Multi-electrolyte-step anodic aluminum oxide method for the fabrication of self-organized nanochannel arrays
© Chen and Chen; licensee Springer. 2012
Received: 22 November 2011
Accepted: 14 February 2012
Published: 14 February 2012
Nanochannel arrays were fabricated by the self-organized multi-electrolyte-step anodic aluminum oxide [AAO] method in this study. The anodization conditions used in the multi-electrolyte-step AAO method included a phosphoric acid solution as the electrolyte and an applied high voltage. There was a change in the phosphoric acid by the oxalic acid solution as the electrolyte and the applied low voltage. This method was used to produce self-organized nanochannel arrays with good regularity and circularity, meaning less power loss and processing time than with the multi-step AAO method.
In recent years, nanochannel arrays with periodic structures have been fabricated by various processes for application in several types of optical devices, such as in optical sensors , 2D photonic crystals , carbon nanotube field emission displays , nanowires [4–6], LED , and nanophotonics . The anodic aluminum oxide [AAO] fabrication technique is one of the key methods for the fabrication of nanochannel arrays [9–11]. AAO nanochannel arrays with an interpore distance ranging from 50 to 420 nm have been obtained by anodizing aluminum in sulfuric, oxalic, and phosphoric acid solutions . The advantages of the AAO process are the large area, high aspect ratio, simple process, and low cost. The self-organized multi-step AAO method has been applied for the fabrication of AAO nanochannel arrays with high uniformity [13–15]. The initial thickness of aluminum used in the multi-step AAO method is greater than 6 μm, especially for high bias voltage, large interpore distance nanochannel arrays. However, if the initial aluminum is the thin-film type, it is very difficult to deposit an aluminum film thicker than 10 μm without defects in the structure. In this paper, a novel process, the self-organized multi-electrolyte-step AAO method, is proposed for the growth of nanochannels with high and low applied voltages in the phosphoric and oxalic acid, respectively. This method can achieve nanochannel arrays with good regularity and circularity with less power loss and processing time than with the multi-step AAO method.
Before the AAO process, we have to make sure that the surface roughness of the aluminum foil is small enough for the growth of the nanochannel arrays . High purity (99.99%) aluminum sheets were degreased in 5% NaOH for 30 s at 60°C and cleaned in a 1:1 volume mixture of nitric acid and deionized water. The aluminum was subsequently annealed at 400°C for 3 h and then electropolished in a mixture of H2SO4:H3PO4:H2O (ratio 2:2:1) at room temperature under a constant input current. After about 30 min of polishing, the mean roughness of the polished surface was measured by atomic force microscopy. The surface roughness was found to be reduced to approximately 3 nm in a 3-μm2 scan area. The aluminum sheet was then mounted on a stainless steel that served as the anode. A graphite bar was used as the counter electrode. The anodization conditions involved different acid solutions as the electrolyte and different applied voltages under a temperature of 3°C. During the anodization process, an Al2O3 layer formed easily on the surface of the aluminum. Al3+ and O2- ions were dissociated due to the adding bias. Al3+ reacts with the acid solution to make sure that Al3+ will not combine with O2-, and an Al2O3 layer again formed on the surface of the aluminum. The repetition of the oxidization reaction and removal formed the self-organized nanochannels following the electrofield from the surface to the bottom of the substrate.
Multi-step and multi-electrolyte-step AAO methods
AAO quality analysis
Parameters of the self-organized nanochannel arrays
approximately 2 μm
approximately 6 μm
approximately 2 μm
approximately 30 min
approximately 72 W
approximately 216 W
approximately 39 W
Circularity ± Δ
0.85 ± 0.1
0.87 ± 0.05
0.87 ± 0.05
Period ± Δ
203 ± 35 nm
206 ± 15 nm
202 ± 20 nm
In this study, nanochannel arrays are fabricated by the self-organized multi-step and multi-electrolyte-step AAO methods. The results show that, with the three-step and multi-electrolyte-step AAO methods, we can achieve better circularity distribution at about 0.87. However, the initial thickness of aluminum for the multi-electrolyte-step AAO method is about 2 μm, which is thinner than the thickness for the multi-step AAO method, 6 μm. Besides, the multi-electrolyte-step AAO method has the advantages of shorter processing time, high quality of nanochannels, and low applied power. Finally, self-organized nanochannel arrays fabricated by the multi-electrolyte-step AAO method show good circularity, large average diameters, and similar periods to those fabricated with the multi-step AAO method.
The authors would like to thank the National Science Council of Taiwan for the financial supports for this research under contract nos. 100-2120-M-008-002, 100-2627-E-008-001, and 100-2221-E-008-111.
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