Aijun Yin

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.

Infrared absorption properties of carbon nanotubes synthesized by chemical vapor deposition

We present an infrared (IR) optical absorbance study of highly uniform nanotubes grown by chemical vapor deposition in the self-assembled porous matrix in alumina. For unambiguous IR spectral measurement, nanotubes were extracted from their growth template, purified, and evenly dispersed on a reflecting substrate. The findings, which are consistent with previous results from conduction studies, reveal that the nanotubes are semiconducting with a band gap of ∼100meV. This suggests the potential of nanotube arrays for IR electro-optical device applications.

Superconducting Pair Correlations in an Amorphous Insulating Nanohoneycomb Film

The Cooper pairing mechanism that binds single electrons to form pairs in metals allows electrons to circumvent the exclusion principle and condense into a single superconducting or zero-resistance state. We present results from an amorphous bismuth film system patterned with a nanohoneycomb array of holes, which undergoes a thickness-tuned insulator-superconductor transition. The insulating films exhibit activated resistances and magnetoresistance oscillations dictated by the superconducting flux quantum h/2e. This 2e period is direct evidence indicating that Cooper pairing is also responsible for electrically insulating behavior.

High-Q mechanical resonator arrays based on carbon nanotubes

We present results of the characterization of a nanoelectromechanical signal-processing device based on arrays of carbon nanotubes embedded in RF waveguides. The design, fabrication, and operation of the device will be discussed, including initial RF measurements. Preliminary tests suggest that transmission of an RF signal through the array is associated with the mechanical resonance of the carbon nanotubes.

Hopping conduction in disordered carbon nanotubes

Abstract We report electrical transport measurements on individual disordered multiwalled carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows an exp [ − ( T 0 / T ) 1 / 2 ] dependence on temperature T , suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T 0 . The electric field dependence of low-temperature conductance behaves as exp [ − ( ξ 0 / ξ ) 1 / 2 ] at high electric field ξ at sufficiently low T . Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at T = 1.7 K . Comparison with theory indicates that our data are best explained by Coulomb-gap variable-range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant.

Observation of giant positive magnetoresistance in a Cooper pair insulator.

Ultrathin amorphous Bi films, patterned with a nanohoneycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper pairs (CP) of electrons between localized states. Here, we show that the magnetoresistance (MR) of this Cooper pair insulator (CPI) phase is positive and grows exponentially with decreasing temperature T, for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.

Fabrication of anodic aluminium oxide templates on curved surfaces

Aluminium anodization provides a simple and inexpensive way to obtain nanoporous templates with uniform and controllable pore diameters and periods over a wide range. Moreover, one of the interesting possibilities afforded by the anodization process is that the anodization can take place on arbitrary surfaces, such as curved surfaces, which has not yet been well studied or applied in nanofabrication. In this paper, we characterize the anodization of Al films on silicon substrates with a curved top surface. The structures of the resultant anodic aluminium oxide (AAO) films are examined by scanning electron microscopy. Unique features including cessation, bending, and branching of pore channels are observed in the curved area. Possible growth mechanisms are proposed, which can also contribute to the understanding of the self-organization mechanism in the formation of porous AAO membranes. The new structures may open new opportunities in optical, electronic and electrochemical applications.

Postgrowth processing of carbon nanotube arrays-enabling new functionalities and applications

Highly ordered carbon nanotube arrays were fabricated by pyrolysis of acetylene using anodic-aluminum-oxide templates. To avoid the natural tendency of the nanotubes sticking together and forming haystack-like bundles when exposing the nanotubes from the growth template, a new postgrowth treatment process using a mixture of 6 wt% phosphoric acid and 1.8 wt% chronium oxide as the etchant, and 0.1 wt% Gum Arabic or 5 wt% polymethacrylic acid as the dispersant, was developed yielding for the first time well aligned and spatially free-standing carbon nanotube arrays. The dispersants can be adsorbed on both the surface of carbon nanotubes and that of the alumina film resulting in the elimination of aggregation of exposed carbon nanotubes, a more uniform dissolution of alumina, and a lower, thereby, more controllable etching rate. The as-prepared carbon nanotube arrays, which are vertically aligned and well separated could be used for many applications such as mechanical oscillators, field emission, and sensors, and the exposed nanotubes offer a good platform for study on collective behavior of electrical and magnetic nano arrays.

Spontaneous formation of ordered nanocrystal arrays in films evaporated onto nanopore array substrates

We have investigated the structure of films thermally evaporated onto anodic aluminum oxide substrates that are patterned with a hexagonal array of 34 and 80 nm diameter holes spaced by 100 nm. Over a range of film thicknesses, Pb and Sn films spontaneously self-assemble into an array of nanocrystals in registry with the underlying hole lattice. The development of the arrays with thickness indicates that surface energies drive coalescing grains to move over the holes. Materials that wet the substrate (Pd, Ge) or whose grains do not coalesce at the substrate temperature (Au) do not form arrays. Potential applications are discussed.

Temperature-dependent studies of Y-junction carbon nanotube electronic transport

Electronic transport in Y-junction carbon nanotubes was examined as a function of temperature. Rectifying behavior was observed from 10to300K with the conductance displaying power-law dependencies on temperature and applied bias. The data are consistent with models for tunneling between two tube segments but display asymmetric exponents depending on bias polarity. The conductance asymmetry is found to be essentially independent of temperature and can be understood from the lack of inversion symmetry of the Y-junction geometry, which creates a unique type of mesoscopic rectification for current flow in the nonlinear response regime.