Low‐Temperature Synthesis of Large‐Area, Free‐Standing Nanorod Arrays on ITO/Glass and other Conducting Substrates

Abstract

Arrays of free-standing nanorods or nanowires are of great interest for applications in data storage, catalysis, sensing, field emission, and optoelectronic devices. [1][2][3][4] Hybrid solar cells, for example, attempt to combine nanostructured semiconducting metal oxides with organic semiconductors to produce efficient photovoltaics at low cost. An ideal architecture proposed for these cells consists of an array of semiconducting nanorods surrounded by a charge-transporting polymer, where the interfacial distance is smaller than the exciton diffusion length in the polymer (approximately 10 nm), resulting in efficient exciton separation at the interface. [4][5][6] The development of such devices has so far been hindered by the inability to reliably produce nanostructures on appropriate substrates, including transparent conducting oxides and crystalline semiconductors. A general method is presented here for producing large-area arrays of free-standing nanorods on supporting substrates. The inclusion of titanium and tungsten adhesive layers has permitted the fabrication of anodic alumina thin film templates of unprecedented quality on indium tin oxide (ITO)/glass, silicon, and flexible substrates over 2 cm 2 areas, of interest for devices. For the first time, free-standing nanorods of a variety of oxides and metals have been reproducibly synthesized over large areas on ITO/glass substrates by electrochemical deposition into the vertically-aligned nanopores of the templates, followed by template removal. A number of techniques have been used to fabricate nanorod arrays, including template synthesis, various forms of vapor deposition, chemical solution growth, electrochemical deposition, and sub-micrometer lithography. [2,7] Vapor deposition and lithographic methods are restricted by slow, expensive, high-vacuum processes, and chemical solution [5] and electrochemical [8,9] growth of free-standing arrays have largely been limited to certain materials, notably zinc oxide. A variety of oxide semiconductors have been proposed for use in nanostructured solar cells and many different metallic, semiconducting, and magnetic materials are of interest for data storage, sensing, catalysis and field emission devices, such that a more universal method is desirable. The most versatile method for producing nanorods in the form of large-area arrays has been the replication of patterns in templates, using filling methods such as pressure injection, electrochemical deposition, and capillary filling with sol-gels. [7] Electrodeposition, in particular, is advantageous for electronic applications as it ensures that there is electrical contact between the deposited nanorods and underlying electrode. It is a low-temperature, inexpensive, scalable technique, which allows growth of a wide variety of metals and semiconductors. [2,[10][11][12] Both anodic aluminum oxide (AAO) and block copolymer [13] templates can be produced with self-assembling, vertically-aligned pores over a large area for nanorod synthesis. Anodic alumina templates in p...

Strong Efficiency Improvements in Ultra‐low‐Cost Inorganic Nanowire Solar Cells

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“…Figure 2 shows a detailed plot of current density versus time (J-t) for the anodization at 60 V in 0.1 M oxalic acid. 0.1 M oxalic acid is used in this study instead of the more common 0.3 M oxalic acid 25 since the anodization process is too fast to stop in time when using the more concentrated acid for anodizing thin Al film. During the first few seconds, a high current is detected while the surface of Al is oxidized.…”

“…25 who overcame these problems by inserting adhesive layers (W and Ti), allowing for the fabrication of large-area free-standing Ni, Cu, Cu 2 O and TiO 2 nanowire arrays. However, fabricating large-area Ag nanowire arrays on ITO glass has been a great challenge, due to poor nucleation on both the pure and W/Ti modified surfaces, resulting in extremely uneven filling of the template.…”

High-Efficiency Electrodeposition of Large Scale ZnO Nanorod Arrays for Thin Transparent Electrodes

“…For the fabrication of ultrathin alumina templates, 10 nm thick Ti and 30 nm thick W adhesion layers were sputtered on ITO in order to relieve the structural stress and to permit a slow oxidation process under the anodization conditions. 23 Further, a 500 nm layer of aluminum is provided on the substrate by electron-beam evaporation (BOC Edwards) by applying 80 mA current for 1 minute.…”