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This atomic force microscopy image shows a nanopore array used as a template to make nanowires. The template shown is only 500 nanometers wide. |
Novel nanostructures wired into the future |
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Unique shapes built on the nanoscale—billionths of a meter—are opening new possibilities in areas as diverse as superconductivity, computer memory media, electrical and thermal transmission, micro-switching devices and highly sensitive free-radical detectors. Until recently, it’s been difficult to create the consistently reproducible nanostructures that researchers need to explore their properties and potential applications, because at this scale even a grain of salt resembles a mountain. But now, Argonne researchers have developed a process using anodized aluminum-oxide (AAO) membranes to synthesize both individual and aligned nanostructures with controlled sizes and shapes. They have built nanowires, nanotubes, nanodots and nanoantidots, or holes, from a range of materials, including lead, nickel, cobalt, bismuth, gold, silver, niobium, vanadium oxide and polymers. “It is a simple, reliable procedure that produces an ordered array with a high level of control,” said Wai-Kwong Kwok, principal investigator and leader of the Materials Science Division’s Superconductivity and Magnetism Group. The electrochemical anodization process starts with aluminum foils 0.25 to 1 millimeter thick. They are placed in an acid solution under a positive electric field and converted to AAO membranes through self-assembly. Their self-organized pore diameters and intra-pore distances can be adjusted by changing the voltage and acid concentration. Anodization time controls the membrane’s thickness, which limits the length of nanowires and nanotubes grown in the pores. The researchers have made AAO membranes with pore diameters ranging from 10 to 400 nanometers and with lengths up to 70 micrometers (millionths of a meter). Different processes are used with the arrays to produce different structures. Electrodeposition grows superconducting, magnetic and quantum nanowires directly into the nanopores of AAO. Wetting or electroless deposition methods—that is, coating methods other than electroplating, such as sputtering or evaporation followed by melting the deposited materials so they wet and coat the inside walls of the AAO nanopores—result in nanotubes that have novel properties. These tubes can be used as starting points for nanocomposites by filling them with other materials, possibly leading to new properties and applications. Sputtering or evaporation techniques lead to highly ordered arrays of magnetic and superconducting holes. Dot arrays are achieved by using AAO membranes thinner than 500 nanometers as shadow masks or by coating the barrier side of the AAO. Magnetic antidot and dot arrays are candidates for recording media with densities in terabits (trillion units of computer information; 1,000 gigabits) per square inch. “One key question is how small a sample can be and still retain its properties,” said Kwok. “We study fundamental effects, and if we find new phenomena, we look into possible applications. For example, in measuring the resistance of nanowire we find great sensitivity to single particles. It doesn’t take a lot of stress to change resistance. This is new science and suggests the possibility of very sensitive sensors.” For more information, please contact Richard Greb. Next: New section - World-Class Research Facilities |