In‐situ synthesis of transparent and conductive carbon nanotube networks

A method for the production of transparent carbon nanotube networks (CNTNs) over transparent substrates was developed. In this method, CNTNs were grown directly in the target surface by applying the catalyst in specific zones of the substrate through lithographic techniques. The networks can be also transferred from the original substrate to other surfaces. The newly grown carbon nanotubes have a very high aspect ratio (>50000). Thus far, networks with an optical transmittance of 94% at 550 nm and a surface resistivity of 3.6 kΩ/sq have been produced. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication of metal nano-structures using anodic alumina membranes grown in phosphoric acid solution: Tailoring template morphology

The influence of experimental parameters on the morphology of the porous structure and on the formation kinetics has been investigated for anodic alumina membranes (AAM) grown in aqueous H₃PO₄ at 160 V. It was found that pore aspect ratio and membrane porosity on the solution-side surface are influenced by tensiostatic charge, bath temperature and the presence of Al³⁺ ions in solution. Morphological and kinetic data, recorded in different conditions, give useful information on the growth mechanism of pore channels in phosphoric acid solution.Nickel nano-structures have been fabricated using AAM as template. Electroless deposition, performed by adding the reducing agent to a suitable bath in several steps, resulted in the formation of short metal nanotubes (about 5 μm long) in the upper part of the channels. Long Ni nanowires (up to 25 μm) with aspect ratio higher than 100 were obtained by pulsed unipolar electrodeposition from a Watt bath. In this case, both the influence of some experimental parameters on the nanowires growth and the fast current transients during the electrodeposition steps were analyzed.     

Field emission from single-walled carbon nanotubes aligned on a gold plate using a self-assembly monolayer

Field emission from single-walled carbon nanotubes (SWNTs) aligned on a patterned gold surface is reported. The SWNT emitters were prepared at room temperature by a self-assembly monolayer technique. SWNTs were cut into sub-micron lengths by sonication in an acidic solution. Cut SWNTs were attached to the gold surface by the reaction between the thiol groups and the gold surface. The field-emission measurements showed that the turn-on field was 4.8 V/μm at an emission current density of 10 μA/cm². The current density was 0.5 mA/cm² at 6.6 V/μm. This approach provides a novel route for fabricating CNT-based field-emission displays. Received: 3 May 2002 / Accepted: 6 May 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr     

Growth of multiwalled carbon nanotube arrays by chemical vapour deposition over iron catalyst and the effect of growth parameters

Multiwalled carbon nanotube (CNT) arrays were grown by catalytic thermal decomposition of acetylene, over Fe-catalyst deposited on Si-wafer in the temperature range 700-750 °C. The growth parameters were optimized to obtain dense arrays of multiwalled CNTs of uniform diameter. The vertical cross-section of the grown nanotube arrays reveals a quasi-vertical alignment of the nanotubes. The effect of varying the thickness of the catalyst layer and the effect of increasing the growth duration on the morphology and distribution of the grown nanotubes were studied. A scotch-tape test to check the strength of adhesion of the grown CNTs to the Si-substrate surface reveals a strong adhesion between the grown nanotubes and the substrate surface. Transmission electron microscopy analysis of the grown CNTs shows that the grown CNTs are multiwalled nanotubes with a bamboo structure, and follow the base-growth mechanism.     

Electrocatalytic oxidation of formaldehyde on platinum well-dispersed into single-wall carbon nanotube/polyaniline composite film

Single-walled carbon nanotubes (SWNTs)/polyaniline (PANI) composite films with good uniformity and dispersion were prepared by electrochemical polymerization of aniline containing well-dissolved SWNTs. The composite films were dispersed Pt by electrodeposition technique. The presence of SWNTs and platinum in the composite film was confirmed by XRD analysis and scanning electron microscopy (SEM). Four-point probe investigation and electrochemical impedance spectroscopy (EIS) revealed that the well arrangement of PANI coated SWNTs in these films enhanced electric conductivity and facilitated the charge-transfer of the composite films. Cyclic voltammogram (CV) and chronoamperogram showed that Pt-modified SWNT/PANI composite film performs higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formaldehyde oxidation. The results imply that the SWNT/PANI composite film as a promising support material improves the electrocatalytic activity for formaldehyde oxidation greatly.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Effect of surface fluorination on the photocatalytic and photo-induced hydrophilic properties of porous TiO2 films

Porous surface-fluorinated TiO₂ (F-TiO₂) films were prepared through PEG modified sol-gel method and surface fluorination. The as-prepared films were characterized with XRD, FTIR, AFM, XPS and UV-vis DRS. The effects of surface fluorination on the photocatalytic activity and hydrophilicity of porous TiO₂ film were studied by photocatalytic degradation of rhodamine B (RhB) as well as water contact angle (CA) on porous TiO₂ film. The results showed that the surface fluorination increased the adsorption of RhB on the porous TiO₂ film and enhanced the photocatalytic degradation of RhB. The concentration and pH of NaF solutions affected much the photocatalytic activity of porous TiO₂ film. Porous F-TiO₂ film prepared in 40 mM NaF solution at pH 4.0 showed the highest photocatalytic activity. Because of its porous structure, the porous F-TiO₂ film had original water CA of 22.7°, which is much smaller than that of normal F-TiO₂ film. Under UV light irradiation, the water CA of porous F-TiO₂ film decreased to 5.1° in 90 min.     

Microstructured horizontal alumina pore arrays as growth templates for large area few and single nanowire devices

We demonstrate the fabrication of horizontally aligned and well‐defined nanopore structures by anodic oxidation of aluminum thin films and micro stripes on a Si substrate. We are able to control both, the pore diameters and interpore distances from 10 nm to 130 nm and from 30 nm to 275 nm, respectively. The anisotropy of the system induces some deviations in the pore configuration from the typical honeycomb structure. By decreasing the dimensions of the Al structures, the final pore diameter and interpore distance remains constant, enabling the transition from multiple to a few nanowire porous structures. Finally, we successfully filled the nanopores by pulsed electroplating, as demonstrated both by Scanning Electron Microscopy and by current–voltage measurements. Having full control over the size, the density, the position and the orientation of the porous structure, our approach is promising for many exciting applications, including nanoelectronics and sensing. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct writing of carbon nanotube patterns by laser-induced chemical vapor deposition on a transparent substrate

Dot array and line patterns of multi-walled carbon nanotubes (MWCNTs) were successfully grown by laser-induced chemical vapor deposition (LCVD) on a transparent substrate at room temperature. In the proposed technique, a Nd:YVO₄ laser with a wavelength of 532 nm irradiates the backside of multiple catalyst layers (Ni/Al/Cr) through a transparent substrate to induce a local temperature rise, thereby allowing the direct writing of dense dot and line patterns of MWCNTs below 10 μm in size to be produced with uniform density on the controlled positions. In this LCVD method, a multiple-catalyst-layer with a Cr thermal layer is the central component for enabling the growth of dense MWCNTs with good spatial resolution.     

Area effect of patterned carbon nanotube bundle on field electron emission characteristics

Patterned carbon nanotube (CNT) bundles were fabricated using thermal chemical vapor deposition (CVD) method. Patterns of different diameters and distances were defined on Si(100) substrates using photolithography. CNT bundle height was controlled using different acetylene (C₂H₂) flow times. The inter-bundle distance of CNTs to CNT bundle height ratio was maintained at approximately 2, a number predicted to have a maximum field emission for CNT, and left the patterned CNT bundle area as a variable parameter. The relationship between CNT bundle area and the field electron emission characteristics was studied. The lowest threshold electric field (E_th) of 0.7 V/μm was obtained when the total area of patterned CNT bundles was approximately 46%. The result shows that there is an optimal CNT bundle area for electron field emission.     

Comparing the weak and strong gate‐coupling regimes for nanotube and graphene transistors

We present an experimental and theoretical comparison of the weak and strong gate‐coupling regimes that arise for carbon nanotube (CNT) and graphene field‐effect transistors (FETs) in back‐gated and liquid‐gated configuration, respectively. We find that whereas the back‐gate efficiency is suppressed for a liquid‐gated CNT FET, the back gate is still effective in case of a liquid‐gated graphene FET. We calculate the gate‐induced Fermi‐level shifts and induced charge densities. In both strong and weak coupling regimes, nonlinearities occur in the gate dependence of these parameters, which can significantly influence the electronic transport. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrochemical synthesis of polypyrrole/carbon nanotube nanoscale composites using well-aligned carbon nanotube arrays

Polypyrrole/carbon nanotube nanoscale composites were successfully fabricated by electrochemical deposition of polypyrrole over each of the carbon nanotubes in well-aligned large arrays. The thickness of the polypyrrole coating can be easily controlled by the value of the film-formation charge. For both thin (low film-formation charge) and thick (high film-formation charge) films, the polypyrrole coating on the surface of each nanotube is very uniform throughout the entire length, as observed by transmission electron microscopy. Received: 2 May 2001 / Accepted: 4 May 2001 / Published online: 20 June 2001     

Viscoelastic properties of multiwalled carbon nanotube solutions

We have prepared solutions of multiwalled carbon nanotubes in very low vapour pressure solvents (a mixture of chlorinated biphenyls). The solutions are stable and show no sign of precipitation for six months. Rheological measurements using a modified Birnboim apparatus with annular and Sogel-Pochetino geometries have been performed. Using time-temperature superposition we obtained the real and imaginary part of the complex viscosity coefficient in a frequency range covering eight orders of magnitude and a temperature range from 5 to 50 ^○C. The data shows unexpected changes in the solution with temperature: for T below 30 ^○C there appears to be some reorganization or clustering. This self-organization could result in a useful technique to improve the electronic properties of polymer/carbon nanotubes composites used in organic electronic devices.     

Synthesis, integration, and electrical properties of individual single-walled carbon nanotubes

High-quality single-walled carbon nanotubes (SWNTs) are synthesized by chemical vapor deposition (CVD) of methane on silicon-dioxide substrates at controlled locations using patterned catalytic islands. With the synthesized nanotube chips, microfabrication techniques are used to reliably contact individual SWNTs and obtain low contact resistance. The combined chemical synthesis and microfabrication approaches enable systematic characterization of electron transport properties of a large number of individual SWNTs. Results of electrical properties of representative semiconducting and metallic SWNTs are presented. The lowest two-terminal resistance for individual metallic SWNTs (≈5 μm long) is ≈16.5 kΩ measured at 4.2 K. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 14 July 1999     

Microstructural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition

By electrochemically controlling the structure of the surface aggregates, the grain microstructure has been optimized to yield mesoporous thin films of tungsten oxide (WO₃) at the electrode-electrolyte interface in a peroxotungstate sol in the presence of a structure-directing agent (Triton) at room temperature. Apart from the dominant ultrafine nanocrystallites and pores (5-10 nm), well-developed abutting grains (25-100 nm) and nanofibrils also constitute an integral part of the film matrix. X-ray photoemission spectra reveal the as-deposited film (WO_3−x) to be constituted by a high proportion of W⁶⁺ states with a low oxygen deficiency (x = 0.02). A relatively high W⁵⁺ content in the film, upon intercalation of 18 mC cm⁻² charge translates into a large coloring efficiency (η_VIS ∼ 70 cm² C⁻¹) and transmission modulation. At a lithium intercalation level of 22 mC cm⁻², in addition to W⁵⁺ and W⁶⁺ states, the film also comprises of W⁴⁺ states. The extremely fast color-bleach kinetics (3 and 2 s, respectively, for a 50% change in transmittance) shown by the as-deposited WO₃ film are repercussions of the mesopore morphology, the multiple nanostructures and the sixfold channels of its hexagonal modification. The film shows a high cycling stability as the switching times do not show any significant decline even after 3500 repetitive cycles. Coloration efficiency over the solar and photopic regions and current density for lithium intercalation for the as-deposited film are superior to that observed for the films annealed at 100, 250 and 500 °C. The abysmal electrochromic response of the annealed films is a consequence of surface defects like cracks and uncontrolled densification and pore shrinkage.     

Microstructure and wear property of carbon nanotube carburizing carbon steel by laser surface remelting

Carbon nanotube was used to carburize the surface of medium carbon steel and mild steel, respectively, by means of laser surface remelting. The slurry of carbon nanotube of ethanol was coated on the surface of the materials prior to laser irradiation. Microstructures, microhardness and wear property of the surface layers treated by different laser performance parameters were studied. Graphite coating was also used for carburizing. The results showed that both carbon nanotube and graphite were dissolved in the surface molten layer, leading a carburized hardening layer on the surface of the substrate. However, different microstructures formed in the carburizing layers, depending mainly on the type of carburization materials, carbon nanotube or graphite. The carbon nanotube hardening layer exhibits a little higher hardness than the graphite hardening layer. The carburized layer greatly increases the wear resistance of the base material.     

Integration and characterization of aligned carbon nanotubes on metal/silicon substrates and effects of water

We report here a facile way to grow aligned multi-walled carbon nanotubes (MWCNTs) on various metal (e.g. gold, tungsten, vanadium and copper)/silicon electrically conductive substrates by aerosol-assisted chemical vapor deposition (AACVD). Without using any buffer layers, integration of high quality MWCNTs to the conductive substrates has been achieved by introducing appropriate amount of water vapor into the growth system. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) determination indicate tidy morphology and narrow diameter distribution of the nanotubes as well as promising growth rate suitable for industrial applications. Raman spectra analysis illustrates that the structural order and purity of the nanotubes are significantly improved in the presence of water vapor. The growth mechanism of the nanotubes has been discussed. It is believed that water vapor plays a key role in the catalyst-substrate interaction and nucleation of the carbon nanotubes on the conductive substrates. This synthesis approach is expected to be extended to other catalyst-conductive substrate systems and provide some new insight in the direct integration of carbon nanotubes onto conductive substrates, which promises great potential for applications in electrical interconnects, contacts for field emitters, and other electronic nanodevices.     

Magnetic properties of CoFeP films prepared by electroless deposition

Structure and magnetization of CoFeP films prepared by the electroless deposition were systematically investigated by varying the bath composition and deposition parameters to optimize soft magnetic properties. The cobalt content in the CoFeP films varies from 40.4 to 94.9 wt% by controlling the bath composition. Increase of the metallic ratio FeSO₄·7H₂O/(CoSO₄·7H₂O+FeSO₄·7H₂O) affects the films’ microstructure, which switches from amorphous to crystalline structure. The magnetic properties of CoFeP films reveal that the coercivity (H_c) values range from 80 up to 185 A/m and the saturation magnetization (M_s) from 82 to 580 eum/g depending on the bath composition, deposition parameters and heat-treatment conditions. Increase of M_s and remanent magnetization (M_r) as well as decrease of H_c are observed for the CoFeP films with bath pH, temperature and the metallic molar ratio increasing. It is also found that the H_c is enhanced with the increase of NaH₂PO₂·H₂O concentration. CoFeP films showing good soft magnetic properties with coercivities less than 140 A/m and M_s close to 600 emu/g can be obtained in high pH bath and thereafter heat treatment. The deposit is found to be suitable as soft magnetic materials for core materials.     

Epitaxial graphene: the material for graphene electronics

The search for an ideal graphene sheet has been a quest driving graphene research. While most research has focused on exfoliated graphene, intrinsic substrate interactions and mechanical disorder have precluded the observation of a number of graphene's expected physical properties in this material. The only graphene candidate that has demonstrated all the essential properties of an ideal sheet is multilayer graphene grown on the SiC(000 ) surface. Its unique stacking allows nearly all the sheets in the stack to behave like isolated graphene, while the weak graphene‐graphene interaction prevents any significant doping or distortion in the band near the Fermi level. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of nanostructured bimetallic films on α-Al2O3 substrates using electroless deposition

The Pt-Pd and Pd-Ag nanostructured bimetallic films on porous α-Al₂O₃ substrates are successfully synthesized by chemical deposition using lyotropic liquid crystalline templating strategy. The co-reduction of two metallic species in the presence of liquid crystalline phase by hydrazine hydrate produces hexagonal nanostructured Pt-Pd and lamellar nanostructured Pd-Ag films. Low-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies show the ordered nanostructure of both Pt-Pd and Pd-Ag films. The energy dispersive X-ray (EDX) and wide-angle XRD analyses of the bimetallic films have verified the coexistence and uniform distribution of constituent metallic species. By taking into account of catalytic activities, well-defined nanochannels and higher surface areas, the nanostructured bimetallic films might have application potential in microreactors.