Electrochemical Fabrication of CdS Nanowire Arrays in Porous Anodic Aluminum Oxide Templates Dmitri Routkevitch, † Terry Bigioni, † Martin Moskovits,* ,† and Jing Ming Xu ‡ Department of Chemistry, Department of Electrical Engineering, and the Ontario Laser and LightwaVe Research Centre, UniVersity of Toronto, Toronto, Ontario M5S 1A1, Canada ReceiVed: September 29, 1995; In Final Form: March 3, 1996 X A technique is described for fabricating arrays of uniform CdS nanowires with lengths up to 1 μm and diameters as small as 9 nm by electrochemically depositing the semiconductor directly into the pores of anodic aluminum oxide films from an electrolyte containing Cd 2+ and S in dimethyl sulfoxide. The nanowire arrays were characterized by powder X-ray diffraction (XRD) and electron microscopy. The deposited material is found to be hexagonal CdS with the crystallographic c-axis preferentially oriented along the length of the pore. The effects of annealing on the crystallinity of the deposited semiconductor were investigated by XRD and resonance Raman spectroscopy. The deposition technique is, in principle, generalizable as a means of fabricating nanowires of a wide range of semiconductors. Introduction The large range of potential new materials applications made possible by the fabrication of uniform nanoscale structures has generated a tremendous amount of interest recently. The technological implications for the design and manufacture of new classes of optoelectronic and electronic devices, as well as for the emergence of new physics, have resulted in this field’s being one of the most active in the physical sciences today. Fundamental to its development is the evolution of novel techniques, among them electrochemical methods, for fabricat- ing uniform structures with dimensions in the nanometer range. Traditionally, electrochemistry has been used to grow thin films on conductive surfaces. Because the growth is controllable almost exclusively in the direction normal to the substrate surface, pseudo-0- and 1-dimensional nanostructures are realiz- able electrochemically only if the deposition is confined within the cells of an appropriate template. Electrodeposition has been used successfully for the formation of ceramic, 1a metallic, 1b,c and semiconductor 1d superlattices. Attempts were also made to use sequential underpotential electrodeposition to grow epitaxial compound semiconductor layers. 2 Anodic aluminum oxide (AAO) films grown in strong acid electrolytes possess very regular and highly anisotropic porous structures 3 with pore diameters, d p , ranging from below 10 to 200 nm, pore lengths, l p , from 1 to 50 μm, and pore densities in the range 10 9 -10 11 cm -2 . The pores have been found to be uniform and nearly parallel, 3 making AAO films ideal templates for the electrochemical deposition of fairly monodispersed nanometer-scale particles. Other porous films such as polymeric membranes, manufactured by etching nuclear tracks, have also been used. 4 It is now feasible to produce AAO films with pore sizes on the scale of the Bohr diameters of bulk semiconductor excitons, suggesting that quantum confinement effects might be observed in the radial, although not necessarily in the axial, direction. In using templates to produce nanostructures, one must take into account the template’s chemical stability, its insulating properties, the minimal diameter and uniformity of the pores, and the pore density. Pore sizes small enough to ensure the observation of quantum size effects in the deposited structures are not readily available with commercial anodic aluminum oxide membrane filters such as those produced by Anopore. The smallest mean pore diameter in nuclear track polycarbonate membranes used for the fabrication of nanowires has been reported to be 18 nm. 4 Recently, membranes with nominal 10 nm diameter pores have become available from Poretics and from other sources. These have been used to fabricate nano- electrode assemblies. 5 However, direct measurement of the diameter distribution function for metallic wires deposited into these pores determined 6 that the mean pore diameters of nominal 10 nm and the nominal 30 membranes were respectively 36 ( 3 and 57 ( 3 nm. The nominal size specified for these membranes corresponds, in fact, to the value of the smallest pores in the distribution, rather than to the mean. Additionally, the nuclear track pores are not parallel, and the pore density (6 × 10 8 cm -2 ) is substantially lower than in AAO films. The pore diameters and densities of anodic aluminum oxide films such as those described in this paper can be easily varied by changing anodization parameters such as the electrolyte used, its concentration, and the anodizing voltage U a . The main constraint in using porous alumina films directly after anod- ization is the insulating, dense oxide barrier layer separating the Al substrate and the porous portion of the aluminum oxide. The thickness of the barrier layer is a function of the anodizing voltage (10-14 Å/V). This imposes a limitation on the use of dc electrodeposition to fill the pores. However, the inherent rectifying properties of the barrier layer allow the pores to be filled uniformly by ac electrolysis without simultaneously depositing material on the surface or into the macroscopic defects of the film. The possibility of electrochemical (EC) deposition of certain metals into these pores resulting in the formation of the highly anisotropic, aligned particle arrays with unusual optical 7 and magnetic 8 properties has been known for some time. These particles faithfully reproduce the shape of the pores. 9 It was also demonstrated recently that porous templates can be filled electrochemically with semiconductor. Cadmium sulfide/se- lenide wires were deposited into the 0.2 μm pores of Anopore membranes, 10 and selenium nanotubules were synthesized in the 2.5 μm pores of nuclear track membranes. 11 In the present paper we report an electrochemical technique for fabricating CdS nanowire arrays based on single step ac electrolysis into the pores of AAO films. Although different in execution, the technique is similar in scope to those used by Chakavarti and Vetter 11 and by Sailor’s group. 10 However, the membranes used in refs 10 and 11 contained pores of consider- * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Electrical Engineering. X Abstract published in AdVance ACS Abstracts, July 15, 1996. 14037 J. Phys. Chem. 1996, 100, 14037-14047 S0022-3654(95)02910-8 CCC: $12.00 © 1996 American Chemical Society