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.     

Electric field-assisted deposition of nanowires on carbon nanotubes for nanoelectronics and sensor applications

Manipulation and control of matter at the nanoscale and atomic scale levels are crucial for the success of nanoscale sensors and actuators. The ability to control and synthesize multilayer structures using carbon nanotubes that will enable the building of electronic devices within a nanotube is still in its infancy. In this paper, we present results on selective electric field-assisted deposition of metals on carbon nanotubes realizing metallic nanowire structures. Silver and platinum nanowires have been fabricated using this approach for their applications in chemical sensing as catalytic materials to sniff toxic agents and in the area of biomedical nanotechnology for construction of artificial muscles. Electric field-assisted deposition allows the deposition of metals with a high degree of selectivity on carbon nanotubes by manipulating the charges on the surface of the nanotubes and forming electrostatic double-layer supercapacitors. Deposition of metals primarily occurred due to electrochemical reduction, electrophoresis, and electro-osmosis inside the walls of the nanotube. SEM and TEM investigations revealed silver and platinum nanowires between 10 nm and 100 nm in diameter. The present technique is versatile and enables the fabrication of a host of different types of metallic and semiconducting nanowires using carbon nanotube templates for nanoelectronics and a myriad of sensor applications.     

Measurement of metal/carbon nanotube contact resistance by adjusting contact length using laser ablation

A technique of measuring contact resistance between an individual nanotube and a deposited metallic film is described. Using laser ablation to sequentially shorten the contact length between a nanotube and the evaporated metallic film, the linear resistivity of the nanotube as well as the specific contact resistivity between the nanotube and metallic film can be determined. This technique can be generally used to measure the specific contact resistance that develops between a metallic film and a variety of different nanowires and nanotubes.     

Photochemical performance and electrochemical capacitance of titania nanocomplexes

Titania nanocomplexes, comprising the disordered nanoribbons or nanowires on the top surface and highly ordered nanotube array on the underlaying layer, has been fabricated by longitudinally splitting off nanotubes in a controlled anodization process. Anatase titania nanocomplexes show higher photovoltage and photocurrent responses and photocatalysis activity than titania nanotube array due to the enhanced light harvesting caused by nanoribbons and nanowires. Furthermore, titania nanowire-nanotube demonstrates a higher photoelectrical performance than nanoribbon-nanotube due to its thicker space charge layer caused by long nanotubes and more effective surface area contributed by nanowires. Cyclic charge-discharge measurements show that titania nanotube array exhibits a much higher electric double layer capacitance than titania nanocomplexes because the surface nanoribbons or nanowires inhibit the free diffusion and transportation of electrolyte ions into the underlaying nanotubes. Therefore, titania nanocomplexes can act as a photoactive material for photocatalysis applications and titania nanotube array can act as an electrode substrate for electrochemical supercapacitor applications.     

Novel fabrication method of diverse one-dimensional Pt/ZnO hybrid nanostructures and its sensor application

A novel low-temperature, solution-phase method for the facile fabrication of a variety of one-dimensional (1D) metal/metal oxide hybrid nanostructures has been developed. This method is based on the wet chemical synthesis of metal oxide nanowires, followed by the surface coating of metal nanoparticles on metal oxide nanowire templates via reduction of metal ions along with controlled etching of metal oxide nanowires at the core, all in a low-temperature liquid environment. As a proof-of-concept, we applied this method to the fabrication of various 1D Pt/ZnO hybrid nanostructures including Pt nanoparticle-coated ZnO nanowires/nanotubes and Pt nanotubes on silicon and polymer substrates. The diverse morphology tuning is attributed to the control of pH in the solution with different metal precursor concentrations and amounts of reducing agent. The change of morphology, crystalline structure, and composition of various 1D Pt/ZnO hybrid nanostructures was observed by SEM, TEM (HRTEM), XRD and ICP-AES, respectively. Further, we have demonstrated a highly sensitive strain sensor (gauge factor = 15) with a Pt nanotube film fabricated by the developed method on a flexible polymer substrate.     

Fabrication of novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes

Novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes (MWCNTs) have been prepared using the organic sol-gel route and the dip-coating technique on magnesium alloy substrates. Homogeneous and dense composite coatings with good reinforcement dispersion were fabricated using low temperature and atmospheric pressure fabrication conditions. The presence of nanotubes caused a substantial enhancement of silica coating fracture toughness on coatings deposited on grounded substrates but it was not as effective on polished substrates because of the low adhesion of the coating to the substrate. Bridging phenomena caused by the MWCNTs was observed, indicating that an effective load transfer between the silica matrix and the nanotube reinforcement was also achieved.     

Adhesion between two radially collapsed single-walled carbon nanotubes

Continuum mechanics analysis and molecular mechanics simulations are performed to study adhesion between two identical, radially collapsed single-walled carbon nanotubes. Not only the inter-adhesion energy between nanotubes but also the inner adhesion energy in a nanotube is considered. A closed-form solution to the adhesion configuration is achieved, which is well consistent with our molecular mechanics simulation. Comparing the potential energy of the adhesion structures formed by two identical single-walled carbon nanotubes, three types of configurations, i.e., circular, deformed, and collapsed shape, will be formed with increasing carbon nanotubes radius and separated by two critical radii of the single-walled carbon nanotube. Furthermore, it is found that the collapsed adhesion structure possesses the highest interfacial adhesion energy. The results demonstrate that, as a potential application in carbon nanotube reinforced composites, arrays formed by collapsed carbon nanotubes will be optimal due to the strong interface strength.     

Raman probing of adhesion loss in carbon nanotube – reinforced composite

Raman spectroscopy of a carbon nanotube – reinforced phenolic resin is used to study the interaction of nanotubes with a host matrix. The observed sublinear dependence of the Raman G-band shift on the matrix strain, accompanied by inhomogeneous broadening of the spectral line, is interpreted as a gradual loss of adhesion between nanotubes and the polymer. An approach to simulate the ensemble-averaged Raman response of the nanotubes in composite is proposed, that takes into account nanotube orientation, angular dependence of the polarized Raman response of nanotubes, and adhesion loss between the nanotubes and the polymer. The comparison of the observed Raman line shapes and Raman shifts with simulation provides interesting insights into the micromechanics of nanotube interaction with polymer.     

Micromachined silicon transmission electron microscopy grids for direct characterization of as-grown nanotubes

Transmission electron microscopy (TEM) is a key technique in the structural characterization of carbon nanotubes. For device applications, carbon nanotubes are typically grown by chemical vapour deposition (CVD) on silicon substrates. However, TEM requires very thin samples, which are electron transparent. Therefore, for TEM analysis, CVD grown nanotubes are typically deposited on commercial TEM grids by post-processing. However, this procedure can damage the nanotubes, and it does not work reliably if the nanotube density is too low. The ability to do TEM directly on as-grown nanotubes on the silicon substrate would solve these problems. For this purpose, we have fabricated micromachined silicon TEM grids with narrow open slits on them. Since the nanotubes grown on these substrates are suspended freely over the open slits, the micromachined substrates form a natural TEM grid for direct imaging of CVD grown nanotubes. Furthermore, the background noise is significantly reduced during micro-Raman spectroscopy, resulting in a better signal-to-noise ratio. As a result, these micromachined Si substrates provide a low cost, mass producible, efficient, and reliable platform for direct TEM, SEM, AFM, and Raman characterization of as-grown nanotubes. These grids can be used for characterizing a wide range of other nanomaterials, including peapods, nanowires, and nanofibres.     

Control of neuronal network organization by chemical surface functionalization of multi-walled carbon nanotube arrays

Carbon nanotube substrates are promising candidates for biological applications and devices. Interfacing of these carbon nanotubes with neurons can be controlled by chemical modifications. In this study, we investigated how chemical surface functionalization of multi-walled carbon nanotube arrays (MWNT-A) influences neuronal adhesion and network organization. Functionalization of MWNT-A dramatically modifies the length of neurite fascicles, cluster inter-connection success rate, and the percentage of neurites that escape from the clusters. We propose that chemical functionalization represents a method of choice for developing applications in which neuronal patterning on MWNT-A substrates is required.     

A study of the size-dependent elastic properties of ZnO nanowires and nanotubes

We present our calculations of the Young's modulus of ZnO nanowires and nanotubes by using the empirical Buckingham-type potential. Our results indicate that the Young's moduli of ZnO nanowires increase as the diameters decrease, and the Young's moduli of ZnO nanotubes increase as the thicknesses decrease. Furthermore, we find that such size-dependent elastic properties mainly arise from the lateral facets of the nanowires and nanotubes. In particular, for a ZnO nanotube with a thin wall, the Coulomb interaction between the ions of the outer and inner atomic layers plays an important role in the Young's moduli of the surface atomic layers.     

Bottom-up growth of carbon nanotube multilayers: unprecedented growth

An unusual growth phenomenon, with no precedent in vapor-phase thin film growth, is described here, for the case of the growth of stacked multiple layers of vertically aligned carbon nanotubes(1-6) on solid substrates. As multiple layers of ordered nanotubes are sequentially deposited from the vapor onto the substrate, each layer nucleates and grows from the original substrate surface at the bottom of the existing multiple stacks of nanotubes. In contrast to conventional understanding of thin film deposition,(7) the mechanism here has similarities to porous oxide film formation on surfaces.(8) The stacked layers of aligned nanotubes act as fully permeable membranes for the downward diffusion of growth precursor vapors, allowing growth to occur at the buried solid interface. The preexisting multiple nanotube stacks lift up to accommodate the vertical growth of fresh layers, allowing the formation of nanotube towers extending in millimeter lengths. Our results provide evidence for a new growth phenomenon, characterized by selective, interface-driven, bottom-up growth of self-assembled nanowires at buried interfaces, covered with weakly adhering thick porous membranes.     

Influence of sterilization methods on cell behavior and functionality of osteoblasts cultured on TiO2 nanotubes

We investigated the adhesion, proliferation and osteogenic functionality of osteoblasts cultured on titanium dioxide (TiO₂) nanotubes in response to different sterilization methods (dry autoclaving vs. wet autoclaving). We prepared various sizes (30–100 nm diameter) of TiO₂ nanotubes on titanium substrates by anodization, sterilized nanotubes by different conditions, and seeded osteoblast cells onto the nanotube surfaces with two different cell seeding densities (10,000 vs. 50,000 cells/well in 12-culture well). The result of this study indicates that the adhesion, proliferation and alkaline phosphatase activity of osteoblasts cultured on only the larger 70 and 100 nm TiO₂ nanotube arrays were dramatically changed by the different sterilization conditions at a low cell seeding density. However, with a higher cell seeding density (50,000 cells/well in 12-cell culture well), the results revealed no significant difference among altered nanotube geometry, 30–100 nm diameters, nor sterilization methods. Next, it was revealed that the nanofeatures of proteins adhered on nanotubular TiO₂ morphology are altered by the sterilization method. It was determined that this protein adhesion effect, in combination with the cell density of osteoblasts seeded onto such TiO₂ nanotube surfaces, has profound effects on cell behavior. This study clearly shows that these are some of the important in vitro culture factors that need to be taken into consideration, as well as TiO₂ nanotube diameters which play an important role in the improvement of cell behavior and functionality.     

Temperature Influence on the Morphology and the Magnetic Properties of Vertically Aligned Fe-filled Carbon Nanotubes

Arrays of vertically aligned Fe-filled multi-wall carbon nanotubes (MWNTs) on oxidized silicon substrates were prepared by pyrolysis of ferrocene in a dual furnace system and characterized by electron microscopy and magnetometry measurement. The effect of the growth temperature on both the filled nanotube morphology and their magnetic behavior was studied. Increasing the growth temperature in the range of 845-1035°C the nanotube alignment becomes worse and the diameter of the encapsulated Fe nanowires increases from 10 to 40 nm. Both the coercivity and the remanence ratio of the arrays of Fe-filled MWNTs decrease with the increase of the growth temperature. Factors causing the observed magnetic behavior are discussed.     

A substrate-integrated and scalable templated approach based on rusted steel for the fabrication of polypyrrole nanotube arrays

We report here a facile, generalizable, and entirely scalable approach for the fabrication of vertically aligned arrays of Fe(2)O(3)/polypyrrole core-shell nanostructures and polypyrrole nanotubes. Our "all electrochemical" approach is based on the fabrication of α-Fe(2)O(3) nanowire arrays by the simple heat treatment of commodity low carbon steel substrates, followed by electropolymerization of conformal polypyrrole sheaths around the nanowires. Subsequently, electrochemical etching of the nanowires yields large-area vertically aligned polypyrrole nanotube arrays on the steel substrate. The developed methodology is generalizable to functionalized pyrrole monomers and represents a significant practical advance of relevance to the technological implementation of conjugated polymer nanostructures in electrochromics, electrochemical energy storage, and sensing.     

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.     

Relation between Growth Parameters and Morphology of Vertically Aligned Fe-filled Carbon Nanotubes

Fe-filled multi-wall carbon nanotubes (MWNTs) were produced by pyrolysis of ferrocene in a dual furnace system. They grew vertically aligned on oxidized silicon substrates placed inside the reaction zone of a chemical vapor deposition reactor. A variation of the growth parameters has been performed in order to evaluate the possibility to control the Fe-filled nanotube growth process and thereby the nanotube- and the filling length, diameter and yield, and also the nanotube alignment. Electron microscopy studies show nanotubes with quite different morphologies. The relation between the aligned Fe-filled MWNTs growth and the most important growth parameters is discussed.     

Template‐Fabricated Gold Nanowires and Nanotubes

Gold nanowires and nanotubes are prepared via electroless deposition of Au onto the pore walls of a porous polymeric membrane. The pores in the support membrane act as a template for the nanostructures. The support is a commercially available nanoporous polycarbonate filter with cylindrical nanoscopic pores. We have shown that by controlling the Au deposition time, Au nanotubes (short deposition times) or nanowires (longer deposition times) can be prepared. The gold nanowires and nanotube membranes can be utilized for nanoelectrode ensembles, molecular filters and as chemical switches.     

平行碳纳米管悬空粘附时临界槽口宽度的计算

根据小形变时的简单梁理论,给出了由碳纳米管悬空部分粘附时的几何形貌来计算平行碳纳米管之间粘附力的理论模型,推导了平行碳纳米管之间粘附力的计算公式以及碳纳米管粘附到沟槽基底时与沟槽基底的粘附力计算公式.由于碳纳米管本身的尺度小,实验上无法直接测量出平行碳纳米管之间的粘附力.本研究给出的计算公式可以由扫描电子显微镜(SEM)下观测到的碳纳米管悬空部分粘附时的几何形貌计算出平行碳纳米管之间的粘附力.根据文献中给出的平行碳纳米管之间粘附力的理论计算结果,估算了不同间距的平行碳纳米管能够粘附在一起所需要的槽口宽度,以及碳纳米管恰好粘附到沟槽基底时不同槽口宽度和深度之间的对应关系.这些结果对制备用于测量平行碳纳米管之间粘附力的悬空结构提供了重要的参考价值.     

Large-area blown bubble films of aligned nanowires and carbon nanotubes

Many of the applications proposed for nanowires and carbon nanotubes require these components to be organized over large areas with controlled orientation and density. Although progress has been made with directed assembly and Langmuir-Blodgett approaches, it is unclear whether these techniques can be scaled to large wafers and non-rigid substrates. Here, we describe a general and scalable approach for large-area, uniformly aligned and controlled-density nanowire and nanotube films, which involves expanding a bubble from a homogeneous suspension of these materials. The blown-bubble films were transferred to single-crystal wafers of at least 200 mm in diameter, flexible plastics sheets of dimensions of at least 225 x 300 mm(2) and highly curved surfaces, and were also suspended across open frames. In addition, electrical measurements show that large arrays of nanowire field-effect transistors can be efficiently fabricated on the wafer scale. Given the potential of blown film extrusion to produce continuous films with widths exceeding 1 m, we believe that our approach could allow the unique properties of nanowires and nanotubes to be exploited in applications requiring large areas and relatively modest device densities.