Sonochemical synthesis of platinum nanowires and their applications as electro-chemical actuators

We report the sonochemical synthesis of platinum nanowires on carbon nanotube templates and their application in electrochemical actuation. The fabrication of platinum nanowires was achieved by suspending well separated single wall carbon nanotubes in isopropyl alcohol and ultra-sonically agitating the solution in the presence of dihydrogen hexachloroplatinate. The platinum nanowires were further processed into micro and macro scale free standing sheets by vacuum filtration. An electrochemical cantilever actuator was constructed using the platinum nanowire sheet which actuated under electrical bias. Displacement of '3 mm was readily achieved when the electrical potential was swept at low voltages between -2 V and 2 V at a scan rate of 200 mV/s. The actuator showed the metallic actuation characteristics instead of that from carbon nanotubes. These results show the applicability of metallic nanomaterials for actuation technologies.     

DNA-linker-induced surface assembly of ultra dense parallel single walled carbon nanotube arrays

Ultrathin film preparations of single-walled carbon nanotube (SWNT) allow economical utilization of nanotube properties in electronics applications. Recent advances have enabled production of micrometer scale SWNT transistors and sensors but scaling these devices down to the nanoscale, and improving the coupling of SWNTs to other nanoscale components, may require techniques that can generate a greater degree of nanoscale geometric order than has thus far been achieved. Here, we introduce linker-induced surface assembly, a new technique that uses small structured DNA linkers to assemble solution dispersed nanotubes into parallel arrays on charged surfaces. Parts of our linkers act as spacers to precisely control the internanotube separation distance down to <3 nm and can serve as scaffolds to position components such as proteins between adjacent parallel nanotubes. The resulting arrays can then be stamped onto other substrates. Our results demonstrate a new paradigm for the self-assembly of anisotropic colloidal nanomaterials into ordered structures and provide a potentially simple, low cost, and scalable route for preparation of exquisitely structured parallel SWNT films with applications in high-performance nanoscale switches, sensors, and meta-materials.     

Nitrogen doping in carbon nanotubes

Nitrogen doping of single and multi-walled carbon nanotubes is of great interest both fundamentally, to explore the effect of dopants on quasi-1D electrical conductors, and for applications such as field emission tips, lithium storage, composites and nanoelectronic devices. We present an extensive review of the current state of the art in nitrogen doping of carbon nanotubes, including synthesis techniques, and comparison with nitrogen doped carbon thin films and azofullerenes. Nitrogen doping significantly alters nanotube morphology, leading to compartmentalised 'bamboo' nanotube structures. We review spectroscopic studies of nitrogen dopants using techniques such as X-ray photoemission spectroscopy, electron energy loss spectroscopy and Raman studies, and associated theoretical models. We discuss the role of nanotube curvature and chirality (notably whether the nanotubes are metallic or semiconducting), and the effect of doping on nanotube surface chemistry. Finally we review the effect of nitrogen on the transport properties of carbon nanotubes, notably its ability to induce negative differential resistance in semiconducting tubes.     

Fabrication of discrete nanoscaled force sensors based on single-walled carbon nanotubes

We present a fabrication technique for discrete, released carbon-nanotube-based nanomechanical force sensors. The fabrication technique uses prepatterned coordinate markers to align the device design to predeposited single-walled carbon nanotubes (SWNTs): Atomic force microscope (AFM) images are recorded to determine spatial orientation and location of each discrete nanotube to be integrated in a nanoscaled force sensor. Electron beam lithography is subsequently used to pattern the metallic electrodes for the nanoscale structures. Diluted hydrofluoric acid etching followed by critical point drying completes the nanosized device fabrication. We use discrete, highly purified, and chemically stable carbon nanotubes as active elements. We show AFM and scanning electron microscope images of the successfully realized SWNTs embedded nanoelectromechanical systems (NEMS). Finally, we present electromechanical measurements of the suspended SWNT NEMS structures.     

Towards pick-and-place assembly of nanostructures

We examine an approach to three-dimensional pick-and-place assembly of wire-like nanoscale components, such as carbon nanotubes and silicon nanowires, on microstructures inside a scanning electron microscope. In this article we demonstrate that microfabricated electrostatically acutuated tweezers can pick up silicon nanowires and show how electron beam deposition of carbon residues can be used to assemble carbon nanotubes on microelectrodes.     

Synthesis and device applications of high-density aligned carbon nanotubes using low-pressure chemical vapor deposition and stacked multiple transfer

For nanotube-based electronics to become a viable alternative to silicon technology, high-density aligned carbon nanotubes are essential. In this paper, we report the combined use of low-pressure chemical vapor deposition and stacked multiple transfer to achieve high-density aligned nanotubes. By using an optimized nanotube synthesis recipe, we have achieved high-density aligned carbon nanotubes with density as high as 30 tubes/μm. In addition, a facile stacked multiple transfer technique has been developed to further increase the nanotube density to 55 tubes/μm. Furthermore, high-performance submicron carbon nanotube field-effect transistors have been fabricated on the high-density aligned nanotubes. Before removing the metallic nanotubes by electrical breakdown, the devices exhibit on-current density of 92.4 μA/μm and normalized transconductance of 13.3 μS/μm. Moreover, benchmarking with the aligned carbon nanotube transistors in the literature indicates that our devices exhibit the best performance so far, which is attributed to both the increased nanotube density and scaling down of channel length. This study shows the great potential of using such high-density aligned nanotubes for high performance nanoelectronics and analog/RF applications.     

Precise control of highly ordered arrays of nested semiconductor/metal nanotubes

Lithographically defined microporous templates in conjunction with the atomic layer deposition (ALD) technique enable remarkable control of complex novel nested nanotube structures. So far three-dimensional control of physical process parameters has not been fully realized with high precision resolution, and requires optimization in order to achieve a wider range of potential applications. Furthermore, the combination of composite insulating oxide layers alternating with semiconducting layers and metals can provide various types of novel applications and eventually provide unique and advanced levels of multifunctional nanoscale devices. Semiconducting TiO₂ nanotubes have potential applications in photovoltaic devices. The combination of nanostructured semiconducting materials with nested metal nanotubes has the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices. Platinum growth by ALD has been explored, covering the initial stages of the thin film nucleation process and the synthesis of high aspect ratio nanotube structures. The penetration depth of the Pt into porous templates having various pore sizes and aspect ratios has been investigated. Several multi-walled nested TiO₂-Pt nanotubes in series have been successfully fabricated using microporous Si templates. These innovative nested nanostructures have the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices in photovoltaic and sensing technologies.      

Separation Techniques for Carbon Nanotubes

Single‐walled carbon nanotubes have considerable potential as building blocks in future nanoscale electronics. The tubes exist in two modifications, metallic and semiconducting, which might be used in a range of electronic applications. One of the grand challenges is to separate metallic from semiconducting tubes in substantial quantities. In this respect, this article gives a brief overview over the separation techniques in this new field of nanotube research.     

Plasmon-assisted local temperature control to pattern individual semiconductor nanowires and carbon nanotubes

We demonstrate a new versatile strategy to rapidly heat and cool subdiffraction-limited volumes of material with a focused light beam. The local temperature rise is obtained by exploiting the unique optical properties of metallic nanostructures that facilitate efficient light-to-heat conversion through the excitation of surface plasmons (collective electron oscillations). By locally heating nanoscale metallic catalysts, growth of semiconductor nanowires and carbon nanotubes can be initiated and controlled at arbitrarily prespecified locations and down to the single nanostructure level in a room-temperature chamber. This local heating strategy can be orders of magnitude (>10(5)) more energy efficient than conventional chemical vapor deposition (CVD) tools in which an entire chamber/substrate is heated. For these reasons, it has great potential for use in process- and energy-efficient assembly of nanowires into complementary metal-oxide-semiconductor (CMOS) compatible device architectures. In general, the high degree of spatial and temporal control over nanoscale thermal environments afforded by this method inspires new pathways for manipulating a range of important thermally stimulated processes and the development of novel photothermal devices.     

Controlled growth of carbon nanotubes

Carbon nanotubes have extraordinary mechanical and electronic properties and hold great promise for future applications. The most important aspects of this structure are its low density, high aspect ratio, one dimensionality, high mechanical strength and high electrical and thermal conductivity. We present a short, state-of-the-art account of tailored nanotube growth. To provide these properties in real devices there exists a need for producing nanotubes on substrates. The challenge in the creation of mesoscale nanotube-based architectures and tailored nanotube networks consisting of thousands of tubes in a predefined order is obviously great. Currently, chemical vapour deposition (CVD) appears to be the most powerful method for achieving such required structures. We describe our work on a new synthesis method, based on catalytic CVD using mainly gas-phase catalyst delivery. Gas-phase catalyst delivery allows us to assemble single-walled and multi-walled carbon nanotubes in predetermined multiple orientations on substrates to build one- to three-dimensional architectures. We are able to control, to a large extent, the types of nanotubes produced, their lengths, locations and their orientations. The ability to make mesoscale architectures with nanotubes should lead us to develop applications in areas such as nano-electromechanical systems.     

Catalytic action of gold and copper crystals in the growth of carbon nanotubes

Multi-wall carbon nanotubes are grown in a chemical vapor deposition process by using bulk gold and copper substrates as catalysts. Nanotube growth starts from a nanometer-sized roughness on the metal surfaces and occurs in a mechanism where the catalyst particle is either at the tip (Au) or root (Cu) of the growing nanotube. Whereas Au leads to nanotubes with good structural perfection, nanotubes grown from Cu show a higher density of defects. High-resolution transmission electron microscopy shows the bonding between Au and carbon at the metal-nanotube interface whereas no bonds between Cu and carbon occur. Highly mobile Au or Cu atoms adsorb at the growing edge of a carbon nanotube from where diffusion along the nanotube wall can lead to the formation of Au or Cu nanowires inside the central hollow of carbon nanotubes.     

基于碳纳米管模板的氮化镓纳米线束合成

通过金属有机物化学气相沉积方法在碳纳米管模板上生长氮化镓纳米线束.对所生长的纳米结构进行了扫描电镜和X射线能谱分析,结果显示氮化镓纳米晶体可以与碳纳米管形成纳米线束状复合物.纳米线束状复合物直径为100～200 nm,长度为1.5～2.5μm,纳米线的两端呈现尖角状.由于氨气很容易吸附在碳纳米管表面,可知所获得的纳米结构的初始生长机制为碳纳米管的表面氮化.该研究也证明金属有机物化学气相沉积将是用于制造化合物纳米结构材料的一项有效的技术.     

Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks

The current percolation in polymer‐sorted semiconducting (7,5) single‐walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field‐effect measurements, atomic force microscopy (AFM), and conductive AFM (C‐AFM) techniques. From AFM measurements, the nanotube length in the as‐processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field‐effect charge transport measurements in the SWNT network using a 2D homogeneous random‐network stick‐percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C‐AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as‐spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field‐effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.     

Carbon nanotubes grafted on silicon oxide nanowires

In this article, we report the grafting of multi-walled carbon nanotubes on silica nanowires by directly growing nanotubes on the surfaces of the nanowires via chemical vapor deposition (CVD) using ferrocene and xylene as Fe catalyst precursor and carbon source, respectively. The grafted carbon nanotubes are a few micrometers long with diameters of 10 to approximately 30 nm, and grow uniformly along the lengths of the nanowires. The distribution density of the grafted carbon nanotubes on the silica nanowires can be tuned by simply adjusting the CVD growth temperature. Our method provides a simple approach for synthesizing nanometer scale grafted heterostructures between nanotubes and nanowires, which could be used to design and construct high-performance filters, chemical sensors and reinforced composites.     

An atlas of carbon nanotube optical transitions

Electron-electron interactions are significantly enhanced in one-dimensional systems, and single-walled carbon nanotubes provide a unique opportunity for studying such interactions and the related many-body effects in one dimension. However, single-walled nanotubes can have a wide range of diameters and hundreds of different structures, each defined by its chiral index (n,m), where n and m are integers that can have values from zero up to 30 or more. Moreover, one-third of these structures are metals and two-thirds are semiconductors, and they display optical resonances at many different frequencies. Systematic studies of many-body effects in nanotubes would therefore benefit from the availability of a technique for identifying the chiral index of a nanotube based on a measurement of its optical resonances, and vice versa. Here, we report the establishment of a structure-property 'atlas' for nanotube optical transitions based on simultaneous electron diffraction measurements of the chiral index and Rayleigh scattering measurements of the optical resonances of 206 different single-walled nanotube structures. The nanotubes, which were suspended across open slit structures on silicon substrates, had diameters in the range 1.3-4.7?nm. We also use this atlas as a starting point for a systematic study of many-body effects in the excited states of single-walled nanotubes. We find that electron-electron interactions shift the optical resonance energies by the same amount for both metallic and semiconducting nanotubes, and that this shift (which corresponds to an effective Fermi velocity renormalization) increases monotonically with nanotube diameter. This behaviour arises from two sources: an intriguing cancellation of long-range electron-electron interaction effects, and the dependence of short-range electron-electron interactions on diameter.     

Significance and systematic analysis of metallic impurities of carbon nanotubes produced by different manufacturers

Commercially available carbon nanotubes (CNT) often contain some quantities of metallic and carbonaceous impurities. These impurities influence their physicochemical properties and performance, and accordingly a number of potential applications. The lack of information of metal impurities may also preclude accurate environmental and health risk assessments for specific CNT materials. To address these needs, a quantitative analysis of the metal contents has been made in a number of commercial carbon nanotubes produced by different manufacturers. More than 20 metals or metalloids were determined by neutron activation analysis. The results indicate arranging from 0.44 to 3 wt% of catalyst residues remained although the producers claim to provide a catalyst-free product. Most of the impurity elements are transition metals, such as Fe, Ni, Mo, Y, Co and Cr. In addition to the expected catalyst residues, other unexpected impurity elements were detected including As, Gd, W, Yb, Sm and so on. Metallic impurities in carbon nanotube materials should come from the large-scale production procedures, post fabrication and post-purification treatments. The analytical results determined by inductively-coupled plasma mass spectrometry show that a further deep purification using conventional acid reflux cannot completely remove the metallic impurities from carbon nanotubes. Post-production clean up is difficult and often incompletely.     

Single-walled carbon nanotubes as excitonic optical wires

Although metallic nanostructures are useful for nanoscale optics, all of their key optical properties are determined by their geometry. This makes it difficult to adjust these properties independently, and can restrict applications. Here we use the absolute intensity of Rayleigh scattering to show that single-walled carbon nanotubes can form ideal optical wires. The spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics, quantum-dot-like optical resonances and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity of approximately 8 e(2)/h, which we derive using an exciton model, suggesting universal behaviour similar to that observed in nanotube conductance. We further demonstrate a radiative coupling between two distant nanotubes, with potential applications in metamaterials and optical antennas.     

Novel vapor phase reactions for the synthesis and modification of carbon nanotubes and inorganic nanowires

Several vapor phase methods have been developed for the preparation and modification of carbon nanotubes and inorganic nanowires. Thus, nebulized spray pyrolysis has been employed for the synthesis of carbon nanotubes and metal nanowires. Multi-walled carbon nanotubes (MWNTs) with fairly uniform diameters and aligned nanotube bundles have been obtained by nebulized spray pyrolysis using solutions of organometallics such as ferrocene in hydrocarbon solvents. Single-crystalline nanowires of zinc, cadmium, cobalt, and lead are obtained by the decomposition of metal acetates. By reacting acid-treated carbon nanotubes with vapors of metal halides, followed by reaction with water and calcination chemically-bonded oxide layers can be obtained on the nanotubes. A similar procedure has been employed to prepare chemically-bonded oxide layers on Al2O3, ZnO, and silicon nanowires by the reaction of the metal halides with the surface hydroxyl groups present on these nanowire surfaces.     

Evidence for metal-semiconductor transitions in twisted and collapsed double-walled carbon nanotubes by scanning tunneling microscopy

The atomic and electronic structure of a twisted and collapsed double-walled carbon nanotube was characterized using scanning tunneling microscopy and spectroscopy. It was found that the deformation opens an electronic band gap in an otherwise metallic nanotube, which has major ramifications on the use of carbon nanotubes for electronic applications. Fundamentally, the importance of the intershell interaction in this double-walled carbon nanotube points to the potential of a reversible metal-semiconductor junction, which can have device applications, as well as a caution in the design of semiconductor components based on carbon nanotubes. Lattice registry effects between the two neighboring walls evidenced by atomically resolved images confirm earlier first principle calculations indicating that the helicity influences the collapsed structure and show excellent agreement with the predicted twisted-collapse mode.     

Conversion of a Bi nanowire array to an array of Bi-Bi2O3 core-shell nanowires and Bi2O3 nanotubes

A template-based heat-treatment method has been developed to convert metal nanowire arrays into arrays of metal-metal oxide core-shell nanowires and single-crystalline metal oxide nanotubes. This process is demonstrated by kinetically controlling the conversion of single-crystalline Bi nanowires to Bi-Bi(2)O(3) core-shell nanowires via a multistep, slow oxidation method, and then controlling their further conversion to a single-crystalline Bi(2)O(3) nanotube array via fast oxidation. This process can conveniently be extended to fabricate a free-standing, easily oxidized metal-metal oxide nanowire and metal oxide nanotube array, which may have future applications in nanoscale optics, electronics, and magnetics.