July 07, 2011

Arrays of indefinitely long uniform nanowires and nanotubes

Nanowire array production from a macroscopic rod by iterative thermal size reduction. Step 1: A macroscopic cylindrical rod (diameter 10 mm, length 200 mm) is fabricated from the material that is to become nanowires upon axial alongation. An thermomechanically suitable polymer sheet (PES, PEI, PSU) is tightly rolled around the rod in a clean room until the final thickness is 30 mm. The composite is then thermally consolidated under vacuum, above the glass transition temperature, in order to fuse the polymer and the cylindrical rod. Finally the composite is drawn in a furnace to obtain hundreds of meters of microwires embedded in a polymer fiber matrix. Step 2: About 100 fibres with 0.5 mm diameters are cut into 200 mm fibres, tightly packed and a polymer cladding is rolled around the fibers, and consolidated; or alternatively cut fibres are inserted inside a pre-consolidated hollow polymer rod and consolidated. Second step drawing results in a submicron wire arrays in the cross section of the fibre. Step 3: The same procedure, followed in the second step is iterated, to obtain hierarchically positioned arrays of arrays of nanowires. The nanowire diameters at each step is geometrically reduced and at each step controlled by monitoring the enwrapping fibre diameter.

Nature Materials - Arrays of indefinitely long uniform nanowires and nanotubes A new nanofabrication technique based on iterative size reduction is able to produce ordered, indefinitely long nanowire and nanotube arrays. Nanowires that are more than 1000 meters long have been made.

Nanowires are arguably the most studied nanomaterial model to make functional devices and arrays. Although there is remarkable maturity in the chemical synthesis of complex nanowire structures their integration and interfacing to macro systems with high yields and repeatability still require elaborate aligning, positioning and interfacing and post-synthesis techniques. Top-down fabrication methods for nanowire production, such as lithography and electrospinning, have not enjoyed comparable growth. Here we report a new thermal size-reduction process to produce well-ordered, globally oriented, indefinitely long nanowire and nanotube arrays with different materials. The new technique involves iterative co-drawing of hermetically sealed multimaterials in compatible polymer matrices similar to fibre drawing. Globally oriented, endlessly parallel, axially and radially uniform semiconducting and piezoelectric nanowire and nanotube arrays hundreds of metres long, with nanowire diameters less than 15 nm, are obtained. The resulting nanostructures are sealed inside a flexible substrate, facilitating the handling of and electrical contacting to the nanowires. Inexpensive, high-throughput, multimaterial nanowire arrays pave the way for applications including nanowire-based large-area flexible sensor platforms, phase-changememory, nanostructure-enhanced photovoltaics, semiconductor nanophotonics, dielectric metamaterials,linear and nonlinear photonics and nanowire-enabled high-performance composites.
Low temperature, multimaterial fibre drawing tower used for the iterative size reduction of composite macroscopic rods down to nanowires and nanotubes. The 2.5 m tower consists of a thermally isolated double zone furnace, feeding, capstan, thickness, and tension meters units. Thermal size reduction takes place at a 2.5 cm hot region of the furnace under applied axial stress (T=50−300 g). Nanowire diameter is controlled using the polymer fibre diameter and tension as a feedback to the down-feed, capstan and furnace temperature controllers. Polymer fibre thickness is measured with a laser micrometer and fibre tension by a transverse tensiometer. Smaller reduction factors (10−20×) are obtained at relatively lower temperatures (260−270 °C) and smaller drawing speeds (0.05 m/min). Higher reduction factors (50−200×) are obtained with higher temperatures (280−300 °C) and higher drawing speeds (1−5 m/min). 10 kilometer-long continous nanowire array is produced from 30-cm long macroscopic rod with a reduction factor 200.

Ordered selenium nanowire arrays obtained by three step iterative size reduction. Selenium is a glass making chalcogenide element with a crystallization temperature of 110 °C and melting temperature of 230 °C. Therefore it is molten at the drawing temperature of 270 °C. XRD diffraction studies indicate that Se is amourphous after each thermal drawing/size reduction. a-b, In the first step a 10 mm Selenium rod is reduced to 250-50 μm single wire (reduction factor 40-200x). Extracted microwires retain their global alignment . c-d, For the second step, ~100 selenium wires are cut and tighly packed and redrawn to 7 μm (reduction factor 34×) to obtain hundreds of meters long ordered selenium microwires. e-f, In the final step, previously obtained selenium microwire arrays are cut, packed and redrawn to 250 nm hierarchically ordered nanowire arrays (reduction factor 28×, total reduction factor is 40,000×).

Within this pencil-wound, 10-meter stretch of nanowire are hundreds of even smaller wires (shown in cross section). Such wires-within-wires might have broad uses in electronics, sensors and other devices.
Credit: M. Yaman et al/Nature Materials 2011

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