Copper nanocrystals have been grown on thin polypyrrole films obtained by electropolymerization on a gold electrode from CuSO 4 solution electrochemically in both potentiostatic (constant potential) and galvanostatic (constant current) modes. A variety of copper nanostructures including fractals, nanowires, and cubic nanocrystals have been observed in the galvanostatic mode, in contrast to a single predominant type of nanostructures obtained by manipulating the under-peak potential (fractals) or over-peak potential (cubic nanocrystals) in the potentiostatic mode. The homogeneous distribution of nanocrystals observed at overpeak potential is consistent with an instantaneous growth mechanism. Depth profiling by X-ray photoelectron spectroscopy further reveals the presence of an ultrathin copper oxide layer on the surface of these nanocrystals.
In the present work, self-assembled nanostructures of copper are grown by electrodeposition on a thin conducting polymer (polypyrrole) film electropolymerized on a gold electrode. The shapes, sizes and the densities of the nanostructures are found to depend on the thickness of the polypyrrole thin film, which provides an easy means to control the morphology of these nanostructures. In particular, for the same applied potential on the gold electrode, smaller nanocrystals with a higher density are observed on thinner polymer films while bigger nanocrystals at a lower density are found on thicker films. The generation of nanostructured materials on a surface is a new thrust in materials science due to the continuing miniaturization of electronic and mechanical devices. In general, feature sizes 30 -300 nm and larger are routinely produced by electron-beam and photolithography techniques, respectively. Important progress has been made over the past few years in the preparation of ordered ensembles of metal and semiconductor nanocrystals to fabricate feature size less than 30 nm [1,2]. Devices fabricated entirely from polymers are now available, opening up the possibility of adapting polymer processing technologies to fabricate inexpensive, large-area devices using non-lithographic techniques [3,4]. Nanostructured materials have attracted much recent attention due to their important roles in many technological areas such as heterogeneous catalysis [5], photonics [6], single electron and quantum devices [7]. The development of nanotechnology requires good understanding of the evolution of the shape, size and distribution of nanoparticles in different growth processes [8,9]. Nanoparticles have been commonly grown in physical [10,11] and electrochemical processes [12 -16]. In the physical deposition process, the deposition rate and surface diffusion coefficient determine the shape, size and density of the nanoparticles on a specific substrate surface [17]. On the other hand, the concentration and pH of the electrolyte as well as the applied potential all play a prominent role in the electrochemical deposition process [18]. The use of an STM tip to develop copper nanostructures on a single-crystal gold electrode electrochemically has also been demonstrated [19]. Two common growth modes can be defined either as progressive or instantaneous, in the electrochemical deposition process, depending on the surface energies of the substrate and the deposited materials as well as their interface energy. In the Volmer -Weber growth process, the growth mode is instantaneous due to the large difference in the surface energies of the substrate and the deposited material [20]. The number of active nucleation sites is proportional to the applied potential because the nucleation barrier becomes lower with increasing applied potential [21].Organic conducting polymers, particularly in the form of thin films, are attractive candidates for microelectronic applications [22,23] due to their unique combination of electrical, mechanical ...
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