Layer-by-layer assembly of rectifying junctions in and on metal nanowires

Alumina membranes containing 200 nm diameter pores were replicated electrochemically with metals (Au and Ag) to make free-standing nanowires several microns in length. Wet layer-by-layer assembly of nanoparticle (TiO2 or ZnO)/polymer thin films was carried out in the membrane between electrodeposition steps to give nanowires that contained rectifying junctions. Concentric structures with similar properties were prepared by first coating the membrane walls with multilayer films, and then growing nanowires inside the resulting tubules, or by growing films on the exposed surface of the nanowires after dissolution of the membrane. The I-V characteristics of nanowires prepared by either technique show current rectifying behavior. The electronic properties of Au(MEA)/(ZnO/PSS)(19)ZnO/Ag (MEA = mercaptoethylamine) devices indicate that rectification is determined by charge injection at the metal/ZnO/PSS-film interface rather than by a tunneling mechanism. In the case of Ag(TiO2/PSS)(9)TiO2/Au devices, switching behavior and hysteresis that could not be described by Schottky or Fowler-Nordheim characteristics was found. The combined replication/layer-by-layer synthetic approach allows one to control both the geometry and the chemical composition of diode nanowire devices.