Fabrication, Characterization and Applications of Magnetic Nanowire Arrays

Open Access
Author:
Sharma, Gaurav
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 25, 2005
Committee Members:
  • Craig A Grimes, Committee Chair
  • Michael V Pishko, Committee Member
  • Elizabeth C Dickey, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • Manetic Nanowires
  • Electrodeposition
  • Nanotechnology
  • Nanoporous Alumina
  • Non-Biofouling
  • Uniform Array
  • Ternary Alloy
Abstract:
Fe-Co-Ni ternary alloy nanowire arrays 32–106 nm in diameter are fabricated within nanoporous alumina membranes using 15 Vrms alternating current electrodeposition at frequencies of 50, 250, 500, 750, and 1000 Hz. The alumina membranes, 10-15 microns thick, are synthesized by anodization of aluminum foil using a two-step technique to increase pore uniformity. The alumina membrane structure is tailored with the end aim being uniform magnetic nanowire electrodeposition. Using an electrodeposition frequency of 1000 Hz, 15 Vrms, consistently and repeatably yield nanowire arrays over membranes several cm2 in extent. Electrochemical Impedance Spectroscopy (EIS) is used to explain the effects of AC electrodeposition frequency. The impedance of the residual alumina barrier layer, separating the underlying aluminum metal and the nanoporous membrane, decreases drastically with electrodeposition frequency facilitating uniform pore-filling of samples several cm2 in area. The magnetic coercivity and hysteresis loop squareness-ratio (Mr /Ms ) were studied as functions of electrolyte composition, nanowire diameter, and nanowire aspect ratio. Anodic polarization studies on thin films having alloy compositions identical to the nanowires display excellent corrosion resistance properties. Two potential applications of the nanowire arrays are investigated. Iron nanowire arrays, oriented perpendicular to the substrate, are fabricated by electrodeposition of iron in nanoporous alumina membranes, followed by precise wet etching of the alumina membrane to partially expose the nanowire array. It is shown that oxidation of standing iron nanowire arrays, at 600 °C in an oxygen ambient leads to standing a-Fe2O3 (hematite) nanowire arrays. These hematite nanowire arrays show a distinct photocurrent response and can be used as photocatalysts. Second, protein adsorption studies on standing Fe-Co-Ni nanowire arrays and flat Fe-Co-Ni thin films show that nanowire array morphology leads to attenuation of protein adsorption. Since protein adsorption is the first step in the biofouling process magnetic nanowire arrays can potentially find applications as non-biofouling surfaces. Placing magnetic nanowire arrays in a sinusoidally varying magnetic field leads to their oscillation that further reduces protein adsorption in comparison to stationary nanowire arrays.