Jimmy Xu

Growing Y-junction carbon nanotubes

The synthesis of connections between two or more different carbon nanotubes is an important step in the development of carbon nanotube-based electronic devices and circuits. But this is difficult to achieve using conventional methods to grow carbon nanotubes because the straight tube structure cannot be controllably altered along its length. Various ideas for post-growth modifications have been suggested, but these have been hard to implement and are prone to defects. Here we use nano-structured template channels to grow individual Y-junction carbon-nanotube heterostructures by the pyrolysis of acetylene with cobalt catalysis.

Highly-ordered carbon nanotube arrays for electronics applications

Highly-ordered arrays of parallel carbon nanotubes were grown by pyrolysis of acetylene on cobalt within a hexagonal close-packed nanochannel alumina template at 650 °C. The nanotubes are characterized by a narrow size distribution, large scale periodicity, and high densities. Using this method ordered nanotubes with diameters from 10 nm to several hundred nm and lengths up to 100 μm can be produced. The high level of ordering and uniformity in these arrays is useful for applications in data storage, field emission displays and sensors, and offers the prospect of deriving computational functions from the collective behavior of symmetrically coupled nanotubes. The fabrication method used is compatible with standard lithographic processes and thus enables future integration of such periodic carbon nanotube arrays with silicon microelectronics.

Fabrication of highly ordered metallic nanowire arrays by electrodeposition

Highly ordered hexagonal arrays of parallel metallic nanowires (Ni, Bi) with diameters of about 50 nm and lengths up to 50 μm were synthesized by electrodeposition. Hexagonal-close-packed nanochannel anodized aluminum oxide film was used as the deposition template. The deposition was performed in an organic bath of dimethylsulfoxide with metal chloride as the electrolyte. A high degree of ordering and uniformity in these arrays can be obtained with this technique by fine-tuning the electrodeposition parameters. Moreover, an unprecedentedly high level of uniformity and control of the wire length was achieved. The arrays are unique platforms for explorations of collective behavior in coupled mesoscopic systems, and are useful for applications in high-density data storage, field emission displays, and sensors.

Nonlithographic nano-wire arrays: fabrication, physics, and device applications

A novel system of nanostructures is described consisting of nonlithographically produced arrays of nano-wires directly electrodeposited into porous anodic aluminum oxide templates. Using this method regular and uniform arrays of metal or semiconductor nano-wires or nano-dots can be created with diameters ranging from /spl sim/5 nm to several hundred nanometers and with areal pore densities in the /spl sim/10/sup 9/-10/sup 11/ cm/sup -2/ range. We report on the present state of their fabrication, properties, and prospective device applications. Results of X-ray diffraction, Raman and magnetic measurements on metal (Ni, Fe) and semiconductor (CdS, CdSe, CdS/sub x/Se/sub 1-x/, Cd/sub x/Zn/sub 1-x/S and GaAs) wires are presented. The I-V characteristics of two terminal devices made from the nano-arrays are found to exhibit room temperature periodic conductance oscillations and Coulomb-blockade like current staircases. These observations are likely associated with the ultra-small tunnel junctions that are formed naturally in the arrays. Single-electron tunneling (SET) In the presence of interwire coupling in these arrays is shown to lead to the spontaneous electrostatic polarization of the wires. Possible device applications such as magnetic memory or sensors, electroluminescent flat-panel displays, and nanoelectronic and single-electronic devices are also discussed.

Periodic array of uniform ZnO nanorods by second-order self-assembly

A nonlithographic second-order self-assembly process for synthesizing uniform and ordered arrays of nanorods and nanodots is presented and applied to the fabrication of ZnO nanorod arrays. Nucleation sites were defined by patterning Au nanodot catalysts with a self-organized array of nanopores formed in anodized aluminum oxide (AAO). The self-assembled vertically aligned ZnO nanorods grown on GaN exhibit hexagonal facets, and have a uniform diameter of 60 nm and a mean length of 400 nm. The growth technique is simple, robust, and offers a direct control over array and single nanorod configurations. The growth temperature is significantly lower than normal, and yet, the resultant defect level is much lower than normal.

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Carbon nanotube probes for single-cell experimentation and assays

Integrating nanotechnology with experimental biology is paramount to advancing fundamental biological science and technology, and, therefore, of high current interest and importance. In this article, we report on a new possibility of utilizing carbon nanotube probes assembled by a modified dielectrophoretic based technique for single-cell experimentation and delivery. The modified approach permits highly reproducible construction of water-stable, highly-aligned, and electrically-conductive probes several hundred microns in length, which hold a great promise for enhancing previously developed molecular-scale intracellular experimental techniques. The results of this work, in particular, indicate that the minimally invasive nanotube probes could be advantageous for studies involving permeabilization and subsequent desorption of molecules into a cell’s interior, thereby obviating permeabilization and diffusion across membranes.

Shell buckling of individual multiwalled carbon nanotubes using nanoindentation

Although the mechanical behavior of carbon nanotubes has been studied extensively in recent years, very few experimental results exist on the shell buckling of nanotubes, despite its fundamental importance in nanotube mechanics and applications. Here we report an experimental technique in which individual multiwalled carbon nanotubes were axially compressed using a nanoindenter and the critical shell-buckling load was measured. The results are compared with predictions of existing continuum theories, which model multiwalled carbon nanotubes as a collection of single-walled shells, interacting through van der Waals forces. The theoretical models significantly underpredict the experimental buckling load.

Highly ordered carbon nanotube arrays and IR detection

Abstract This review highlights the relevant fabrication process, physical features, electronic properties of carbon nanotube (CNT) arrays in the context of potential infrared (IR) applications. Although experimental demonstration of the CNT array’s in IR detection operation is necessarily needed to pave the way for full scale development, it is perhaps more important, at this early stage of the exploration, to gain a better understanding of the fundamental aspects of the material, the underlying physics, and the base fabrication technology. Theoretical analysis and experimental evidences will be presented to show that the CNT array as an IR material can have a semiconducting bandgap that should be and is tunable with the tube diameter and can cover a broad range of spectral response from 1 to 10 μm. Moreover, the technology offers natural normal incident detection, a detection surface area that is not wafer-size limited and is conformable to curved surface, and low manufacturing cost. The highly ordered and uniform array and tube-to-tube insulation helps to suppress spatially random thermal noise, and gives rise to high resilience to defect-induced failure.

Buckling instabilities in multiwalled carbon nanotubes under uniaxial compression

We report experimental observations of shell buckling instabilities in freestanding, vertically aligned multiwalled carbon nanotubes subjected to uniaxial compression. Highly ordered and uniform arrays of carbon nanotubes embedded in an alumina matrix were fabricated and subjected to uniaxial compression using a nanoindenter. The buckling load was found to be on the order of 2μN for nanotubes with 25nm outer radius, 13nm inner radius, and heights of 50 and 100nm. Good agreement was found between the experimental observations and the predictions of linear elastic shell buckling theory.