Nanopores and nano wires: a challenge for new properties and applications

Filling the nanosized holes of polymer membranes with various combinations of metals or conductive polymers can produce nanocomposite materials with tailor-made electrical, magnetic or chemical properties. As sophisticated filter devices, these have many potential applications such as use in shielding microwaves ovens and mobile phones in artificial noses or in lab-on-a-chip. Nanocomposites are materials which contain one or more components in shapes such as cylinders, rods or networks with nano-size dimensions. The enormous versatility of possible combinations of such composites, which can have both electrical conductivity and non-conductivity components, offers a real possibility of obtaining materials that combine metal-like properties with non-metallic characteristics. In addition, changes in the features and type of nanostructured component can substantially affect the composite's electrical and chemical properties. If a thin polymer film such as polycarbonate is bombarded with energetic heavy ions (accelerated by a cyclotron), holes are burned out and after track-etching a membrane is produced. As filters, most membranes rely on several different mechanisms to enhance contaminant removal. Track etched membranes, in contrast, capture particles by simple, direct sieving, so particles larger than the pore size cannot pass through. The existing production technology for these membranes has been significantly enhanced to allow the reliable and reproducible manufacture of nanoporous membranes with nanosized holes. They are currently used within the ultra-filtration market as molecular filters for biological applications such as water purification. Now these polymer membranes can be made with a well-controlled pore size and shape in the form of regular, periodic arrays. The nanopores can be filled with another organic or inorganic material, such, as a polymer or metal. If a different material, such as an electrically conductive polymer, is deposited in the holes, nanotubules with enhanced conductivity are obtained. Filling the nanoholes with a magnetic material such as iron, cobalt or nickle - or a combination of these metals - by electrochemical deposition produces a membrane with nanoparticles or wire of either one metal or with multi-layers of different metals. Various combinations of polymers and metals are produced and characterised with the aim of using such membranes to design devices in which the electrical and magnetic properties can be studied controlled and so optimised. The nanoparticles produced by using these templates take the form of rods or tubes. The metal nanowires embedded in these polymer films are regularly spaced and densely packed. Since the valence and degree of their electrical insulation and magnetism is controllable and can be fine-tuned, they have good potential for use in high-density data storage. They also absorb certain wavelengths of electromagnetic radiation - similar to those use in microwave ovens and communications. Consequently, potential applications include sophisticated devices such as microwave filters for making shields for microwave ovens and mobile phone. The deposition of conductive polymer nanotubules in the membrane will lead to the preparation of chemical detectors (artificial nose) and biosensors. (Author)