interactions for isotropic (in two dimensions) arrays of SWNTs grown on Si/SiO2 substrates and horizontally (on substrate) aligned arrays of SWNTs grown on single crystal quartz substrates. The anisotropic interactions associated with the quartz lattice between quartz and SWNTs that allow near perfect horizontal alignment on substrate along a particular crystallographic direction is examined by Raman spectroscopy, and shown to lead to uniaxial compressive strain in as-grown SWNTs on single crystal quartz. This is the first experimental demonstration of the hard-to-achieve uniaxial compression of SWNTs. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is non-uniform and can be larger than 1% locally at room temperature. Effects of device fabrication steps on the non-uniform strain are also examined and implications on electrical performance are discussed. Based on our findings, there are discussions about device performances and designs included in this dissertation. The channel length dependences of device mobilities and on/off ratios are included for SWNT TFTs. Time response of polymer-electrolyte gated SWNT TFTs has been measured to be ~300 Hz, and a proof-of-concept logic inverter has been fabricated by using polymer electrolyte gated SWNT TFTs for macroelectronic applications. Finally, I dedicated a chapter on scalable device designs based on aligned arrays of SWNTs, including a design for SWNT memory devices.

Nanostructured ferroelectric materials

Nanostructured materials are central to the evolution of future electronics and information technologies. Ferroelectrics have already been established as a dominant branch in the electronics sector because of their diverse application range such as ferroelectric memories, ferroelectric tunnel junctions, etc. The on-going dimensional downscaling of materials to allow packing of increased numbers of components onto integrated circuits provides the momentum for the evolution of nanostructured ferroelectric materials and devices. Nanoscaling of ferroelectric materials can result in a modification of their functionality, such as phase transition temperature or Curie temperature (TC), domain dynamics, dielectric constant, coercive field, spontaneous polarisation and piezoelectric response. Furthermore, nanoscaling can be used to form high density arrays of monodomain ferroelectric nanostructures, which is desirable for the miniaturisation of memory devices. This thesis details the use of various types of nanostructuring approaches to fabricate arrays of ferroelectric nanostructures, particularly non-oxide based systems. The introductory chapter reviews some exemplary research breakthroughs in the synthesis, characterisation and applications of nanoscale ferroelectric materials over the last decade, with priority given to novel synthetic strategies. Chapter 2 provides an overview of the experimental methods and characterisation tools used to produce and probe the properties of nanostructured antimony sulphide (Sb2S3), antimony sulpho iodide (SbSI) and lead titanate zirconate (PZT). In particular, Chapter 2 details the general principles of piezoresponse microscopy (PFM). Chapter 3 highlights the fabrication of arrays of Sb2S3 nanowires with variable diameters using newly developed solventless template-based approach. A detailed account of domain imaging and polarisation switching of these nanowire arrays is also provided. Chapter 4 details the preparation of vertically aligned arrays of SbSI nanorods and nanowires using a surface-roughness assisted vapour-phase deposition method. The qualitative and quantitative nanoscale ferroelectric properties of these nanostructures are also discussed. Chapter 5 highlights the fabrication of highly ordered arrays of PZT nanodots using block copolymer self-assembled templates and their ferroelectric characterisation using PFM. Chapter 6 summarises the conclusions drawn from the results reported in chapters 3, 4 and 5 and the future work.

Chain extension in electrospun polystyrene fibres: a SANS study

Small angle neutron scattering techniques were used to quantify the size and shape of the chain conformation in electrospun fibres of atactic polystyrene prepared from solutions in methyl ethyl ketone. Aligned arrays of fibres were collected onto a rotating collector with tangential velocity varying between 0 ms-1 and approximately 15 ms-1. The measured radii of gyration of the polystyrene chains were found to be slightly higher than that expected for samples prepared from solutions in the concentrated regime. The ratio of the radius of gyration parallel and perpendicular to the chain axis was found to be approximately 1.05 in contrast to the substantial macroscopic shape transformation intrinsic to electrospinning. When the tangential velocity of the rotating collector was greater than the flight velocity of the fibres (ca. 4 ms-1), a further extension of the polymer chains was observed with a ratio of the radii of gyration increasing to 1.20 at the highest collector speeds. It is proposed that the heterogeneous processes involved, particularly solvent evaporation and the formation of a polymer skin during electrospinning play a significant role in determining the level of molecular anisotropy in the fibres.

Carbon nanotubes in electrospun polyethylene oxide nanofibres: a potential route to conducting nanofibres

Polyethylene oxide solution containing multi-walled carbon nanotubes have been electrospun onto a rotating collector to produce highly aligned arrays of electrospun nanofibers ranging in diameters from (200 – 360) nanometres. The addition of a surfactant (Triton X-100)is highly effective in dispersing carbon nanotube within an aqueous solution of polyethylene oxide and the resulting mixture can be electrospun without excessive clumping to produce nanofibers containing high loadings of nanotubes; in this case up to 5% wt thereby providing an effective route to electrically conductive nanofibres.

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.

Uniform, dense arrays of vertically aligned, large-diameter single-walled carbon nanotubes

Precisely controlled reactive chemical vapor synthesis of highly uniform, dense arrays of vertically aligned single-walled carbon nanotubes (SWCNTs) using tailored trilayered Fe/Al2O3/SiO2 catalyst is demonstrated. More than 90% population of thick nanotubes (>3 nm in diameter) can be produced by tailoring the thickness and microstructure of the secondary catalyst supporting SiO2 layer, which is commonly overlooked. The proposed model based on the atomic force microanalysis suggests that this tailoring leads to uniform and dense arrays of relatively large Fe catalyst nanoparticles on which the thick SWCNTs nucleate, while small nanotubes and amorphous carbon are effectively etched away. Our results resolve a persistent issue of selective (while avoiding multiwalled nanotubes and other carbon nanostructures) synthesis of thick vertically aligned SWCNTs whose easily switchable thickness-dependent electronic properties enable advanced applications in nanoelectronic, energy, drug delivery, and membrane technologies.