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Metal−CdSe−metal (metal = Au, Ni) nanowires were grown by electrochemical replication of porous aluminum oxide and polycarbonate track etch membranes with pore diameters of 350 and 70 nm, respectively. The lengths of the individual segments of the nanowires were controlled by varying the amount of charge that was passed. The composition of the CdSe segments was characterized by energy-dispersive X-ray spectroscopy. A 1:1 ratio could be obtained, and Cd- and Se-rich stoichiometries were also made by adjusting the concentrations of Cd²⁺ and SeO2 in the aqueous plating solutions. X-ray powder diffraction showed the presence of both zinc blende and wurzite phases, and grain sizes on the order of 10 nm were observed by TEM. The nanowires were resistive in the dark but showed pronounced visible light photoconductivity.

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)19ZnO/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)9TiO2/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.

We demonstrate that the electrical quality of junctions fabricated in lattice-mismatched In0.75Ga0.25As on InP grown by molecular beam epitaxy can be improved with the addition of in situanneals in the buffer layer that separates the substrate from the In0.75Ga0.25As device layers. Near infrared photodetectors fabricated using this material had dark current densities of approximately 2.5 mA/cm2at a reverse bias of 1 V, which is more than one order of magnitude smaller than commercially available photodetectors grown using vapor phase epitaxy. Transmission electron microscopy revealed that dislocations due to the lattice mismatch between the substrate and the epitaxial layer are confined primarily to the buffer layer for all samples studied. No significant differences in x-ray diffraction spectra or dislocation distribution were observed on samples with and without in situannealing. Atomic force microscopy indicated that all samples had a crosshatch pattern, and that the average surface roughness of the sample that contained in situanneals is a factor of three greater than the sample without in situanneals.